Jig, fixture & guages theory

TRAINER GUIDE - II

JIGS, FIXTURES & GAUGES

(1
ST
SEMESTER)

PGTD / PDTD

VERSION - 0

MSME TOOL ROOM
INDO GERMAN TOOL ROOM
AHMEDABAD

TRAINER GUIDE -II
FOR
ADVANCE DIPLOMA IN
TOOL & DIE MAKING
Subject Area:
Tool Design Theory –
“Jigs, Fixtures & Gauges”
( Part –I & II)

CONTENTS
Chapter
No.
DESCRIPTION
Page
No.
A1. Introduction To Production Toolings
A1.1 Introduction Of Tools Used In Mass Production…………...1
A2. Introduction To Jigs & Fixtures………………………………… 4
A3 Elements Of Jigs & Fixtures
A3.1 Locators, Locating Methods & Devices……………………. 7
A3.2 Clamps, Clamping Methods & Devices……………………. 35
A3.3 Guiding Elements (Jig Bushings)…………………………... 60
A3.4 Tool Bodies (Jig & Fixture)………………………………….. 81
A3.5 Fasteners (Jig & Fixture)……………………………………. 84
A4 Limit, Fit & Tolerance
A4.1 Introduction…………………………………………………… 96
A4.2 Advantages Of Limits & Fits………………………………… 97
A4.3 Tolerances …………………………………………………… 98
A4.4 Limits………………………………………………………….. 100
A4.5 Fits…………………………………………………………….. 101
A4.6 Types Of Assembly………………………………………….. 108
A4.7 Allowances …………………………………………………… 111
A4.8 Deviation ……………………………………………………... 112
A4.9 Maximum & Minimum Material Condition…………………. 115

CONTENTS
Chapter
No.
DESCRIPTION
Page
No.
A5 Design
A5.1 Design Of Jigs & Fixtures…………………………………… 118
A6 Jigs
A6.1Introduction…………………………………………………… 137
A6.2 Function Of Jigs & Fixtures…………………………………. 138
A6.3 Factor Characteristics In Jig Design……………………….. 138
A6.4 Jig Support……………………………………………………. 140
A6.5 Jig Bodies And Rigidity……………………………………… 140
A6.6 Classification Of Jigs………………………………………… 140
A6.7 Types Of Jigs & Their Description…………………………. 151
A6.8 Maintenance, Storage & Safety Of Jigs…………………… 152
A7 Fixtures
A7.1 Introduction…………………………………………………… 155
A7.2 Basic Design Consideration………………………………… 155
A7.3 Factors In Fixture Design…………………………………… 156
A7.4 Classification Of Fixture…………………………………….. 158
A7.5 Maintenance, Safety & Storage Of Fixtures……………….181
A8 Estimation
A8.1 Introduction…………………………………………………… 183
A8.2 Purpose Of Cost Estimating………………………………… 183
A8.3 Elements Of Cost…………………………………………….. 184
A8.4 Cost Structure………………………………………………… 187
A8.5 Estimation Of Cost Elements……………………………….. 188
A8.6 Estimating Tool Cost………………………………………… 192

Chapter
No.
DESCRIPTION
Page
No.
A8.7 Steps In Making A Cost Estimation………………………… 194
A8.8 Chief Factures In Cost Estimation…………………………. 194
A8.9 Numerical Examples………………………………………… 195
A9 Gauges
A9.1 Introduction…………………………………………………… 199
A9.2 Classification Of Gauges……………………………………. 201
A9.3 Design Of Gauges…………………………………………… 219
A9.4 Sub Zero Treatment…………………………………………. 229
A9.5 Maintenance, Safety & Storage Of Gauges………………. 229
A9.6 Numerical Examples………………………………………… 230
\

INTRODUCTION
A1.1 Introduction of Tools used in Mass Production
Production of quality goods in large quantities at high speeds is the requirement of
the day. To meet this, there have been considerable changes and developments in the
manufacturing industries, with an empha sis on increased efficiency and productivity. As a
sequel to these changes the tool technology has also undergone changes, leading to the
designing and development of special tools, methods and techniques for the benefit of
industry, to ensure quality products at economical rates.
Jigs and fixtures are the special production tools which make the standard machine
tool, more versatile to work as specialised machine tools. They are normally used in large
scale production by semi -skilled operators, however t hey are also used in small scale
production, when interchangeability is important. Manufacturing industries in India, on par
with their counterpart elsewhere, have brought lot of revolution in manufacturing technology,
during the (last 20 years, as a consequence of which several developments like CNC
Lathes, CNC Machine Centers, Flexible Manufacturing Systems, Fabrications Centre,
Transfer Machines, Robotics, etc. took place). Our Engineers and Technologists are deeply
involved in devising innovative 7 techniques. Lot of modernisation has taken place in Indian
Industry. Even with these advancements in the manufacturing indsutries, there is a
continued use of jigs and fixtures in some form or the other either independently or in
combination with other systems.
CHAPTER OUTLINE
A1.1 –Introduction of tools used in
Mass Production
TOPIC OUTLINE
A1.1a Jigs
A1.1b Fixtures
A1.1c Gauges
A1.1d Press Tools
A1.1e Moulds

The work tooling refers to the hardware necessary to produce a particular product. The most
common classification of types of tooling is as follows :
1. Sheet metal press working tools.
2. Moulds and dies for plastic moulding and die casting.
3. Forging dies for hot and cold forging.
4. Jigs and fixtures for guiding the tool and holding the workpiece.
5. Gauges and measuring instruments.
6. Cutting tools such as drills, reamers, milling cutters, broaches, taps etc.
The tool maker manufactures the above item from the design supplied to him. On gaining
experience the tool maker will be able to design and manufacture simple tools.
A1.1a Jigs
A jig is a device that locates and holds the workpiece. It also guides and controls
one or more c utting tools. Jigs are fitted with hardened steel bushings for guiding drills or
other tools. Small jigs are not usually clamped to the machine. For holes above 6mm jigs
are usually clamped. Drill jigs are used while drilling reaming counter boring, tapping,
chamfering etc.
There is hardly a product produced that does not contain one or more holes. The
location finish and size of these holes may be critical as in the case of a component for a
missile or they may be holes like those punched in a templat e for the purpose of hanging it
on the wall when not in use.
Holes are produced and finished in a number of ways. They are drilled, reamed,
bored, punched, ground, flame cut etc. Drilling is by far the most common method.
A1.1b Fixtures
A fixture is a device that locates and holds the workpiece. Setting blocks and feeler
gauges are used for setting the cutter in relation to the workpiece. Fixtures designated for
machining operations always clamped on to the machine.
A fixtures is a device for holding a workpiece during machining operations. The
name is derived from the fact that a fixture is always fastened to a machine or bench in a
fixed position.
Many machining operations can be performed by clamping the workpiece to the
machine table without using a fixture, especially when a few parts are to be machined.

However when the number of parts is large enough to justify its cost, a fixture is used for
holding and locating the work. Further, when the profile of the Component is not regular or
when machining has to be done w.r.t. a reference face or bore, application of fixture will be
necessary.
A1.1c Gauges
Modern manufacturing requires extensive uses of gauges for shop work, inspection
and reference.
A gauge can be defined as a device for investigating the dimensional fitness of a part
for a specified function.
A1.1d Press Tools
Press tools are special tools custom built to produce a particular component mainly
out of sheet metal.
The principle op erations of sheet stampings include cutting operations (Shearing,
blanking, piercing etc.) and forming operations (bending, drawing etc.).
Sheet metal items such as automobile parts (roofs fenders, caps etc.) components of
aircraft, parts of business mach ines, household appliances, sheet metal parts of electronic
equipments, precision parts required for horological industry etc. are manufactured by press
tools.
A1.1e Moulds for Plastics
Plastics did not enter our lives with the fanfare of other revolu tionary inventions, but
more by the process of infiltration. Plastics being synthetic materials were at first considered
to be cheap substitute for the better known and more expensive materials. Plastic articles
are not only replacing wood, metal and other materials but because of their particulars
qualities they function better than other materials for specific purposes. Through the years
plastics have carved the right as materials themselves and not as substitute for other
materials. Not only are plastics more useful, adaptable and practical than the materials they
have supplemented, but uses for plastics have been found for which no other material can
be used.

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Maximum productivity at minimum cost is the demand of modern industry. To meet this
requirements designing of efficient and accurate jigs and fixtures is required. Quality,
simplicity and economy from the important criteria from the design of jigs and fixtures.
To meet this requirement the designer will have to made an economic analysis for using
jigs and fixtures and has to device certain principles of design, and finally develop a checklist
for the jigs and fixture design.
A5.1a Tool Design Objectives
The main objective of tool design is to lower manufacturing costs while maintaining
quality and increased production. To accomplish this, the tool designer must satisfy the
following objectives:
Design Economics
Maximum productivity at minimal cost is t he demand of the day. Tool designer has
therefore an additional consideration of keeping the cost of these special tools as low as
possible apart from developing designs for efficient and accurate jigs and fixtures. For this
he has to apply the design economy. i.e. to reduce the cost without sacrificing the quality.
The following are some of the considerations involved in the economy design.
1. Simplicity
2. Preformed components
3. Standard components
4. Secondary operations
5. Tolerance and allowances
6. Simplified drawings
TOPIC OUTLINE
A5.1a Tool Design Objectives
A5.1b Design Principles
A5.1c Major factors in design of jigs & fixtures
A5.1d Elements of design (jigs & fixtures)
A5.1e Flow chart for development of design solution
A5.1f Check list for the design of jigs & fixtures
CHAPTER OUTLINE
A5.1 Design of Jigs & Fixtures

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1. Simplicity
Simplicity is essential in the tool design. Every element in the design of jigs and
fixtures should be considered for possible savings in time and materials.
2. Preformed Materials
These materials greatly reduce tooling costs by the elimination of many machining
operations. Wherever practicable, preformed materials, such as drill rods, structural
sections, pre machined bracket materials etc. should be included in the design.
3. Standard Components
Commercially available standard components such as clamps, locators, supports,
drill bushings, pins, screws, bolts, nuts etc. would contribute greatly in improving the tool
quality besides effecting considerable savings in labour cost and time.
5. Secondary Operations
Secondary operations such as grinding, heat treating and some machining should be
as far as possible be eliminated as they involve additional time and cost. If they cannot be
totally eliminated they should be limited to areas necessary forefficient tool operations.
5. Tolerances and Allowances
Generally the tolerances of a jig or a fixture should be between 20 percent and 50
percent of the part tolerance, as unnecessarily close tolerances will be add up to the higher
cost of the tool.
6. Simplified Drawings
Tool drawings will for a sizable part of the total tooling cost, hence it is necessary to
keep them low. This is accomplished by simplifying the drawings as follows :
a) Wherever practicable words should replace drawn details.
b) Elimination of redundant views, projections or details.
c) When possible, replace drawn details with symbols.
d) Reduce the drawing time by using templates and guides.
e) Standard parts should only be drawn for clarity, not detail refer to these by part numbers
or named.

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A5.1b Design Principles
After the economic and design analysis the tool designer must comply with the
following in designing the jigs and fixtures.
1. He has to thoroughly understand the component details, its pre -machined conditions,
reference surface dimensions, accuracies and tolerances to be achieved.
2. He has to know on which machine the operations is likely to be performed. A check has
to be made for constraints on the design parameters.
3. He has to make provision for easy loading and unloading of the work piece.
4. Facility for quick and accurate positioning of work piece be provided.
5. Fool proof method has to be incorporated to avoid wrong position while loading the work
piece.
6. Designer has to take into account the optimum clearance with swarf removal and
cleaning facility.
7. Machined surface are be taken as locating surface preferably.
8. Sharp corners in the locating surface must be avoided.
9. Adjustable locations are to be provided for right surfaces.
10.Locating surfaces should be as small as possible.
11.Locating pins should be tapered and easily accessible and visible to the operator.
12.Designer must have the economic approval to the design considerations.
13.As many degrees of freedom of movement should be arrested as necessary to achieve
the required accuracies. In general 3-2-1 principle to be adopted.
i.e.
3Points in the first plane
2Points in the second plane
1Points in the third plane
14.Make the layout always to a scale, whenever possible.
15.The use of standard items in clamping, locating and fastening elements should be made,
whenever possible.
16.Total engineering data in the drawing to be provided. I.e. material, heat treatment of the
component, geometrical accuracies, toler ances, surface roughness for manufacturing
and inspection purpose etc.
17.Stress to be given to minimise the weight of the jig or fixture for easy handling and to
reduce the fatigue on the operator.
18.Care has to be taken for providing suitable support or guidance for preventing work piece
bending or movement while operation and clamping.

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19.Attention to be given to tightening up of loose items of jig or fixture.
20.In jig and fixture layout a distinction between work piece and jig or fixture component to
be brought by means of chain dotted lines for work piece and full lines for jig and fixture
components.
21.Provision to be made for the setting gauges in fixture.
22.Machining table mounting requirements are to be considered while designing.
23.Bill of material to be provided.
The good design of jigs / fixtures is that which satisfies the following
a) Functional aspect
b) Quality
c) Cost
d) Production schedule
e) Safety
f)Adaptability to the machine
A5.1c Major Factors in the Designof Jigs & Fixtures
In planning jigs and fixtures, it is essential to consider three major factors, which have
a definite influence upon the design of tools.
i) The tool should be designed for efficient operation and for easy manipulation by the
operator.
ii)The tool should be designed so that it will be produce accurate workpieces on a
repetitive basis.
iii)The cost of the tool should usually be governed by the number of parts to be
produced.
1. Efficient Operation
In determining how a jig or fixture can be best designed for its most efficient use by
the operator, the following should be considered :
i) Type of jig or fixture required for the specific part.
ii)Locating and loading of the work
a. Clearances necessary for locating the work.
b. Methods of foolproofing against improper loading
c. Unloading the work
iii)Rapid methods of clamping the work.

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iv)Methods of handling the tool especially when it is large and heavy.
v) Chip clearances and chip removal.
vi)Wearing surfaces and replacement of worn parts.
vii)Selection of materials for the special tool.
viii)Safety in operation.
2. Accurate Workpieces
The design of jigs and fixtures is influenced by the degree of accuracy requiredin the
workpiece. The features of design will vary as the requirements for accuracy vary for a
workpiece. This is one of the major factors to consider in the design of special tools.
When multiple or subsequent operations are necessary, the same locating surface or
surfaces on the workpiece should be used in each of the special tools required for the
manufacture of that part. The accuracy necessary to obtain the propre relationship between
a workpiece and other parts in an assembly is an important consideration in design. Some of
the factors to be considered in this respect are :
1.The accurate relationship of operating surfaces on different parts when they are
assembled?
2.Adequate rigidity to maintain accuracy in jig or fixture.
3. Economy & Cost
The cost of the tool and the number of parts to be produced are other factors in
determining the design. If a small quantity of parts is to be produced, a simple low cost tool
may be satisfactory.
The necessity of keeping the manufacturing cost of a new article as low as possible, or
reducing the present cost of an existing article, usually determines the type of jig or fixture
that is to be made. In some other instances, the cost of an operation may be reduced by
using a more efficient though more expensive tool. Increased accuracy and
interchangeability secured through the use of a more elaborate tool frequently warrants its
greater coat.
The use of standard accessories requires serious consideration in the economical
construction of jigs and fixtures. The designer should familiarize himself with the possible
used of all types of standard accessories. Pre-fabricated units such as tool bodies, locating
and clamping devices, drill jig bushings, and tool body supports should be used. Standard
parts may be purchased or made in quantities and kept in stock.

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A5.1d Elements of Design (Jigs & Fixtures)
1. The work must be Located Properly
The ease and rapidity with which the workpiece can be located and removed is an
important consideration in the design of special tools. Therefore, the designer should
become thoroughly familiar with the various methods of locating and clamping before a
design is definitely decided upon.
Parts having rough or irregular surfaces, and parts which vary in size, usually present
special problems in locating. To compensate for such irregularities or variations, adjustable
locators should be used. The design of adjustable locating stops and supports should
provide for positive location and for simple and easily accessible means of adjustment and
locking.
Locating points on jigs and fixtures should be designed so that incorrect loading of
the workpiece is impossible, further more to obtain the proper balance, the locators should
be place as far apart as the shape of the workpiece will allow. Consideration must also be
given to the position of the locators to allow for the necessary clearance in loading and
unloading. The locating points should be made wear resistant in order to maintain accuracy,
especially when non-adjustable stops are used.

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Accuracy
Location should be done on the most accurate surface of the workpiece. A machined
surface is preferable to an unmachined one. When more than one machined surfaces are
available, locate from the most accurate surface. For example, the center of the turned part
can be located from outside diameters 110 or 80 or form central 50 fbore 80fhas the
minimum tolerance of 0.05, so the workpiece can be located most accurately from outside
diameter 80f. Location form 50 fbore would be less accurate than location from 80fbut
more precise than location from outside diameter 110fwhich has a much wider tolerance of
1mm (±0.5mm).
2. The work must be clamped properly
The method of clamping and the design of the clamps depend upon the shape of the
workpiece.
The designer should consider the following fundamentals :
i. The clamps should be positioned to resist the maximum pressure of the cutting
tools.
ii.The clamps should be located over or as near as possible to some bearing point
of the workpiece. This must be done to avoid springing the part.
iii.The clamps should be designed so that they can be quickly and easily unlocked
and shifted out of the way of the workpiece when it is unloaded.
iv.Complicated clamping devices should be avoided if possible. A simple device
has fewer wearing surfaces and will stay in working condition for a longer time.
v. The kind of material in the workpiece should be considered in choosing a design
for the clamps. For example, finished surfaces or soft material require a larger
clamping area than surfaces of hard material. The larger clamping face
distributes the pressure so that the workpiece is not deformed or spoiled. The
work supporting devices opposite the clamps should be large enough to support
the pressure of the clamps.
3. Large tools –Weight & Handling
Special tools designed for large workpieces are often unduly heavy. It is therefore
desirable to make them as light as possible for easy manipulation without impairing their
strength. Large cast iron tool bodies can be made lighter by coring out the metal. It is often
possible to reduce weight by the use of well designed, fabricated and welded tool bodies,
where heavy castings are to be drilled, reamed or bored on several sides, trunnions, thereby

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eliminating the otherwise difficult problem of lifting and turning jig faces up into the operating
position. All corners should be well filleted for strength.
Heavy tools should be provided with handles or holes for bars or eye bolts to facilitate
lifting. In some cases it is often good design to provide smaller jigs with handling devices for
convenience in holding them while the enclosed workpiece is machined. Sharp edges and
corners which might injure the operator should be avoided.
4. Chip clearance
Clearance must be allowed so that the chips will not accumulate and interfere with
the cutting operation or workpiece location. The jigs should be designed so that it ca n be
easily cleaned. Holes or escapes for draining the coolant or cutting lubricant should be
provided.
5. Materials for Jig & Fixture
Jigs and fixtures are made from a variety of materials, some of which can be hardened to
resist wear. It is sometimes necessary to use nonferrous metals like phospher bronze to
reduce wear of the mating parts, or nylons or fibre to prevent damage to the workpiece.
Given below are the materials often used in jigs, fixtures, press tools, collects, etc.
a.High Speed Steels (HSS)
These contain 18% (or 22%) tungsten for toughness and cutting strength, 5.3%
chromium for better hardenability and wear resistance and 1% vandadium for retention of
hardness at high temperature (red hardness) and impact resistance.HSS can be air or oil
hardened to RC 65-65 and are suitable for cutting tools such as drills, reamers and cutters.
b. Die Steels
These are also called high carbon (1.5-2.3%) high chromium (12%) (HCHC) cold
working steels and are used for cutting press tools and thread forming rolls. Hot die steels
with lesser carbon (0.35%) and chromium (5%) but alloyed with molybdenum (1%) and
vanadium (0.3-1%) for retention of hardness at high temperature are used for high
temperature work like forging, casting and extrusion.

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c.Carbon Steels
These contain 0.85-1.18% carbon and can be oil hardened to RC62-63. These can be
used for tools for cutting softer materials like wood work, agriculture, etc. and also for hadn
tools such as files, chisels and razors. The parts of jigs and fixtures like bushes and locators,
which are subjected to heavy wear can also be made from carbon steels and hardened.
d. Collet Steels (Spring Steels)
These contain about 1% carbon and 0.5% Manganese. Spring steels are usually
temperedto RC 57 hardness.
e. Oil Hardening Non-Shrinking Tool Steels (OHNS)
These contain 0.9-1.1% carbon, 0.5-2% tungsten and 0.55-1% carbon. These are used
for fine parts such as taps, hand reamers, milling cutters, engraving tools, and intricate press
toolswhich cannot be ground after hardening (RC 62).
f. Case Hardening Steels
These can be carburised and case hardened to provide 0.6-1.5 thick, hard (RC 59 -63)
exterior. 17Mn1Cr95 steel with 1% manganese and 0.95% chromium is widely used.
15Ni2Cr1Mo15 steel with additional nickel (2%) reduces thermal expansion up to 100
0
C.
Case hardening steels are suitable for parts which require only local hardness on small
wearing surfaces where costlier, difficult to machine full hardening tool steels are not
warranted.
g. High Tensile Steels
These can be classified into medium carbon steels with 0.55% - 0.65% carbon (En8-9)
and alloy steels like 50 Ni2Cr1m028 (En25). The tensile strength can be increased up to
125 kg/mm
2
(RC50) by tempering.
Medium carbon steels are used widely for fasteners and structural work while alloy
steels are used for high stress applications like press rams.
h. Mild Steel
It is the cheapest and most widely used material in jigs and fixtures. It contains less than
0.3% carbon. It is economical to make parts which are not subjected to much wear and are
not highly stressed from mild steel.

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i.Cast Iron
It contains 2-2.5% carbon. As it can withstand vibrations well, it is used widely in milling
fixtures. Self lubricating properties make cast iron suitable for machine slides and guide
ways. The ingenious shaping of a casting and the pattern can save a lot of machining time.
Although, the strength of cast iron is only half the strength of mild steel, a wide variety of
grades have been developed. Nodular cast iron is as strong as mild steel, while meehanite
castings have heat resistant, wear resistant, and corrosion resistantgrades.
j.Steel Castings
These combine the strength of steel and shapabilly of a casting.
k. Nylon and Fibre
These are usually used as soft lining for clamps to prevent denting or damage to the
workpiece under high clamping force. Nylon of fibre pads are screwed of stuck to mild steel
clamps.
l.Phospher Bronze
It is widely used for replaceable nuts in screw operated feeding and clamping systems.
Generally screw making process is time consuming and costly. So, their wear is minimised
by using softer, shorter phospher bronze mating nuts. These can be replaced periodically.
Phospher bronze is also used in applications calling for corrosion resistance, like boiler
valves.
6. Construction of Jigs and Fixtures
Jigs and fixtures bodies may be made of c ast iron, or they may be built up of steel
plates or structural forms held together by screws and dowels or welded joints. Welded
constructions are proving desirable because the bodies are strong and light, and addition
alterations and additions to the tool can be effectively accomplished.
The size of a tool, the quantity of workpieces to be made, and the cost of construction
are important considerations in planning a design.
7. Replacement of Worn Parts

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Some parts of tools are subjected to so much wear that the accuracy of the tool may
be impaired. For such tool parts, material that can be made wear resistant must be selected
and the tool must be constructed so that worn out parts are easily replaceable.
8. Safety in Operation
One of the most important considerations affecting the design of tools is the safety of
the operator. Any features, which might cause injury, must be eliminated. Adequate
operating accessories, such as suitable and efficient levers and locks are essential for safety
in operation. The design of jigs and fixtures should provide means of clamping to the
machine table if large tools are used or when tooling need not be shifted. Convenient
holding devices should be provided as a safety factor whenever necessary.
The tool must be designed so that it can be easily set up, adjusted, operated, and
cleaned. Features, which safeguard the equipment against misuse, are also very important
elements of design.
A5.1e Flow chart for development of design solution

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FLOW CHART FOR DEVELOPMENT OF DESIGN SOLUTIONS
Initial Design Concept
Design procedure
1. Statement of the problem
eg. To design a drill jigto hold a support bracket while drilling 3 –6mm holes
To design a lathe fixture for holding a pump housing for drilling and boring of bearing
holes.
Part Details Operation
Classification
Equipment
Selection
Operator
Criteria
Select Pertinent
Items
Select Pertinent
Items
Select Pertinent
Items
Select Pertinent
Items
Discarded
ideas
Preliminary
Tool Design
Cost Analysis &
Evaluation
Primary Tool
Design
Alternate 1 Tool
Design
Alternate 2 Tool
Design
Evaluation and Final Decision
Completion of Design, Execution of Shop Drawings
Phase 1
Phase 2
Phase 3

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2. Need Analysis: (Who, why, how, when, what and where about functional
requirements)
3. Ideation (sketches)
Information collection.
4. Analysis and synthesis
5. Tentative design solutions
6. Evaluation and testing
7. The finished design
A5.1f Checklist for the Design of Jigs & Fixtures
The following list of checkpoints should be considered before any design of jig or fixture is
released for manufacturing.
¨Check List for Tooling Layout
1. Is the tool layout the latest issued?
2. Is the part drawing the latest issued?
3. Is the part correctly shown on the layout?
4. Are the locating points provided using the thumb rule 3-2-1--Three points in first plane
-Two points in second plane
-One point in third plane
5. Are practical considerations made in locating and clamping a part in jig or fixture?
6. Can locators be easily cleaned or replaced?
7. Is the jig or fixture of sound design? Are rigidity and simplicity taken into consideration?
8. Are the locators accessible for cleaning?
9. Are all the clamping requirements properly considered?
10.Are the individual components designed from the point of minimizing machining?
11.Is the layout made to scale?
12.Are the standard items used indicated?
13.Does using separate numbers with leaders and arrows pointing to the details identify the
different parts?
14.Is the bill of materials provided in the drawing?
15.Are the details of operations, such as heat treatment and the surface roughness
indicated in the drawings?

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¨The machine and Set Up
1. Will the jig or fixture fit into the machine for which it is intend?
2. Will the clamping slots or holes in the jig or fixture line up with the T-slots in the table?
3. Will the fixture, when in place, overhand at the end of the table?
4. Will the jig or fixture interfere with any other fixture next to it in the case of multispindle
machine?
5. Can a set up operator see whether the cutter/drill are correctly set?
6. Can the cutting tools be adjusted and removed easily for sharpening when the fixture is
in place?
7. Can setting blocks, bushings, stops, or collars be used in setting up the cutting tools?
8. Are suitable locating plugs for setting up, provided?
9. Does the set up operator need more than one size of wrench?
10.Are the hold down bolts to make tightening easy provided?
¨Method of Location
1. Are the locating points widely placed?
2. Are the centralized means required to compensate for variations in the work piece.
3. Is the tolerance on locating points sufficiently close to obtain the specified operational
accuracy?
4. Are the locating points as small in area as possible?
5. Are locators safe from damage by cutters?
¨Method of Clamping
1. Are the loads static or dynamic?
2. Is the work piece supported as closely as possible to the point of load applications?
3. Is the cutting force resisted by a solid support and not by the clamp?
4. Can the cutting force be used conveniently to help securing the work piece?
5. Has the clamp sufficient range to accommodate allowable work piece variations?
6. Is the work piece directly supported under clamping points?
7. Will the clamping force unduly distort the work piece?
8. Will the clamp tend to loosen under cutter chatter or vibration?
9. Can it be planned to have a single standard wrench to tighten all the clamps?
10.Does the work piece size, the required clamping force, or the required speed of action
warrant used of pneumatic or hydraulic clamping?

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¨Handling
1. Is the part within handling capacities by hand?
2. Is by hoist, are the facilities and necessary sling clearance, loading skills provided to
ensure easy handling?
3. If by conveyer, is the correct height maintained?
¨Loading / Unloading Work Pieces
1. Will cutters such as long drills interfere with work piece while loading or locating?
2. Will clamp interfere while loading or unloading?
3. Is the clearance sufficient to permit the work piece to be easily lifted over or into locating
and centering devices?
4. Have any sliding pins or other hand-operated locators been provided with comfortable
handles?
5. Are movable locators and adjustments on the side of the fixture nearest the operator?
6. Can the fixture be loaded with one hand while the other hand is used for loading the
completed work piece?
7. Are any burrs likely to interfere with unloading?
8. Should an ejector be provided?
¨Thrust and Torque
1. Can satisfactory blockings be arranged to withstand cutter feed strains and distortion?
2. Are clamps carrying thrust load avoided?
¨Chips
1. Are channels to allow the collate to wash the chips away provided?
2. Are blind spots and traps avoided?
¨Capacity
1. Will the proposed design come within the column clearance capacity, table spindle travel
of machine?
2. Are the jigs fit large enough to span T-slots in the machine table?
3. Is there sufficient clearance between tools and the work piece for easy gauging?

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¨Lubrication
1. Is the machine equipped for coolants?
2. Can suitable coolant reservoir be provided?
¨Tool
1. Is the design ensures the use of standard tools (drills, reamers, milling cutters)?
2. Can use of existing special tools be incorporated?
3. Is provision made to secure all special tools needed?
4. Can the cutter flutes discharge chips even when covered up?
¨Standard Parts
1. Does the proposed design incorporated fully the use of all standard parts carried I stock
for jigs or fixtures?
¨Loose Pieces
1. Can the loose parts (if inavoidable) be attached to the fixture with keeper screws or light
chains?
2. Is the proper identification or storage facilities provided for the loose parts (Clamps,
removable locating plugs and adapters)?
¨Balance
1. Is the uniform mass distribution in design ensured?
¨Progressive Experience
1. Does similar type of equipment exist if so what is your experience?
2. Are they suggestions from machine shop or tool room been considered and
incorporated?
¨Manufacturing Considerations
a) Built up
1. Can the jig or fixturebe built? Does the proposed design includes any impossible
(difficult impractical) machining problems?
2. Does it confirm to all known machine capacities?
b) Castings
1. Does the design land itself to good pattern making practices and economical castings?

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c) Welded Construction
1. Would welded steel construction offer any advantages?
d) Material
1. Have proper steels / materials been selected to make the miscellaneous details?
¨Production Requirement
1. Is the jig/fixture best suited to meet the requirements?
2. Will available machine burden, require manual semi or full automatic, single or multiple
set up?
¨Safety
1. Does the fixture design protect the operator from coolant spray or flying chips?
2. Is the designed tool safe to operate with?
¨Sundry Requirements
1. Will the fixture design keep the length of the cutter travel to a minimum?
2. Will the operator, when positioning the jig, clearly able to see all bushings or cutter
guides?
3. Are the bushings long enough to provide adequate tool support?
4. Do the tool need guiding for a second operations?
5. Can the use of slip bushings be avoided by used of stepped drills?
6. If the slip bushings are must, are the heads large enough, fluted for easy gr ipping, and
provided with locking means?
7. Do all supporting pads and pins stand well clear of chip collecting surfaces?
8. Is the chip fouling the clamp lifting springs?
9. If the work piece is to be measured while still in the fixture can it be easily cleaned?
10.Is there sufficient clearance between tools and the work piece for easy gazing?
11.Can the tools be damaged or made inaccurate through incorrect insertion of the work
piece?
12.Have breather holes been provided to allow escape of air from close firing plunger
holes?
13.Provided the production and economics warrant, have all wearing parts been specified to
be hardened?
14.Have means of setting the cutter or cutters in the correct position been provided?
15.In case of revolving features

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a) Has sufficient metal been left on jig to form an integral balance weight or provision for
balance weight?
b) Have arrangements been made for swarf to be shaken out or be blown out from
interior of jig?
c) Are projecting screws etc. covered in order to eliminate risk of injury to the operator
(counter bored/counter sunk)?
d) Have pilot made fool proof, if arranged so that the work cannot be inserted except in
the correct way?
16.Is core out of unnecessary metal, making the fixture as light as possible, consistent with
rigidity and stiffness made?

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Summary
Quality and higher productivity are aimed through the use of jigs and fixtures in any
production process. The jigs and fixtures add to the cost of tooling, as such their use in
production should be justified economically. The design of jigs and fixtures need to satisfy,
functional, qualitative, safety and adaptability aspects toa production techniques. Tool
engineer has to adopt certain design principles for the jigs and fixtures. A check list for the
design of jigs and fixtures before releasing it for manufacture will greatly help the production
process with considerable saving in production time and cost.
Questions
1. What are the different elements of design?
2. Explain the design steps for designing of jigs and fixtures?
3. How does the checklist help the tool designer in designing of good jig or fixture?
4. What are the major factors to be considered in design of jigs & fixtures?
5. Explain, why jig has a four feet not three?
6. What are the design principles to be followed while designing of jigs & fixtures?

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A8.1 Introduction
Cost estimating may be defined as the process of forecasting the expenses that must
be incurred to manufacture a product. These expenses take into consideration all
expenditures involved in design and manufacturing, with all the related service facilities such
as pattern making, tool making, as well as a portion of the general administration and selling
costs.
Cost estimates are the joint product of the engineer and the cost accountant, and
involves two factors : physical data and cost ing data. The engineer as part of his job of
planning manufacturing determines the physical data. The cost accountant compiles and
applies the costing data.
A8.2 Purpose of Cost Estimation
Cost is the background of almost every decision the tool engineer makes in
organizing manufacturing operations and in selecting materials, methods, tooling and
facilities. An understanding of cost determination is essential to ensure that these decisions
are based on sound and dependable estimates of cost.
Estimates of cost must be reasonably accurate if a venture is to be successful
(realistic cost estimate). If a job is overpriced, it is lost to a competitor. If it is underestimated,
it results in financial loss.
Detailed cost estimates are prepared to:
CHAPTER OUTLINE
A8.1 Introduction
A8.2 Purpose of cost estimation
A8.3 Elements of cost
A8.4 Cost structure
A8.5 Estimation of cost elements
A8.6 Estimating tool cost
A8.8 Steps in making of cost
estimation
A8.8 Chip factors in cost estimation
A8.9 Numerical examples
CHAPTER OUTLINE
A8.3a Material cost
A8.3b Labour cost
A8.3c Expenses
A8.5a Direct Material cost
A8.5b Direct labour cost
A8.5c Indirect expenses
A8.5d Direct expenses

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1. Determine the selling price of a product for a quotation or contract, so as to ensure a
reasonable profit to the company.
2. Check the quotations supplied by the vendors.
3. Decide whether a part or assembly is economical to be manufactured in the plant or
is to be purchased from outside.
4. Determine the most economical process or material to manufacture a product.
5. Initiate means of cost reduction in existing production facilities by using new
materials, which result in savings due to lower scrap loss and re vised methods of
tooling and processing.
6. To determine standards of production performance that may be used to control
costs.
A8.3 Elements of Cost
The constituents of cost of a product or the “cost elements”are : Material cost,
Labour cost and Expenses. We shall discuss each element in turn.
a) Material cost
Material is divided into two basic categories: (a) material for fabricated parts (b)
standard purchased parts. The total cost of thesetwo will give the material cost. Again there
are two kinds of materials, which comprise the factory cost of a product. These are : Direct
material and Indirect material.
i) Direct Material
The direct material is the raw material, which is processed in the plant and finally forms
the finished product. Any standard part, which also becomes a part of the finished product,
will also come under the category of direct material.
ii)Indirect Material
Indirect materials are those, which hel p in the processing of direct materials into the
finished product. These materials don’t form a part of the finished product. Indirect
materials include: Shop supplies such as cotton waste, lubricating oil, cutting fluids, coal, oil,

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gas, shielding gasesused in Arc welding, Emery paper used for polishing, quenching oils for
heat treatment etc. Indirect materials form the part of on cost or overheads.
b) Labour Cost
Labour, which enters into the manufacture of a product, is of two categories : Direct
Labour and Indirect Labour.
i) Direct Labour
The operator or operators, which actually process the raw materials either on machines
or manually, form the direct labour.
ii)Indirect Labour
All the staff excepting administrative and sales office staff, wh ich helps in running the
plant, comes under the category of indirect labour. Indirect labour includes: Foremen,
supervisors, maintenance staff, stores personnel, time office staff, drawing office staff, etc.
Indirect labour forms a part of overheads.
c) Expenses
Total cost of the product minus the costs of direct material and direct labour constitutes
the ‘Expenses’. Expenses may also be either direct or indirect.
i) Direct Expenses
These expenses like the direct material and direct labour are directly chargeable to
the finished product. These are also known as “chargeable expenses”. These include:
a) Cost of patterns, jigs, fixtures, dies, drawings or designs specifically prepared for a
particular product, which cannot be used for other purposes.
b) Cost of any experimental work done specially for a particular product.
c) Cost of inward carriage or freight incurred on supply of special material needed for the
particular product.
d) Hire of special or single purpose tools or equipment for a particular product.

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ii)Indirect Expenses
These are also called “oncosts” “overheads” or “burden”. These include: cost of
indirect material, cost of indirect labour and other expenses that cannot be conveniently
charged directly to a particular job. Indirect expenses may be divided into:
a) Factory expenses or overheads.
b) Office and Administrative expenses or overheads.
c) Selling and Distribution expenses or overheads.
¨Factory expenses
These expenses include : indirect materials, indirect labour, expenses, insurance,
maintenance and depreciation of machine, power etc.
¨Office and administrative expenses
These expenses consist of all expenses incurred in the direction, control and
administration of an undertaking. These expenses include : rent and rates of office
premises, salaries of office staff, printing and stationery, postage, salaries of high officers,
depreciation of office equipment and insurance on office equipment.
¨Selling and distribution expenses
These expenses include : salaries of sales staff, publicity and advertisement,
catalogues, leaflets and price lists, packing and forwarding charges, godown rent,
commission to salesmen etc.
The overheads may be grouped into two main categories:
1. Fixed overheads or constant overheads
These are such items of indirect expenses, which remain constant or fixed irrespective of
volume of production. These items include : salaries of higher officers (administrative and
management executives), capital taxes, insurance charges, depreciation of building, plant
machinery etc., rent of buildings.

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2. Variable or floating overheads
These are such items of overheads, which vary, with the volume of production. Such
items are : internal tran sport charges, power, fuel, stores expense, factory lighting and
heating and sales office expenses and repairs of machine tools.
Since fixed overheads remain constant irrespective of volume of production, production
should be increased to reduce the cost of the part. There should be some minimum
production to meet the fixed expenses and start earning profit.
A8.4 Cost Structure
The elements of cost can be combined to give following types of cost:
1. Prime cost. Prime cost or direct cost is given as :
Prime cost = Direct material + Direct labour + Direct expenses (if any)
2. Factory cost. This cost is given as:
Factory cost = Prime cost + Factory expenses.
Factory cost is also called as “Works cost”.
3. Manufacturing cost. Manufacturing cost or costof production is given as:
Manufacturing cost = Factory cost + Administrative expenses.
4. Total cost. Total cost is given as:
Total cost = Manufacturing cost + Selling and Distribution expenses.
5. Selling price. Selling price is given as:
Selling price = Total cost + Profit.
The above mentioned cost structure is explained with the help of a block diagram as follows:

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Selling price
Total cost Profit
Manufacturing
cost
Selling
expenses
Factory cost
Administration
expenses
Prime cost
Factory
expenses
Direct material
+
Direct labour
+
Direct expenses
(if any)
A8.5 Estimation of Cost Elements
Directly material cost, direct labour cost and directs expenses can be found out most
accurately by the estimat ing procedure. The indirect expenses items, which are so
numerous, are determined by cost accounting section only, which furnishes the figure
department wise to the estimator. The various cost elements are estimated in the manner
give below.
a) Direct Material Cost
The cost of standard purchased parts can be obtained from the purchasing section. The
raw material chargeable to a product is that in the rough state and includes all scrap
removed. Material can be in the form of sheet metal, bar stock, forgings or castings, plastic
etc. The weight of the material can be determined from the drawing of the part. An irregular
part is divided into simple sections to calculate its volume. Volume is multiplied by density of
the material to find its weight. Theweight of a part multiplied by the unit cost of the material
gives the material cost per piece. If the unit cost covers only the purchase price of the
material, the material cost is multiplied by one or more additional factors to account for bulk
losses,purchasing and handling costs.
b) Direct Labour Cost

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For estimating the direct labour cost of a product, the job is divided into operations
needed to machine it, and then estimating the operation time for each operation. Total time
multipliedby a labour rate gives the direct labour cost. The total time required to perform an
operation may be divided into the following parts:
i) Set-up time
This is the time needed to prepare for the operation and may include : time of study the
blueprint or to do paper work, time to get tools from the crib and the time to install the tools
also on the machine.
ii)Man or handling time
This is the time the operator spends loading and unloading the work, manipulating the
tools and the machine and making measurements during each cycle of operation.
iii)Machining time
The elements comprising the machining time are those, which are performed by the
machine. This is the time during each cycle of operation that the machine is working or the
tools are cutting.
iv)Tear down time
This is the time required to remove the tools from the machine and to clean the tools and
the machine after the last component of the batch has been machined.
Tear down time is usually small. It will seldom run over 10 minutes on the average
machine in the stop. It may require only a few minutes to tear down a set up on a drilling
press and 10 to 15 minutes on the average miller or turret lathe. In exceptional cases, it may
go upto as high as 30 minutes on very large boring mills and large milling machines.
v) Down or lost time
This is the unavoidable time lost by the operator due to breakdowns, waiting for the tools
and materials etc.
vi)Allowance

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The total time to perform an operation also includes time for personal needs of the
operator, time to change or resharpen the tools etc. The time all these allowances are taken
to be about 20 percent of the sum of all other times and then the total time for the operation
is obtained.
(a) Personal allowances. This is time taken by the operator to attend to his personal
needs such as going to lavatory, taking a cup of tea, smoking etc. The time for this is usually
taken to be about 5 percent of the total time.
(b) Fatigue. The efficiency of the worker decreases due to fatigue or working at a
stretch and also due to working conditions such as poor lighting, heating or ventilation. The
efficiency is also affected by the psychology of the worker, which may be due to domestic
worries, job security etc. For normal work, the allowance for fatigue is about 5 percent of the
other times. This allowance can be increase depending upon the type and nature of work
and working conditions.
(c)Time to change or resharpen tools. Some allowances should also be provided for
the time taken by the operator to get the tools changed or to resharpen the tools. This time
varies from machine to machine.
(d) Inspection or checking allowance. To maintain the uniform quality of the parts,
the dimensions of the parts should be checked or inspected at regular intervals depending
upon the closeness of tolerances. The checking times for the various instruments are given
below, to check one dimension :
With rule 0.10 minute
Vernier caliper 0.50 minute
Inside caliper 0.10 minute
Outside caliper 0.05 minute
Inside micrometer 0.30 minute
Outside micrometer 0.15 minute
Depth micrometer 0.20 minute
Dial micrometer 0.30 minute
Thread micrometer 0.025 minute
Plug gauge 0.20 minute
Snap gauge 0.10 minute

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Set up time and tear down time are performed usually once for each lot or batch of
parts. Set up time per piece is obtained by dividing the set up time for the machine by the
number of pieces produced in lot. Set up time, handling time and tear down time are
estimated from previous performanceson similar operations. All work on a particular type of
machining tool consists of a limited number of elements. These elements can be
standardized, measured and recorded. This is done under Time and Motion study.
Standard data is available for set uptime, tear down time and handling time. Machining time
is obtained with the help of formulas for each machining operation, which takes into account
speeds of cutting, feeds, and depth of cut and tool travel. The actual amount of down or lost
time that will occur in a particular operation can scarcely be predicted. Some operations will
run smoothly, others may be beset by troubles. The sum of machining time and the handling
time is called ‘run time’ or ‘unit of operation time’.
The total time to manufa cture a product (from which the direct labour cost will be
estimated) may be divided into the following major groups :
1. Set up time
2. Machining time
3. Non machining time
4. Down time
The tear down time discussed earlier may be included in the set up time itself. The
non-machining times will be man or handling time, personal needs, fatigue, cutter or tool
sharpening and inspection. The man or handling time, as already discussed, includes :
loading and clamping the part, unloading the part, advancing or retracting the cutting tool,
tightening a chuck, a trial cut, trial gauging, debarring the machine, cleaning the fixture etc.
c) Indirect Expenses
Indirect expenses or overheads are those charges which vary in proportion to the
production rate, but which are not easily attributable directly to a given operation or part.
These expenses are apportioned among the operational units (machines, plants etc.)
according to some weighting factor.
1. Percentage of direct labour cost
2. Percentage of direct labour hours
3. Machine hour method
4. Direct material method

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5. Unit of production method
6. Space rate method
d) Direct Expenses
The direct expenses are estimated in the following manner :
The engineering (preparation of drawings, blue prints, drafting etc.) and design cost of a
product is calculated as a flat hourly rate for each estimated hour of design and engineering
time. On the same lines, the cost of any experimental work done specifically for a particular
product, can be estimated. The cost of patterns and special tools such as jigs, fixtures, dies
and gauges can be estimated as outlined below:
¨Tool cost
Generally tool cost estimating is concerned with tool and other special equipment to
be used for production. Much product cost estimating depends upon tooling cost estimating.
Therefore, tool costs are often treated separately during a product cost study.
Tooling costs are estimated to :
1. Determine how much must be invested i n tools and equipment to manufacture a
product.
2. Determine the cost of alternate methods of tooling to help in selecting the more
economical method.
3. Find the cost of a proposed machine or tool that promises to produce more
economically method.
4. Determine the reasonable cost of a special machine or tool to gauge whether tool
room performance is efficient or vendor’s prices are reasonable.
The cost of cutting tools, both special and standard, jigs, fixtures, dies and gauges and
other special equipment is often separated from the cost of machine tools. This is done to
determine what portion of the tooling cost is to be charged directly to the project or proposal
and what portion is to be capitalized.
A8.6 Estimating tool cost
Tooling cost is estimatedin the same manner as the cost of manufacturing of a product
is estimated. There are four major factors in the cost of any tool. These are: material, labour

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and overheads or burden as in manufacturing costs. The fourth factor is the cost to engineer
and design the tool, that is, the cost of designing and drafting the tool.
i) Engineering and design cost
The engineering and design cost represents a large portion of the total cost of the tool,
often as high as 20 to 30 percent. This cost can be directly charged to the individual tool and
as a consequence are always considered first in establishing an estimate, the engineering
and design cost is applied to the tool cost estimate as a flat hourly rate for each estimated
hour of design and engineering time.
ii)Tool materials cost
The cost of the material for a proposed tool may be calculated and therefore becomes
more of an actual cost than an estimated cost. It is the most accurate item in the tool cost
estimate and is determined as discussed before. Standard parts such as knobs, hand
wheels, bushings, bolts, screws, springs, and similar items that complete the tool bill of
material can be accurately priced from catalogues, price lists or invoices.
iii)Tool labour cost
The labour involved in the machining, assembling, fitting, and tryout of tools is always
difficult to estimate accurately, even for the most experienced estimator. The machining
time can be calculated. The other times cannot be calculated and must b esti mated. It is
difficult to foresee all the problems that may develop in fitting assembly and try out
operations, even under most favourable conditions. Therefore, the estimate must include a
liberal factor of safety for lost time, which is impossible to anticipate and will always be
present. The sum of all the times will be the labour time for the tool. When multiplied by the
toolmakers hourly rate, the estimated labour cost is determined.
iv)Burden or overhead
The tool room may be considered as a department, and therefore may have an
established burden rate, just as production department has. It general, the man-hour or
direct labour cost method of burden distribution is applied as discussed under point 3 above.
A8.8 Steps in making a cost estimation

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The cost of a new product may be estimated by following the basic steps given below :
1. Make a complete and thorough analysis of the cost request to understand it fully.
2. Make an analysis of the part or product and make separate lists of standard partsand the
parts to be fabricated within the plant.
3. Make a manufacturing process plan for the parts to be fabricated.
4. Determine the material costs for the standard and the fabricated parts.
5. Estimate the total production time for each operation listed in step 3.
6. Apply the labour and burden rates to each operation.
7. Add the material costs (step 4) and the labour and burden costs (step 6). This will give
the total manufacturing cost.
8. Apply the profit factors to arrive at the selling price.
A8.8 Chief factors in cost estimation
Each cost estimate may not be exactly the same as the actual manufacturing cost. The
most significant causes for the cost deviations can be : Fluctuations in ma terial and labour
costs, incomplete design information at the time of estimate, unexpected delays resulting in
premiums paid for overtimes and materials and the unexpected machining or assembly
problems. However, the average of cost estimates over a period of time should be
reasonably close to the actual manufacturing costs. For this, the following factors should be
considered for arriving at an accurate and complete cost estimate :
1. Each estimate should contain complete costs of direct material, directlabour, factory
overheads, spoilage, engineering, administration and selling.
2. If the cost of a new product is estimated on the basis of previous estimates of
comparable parts, detailed estimating should be used. It is necessary to make
substitutes in the past estimates for individual operations, individual parts or individual
sub assemblies.
3. The period of time between the cost estimating and the actual production of the part
affects the determination of unit prices. During the intervening period, the basic material
and labour rates may rise or fall. Therefore, an estimate of what the cost will be at the
time of actual production is what is really needed. Thus the estimator should have the
ability to project thinking and reasoning into the future.

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4. The volume of the pieces to be produced also affects the costing rates since the time and
therefore the cost of performing an operation decreases as the number of units produced
is increased.
5. The addition of new type of equipment and special buildings require the development of
new overhead rates etc.
A8.9 Numerical Examples
From the following data, calculate the total cost and selling price for a job :
Direct material = Rs. 5500
Manufacturing wages = Rs. 3000
Factory overheads to manufacturing wages = 100%
Non manufacturing overheads to factory cost = 15%
Profit on total cost = 12%
Solution. Direct material = Rs. 5500
Manufacturing wages (Direct labour) = Rs. 3000
Factory overheads = 100% of Rs. 3000
= Rs. 3000
\Factory cost = Direct material + Direct labour + Factory overheads
= Rs. 5500 + 3000 + 3000
= Rs. 11,500.
Non-manufacturing overheads, i.e., administrative and selling overheads
= 15% of Rs. 11,500
= Rs. 1825
Total cost= Factory cost + Rs. 1825
= Rs. 11,500 + Rs. 1825
= Rs. 13,225
Profit = 12% of total cost
= Rs. 1588
\Selling price = Total cost + Profit
= Rs. 14,81

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Table 1 -Weight of Rolled Steel (8.843 gm/cc)
Size S
Round S fkg/metre
length
Square S Sq.
Hexagonal S
A/F
Sheet S Thick
kg/sq. metre
5
5.5
6
8
8
9
10
11
12
14
16
18
18
19
20
22
25
28
28
32
35
36
38
40
41
45
50
55
56
60
63
65
80
81
85
80
0.154
0.19
0.222
0.302
0.395
0.50
0.62
0.85
0.89
1.21
1.58
2.00
2.48
2.98
3.85
4.83
6.31
8.99
9.86
12.49
15.41
18.8
19.34
22.2
24.48
26.0
30.2
31.08
34.8
39.46
0.20
0.24
0.28
0.38
0.502
0.64
0.885
0.95
1.13
1.54
2.01
2.54
2.83
3.14
3.80
4.91
5.82
6.15
8.04
10.18
12.56
15.90
19.62
23.8
24.62
28.3
31.16
33.2
38.5
39.58
44.2
50.24
0.180
0.206
0.245
0.333
0.435
0.551
0.68
0.823
1.33
1.96
2.45
3.29
4.96
6.96
8.81
11.4
18.0
20.6
24.5
28.8
33.3
38.2
43.5
39.2
55
88.5
94.2
109.9
125.6
141.3
182.8
196.2
219.84
251.14
284.68
298.22
313.92
331.36
364.46
398.63
430.88
463.9
498.04
530.18

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Table 2 -Thumb Rules for Estimation
Group
No.
Operations involved Total costs
1. Only shaping, turning drilling and
fitting.
Three times the raw materials cost.
2. Shaping / turning, drilling, fitting
and milling.
Four times the raw materials costs.
3. Above operations plus heat
treatment.
Five times the raw materials costs.
4. Above operations plus precision
grinding or lapping.
Six times the raw materials costs.
Example
Find the manufacturing cost of 14 fbore collared bush shown in figure. The bush is to be
manufactured form 28 fx 35 long alloy steel bar which costs Rs.80 / kg. After rough turning,
the bush is to be hardened and finished by grinding.
Solution: Referring to Table1, we note that 28 f
Steel bar weights 4.83 kg/metre.
\Wt. Of a 35 long piece = 83.4x
100
35
= 0.169 kg.
Cost of material at Rs.80 = 0.169 x 80
kg = Rs. 13.52
Machining involves turning, hardening, and grinding.
Referring to table 2, we notice that the bush falls in Group 4, for which the total cost is
approximately six times the raw material cost.
\ Total cost = 13.52 x 6
= Rs. 81.12
If the bush is to be sold, profit should be added.
Selling cost at 30% profit= 81.12 x 1.3
= Rs. 105.5 (min.)
Selling cost at 100% profit = 81.12 x 2
= Rs. 162.24 (max.)

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SUMMARY
Quality and higher productivity are aimed through the use of jigs and fixtures in any
production process. The jigs and fixture s add to the cost of tooling, as such their use in
production should be justified economically. The design of jigs and fixtures need to satisfy,
functional, qualitative safety and adaptability aspects to a production techniques. Tool
engineer has to adopt certain design principles for the jigs and fixtures. A check list for the
design of jigs or fixtures before releasing it for manufacture will greatly help the production
process with considerable saving in production time and cost.
Questions
1) Definecost estimating.
2) What is the purpose of cost estimating?
3) Name the various constituents of cost.
4) Name the indirect material and indirect labour.
5) What are : direct expenses, indirect expenses, factory expenses, office and
administrative expenses, selling and distribution expenses.
6) Define : Prime cost, Factory cost, Manufacturing cost. Total cost and Selling price.
7) Define : Set up time, Handling time, Machining time. Tear down time and down or
lost time.
8) List the various steps of cost estimating.
9) Discuss the chief factors in cost estimating.
10)Find the manufacturing cost of 14 fbore collared bush shown in figure. The bush is
to be manufactured form 28 fx 35 long alloy steel bar which costs Rs.80 / kg. After
rough turning, the bush is to be hardened and finished by grinding.
11)From the following data, calculate the total cost and selling price for a job :
Direct material = Rs. 5500
Manufacturing wages = Rs. 3000
Factory overheads to manufacturing wages = 100%
Non manufacturing overheads to factory cost = 15%
Profit on total cost = 12%

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A7.1 Introduction
Maximum productivity at minimum cost is the demand of modern industry. To meet
this requirement designing of efficient and accurate jigs and fixtures is required. Quality,
simplicity and economy from the important criteria for the design of jigs and fixtures.
To meet this requirement the designer will have to make an economic analysis for
using jigs and fixtures and has to device certain principles of design, and finally develop a
checklist for the jigs and fixture design.
A7.2 Basic Design Considerations
In addition to locating and holding the part, the designer must also consider several
other factors before a welding jig or fixture can be designed.
Heat dissipation is an important consideration with any welding tool. Several
methods can be used to insure that proper heat is maintained in the weld area. The primary
factor that determines the amount of heat required is the metal being joined.
When metals such as steel and other poor heat conductors are joined, the excess
heat should be carried of f to prevent overheating the weld. To do this, backup bars of
copper, titanium, or beryllium can be used. For metals that are good conductors or heat,
such as copper or aluminum, too rapid cooling becomes the problem. To prevent this, the
fixture or jigmust be made to contact the part in as small an area as possible.
Clamping supports must be provided to prevent distorting the work while it is in a
heated condition. Whenever possible, place clamps directly over the supporting elements.
Locators should be positioned so that the distortion will cause the part to loosen
rather then tighten against the locators. If this is not possible, either power or manual
ejectors should be built into the tool.
Foolproofing is one feature that is necessary for any t ype of welding jig or fixture.
Each tool must be designed so the part will only fit into its proper position.
CHAPTER OUTLINE
A7.1 Introduction
A7.2 Basic Design consideration
A7.3 Factors in fixture design
A7.4 Classification of fixtures
A7.5 Maintenance, Safety and Storage
TOPIC OUTLINE
A7.4a Types of fixtures based on how the
tool is built
A7.4b Types of fixtures based on the type of
machine on which they are used
A7.4c Types of fixtures & their descriptions

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A7.3 Factors in Fixture Design
The major difference between a drill jig and a fixture is that a jig has hardened
bushings which guide the drill, while a fixture for a machining operation is attached to the
work table of a machine to hold the workpiece in a fixed position for the action of milling
cutters, broaches, or other of cutting tools.
Work may be entirely enclosed in a drill jig and is r eached for the drilling or boring
through bushings provided for that purpose. However, to enclose the work in a milling fixture
would defeat the purpose for which the fixture designed that of securely holding the work in
position for the action of cutting tools. The clamps used in fixtures are applied so that the
work surface is clear for machining.
Milling fixtures are usually fastened to the table of the machine upon which they are
to be used. As a rule, they are flat on the bottom so that they will rest securely against the
table upon which they are clamped. Clamping lugs or other clamping surfaces are generally
provided. The cutter operating with a fixture is usually in a fixed position. The work held in
the fixture is “fed” to the revolving cutter by means of the movable table. Broaching fixtures,
on the other hand, are usually stationary with the broach travelling through the work.
Cutting tool chatter can be greatly reduced by carefully designed work holding
devices. These must be designed so that they are properly proportioned and sufficient in
number to support and hold the work rigidly in the fixture.
One of the major factors in the design of a milling fixture is providing a place in the
fixture for the workpiece to resist the thrust of the cutter. The thrust of the cutter should be
against the body of the fixture, rather than against the clamps. The direction of rotating of
the cutter often governs the placing of the work. An attempt must be made, therefore, to
design the tool so that the body of the fixture takes this thrust. The design of fixtures should
also permit the use of the clamping collars used on the cutter arbor. Often, though, it is not
good economy to use small diameter cutters that allow only a minimum clearance of 3mm or
less between the arbor and the work or a projecting part of the fixture, because of the
necessity of sharpening the cutter which will reduce the clearance.
¨Design Points
Are all parts well designed to take the loads imposed on them in service?
Is the tool study enough to stand considerable abuse?
Is the fixture amply proportioned to damp out vibration and chatter? This applies especially
to milling fixtures.

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Is the design of all parts and mechanisms as simple as possible?
Have cylindrical plungers and holes been used in preference to square or polymer once?
Have holes for headed pressed in parts (such as for accurate location of resigns) been
countersunk to allow any excess press lubricant to collect in the countersink (allowing the
rest pin to vibrate slightly in service) instead of gradually squeezing out under the head?
Have spring pocket holes been countersunk on their open ends?
Are the dowel pins in each part as widely spaced as practicable?
Where detachable parts need very accurate location, have register keys or pins been used
instead of dowels?
Is the accuracy of the operation such that the base of the fixture should be scraped to fit the
machine table?
Have breather holes been drilled to allow air to escape from lose fitting plunger holes?
Is it possible to forecast any part design changes and to make allowance for them in the
design of the fixture?
¨The procedure in developing designs for fixtures is similar to the procedure
followed in designing jigs.
A7.4 Classification of Fixture
Fixture
Types of Fixtures based on how
the tool is built
Types of Fixtures based on the type of
machine on which they are used

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The jigs and fixtures used for welding can generally be limited to three basic types;
tacking, welding, and holding.
Types of Fixtures based on
how the tool is built
Plate fixture
Angle plate fixture
Vice jaw fixture
Indexing fixture
Multistation fixture
Profile fixture
Types of Fixtures based on the type
of machine on which they are used
Turning
fixture
Milling
fixture
Planning
fixture
Broaching
fixture
Grinding
fixture
Shaping
fixture
Shaving
fixture
Forming
fixture
Stamping
fixture
Welding, brazing,
soldering fixture
Assembly
fixture
Inspection
fixture
Testing
fixture
Heat treatment
fixture
Honing
fixture
Lapping
fixture

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Tacking jigs and fixtures are used to hold the parts of an assembly in their proper
position so they canbe tack welded together. These tools are generally used for assemblies
that must be held together in several places to prevent warping or distortion when welding is
complete. Parts assembled in a tacking jig or fixture are removed after tacking and eith er
finished without special tools or transferred to a holding jig or fixture.
Welding jigs or fixtures are used to hold the parts of an assembly in position for
welding. The difference between welding and tacking is the amount of welding performed.
The tacking tool is used only when the part is to be tack welded. When the part is to be
completely welded together, a welding jig or fixture is used. Welding jigs and fixtures are
normally built heavier than tacking tools to resist the added forces caused by the heat within
the part.
Holding jigs and fixtures are used to finish tack welded assemblies. Like welding
tools, holding jigs and fixtures must be made rigid enough to prevent distortion and warping.
Generally fixtures are classified as -
1) How tool is built
a) Plate Fixture
b) Angle plate
c) Vice jaw
d) Indexing
e) Multistation
f) Profiling
2) Machine on which they used
a) Turning
b) Milling
c) Planning, shaping & slotting
d) Broaching
e) Grinding
f) Holding, Broaching and soldering
g) Assembly
h) Inspection
Types of Fixtures
The fixtures are classified based on
i. How the tool is built

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ii.The type of machine on which they are used
Though jigs and fixtures are made basically the same way, due the increased tool forces the
fixtures are built stronger a heavier than jigs.
7.4a Types of Fixtures based on how the tool is built
1.Plate Fixtures
These are the simplest form of fixtures used for most machining operations. This fixtures
consists of flat plate with variety of clamps and locators. Its adaptability to several machining
operations makes it a popular type of fixtures.
2. Angle Plate Fixtures
These fixtures are used to machine at right angles to its locator. If machining is to be
carried out at other angles a modified angle plate fixtures can be used.

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3. Vise jaw Fixtures
These are used for machining small parts. With this type of tool, the standard vise jaws
are replaced with jaws which are formed to fit the part. These are least expansible and are
limited by the size of vises available.

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4. Indexing Fixtures
These are similar to indexing jigs and are used for machining parts which must have
machined details evenly placed.

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5. Multistation Fixtures
These are intended for high speed, high volume production runs, where the machining
cycle is continuous.
Duplex fixtures are two station fixtures and are the simplest of the multistation fixtures.
This form of fixture allows both the loading and unloading operations while the machining
operation is in progress.
For example, once the machining operation is complete at station one, the tool is
revolved and the cycle repeated at station two. At the same time, the part is unloaded at
station one and a fresh part loaded.
6. Profiling Fixtures
These are used to guide tools for machining contours which the machine cannot
normally follow. These contours can be either internal or external. Since the fixtures
continuously contacts the tool an incorrectly cut shape is almost impossible.

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7.4b Types of Fixtures Based on The Type Of Machine on Which They Are Used
Fixtures are generally classified based on the machining on which they are used.
The following are some of the common production operations in which fixtures are used :
1.Turning
2.Milling
3.Grinding
4.Welding, brazing, soldering fixtures
5.Assembly fixtures / Inspection fixtures
1. Turning Fixtures
These fixtures are used for turning, facing and boring operations, and mainly consist of
workpiece locating and clamping elements. The standard fixtures that are used in a Lathe
machines are, three jaw and four jaw chucks, collects face plate etc. These jaw chucks are
used for holding round hexagonal or other symmetrical works. Collects are used for bar
stock. Special jaw chucks, face plates with clamping devices are used for holding irregular
shaped turning jobs.

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CLASSIFICATION OF LATHE FIXTURES
Following points are to be noted while designing turning fixtures.
i) Grip the rotating work piece to the fixture to resist torsional forces.
ii)The fixture should be rigid with minimum possible overhang.
iii)Locate the work piece on critical surfaces, which are the areas from which all or
major dimensional or angular tolerances are taken.
iv)Provides adequate support for frail sections or sections or sections under pressure
from turning tools.
v) Balance the fixtures to avoid vibrations.
vi)Fixtures should not have any projections, as they will cause injury to the operator.
vii)A pilot bush for supporting tools should be provided where extreme accuracy is
required in boring operations.
Figure shows a typical turning fixtures. The fixture body is located on the machine spindle
and bolted in position, it carries the work piece location and clamping system.
Figure shows yet another special turning fixture in which work piece is located and clamped
to a shelf that projects from the fixture body. The fixture incorporates a balance weight (the
fixture would other wise be out of balance) and a pilot bush to guide the boring bar.
Lathe Fixtures
Chuck Type Face Plate Arbor Special
Collet Type Pot TypeMandrel

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2. Milling Fixtures
Milling fixtures are the work holding device which arefirmly clamped to the table of
milling machine. They hold the work piece in correct position as the table movement carries
it past the cutter or cutters.
The essential features of a milling fixture are
a) Base
b) Location elements
c) Clamping elements and
d) Setting blocks
These fixtures are classified based on
i) Type of operation performed
ii)Method of milling
iii)Method of clamping the work piece
MILLING FIXTURE
Cradle
fixture
Rotary
fixture
Drum
fixture
Indexing
fixture
Magnetic
chucks
Vacuum
chucks

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Following design principles be adopted for milling fixtures
1. Pressure of cut should always be against the solid part of the fixtures.
2. Clamps should always operate from the front of the fixture.
3. Work piece should be supported as near the tool thrust as possible.
A milling fixture is located accurately on machine tube and then bolted in position. The tube
is positioned relative to the cutter with the air of setting blocks. The location an d clamping
systems are similar to those used for drill jigs, but as the cutting forces are high, interrupted
and tend to lift the workpiece, the clamping forces must be big, hexagonal nuts are usually
used to clamp the work piece rather than hand nuts.
Fig. show a simple milling fixture and a line or string milling fixtures. The line or string milling
fixture shown is used to mill a slot in the end of each of the five cylindrical work pieces
arranged in line. This arrangement facilities all the work pieces to be located as required and
clamped with one screw.
Fig. shows an index milling fixture having a number of surfaces to be milled by successive
positioning of a single fixture provided with an indexing arrangement.

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3. Grinding Fixtures
Fixtures used in grinding depend upon the type of grinding operation and the machine
used.
GRINDING FIXTURE
Angle Plat Fixture Automotive
Grinding Fixture
Magnetic Chuck

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Following table gives some commonly used grinding fixtures
Sr.
No.
TYPE OF OPERATION FIXTURES USED
1 External Grinding Mandrel -Taper
-Straight
-Combined
-Straight
-And taper
2 Internal Grinding Chucks, special jaw chucks or special fixtures
as the lathe fixtures.
3 Surface Grinding Clamped on machine table, held in vise, held in
magnetic, vacuum and special features.
4. Welding Brazing and Soldering Fixtures
These fixtures comprise of usual locating and clamping elements used in other fixtures.
However the effects of heat and prevalence of welding spatter will have to be taken into
account while designing them.
Some of the consideration are as follows :
i) Expansion of heated work pieces and resulting distortion should be taken care of by
providing adequate clearances between work piece and locators.
ii)Handles subject to heating should be properly insulated.
iii)Thewelding spatter should not be allowed to face on the threaded parts of clamping
elements.
iv)Parts near the welding area should not be threaded.
v) Spatter grooves must be provided below the line of welding of work pieces to the
base plate with the weld spatter.
vi)Care should be taken to prevent locking of joined work pieces in the welding fixture
after welding.
vii)Provision for easy tilting or rotation be made to ease welding from various dies.
Toggle clamps, without threaded elements are widely used in welding fixtures.
Welding and inspection are everyday operations in manufacturing. Like many other areas,
these operations can be simplified and improved through the use of appropriate jigs and
fixtures. Although welding is specified in the examples, the methods and techniques listed
will apply equally well to other assembly operations such as brazing, soldering, riveting, and
stapling.

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5. Broaching Fixtures
As broaching is a fast and accurate method of metal cutting involving high cutting forces,
broaching fixtures are required to perform one or more of the following functions:
a) Hold the workpiecerigidly.
b) Locate the workpiece in correct position relative to the tool or the machine table.
c) Guide the broach in relation to the workpiece.
d) Move the workpiece into and out of the cutting position.
e) Index the workpiece between the cuts.
Fixturesare used for both internal and external broaching. The fixtures used for internal
broaching are the simplest and for many operations consist of a face plate or support place
on the broaching machine. The fixtures for external broaching are made quite rigid so that
the workpiece does not move during the broaching action.

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6. Assembly Fixtures
These are used to hold various components in their correct position, while they are
assembled; Assembly operations often involve pressing interference fit pins, bushes and
other parts in housings. The assembly fixtures need to be of light construction with adequate
rigidity to ensure relative positional relationships of the various components. They may be
built up from light castings, steel section or completely from steel.
Assembly fixtures generally are of two types :
a) Mechanical assembly fixtures used for operations generally performed at ordinary room
temperatures with mechanical means.
Eg, Reverting Fixtures.
b) Fixtures for hot joining methods of assembly work using energy in the form of heat.
Eg. Welding, brazing and soldering fixtures.

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7. Inspection Fixtures
Every part made must meet a standard for size and shape if it is to perform its design
function. While it is quite possible to measure each dimension separately, this is not the
most cost effective means to insure part quality and conformity to dimension.To satisfy the
requirements of speed and accuracy, gauging or inspection fixtures are used.
The main requirement of an inspection fixture is accuracy. Each inspection fixture
should contain only those elements needed to check the specified sizes of forms. Individual
gauges that only check one size are preferred over complicated tooling if the dimension
being gauged is independent of other part features. An example of this is the size of the
threads in a hole.
While the location of the hole is importantto the part, the size of the thread is
independent of the location.
There are two general types of inspection fixtures; gauging and measuring.

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A7.4c Types of fixture & there descriptions
Types of fixture Description
Vise Fixtures
Standard machine vises adopted with special jaws provided
on easy way of holding parts for machining
Lathe Fixtures
Are used on vertical and horizontal turret lathes and
high-speed production lathes
Chuck fixtures
The cheapest type of lathe fixture is the standard lathe
chuck with special jaws or inserts machined to fit the part
Face Plate Fixtures Is used to machine large diameter parts on the vertical lathe
Mandrel and arbor type
fixtures
These fixtures will centre, locate and grip the work from the
inside and are normally used for parts that already have
machined internal surface
Miscellaneous fixtures
Lathe operations on parts that are unusual because of their
shape or dimensions, at the time fixtures are complicated
and expensive yet they are always efficient with respect to
the saving of time and the improvement of quality
Milling fixtures
A mill fixture holds the part in the correct relation to the
milling cutter as the table movement carries the part
through cutters
a) Cradle fixture
The work piece is rocked or rotated within a given angle
during milling
b) Rotary fixture The work piece is rotated under the cutter
c) Drum fixtures
The work piece is mounted on the periphery of a rotating
drum
Indexing fixtures
Where the work piece is indexed in to the next position
during the machining cycle of mill
Rise and fall fixtures
Which allow raising and lowering of the work piece in
conjunction with the mil feed
Magnetic chucks
Are used to hold ferromagnetic materials in production
milling operation
Types of fixture Description

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Vacuum chuck
Are being used for holding nonferrous and non magnetic
parts for milling operations
Boring fixtures
Differ from drill jigs in that they are to be used with boring
bars
Line boring fixture
The distance between the holes requires a line boring bar,
consequently their type of fixture is called a line boring fixture
Stationery fixture
Is designed to mount on the table and present the work piece
to the boring bar in proper location for matching
Universal fixture
May be obtained commercially from the manufacturers of
boring machines.
Indexing fixture
Consist of a base with a rotary table or rotating indexing
plate mounted on it
Automatic loading fixture
Is suitable only for long production tuns of a particular work
piece
Broaching fixture
External broaching usually requires a special fixture for each
job
Grinding fixture
Must allow for the unrestricted access of coolant to the work
The structural design of grinding fixtures is very similar to
that of other fixtures
Angle plate fixtures Is used for internal grinding
Automotive grinding
fixtures
Are used in the automotive industries
Trunk pin grinding fixtures, cam grinding fixtures, cam shaft
grinding fixtures etc.
Magnetic chuck Is used to hold work pieces in surface grinding operation
Planning fixtures
Can be economically applied to smaller parts when they are
clamped in a gang fixture
Welding fixtures
Their purpose is to locate and hold the parts in correct
relative position for joining to reduce distortion
A7.5 Maintenance, Safety & Storage
Provision for Maintenance
Has provision been made for lubricating the tool mechanisms?
55

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Have all wearing parts been hardened?
Are these parts easily made and replaced?
Have correct materials and heat treatment been specified?
Has provision been made for easy removal or pressed in parts?
Can vulnerable parts be removed and replaced quickly without disturbing the set up of the
fixture on the machine?
Safety
1. Does the fixture design protect the operator from coolant spray or flying chips?
2. Is the designed tool safe to operate with?
Handling and Storage
Lifting Aids
Have lifting lugs, eyebolts, or chain slots been provided for slinging heavy tools?
Have lifting handles been attached to all awkward or heavy loose parts of the fixture?
Loose Parts
If loose parts such as spacing pieces, wrenches, or locating pins are unavoidable, can they
be attached to the fixture with keeper screws or light chains toprevent loss in storage?
Fragile Parts
Is there any fragile part of the jig which needs a protective cover in storage?
Is the tool so delicate or highly finished as to require a special case, cover, or box to protect
it in storage?
Identification
Has the tool, and all loose items belonging to it, been marked clearly with identification
numbers or symbols?
Storage Aids
Can the tool be stowed safety without danger of tipping over?
Is a special storage stand or rack desirable for safe and convenient storage?
SUMMARY:

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The use of fixtures is extending and developing very fast. The quality, type and
complexity of fixtures used depend upon the type of job and its method of production.
Fixtures are classified into two types depending on how they are built and based on the type
of machine on which they are used.
QUESTIONS:
1. What are the two types of classifying fixtures?
2. What is an indexing fixture?
3. What type of fixture is used for machining contours, which the machine cannot
normally follow?
4. What is an assembly fixture?
5. What are the rules for selecting clamps of work piece in fixtures?
6. What are the principles to be following in designing of fixtures?
7. Describe the various grinding fixtures.
8. Describe the design principles for a lathe fixture.
9. Name the various work holding devices use on a lathe.
10.How are cutters set in relation to the work in milling fixture?
11.Name the essential features of a milling fixture.
12.Why the proper disposal of swarf or burr is very important in jigs & fixtures design?
13.What provisions can be made to ease the handling of heavy jigs & fixtures?
14.Explain the advantages to be obtained from the use of pneumatic & hydraulic
clamping devices.
15.How can a lathe fixture be clamped to the lathe?
16.Write short notes on “Broaching fixtures”, “Assembly fixtures”.
17.What are the checks to be made for fixtures for (a) provision for maintenance,
(b) manufacturing & maintenance cost, (c) handling, (d) loading & unloading, (e)
storage, (f) human factors?

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A9.1 Introduction
Gauges are inspection tools of rigid design, without a scale, which serve to check the
dimensions of manufactured parts. Gauges do not indicate the actual valueof the inspected
dimension on the work. They can only be used for determining as to whether the inspected
parts are made within the specified limits. A workman checking a component with a gauge
does not have to make any calculations or to determine the a ctual dimensions of the part.
Gauges are easy to employ. This is one reason for their wide application in engineering.
Gauges differ from measuring instruments in the following respects :
(a) No adjustment in necessary in their use.
(b) They usually are not general-purpose instruments but are specially made for some
particular part, which is to be produced in sufficiently large quantities.
Gauging is used in preference to measuring when quantities are sufficiently high,
because it is faster and easier with resulting lower costs.
A9.1a Advantages and Disadvantages
Modern manufacturing requires extensive use of gauges for shop work, inspection, and
reference. Shop gauges are used by workmen. Inspection gauges are used by inspectors
to check manufactured product, and reference gauges are reserved for checking the other
two types.
A gauge is defined by the Sheffield Corporation as “a device for investigating the
dimensional fitness of a part for a specified function”. Gauging is defined by the ANSI as “a
process of measuring manufactured materials to assure the specified uniformity of size and
contour required by industries.”
CHAPTER OUTLINE
A9.1 Introduction
A9.2 Classification of Gauges
TOPIC OUTLINE
A9.1a Advantages & disadvantages
of gauges
A9.2a Fixed gauges
A9.2b Advantages of fixed gauges
A9.2c Classification of fixed gauges
A9.2d Indicating gauges
A9.2e Special gauges
A9.2f Classification of Plain Gauges

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Basically, gauging accomplishes two things: (1) it controls the dimensions of a product
within the prescribed limitations, and (2) it segregates or rejects products that are outside
these limitations.
Gauging devices and gauging methods, like other phases of tooling in modern
manufacturing, have become standardized. Generally speaking, standardized components
that can be obtained commercially are assembled into a unit to gauge a particular product. It
is therefore quite important that the tool designer be familiar with gauging equipment and
practice.
It may be necessary to design special gauges for checking dimensions that do not
readily adapt to standard gauges. A gauge of this type may be quite simple, as shown in Fig.
Frequently time can be saved by the use of a simple length gauge in place of a machinist’s
rule when a quantity of workpieces is involved. It should not be assumed that special
gauges are necessarily elaborate or that they are used only to measure close tolerances.
The tolerance of the workpiece in Figure may be as large as ±1/64 in., and a machinist’s
scale would be sufficiently accurate for the job; however, it would take a little longer to read.
There are many gauging methods used to determine when a product conforms
dimensionally with drawings, specifications, or other prescribed requirements. However,
when these methods are analyzed, it will be found that they are designed to check one of the
seven basic elements of workpiece geometry :
Distance
Used to specify the relative location of the various components and elements of the
workpiece. Distance is measured by comparison to a known standard.
Flatness
Used to ensure that every element of a surface is within a specified distance from a nominal
surface plane. Determines straightness and alignment of a product.
Parallelism
Used to ensure that two flat surfaces are parallel to each other.
Perpendicularity (squareness)
Used to determine that two flat surfaces are normal to each other.

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Angularity
Used to specify the angle between two flat surfaces, other than 90
0
.
Concentricity
Used to ensure that points on a cylindrical surface are concentric to a common axis.
Surface texture
Similar to flatness but concerned with irregularities in a surface rather than straightness and
alignment.
These elements of workpiece geometry are theoretical and are unattainable in actual
machining practice. It is therefore necessary to specify the degree of variation (tolerance)
that is acceptable for the product to function properly. Gauges are the means by which the
products are checked to determine whether the elements of geometry fall within the
specified variation.
A9.2 Classification of Gauges
Classification of gauges on basis of accuracy.
(With wear allowance) (1/3 wear allowance)(More accurate)
For shop purpose for inspection purpose Calibration purpose
¨Classification on use
A9.2a Fixed Gauges
Gauges
Shop Gauges Inspection Gauges Reference Gauges
Gauges
Fixed Gauges Indicating Gauges Special Gauges

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Fixed gauges may be classified as fixed gauges or fixed limit gauges. the master gauge
is made to represent the workpiece dimension in its nominal condition and is used as a
setting gauge for setting up comparator type measuring instruments or as a reference
standard for calibrating direct measuring tools, which require periodic readjustment. Master
gauges are dimensioned to represent the dimension to be gauged. The dimension may be
the basic size or the median size of the tolerance zone.
Fixed limit gauges are used to determine whether the product is within prescribed limits
and are intended for use as inspection gauges. Since there is a high and low limit on the
product, two gauges are usually required, which are made to constitute the design limits of
that dimension.
A9.2b Advantages of fixed gauges
The various advantages of fixed gauges in comparison to comparator type gauges
are :
(1) Fixed gauges are essentially free from errors due to drift and the original adjustment,
non-linear response, effect of power supply variations and other extraneous factors,
which necessitate regular calibration and occasional correction on comparator type
gauges.
(2) These provide positive dimensional information.
(3) These are portable and independent of power supply availability.
(4) It involves no other auxiliary equipment and set-ups.
(5) These can be designed to check combinations of several dimensions comprising
lengths, diameters and angles.
(6) These can be designed to inspect interrelated features, for size, location, for, alignment
etc. so as to check the virtual size of a member (combined effect of all parameters with
regard to the functional adequacy of the inspected features).
(7) These are particula rly useful in the checking of part members whose meaningful
geometric irregularities can’t be readily detected by gauges, which do not provide
complete reverse replica of the critical part portions.
(8) These provide uniform reference standards.
(9) These are not expensive.
(10)Comparators have to be set from time to time using master fixed gauges.
A9.2c Fixed gauges are further classified as :
Fixed Gauges
Fixed limit Gauges Fixed Master Gauges

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A9.2d Indicating Gauges

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Indicating gauges detect variations in a specific distance and display them on a dial or
graduated scale. They have the ability to detect minute errors because the variation shows
on the scale in an amplified version.
The majority of indicating gauges compare the actual dimension of the workpiece with
the dimension of a master setting gauges. For this reason, indicating gauging methods are
often referred to as comparative gauging. The master setting gauge is equal to the nominal
dimension to be gauged. The indicating gauge actually measures only the amount and
direction of any deviation, which exists in relation to the nominal size.
The amplification of indicating gauge movement may be mechanical, pneumatic, optical,
or electric. The tool designer will seldom be required to design the amplification system, as
this is generally done by a company specializing in the manufacture of indicating gauges.
The tool designer’s job will be to design fixtures to adapt the particular amplification system
to a practical inspection setup.
The following figure shows a series of levers, which is an extension of the simple lever
movement making a compact device with greater magnification. However, in this type the
errors due to wear, friction, and inertia must be carefully considered.
The following figure shows the lever movement applied to a simple depth gauge.
The device quickly checks the depth of surface, A, and if the workpiece is resting on a flat
surface such as a surface plate, the uniformity of surface A can be checked by rotating the
workpiece. The spring ensures that the spindle will remain in contact with the surface, and
the pin located in the plunger slot prevents the plunger from falling out when the workpiece is
removed.

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A9.2e Special Gauges
Special gauges are specially designed and manufacture for special purpose.
A9.2f Classification of Plain Gauges
Plain gauges are used in checking plain, that is, unthreaded holes and shafts. Plain
gauges are classified as following ways:
1. According to form of tested surface
2. According to purpose
3. According to type
4. According to design
1. According to the form of tested surface
According to the form of the tested surface, the gauges are of two types : Gauges for
checking the holes and gauges for checking the shafts. Gauges for checking the holes are
called “Plug Gauges” and those for checking the shafts arecalled “Snap or Gap gauges and
Ring gauges”.
a) Plug Gauge
Plug gauge are used to check the holes. The ‘go’ plug gauge is the size of the low limit of
the hole while the ‘not go’ plug gauge corresponds to the high limit of hole.
These are used to check the uniformity of holes. A plug gauge may be straight or tapered
and of any cross section. It may have either an integral or replaceable handle.

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b) Pin gauges
When the holes are larger than 3 in. (as, for example, automobile cylinder bores),
plug gauges are very heavy. In these cases, it is more convenient to use pin gauges. In
using a pin gauge, the gauge is placed lengthwise across the cylinder bore, and the
measurement is made in a manner similar to that of an inside micrometer. Pin gauges are
also used to measure the width of slots or grooves. In this connection, they are sometimes
called width gauges. Figure shows a typical pin gauge.
c) Snap, Gap or Ring Gauge
These gauges are used for gauging the shafts and male components. The ‘Go’ snap
gauge is of a size corresponding to the high (maximum) limit of the shaft, while the ‘Not Go’
gauge corresponds to the low (minimum limit).

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Double ended type snap gauges can be conveniently used for checking sizes in the
range of 3 mm to 100 mm and single ended progressive type snap gauges are suitable for
size range of 100 to 250 mm. The gauging surfaces of the snap gauges are hardened upto
720 HV and are suitably stabilis ed, ground and lapped. The other surfaces are finished
smooth.
d) Ring Gauges
These are gauges whose inside measuring surfaces are circular in form. Ring
gauges are used to measure cylindrical surfaces, tapers on shafts, and similar work pieces,
as well as external threads.

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Plain Ring Gauges
The plain ring gauges are made of suitable wear resisting steel and the gauging
surfaces are hardened to a hardness of about 720 H.V. The gauging surfaces are first
suitably stabilised using proper heat treatment process and then ground and lapped and
other surfaces are finished smooth. These are protected against climatic conditions by
applying a suitable anti-corrosive coating.
These are available in two designs, ‘Go’ and ‘No Go’. These are designated by ‘Go’
and ‘No Go’ as may be applicable, the nominal size, the tolerance of the work piece to be
gauged, and the number of the standard followed.
2. According to Purpose
According to purpose, the gauges may be classified as :
(a) Workshop gauge or Working gauge
(b) Inspection gauge
(c) Purchase inspection gauge
(d) Reference or Master gauge

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(a) Workshop Gauge
Workshop gauge or the manufacturing gauge is used by the machine operator to check
the dimensions of the parts as they are being produced. These gauges usually have limits
within those of the component being inspected. They are designed so as to keep the size of
the part near the centre of the limit tolerance.
(b) Inspection Gauge
Inspection gauges are those used by inspectors in the final acceptance of manufactured
parts when finished. These gauges are made to slightly larger tolerances than the workshop
gauges so as to accept work slightly nearer the tolerance limit tha n the workshop gauges.
This is to ensure that work, which passes the working gauge, will be accepted by the
inspection gauge also.
(c) Purchase Inspection Gauge
The need of such gauges arises when the products of other plants are to be accepted.
The purchaser must remember that the parts may have been made and checked by working
gauges worn to the maximum permissible degree. Therefore the ‘Go’ side of the purchase
inspection gauge must be designed accordingly. Thus, nominal size of ‘Go’ purchase
inspection gauge will be equal to the lower limit of the hole. ‘No -Go’ purchase inspection
gauge design is similar to ‘No-Go’ working gauge.
(d) Reference or Master Gauges
Reference or master gauges are used only for checking the size and condition of other
gauges. Reference gauges are the reverse or opposite in form to working or inspection
gauges. Due to the expenditure involved, reference gauges are seldom used and gaug es
are checked by universal measuring instruments optimeters, comparators etc. or gauge
blocks (for snap gauges).
3. According to type
(a) Standard gauges
(b) Limit gauges
(a) Standard Gauges
Every gauge is almost a copy of the part example, a bushing is to be made which is to
mate with a shaft. In this case, shaft is the mating part. The bushing is checked by a gauge,

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which in so far as the form of its surface and its size is concerned, is a copy of the mating
part, that is, the shaft.
If a gauge is ma de as an exact copy of the opposed (mating) part, in so far as the
dimensions to be checked are concerned, it is called a “standard gauge’. The first gauges to
be developed were the standard gauges. The first standard gauges were the opposed
(mating) parts themselves. When a component is assembled with its mating part, a (mating)
part itself. However, such individual fittings are not convenient or even possible in mass
production conditions. Moreover, the two parts to be assembled might be in productio n in
two different shops or even at two different plants. Therefore, it is more proper to use, as a
checking tool, not the mating part, but its exact copy as far as the tested dimension is
concerned.
Such a standard gauge has two drawbacks:
i) The quality of the manufactured part will depend upon the freedom with which it
mates with the standard gauge. The judgment of this freedom is a relative thing and it
usually creates misunderstanding between the purchaser and the manufacture.
ii)A standard gauge ca nnot be used to check an interference fit. For example, if a
bushing of 50 mm diameter is to be made for assembly with a shaft of 50.1 mm diameter,
then the standard gauge diameter will be 50.1 mm of opposed part. Such a gauge will
not pass into a properly produced bushing of diameter 50 mm.
(b) Limit Gauge
The system of limit gauges is very widely used in industries. Limit gauges are made to
the limits of the dimensions of the part to be tested. As there are two limits of the dimensions
of a part, high and low, two gauges are needed to check each dimension of the part. The
part is checked by successively assembling each of the gauges with it. Since the
dimensions of a properly manufactured part must be within the prescribed limits, one of the
gauges called a “Go Gauge” should pass through or over the part, while the other gauge
called a “Not Go Gauge” should not pass through or over the part. Gauges should pass
through or over a part under their own weight and the part and the gauge must be at the
same temperature.

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4. According to Design
According to design, the gauges may be classified as:
(i) (a) Single limit(b) Double limit
(ii) (a) Single ended(b) Double ended
(iii) (a) Fixed (b) Adjustable
(iv) (a) Integral end(b) Renewable end
(v) (a) Solid end (b) Hollow end
5. Common types of fixed gauges
a) Threaded or screw Gauges
These gauges are designed along the same general principles as all other types of
gauges. These gauges are of Plug and Ring type and are made in the ‘Go’ and ‘Not Go’
models. These are of special quality gauge steel, hardened and seasoned before the
threads are ground and finally lapped to dimensions. Nuts or internal threads are che cked
with Plug thread gauges and screws or external threads, with ring thread gauges.
Thread gauges similar to plain gauges are designed with manufacturing tolerance for
new gauges and wear allowance. Both tapered and trilock methods securing the plug
gauges into their holders are employed. In case of ‘Go’ and ‘Not Go’ plug gauges, it is

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common practice to make the wearing side, i.e., the ‘go end’ at least twice as long as the ‘not
go end’. The maximum wear in the latter case occurs on the end thread or threads.
In screws threads, there are three classes of fit : (a) Close fit (b) Medium fit (c) Free
fit. Tolerances for major, minor and effective or pitch diameters for all these three types of fits
are given in national standards. Screw or thread gauges take the form of the mating thread
and are assembled with the thread to be checked. By suitable designing the gauges, it is
possible to provide a limit gauging system, which will control and complex dimensions of the
threads within the tolerance limit.
There are two types of thread gauges :
1. Plug screw gauges 2. Ring screw gauges
b) Receiving gauges
These are similar to ring gauges but are used to verify the specified uniformity of size
and contour of noncircular holes. They are extensively used to check splined shafts. Three
receiving gauges are shown in Fig.

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c) Flush Pin Gauges
These types of gauges are mainly used to check the depths of slots, but can also be
used for gauging lengths and tapers. A flush pin is defined as : a gauge for checking the
distance between two surfaces, comparising a body having a through hole, and a pin in the
hole which projects from a face of the body, a distance equal to the dimension to be gauged
when the opposite or indicating end of the pin is flush with the opposite face of the body. The
indicating end of the pin, or the adjacent face of the body, has a step of a depth equal to the
tolerance on the dimension gauged.
d) Form Gauges
These are specially designed to check the form or contour of a workpiece.
Consequently this type of gauge is of particular interest to the tool design draftsman.
Gauges used to check a cutting tool. Another form gauge is also used to check the contour
of railroad wheel threads. Pratt & Whitney makes a series of similar gauges to the
specifications of the American Association of Railroads. These gauges not only check the
contour of a tire or flange when new but show the amount of wear and indicate when wheels
should be reconditioned.

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e) Template Gauge
These are similar to form gauges but are designed to check the position and
dimensions of two adjacent surfaces. Template gauge designed to check two shoulders of a
shaft. Template gauge designed to check the ways (or slides) of a machine tool.
f) Gauge Blocks
Length dimensions used in industry are based on the standard unit of dimensional
measurement, such as the international inch or meter. Until recent times, the basic
reference unit has been the International meter bar or its official duplicates. More recently,
the international basic standard of length has been defined in terms of a specified
wavelength of light. In theory, all gauges should be checked against the basic international
standards, but when millions of gauges are considered, such a procedure would obviously
be impractical. Large companies may be able to stand the expense of special equipment
needed for achieving reliable length measurements based on wavelength, but the majority of
companies cannot. The logical solution is to use the basic standard to calibrate many
secondary standards, which are close replicas of the former. The secondary standard would

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be manufactured in quantity by mass production methods in order to be relatively
inexpensive.
The secondary standards that have emerged are known as gauge blocks and have
become commonplate in modern plants. On first thought one tends to classify gauge blocks
as a master fixed gauge, but in reality they are much more than this. A single gauge block
may be classed as a master fixed gauge, but a single gauge block has little use except in
special cases. Gauge blocks must be viewed from the concept of several single master
gauge blocks being combined into a single gauge bar. The combined single blocks result in
a bar whose actual dimension truly represents, within specific limits, the nominal dimension
sought for a particular application. A typical set of gauge blocks is shown in figure.

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The tool designer will not be responsible for the manufacture of gauge blocks, but the
needs to understand how they are used. The gauges the designs will be calibrated and
checked with gauge blocks before the gauge is placed in use.
Gauge blocks are made of steel, chrome plated steel, stainless steel, tungsten
carbide, or chrome carbide, depending upon the manufacturer and the price the purchaser is
willing to pay. They are stabilized for minimum dimensional change and are hardened to
RC63 to 64. Standard blocks may be rectangular (approximately 1/8 x 1¼ in.) or square
(approximately 1 by 1 in.) in shape. The gauging surfaces are lapped to a very high finish.
Gauge blocks set are classed in grades according to accuracy.
When individual gauge blocks are combined, or built up, to provide a specific
measurement, they must be wrung together. This is accomplished by a twisting and sliding
motion that squeezes out the air between the almost geometrically flat gauging surfaces.
When properly wrung together, the blocks adhere to each other to the extent that
considerable force must be exerted to pull them directly apart. This phenomenon is usually
explained as a combination of molecular attraction and the cementing action of oil or
moisture film on the gauging surfaces. Steel age blocks should not be wrung together any
longer than nece ssary, as moisture trapped between the blocks may cause corrosion.
Gauge block sets are available with various numbers and combinations of blocks.
A particular size can be built up by wringing individual slip gauges together.
6. Wringing :
Wringing is the act of joining the slip gauges together while building up to sizes. It is due
to molecular attraction & cementing action of moisture.
Some sets of slip gauges also contain protector slips of some standard thickness
made from higher wear resistance steel or tungsten carbide. These are used for protecting
the exposed faces of the slip gauge pack from damage.

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7) Classification of block gauges on basis of grades.
¨Grades
a) Grade ‘00’ accuracy : It is a calibration grade used as a standard for reference to test all
the other grades.
b) Grade ‘0’ accuracy : It is an inspection grade meant for inspection purposes.
c) Grade i accuracy : Workshop grade for precision tool room applications.
d) Grade ii accuracy : For general workshop applications.
Block Gauges
Grade 00accuracy Grade 0 accuracy Grade iaccuracy Grade iiaccuracy

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A9.3 Design of Gauges
A9.3a Manufacturing Tolerances
We know that is any other manufacturing process, in gauge making also it is
economically impractical to attempt to make ‘Go’ and ‘Not Go’ gauges exactly to the two
limits of the work tolerance. Thus it is necessary that permissible deviations in accuracie s
must be assigned for gauge manufacture. Gauge maker’s tolerance or manufacturing
tolerance should be kept as small as possible so that a large proportion of the work tolerance
is still available for the manufacturing process. However the small the gauge tolerance, the
more the gauge will cost.
There is no universally accepted policy for the amount of gauge tolerance. However,
the following norms are generally accepted : Limit gauges are made 10 times more accurate
than the tolerances they are going tocontrol. That is, the tolerance on each gauge whether
‘Go’ or ‘Not Go’, is 1/10
th
of the work tolerance. For example, if the work tolerance is 10
units, then the manufacturing tolerance for ‘Go’ and ‘Not Go’ gauge each will be 1 unit. This
makes it possible, although the probability is small, for the work tolerance available in the
shop to be cut down to 80% of the specified tolerance. The amount of tolerance on
CHAPTER OUTLINE
A9.3 Design of gauges
A9.4 Sub zero treatment
A9.5 Maintenance, safety & storage
A9.6 Numerical examples
TOPIC OUTLINE
9.3a Manufacturing tolerance
A9.3b Wear Allowance
A9.3c Taylor’s Principle
A9.3d Fixing of gauging elements
(ends) with handle
A9.3e Provision of Pilot
A9.3f Correct Centering
A9.3g Materials
A9.3h Hardness and Surface finish
A9.3iRigidity
A9.3j Alignment
A9.3k Gauge members
A9.3l Gauge marking
A9.3m Essential features
A9.4a Characteristics of subzero
treatment

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inspection gauges is generally 5% of the work tolerance. Tolerance on reference or master
gauges is generally 10% of the gauge tolerance.
A9.3b Wear Allowance
Mostly the measuring surfaces of ‘Go’ gauges, which constantly rub against the
surfaces of the parts in inspection, are subjected to wear and loose their initial size. ‘Not Go’
gauges are not subjected to so much wear as ‘Go’ gauges and there is considerable wear on
‘Go’ gauges only. The size of go plug gauge is reduced while that of go snap gauge
increases. It is of course desirable to prolong the service life of the gauges, and therefore a
special allowance of metal, known as wear allowance is added to the go gauge in a direction
opposite to wear. Wear allowance is usually taken as 5% of work tolerance. Wear
allowance is applied to a nominal go gauge diameter before gauge tolerance is applied.
A9.3c Taylor Principle
This principle is based on the use of a correct system of limit gauges to inspect shafts
and holes. According to Taylor it is not adequate to use simple Go gauge on outer
dimension only but the shape is an importan t factor, i.e. the Go gauge should be full form
gauge and it should be constructed with reference to the geometrical form of the part being
checked, in addition to its size. In other words, Go gauge should check all the dimensions of
a work piece in the maximum metal condition. As regards Not Go gauges, Taylor was of the
view that it was useless for Not Go gauge to be full form, and each feature being dealt should
be checked with a specific Not Go gauge. In other words, Not Go gauge shall check only
one dimension of the work piece at a time, for the minimum metal condition. Thus according
to it, a hole should completely assemble with a ‘Go’ cylindrical plug gauge made to the
specified ‘Go’ limit of the hole, having a length at least equal to the length of engagement of
the hole and shaft. In addition, the hole is measured or gauged to check that its maximum
diameter is not larger than the ‘No Go’ limit. The Taylor principle interprets the limit of size
for gauging holes and shafts as follows :
1. For holes
If the Taylor principle is followed then the diameter of the largest perfect imaginary
cylinder which can be inscribed within the hole so that it just contacts the highest points of
the surface, shall not be a diameter smaller than the ‘Go’ limit ofsize. Further, the maximum
diameter at any position in the hole should not exceed the ‘No Go’ limit of size.

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2. For shafts
If the Taylor principle is followed than the diameter of the smallest perfect imaginary
cylinder which can be circumscribed about the shaft so that it just contacts the highest points
of the surface, should not be a diameter larger than the ‘Go’ limit of size. Further the
minimum diameter at any position on the shaft should not be less than the ‘No Go’ limit of the
size.
It may be noted the Taylor principle does not take care of the error of form, circularity
or straightness etc., the tolerances for which should be specified separately.
Thus according to Taylor principle, we require a plug ring gauge having exactly the
‘Go’ limit diameter and a length equal to the engagement length of the fit to be made for
checking the ‘Go’ limit of the work piece and this gauge must perfectly assemble with the
work piece to be inspected. The other gauge needed is the ‘No Go’ gauge, which contacts
the work piece surface only in two diametrically opposite points and at those two points it
should have exactly ‘No Go’ limit diameter. This gauge should not be able to pass over in
the work piece in any consecutive position in the various diametric directions on the work
piece length.
In certain applications, the Taylor principle cannot be strictly and blindly followed.
The following deviations are allowed which basically do not deviate f rom the principle as
such.
3. For ‘Go’ Limit
(i)In case the manufacturing process assures that the error of straightness will not
affect the character of fit of the assembled work pieces, it is advisable to go for
standard gauge blanks instead of using full form and length of engagement and
make the gauge unnecessarily bulky and cumbersome and avoid the special gauge
of exactly same working length for one work piece.
(ii)If gauge happens to be too heavy, only segmental cylindrical bar could be used
provided the manufacturing process ensures that errors of roundness will not have
any effect on the character of fit of the assembled work pieces.
(iii)For shafts, particularly heavy ones; it is generally not desirable to use full form ring
gauges but only gap gauges. But for this purpose the manufacturing process used
should take care of the error of roundness (especially lobbing) and the error of
straightness.

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4. For ‘No Go’ Limit
The two point checking devices are not feasible and practicable in actualpractice
because these are subject to rapid wear etc.; these can be safely replaced by small
planes/cylindrical surfaces/spherical surfaces. For gauging very small holes and in cases
where work pieces may be deformed to an oval by a two point mechanical contact device,
the ‘No Go’ gauge of full form may have to be used.
5. Allowable Deviation from the Taylor’s Principle
In some applications, difficulties are experienced in conveniently using gauges if they
are strictly based on Taylor’s principle. Accordingly some deviations may be permitted.
According to Taylor’s principle, a Go gauge should be of full form having length equal
to engagement length of fit, but this is not always necessary. For example, if it is known that
the manufacturing process ensures that error of straightness of hole or shaft is so small that
it would not affect the desired fit of assembly, then length of Go cylindrical plug or ring gauge
may be less than the length of engagement.
For very big holes, the full form gauge may be too heavy and inconvenient to use.
Therefore, segmental cylindrical bar or spherical gauge may be used if it can be assumed
that the manufacturing process would not produce the error of roundness outside the
permissible limits to affect the character of fit.
Similarly ‘Go’ gap gauge can be used in place of Go cylindrical ring gauge (which is
often inconvenient for gauging shafts) provided manufacturing process can ensure the
errors of roundness (lobbing) and straightness of shaft within permissible limits. The
straightness of long shafts of small diameter should be checked separately. Similarly,
though Taylor’s principle desires use of two point checking device to check No Go limit, but
same is not always necessary in following cases.
Since point contacts are subject to rapid wear, these can be replaced by small plane,
cylindrical or spherical surfaces whenever appropriate. The two point checking device is
also found difficult to design and manufacture for gauging very small holes and for such
cases No Go plug gauge of full cylindrical form can be used. However possibility of
accepting work pieces having diameters outside the No Go limit should be checked.
In certain case the non -rigid workpiece may be deformed by 2 points mechanical
contact device and in such cases No Go ring or plug gauges of full cylindrical form have to be
used.

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No Go gauges of full cylindrical form can also be used for thin walled work piece,
which may be out of round due to heat treatment, but would become circular when such
gauges are applied with force just sufficient to convert the elastic deformation into circularity.
A9.3d Fixing of gauging elements (ends) with handle
Plug gauges can be of solid type in which the gauging members are integral with the
handle or the gauging elements can be separated from the handle and suitable fixed
together. There are known as “Renewable” type of gauges. Below 50 mm diameter, solid
type gauges are mostly used. For larger diameters, the renewable end types of plug gauges
are used.
A9.3e Provision of Pilot
In case of very closely toleranced parts, sometimes it happens that the plug gauge
does not easily enter the hole. This requires skill and practice. Since the gauges are
generally used by semiskilled workers in industry, therefore, to solve this problem, we use

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what is known as piloting of plug gauge. The diameter of land is the same as that of the
plug-gauging portion.
A9.3f Correct Centering
For the purpose of fabrication of plug gauges, grinding is very extensively used, and
sometimes lapping may be used. In grinding, the centre of job and centre of machine plays
an important role. Centers for high -grade job should be very perfect. Sometimes before
grinding, the centers are even honed after heat treatment. After making the centre in the
plug gauges, for heat treatment purposes these centers shoul d be protected so that the
same may not get spoiled due to heat and burn. A typical centre in the gauge is shown in
figure. The recess and groove in the centre protect the centre from external contacts and
burns etc.
A9.3g Materials
Most gauges are s ubjected to considerable abrasion during their use and must,
therefore, be made of wear resistant materials. Hence, the materials for limit gauges should
meet of the following requirements :
(i)Uniformity of structure and required co-efficient of linear expansion.
(ii)Proper workability, especially in grinding and polishing.
(iii)Stability of dimensions and forms of parts in the process of operation and
possibly lower deformation in heat treatment and during manufacture.
(iv)High resistance to mechanical wear and corrosion.
(v)Optimal hardness which is a property characterizing a high durability and
resistance to damage in use.

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High Carbon and alloy steel have been used as gauge materials because of their
relatively high hardenability and abrasion resi stance. For high volume production runs,
gauge wear surfaces are often chrome plated. The durability of steel gauges coated with a
layer of chrome of 5 to 8 um is 10 to 12 times that of uncoated ones. For economy in
material as well as hardening costs, gauges are designed in such a way that only the parts
subjected to wear are made of hardened steel. Handles are made of cheaper MS. For high
degree of accuracy, long production runs and excessive wear conditions and particularly in
bigger gauges, the ent ire body is made of M.S. and only wear contact surfaces are
deposited with welded layer of hard materials such as cemented carbides, Satellite or
weartrode. Some gauges are made entirely of cemented carbides or they have cemented
carbide inserts at certainwear points.
A9.3h Hardness & Surface Finish
Recommended hardness for gauges is 60 to 64 Rock well C for plain gauges and 56
to 62 Rock well C for screw gauges. The recommended surface finish is : 0.127 to 0.254 um,
Ra for ground gauges and 0.05 to 0.20 um, Ra for lapped gauges.
A9.3i Rigidity
Rigidity is one of the most important features of gauge design. Gauges such as gap
gauges must always be designed with adequate rigidity as well as with robustness suitable
for use in shop where they seldom meet with the treatment they deserve.
A9.3j Alignment of Gauge Faces
In a normal gap gauge, the faces must be parallel and opposite to each other and the
points of contact with the work at each face must lie on a line normal to the gauging face.
A9.3k Gauge members
The gauges and gauges members shall be well finished and free from defects, which
may affect their serviceability and shall be consistent with the grade of the product.
The gauges and gauge members shall be demagnetized. All sharp corners and
edges of measuring surfaces shall be broken. Incomplete starting threads on thread gauges
shall be removed and blunt start shall be provided. The gauges shall be free from burrs.
Whenever grooves or recesses are provided for colour coding, they shall be of
0.3mm depth and 2mm minimum width and the ir bottom corner shall be suitably rounded.
The bottom corners of the grooves on gauges members shall be rounded off.

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Feature Size Handle Nos. as per
standard
Double ended handle
1 ~ 3mm
6 ~ 50mm
Handle No. 1 ~ 7
40 ~ 65mm Handle No. 8
Single ended handle 65 ~ 80mm Handle No.9 for Go gauge
Handle No.10 for No Go
There are two types of replaceable handles : (1) the taper lock (used for gauges up to 1.5
in. in diameter) and (2) the trilock (used for gauges above 1.5 in. in di ameter). The former
has a gauge member made with a taper shank that fits tightly into a mating taper in the
handle, while the latter has a reversibly gauging member, which has a central hole and three
adjacent grooves. A screw passes through the gauge and fastens this to the handle while at
the same time ensuring a snug fit by forcing three wedge shaped prongs into the
corresponding grooves of the gauge.
Plain Plug Gauges
Generally the gauging members of the plain plug gauges are made of suitable wear
resisting steel and the handles can be made of any suitable steel e.g. handles may be made
of light metal alloys for heavy plain plug gauges, or suitable non -metallic handles may be
provided for smaller plain plug gauges. The gauging surface of plain plug gauges are
normally hardened to not less than 750 H.V. and suitably stabilised and ground and lapped.
The plain plug gauges are normally of double-ended type for sizes up to 63 mm and
of single ended type for sizes above 63 mm.
The usual way of designati ng the plain plug gauges is by ‘Go’ and ‘No Go’ as
applicable, the nominal size, the tolerance of the work piece to be gauged and the number of
the standard followed e.g., a double ended ‘Go’ and ‘No Go’ plain plug gauge for gauging a
bore of 10 mm with tolerance H7 and if designed according to Indian Standard (IS : 3484 -
1966) shall be designated as :
‘Go’ and ‘No Go’ plain plug gauge 10 H7, IS : 3484.
The various types of plain plug gauges in common use are as below :
(i)‘Go’ and ‘No Go’ plain plug gauges for sizes up to 10 mm, (solid type).

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(ii)‘Go’ and ‘No Go’ plain plug gauges for sizes over 10 and up to 30 mm (Taper
Inserted Type).
(iii)‘Go’ and ‘No Go’ plain plug gauges for sizes over 30 mm and up to 63 mm (Fastened
type).
(iv)‘Go’ and ‘No G o’ plain plug gauges for sizes over 63 mm and up to 100 mm
(Fastened type).
(v)‘Go’ and ‘No Go’ plain plug gauges for sizes over 100 mm and up to 250 mm (Flat
type). This is a shell form plug gauge. Each plug is relieved to reduce weight.
For still further bigger holes and to restrict weight, use can be made of segmental
cylindrical ended gauges.
Spherically ended rods are used for very large holes. It would be noted that with
such types of gauges the full form of the gauge is lost and the errorsof holes like ovality may
not be detected. It is the general practice not to use cylindrical plugs above 100 mm
diameter but to use cylindrically ended bars or spherically ended rods. Similarly Go gauges
between the sizes of 100 and 200 mm diameter can take the form of a cylindrically ended
bar.
The plug gauges in order to protect them against climatic conditions. In order to
prevent damage in handling and transit, these are packed in suitable cases. It may be
mentioned that gauges with the gauging portion integral with the handle are now becoming
obsolete and gauges with renewable ends are gaining popularity because of the following
advantages :
(i)Worn or damaged end can be replace conveniently.
(ii)In the event of scrapping of gauge, handle can be used for other gauge.
(iii)To reduce the weight, handle can be made of plastic, which also facilitates in
handling the gauge, reduces cost and minimises risk of heat transfer.
For smaller through holes, another useful renewable end plug gauge is the
progressive type of gauge in which both the GO and NOT GO gauging members are
provided on same side separated by a small distance. First Go portion is inserted in the hole,
which would be further obstructed byNOT GO portions if hole is of tolerable size.
A9.3l Gauge marking
The plain gauges are marked with the following on their handles for their identification :

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(i)Nominal size.
(ii)Class of tolerance.
(iii)The word ‘Go’ on the ‘Go’ side.
(iv)Thewords ‘No Go’ on the ‘No Go’ side.
(v)The actual values of tolerance.
(vi)Manufacturer’s name or trademark.
The ‘No Go’side is always painted with a red band. A typical example of the marking
is shown in Fig. It is usual practice to apply suitable anti-corrosive coating to
A9.3m Essential features
These gauges are easy to handle and are accurately finished. They are generally
finished to one tenth of the tolerance they are designed to control. For example, if the
tolerance to be maintained is at 0.02 mm, then the gauge must be finished to within 0.002
mm, of the required size.
These gauges must be resistant to wear, corrosion and expansion due to
temperature. The plugs of the gauges are ground and lapped.
The Go end is made longer than the ‘No Go’ end for easy identification. Sometimes
a groove is cut on the handle near the ‘No Go’ end to distinguish it form the ‘Go’ end.
The dimensions of these gauges are usually stamped on them.
¨Surface finish Ra --- 0.04 –0.16 mm
¨All gauges and gauge members shall be demagnetized
¨Groove depth 0.3mm X 2mm width.
¨Taper used is 1:50
A9.4 Sub-Zero Treatment
Introduction of one or more cooling periods at a temperature well below the freezing
point of water, in the normal heat treatment process of steels is called sub-zero treatment.
Generally sub -zero treatment is carried out for gauges, as maintaining their
dimensional stability is very important. It’s objective is to ensure complete transformation of
austenite into marten site structure which is much compact and hard than austenite. Cooling
at 100°F is sufficient for plain carbon and low alloy steels. But for high alloy steel, several
cooling may be required, accompanied by tempering and air-cooling cycle between each.

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On performing this treatment on hardened work pieces, further hardening takes place by the
conversion of austenite to martensite. The deeper the cooling, higher the hardening effect.
Characteristics:
¨Even on cooling to cryogenic temperature, the austenite does not completely change to
martensite.
¨Repeated heat treatment at sub-zero temperature causes additional transformation of
austenite.
¨Variations in quenching rate within limits have virtually no effect on austenite
transformation.
¨Holding the hardened steel at +20°c for more than 15 minutes or heating to 150~170°F
before next cooling before 0°c can cause (sat) stabilization of austenite. The degree of
stabilization depends upon degree of alloying or time of holding. But practically it cannot
be completely transformed to martensite.
Sub-zero treatment produce high hardness, higher stresses. This increases deformation
and risk of cracking in sharp tools.
A9.5 Maintenance, Safety & Storage
Gauges should be used and cared for properly to ensure their maximum useful service
life. Some suggestions for their use and care are:
¨Master, inspection and working gauges should be applied only to the uses for which they
are intended, i.e., a master gauge shall be used to check inspection and working gauges,
an inspection gauges will be used to check the finished product and a working gauge
should be used to check the product as it is being manufactured.
¨A plain cylindrical gauge should be cleaned and a thin film of light oil should be applied to
the gauging surface before it is used. The work should also be cleaned. Then the gauge
should be aligned with the hole to be measured and given a forward motion combined
with a slight rotation. The ‘go plug’ gauge will enter the hole if the latter is of correct size
but if not so, then the gauge will not enter it. Keep the gauge moving into and out of the
hole when the fit is closed, to prevent seizure of the parts. The same sugges tions are
applicable to the use of plain cylindrical ring gauges.
¨Don’t force a snap gauge over work, because forcing will cause the gauge to pass
oversized parts and it may also spring the frame of the gauge. In fact force should be
avoided in any gauging operation as it tends to harm the gauge, the work or both.

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¨A gauge should be cleaned after use and prepared for storage. If it is to be stored for a
short time only, it should be coated with rust preventive oil. If it is to be stored for a long
period of time, however, it should be dipped in a molten plastic material designed as a
protection coating for tools and gauges.
A9.6 Numerical Examples
¨A 25 mm H8 – f7 fit is to be checked. The limits of size for H8 hole are : High limit
25.033mm, low limit 25.000 mm. The limits of size for the f7 shafts are : High limit 24.980
mm, low limit 24.959 mm. Taking gauge maker’s tolerance to be 10% of the work
tolerance, design plug gauge and gap gauge to check the fit.
Solution
Tolerance for hole = H.L. –L.L.
= 25.033 –25.00 = 0.033 mm.
Tolerance for shaft = H.L. –L.L.
= 24.980 –24.959 = 0.021 mm.
\Gauge makers tolerance for plug gauge = 0.1 x 0.033mm = 0.003mm (rationalised)
Gauge makers tolerance for gap gauge = 0.0021 mm = 0.002 mm (rationalised)
As the work tolerances are less than 0.09 mm, wear allowance may not be provided.
(i)Plug Gauge
Basic size of ‘Go’ plug gauge = L.L. of the hole (MMC) = 25.000 mm.
\In unilateral system,
Dimensions of ‘Go’ plug gauge =
000.0
003.0
mm00.25
-
+
That is,
High limit of ‘Go’ plug gauge = 25.000 + 0.003
= 25.003 mm
Low limit of ‘Go’ plug gauge = 25.000 mm
Now,
Basic size of ‘Not Go’ plug gauge = 25.033 mm
\Dimensions of ‘Not Go’ plug gauge =
003.0
000.0
mm033.25
-
+
(ii)Gap Gauge
‘Go’ side = H.L. of shaft (MMC)

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= 24.980 mm.
\Dimensions of ‘Go’ gap gauge =
002.0
000.0
mm980.24
-
+
‘Not Go’ side = L.L. of shaft = 24.959mm
\Dimensions of ‘Not Go’ gap gauge =
000.0
002.0
mm959.24
-
+
¨Shafts of 75 ±0.02 mm diameters are to be checked by the help of a Go, Not Go snap
gauges. Design the gauge, sketch it and show its Go size and Not go size dimensions.
Assume normal wear allowance and gauge maker’s tolerance.
Solution
High limit of shaft = 75.02 mm
Low limit of shaft = 74.98 mm
Work tolerance = 75.02 –74.98 = 0.04 mm
\Gauge makers tolerance (10%) = 0.004 mm
wear allowance = 0.002 mm
‘Go side’ of snap gauge = H.L. of shaft, (MMC) = 75.02 mm
‘Not Go’ side of snap gauge = 74.98 mm
Wear allowance is to be applied first to ‘Go’ side, before gauge maker’s tolerance is
applied.
‘Go’ side of snap gauge after considering the wear allowance
= 75.02 –0.002 = 75.018 mm
\Dimensions of snap gauge are given as:
Unilateral System
‘Go’
004.0
000.0
mm018.75
-
+
‘Not Go’
000.0
004.0
98.74
-
+
Bilateral System
‘Go’
002.0
002.0
mm018.75
-
+
‘Not Go’
002.0
002.0
mm98.74
-
+
Example

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Calculate the dimensions of plug and ring gauges to control the production of 50 mm
shaft and hole pair of H7d8as per I.S. specification. The following assumption may be made
: 50 mm lies in diameter step of 30 and 50 mm and the upper deviation for ‘d’ shaft is given
by –16 D
0.44
and lower deviation for hole H is zero. Tolerance unit i (microns) = 0.45
3
√D +
0.001 D and IT6 = 10i and above IT6 grade the tolerance magnitude is multiplied by 10 at
each fifth step.
Solution
For calculation of tolerance, value of diameter is taken as the mean of range in which
it lies.
= 38.7 mm
value of tolerance unit
D001.0D45.0i
3
+=
7.38x001.07.3845.0
3
+=
= 0.45 (3.38) + 0.0387
= 1.521 + 0.0387 = 1.5597 microns
= 0.00156 mm.
Now hole is of type H and grade 7
IT7 = IT6 X10
.2
= 10i x 10
0.2
= 10i x 1.585 = 15.85i
and for H hole, fundamental deviation = 0
and value of tolerance = 15.85 x 0.00156 = 0.02475 mm
\For hole H7disposition of work tolerance will be as shown in Fig.
For shaft d8
Tolerance = IT8 = 10
0.2
x IT7
= 1.585 x 15.85i = 25i (approx.)
= 25 x 0.00156 = 0.0391 mm.
Fundamental deviation for ‘d’ shaft
= 16D
0.44
= -16 x 38.7
0.44
= 16 x 5 = -80 microns = -0.1191 mm.
Upper deviation = -0.08 mm
150050X30D ==
mm68.381510 ==

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And work tolerance = -0.0391 mm.
\Lower deviation = -0.08 –0.0391 = -0.1191 mm.
Considering for Gauges
(1) Plug gauges
According to new system, ‘Go’ gauges are given 1/10
th
of work tolerance in the
tolerance zone and ‘No Go’ gauges, outside it.
Since work tolerance is less than 0.09 mm.
\Effect of wear in ‘Go’ gauges is not considered.
Work tolerance = 0.02475 mm.
\Gauge tolerance = 0.02475 mm
\Limits for ‘Go’ gauges are
50.0000 mm.
50.0025 mm.
And for ‘No Go’ gauge, dimensions are
50 + 0.0248 = 50.0248 mm.
50.0248 + 0.0025 = 50.0273 mm.
(2) Ring gauges
The various dimensions of Gap gauges are shown in Fig.4.56 and calculations made
in a similar manner as for plug gauges.
Dimensions for ‘Go’ gauge are
50 –0.08 = 42.92 mm
and 49.92 –0.0039 = 49.9161 mm.
Dimensions for ‘No Go’ gauge are
50 –(0.38 + 0391) = 49.8809 mm.
And 49.8809 –0.0039 = 49.8770 mm.
Answers : Plug Gauge.Dimensions for
‘Go’ gauge :
50.0009 mm.
50.0025 mm.
‘No Go’ gauge :
50.0248 mm.

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50.0273 mm
Ring Gauge. Dimensions for
‘Go’ gauge :
49.9161 mm.
49.9200 mm.
‘No Go’ gauge :
49.8770 mm.
49.8809 mm.
Gauge Design –Explanation for the abbreviations used
G = High limit of work piece (Hole)
H = Tolerance on cylindrical plug or cylindrical bar gauge
Hs = Tolerance on spherical gauges
Z = Distance between Centre of tolerance zone, of go gauges for holes and go
workpiece limit.
K = Low limit of workpiece
Y = Margin, out side of the Go workpiece limit of the ware limit of the gauges of
holes.
a = Safety zone provided for compensating measuring uncertainty of gauge for
holes.

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Y1 = Margin out side of the Go workpiece limit of the ware limit of gauges of
shafts.
Z1 = Distance between centre of tolerance zone of go gauges for shafts and go
workpiece limit.
H1 = Tolerance on gauges for Shafts.
HP = Tolerance on reference disc for gap gauges.
a1 = Safety zone provided for compensating measuring uncertainities of gauges
for shafts.
T = Workpiece tolerance = G – K
ES = Upper deviation of a hole (E’ cart superior)
EI = Lower deviation of a hole (E’ cart Inferior)
es = Upper deviation of a shaft
ei = Lower deviation of a shaft.
¨Gauges for Holes upto 180mm diameter (IT 6 to IT 16)
For all grades of work tolerances, the manufacturing tolerance (H) on the go gauge is
placed by an amount ‘Z’ inside the work limits. The wear allowance ‘Y’ is arranged for
grades IT 6 to IT 8 outside the work limits and no wear allowance on the gauges is specified
for grades IT 9 to IT 16. The manufacturing tolerance is symmetrical for GO gauges and NO
GO gauges. For NO GO gauges, manufacturing tolerance is placed above the upper limit of
workpiece.
¨Gauges for holes above 180mm diameter (IT 6 to IT 16)
As the tolerance zone above 180mm is comparatively great, it can be afforded to
guarantee the work limits and to shift the manufacturing gauge tolerances more inside the
work limits. A safety zone (a) is therefore introduced. This serves also as a safety zone
provided for compensating the measuring uncertainties of gauges for holes.
The basic size of No Go gauge for all grades of workpieces is reduced by a. On Go side,
for grades IT 6 to IT 8, permissible worn limit is increased by an amount a.
¨Gauges for shafts upto 180mm diameter

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The manufacturing tolerance H1 on the ‘Go’ gauge is placed by a distance Z, insidethe
limit of the workpiece. The wear allowance ‘Y’ is arranged outside the work limits for grades
IT 5 to IT 8 and no wear allowance are specified for grades IT 9 to IT 16.
Manufacturing tolerance for Go and No Go gauge is symmetrically positioned. For No
Go gauge, the basic size is the lower limit of workpiece.
¨Gauges for shafts above 180mm diameter
Similar to hole gauges, a safety zone ais provided on workshop gauges, by which the
basic size for ‘No Go’ side for all gra des of workpieces is increased above the low limit of
workpiece. On the Go side, for grades IT 5 to IT 8, the permissible worn out limit is
decreased by an amount a.
¨Steps in gauge design
Gauges for inside measurements (pluggauges)
Data given –Dimension of a hole with tolerance.
(Ex f20 H 7)
-Find out the value of tolerance for f20 H7 from standard chart.
-Corresponding to ‘T’the value of the following can be calculated or found out.
-Manufacturing Tolerance for gauges (H). From Table (1) corresponding to the shape of
the gauge chosen and tolerance grade of the workpiece, the grade of tolerance for size
and form of the gaugecan be found out.
-Form Table 2 the actual value of H, Y and Z, can be taken.
-Corresponding to the disposition diagram the formula for Go and No Go gauges can be
written. See table 4 for formula.
Similar way, the steps can be followed for gauges for shafts.
Examples
¨Design a plug gauge for dia 30 H7
30 H7
E S = +21 microns
E I = 0
Tol. = 21 microns
K (low limit size of hole) = 30.00
G (high limit size of hole) = 30.021

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Go gauge New = (K + Z)
2
H
±
Go gauge worn out = K – Y
From table 2
Z = 3 microns
Y = 3 microns
2
H
= 2 microns
Go gauge New = (30 + 0.003) ±0.002
= 30.003 ±0.002
Go gauge worn out = 30 –0.003
= 29.997
No Go gauge = 3.021 ±0.002
¨Design a plug gauge for a hole
1.0
2.0
40
+
+
K = Low limit = 40.1
H = High limit = 40.2
T = Tolerance = 0.1
From table 2
For T = 1 and size = 40
2
H
= 0.002
Y = 0
Z = 0.011
Go Gauge New = (K + 2)
2
H
±
= (40.1 + 0.011) ±0.002
= 40.111 ±0.002
Go gauge worn out = K – Y
= 40.1 –0 = 40.1

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No Go gauge = G
2
H
±
= (40.2) ±0.002
= 40.2 ±0.002
¨Design a plain ring gauge for diameter 40 K7
For diameter 40 K7
e s = +27 micron = 0.027
e i = +2 micron = 0.002
G = 40.027
K = 40.002
T = 0.0025
From table 3
002.0
2
H
1
=
Y1= 0.003
Z1= 0.0035
From table 4
Go gauge new = (G –Z1)
2
H
1
±
= (40.027 –0.0035) ±0.002
= 40.0235 ±0.002
Go gauge worn out
= G + Y1
= 40.027 + 0.003
= 40.030
No go gauge = K
2
H
1
±
= 40.002 ±0.002
¨Design a snap gauge for diameter 260 –0.05
G = 260
K = 259.95
T = 0.05

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From table 3
2
H
1
= 0.006
Y1= 0.006
Z1= 0.008
a1= 0.003
Go gauge new = (G –Z1)
2
H
1
±
= (260 –0.008) ±0.006
259.992 ±0.006
Go gauge worn out = G + Y1-a1
= 260 + 0.006 –0.003
= 260.003
No Go gauge = (K + a1)
2
H
1
±
= 259.95 ±0.006
TABLE-1 MANUFACTURING TOLERANCES FOR GAUGES
Tolerance
Grade for
workpiece
IT4 IT5 IT6 IT7 IT8 to IT10
IT11 to
IT12
IT13 to
IT16
Size
IT
Form
IT
Size
IT
Form
IT
Size
IT
Form
IT
Size
IT
Form
IT
Size
IT
Form
IT
Size
IT
Form
IT
Size
IT
Form
IT
Tolerance
Grade for
Cylindrical
Plug
Gauges
0+0+1+1+ 2 1 3 2 3 2 5 4 7 5
Tolerance
Grade for
Cylindrical
Bar Gauges
- - - - 2 1 3 2 3 2 5 4 7 5

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Tolerance
Grade for
Spherical
Plug or Dist
Gauges
- - - - 2 1 2 1 2 1 4 3 6 5
Tolerance
Grade for
Spherical
Ended Rod
Gauges
- - - - 2 1 2 1 2 1 4 3 6 5
Tolerance
Grade for
Cylindrical
Ring
Gauges
- - - - 3 2 3 2 4 3 5 4 7 5
Tolerance
Grade for
Gap
Gauges
- - - - 3 2 3 2 4 3 5 4 7 5
Tolerance
Grade for
reference
Disks for
Gap
Gauges
- - - - 1 1 1 1 2 1 2 1 3 2
Tolerance
Grade for
reference
cylindrical
Setting Plug
Gauges
- - - - 1 1 1 1 2 1 2 1 3 2
Tolerance
Grade for
reference
cylindrical
setting Ring
Gauges
- - - - - 1 1 1 2 1 2 1 3 2
Table –2 Gauge tolerances & their location for gauges. For inside measurements (Holes)
All values are in micron (um)
Nominal sizes
mm
Symbols
Work Tolerance Grades as per ISO
Over
Upto &
Incl.
6 7 8 9 10 11 12 13 14 15 16
-
3
T 6 10 14 25 40 60100140250400600
H/20.6 1 1 2 5 5
Y 1 1.53 0 0 0 0
Z 1 1.52 5 10 20 40
3 6
T 8 12 18 30 48 75120180300480750
H/20.75 1.25 1.25 2.5 6 6

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Y 1 1.53 0 0 0 0
Z 1.5 2 3 6 12 24 48
6 10
T 9 15 22 36 58 90150220360580900
H/20.75 1.25 1.25 3 7.5 7.5
Hs/20.75 0.75 0.75 2 4.5 4.5
Y 1 1.53 0 0 0 0
Z 1.5 2 3 7 14 28 56
10 18
T 11 18 27 43 701101802704307001100
H/2 1 1.5 1.5 4 9 9
Hs/21 1 1 2.5 5.5 5.5
Y 1.5 2 4 0 0 0 0
Z 2 2.54 8 16 32 64
18 30
T 13 21 33 52 841302103305208401300
H/21.25 2 2 4.5 10.5 10.5
Hs/21.25 1.25 1.25 3 6.5 6.5
Y 1.5 3 4 0 0 0 0
Z 2 3 5 9 19 36 72
30 50
T 16 25 39 6210016025039062010001600
H/21.25 2 2 5.5 12.5 12.5
Hs/21.25 1.25 1.25 3.5 8 8
Y 2 3 5 0 0 0 0
Z 2.53.56 11 22 42 80
50 80
T 19 30 46 7412019030046074012001900
H/21.5 2.5 2.5 6.5 15 15
Hs/21.5 1.5 1.5 4 9.5 9.5
Y 2 3 5 0 0 0 0
Z 2.5 4 7 13 25 48 90
Nominal sizes
mm
Symbols
Work Tolerance Grades as per ISO
Over
Upto &
Incl.
6 7 8 9 10 11 1213 14 15 16
80 120
T 22 35 54 8714022035054087014002200
H/2 2 3 3 7.5 17.5 17.5
Hs/22 2 2 5 11 11
Y 3 4 6 0 0 0 0
Z 3 5 8 15 28 54 100
120 180
T 25 50 63100160250400630100016002500
H/22.5 4 4 9 20 20

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Hs/22.5 2.5 2.5 6 12.5 12.5
Y 3 4 6 0 0 0 0
Z 4 6 9 18 32 60 110
180 250
T 29 46 72115185290460720115018502900
H/23.5 5 5 10 23 23
Hs/23.5 3.5 3.5 7 14.5 14.5
Y 4 6 7 0 0 0 0
Z 5 7 12 21 24 40 4580100170210
a 2 3 4 4 7 10 1525 45 70 110
250 315
T 32 52 81130210320520810130021003200
H/2 4 6 6 11.5 26 26
Hs/24 4 4 8 16 16
Y 5 7 9 0 0 0 0
Z 6 8 14 24 27 45 5090110190240
a 3 4 6 6 9 15 2035 55 90 140
315 400
T 36 57 89110230360570890140023003600
H/24.5 6.5 6.6 12.5 28.5 28.5
Hs/24.5 4.5 4.5 9 18 18
Y 6 8 9 0 0 0 0
Z 7 10 16 28 32 50 65100125210280
a 4 6 7 7 11 15 3045 70 110180
400 500
T 40 63 97155250400630970155025004000
H/2 5 7.5 7.5 13.5 31.5 31.5
Hs/25 5 5 18 20 20
Y 7 9 11 0 0 0 0
Z 8 11 18 32 37 55 70110150240320
a 5 7 9 9 14 20 35 55 90140220
Table –3 : Gauge tolerances & their location for gauges for outside measurement (Shafts )
All values are in micron
Nominal sizes
mm
Symbols
Work Tolerance Grades as per ISO
Over
Upto
& Incl.
5 6 7 8 91011121314 15 16
-
3
T 4 61014254060100140250400600
H1/20.6 1 1.5 1.5 2 5 5
Hp/20.4 0.4 0.6 0.6 0.6 1 1
Y1 1 1.5 3 0 0 0 0
Z1 1 1.5 2 5 10 20 40
3 6 T 5 81218304875120180300480750

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H1/20.75 1.25 2 2 2.5 6 6
Hp/20.5 0.5 0.75 0.75 0.75 1.25 1.25
Y1 1 1.5 3 0 0 0 0
Z1 1 2 3 6 12 24 48
6 10
T 6 91522365890150220360580900
H1/20.75 1.25 2 2 2 7.5 7.5
Hp/20.5 0.5 0.75 0.75 0.75 1.25 1.25
Y1 1 1.5 3 0 0 0 0
Z1 1 2 3 7 14 28 56
10 18
T 8 11182743701101802704307001100
H1/21 1.5 2.5 2.5 4 9 9
Hp/20.6 0.6 1 1 1 1.5 1.5
Y1 1.5 2 4 0 0 0 0
Z1 1.5 2.5 4 8 16 32 64
18 30
T 9 13213352841302103305208401300
H1/21.25 2 3 3 4.5 10.5 10.5
Hp/20.75 0.75 1.25 1.25 1.25 2 2
Y1 2 3 4 0 0 0 0
Z1 1.5 3 5 9 19 36 72
30 50
T 111625396210016025039062010001600
H1/21.25 2 3.5 3.5 5.5 12.5 12.5
Hp/20.75 0.75 1.25 1.25 1.25 2 2
Y1 2 3 5 0 0 0 0
Z1 2 3.5 6 11 22 42 80
50 80
T 131930467412019030046074012001900
H1/21.5 2.5 4 4 6.5 15 15
Hp/21 1 1.5 1.5 1.5 2.5 2.5
Y1 2 3 5 0 0 0 0
Z1 2 4 7 13 25 48 90
Table 3 Continued
Nominal sizes
mm
Symbols
Work Tolerance Grades as per ISO
Over
Upto
& Incl.
5 6 7 8 9 10111213 14 15 16
80 120
T 152235548714022035054087014002200
H1/22 3 5 5 7.5 17.5 17.5
Hp/21.25 1.25 2 2 2 3 3
Y1 3 4 6 0 0 0 0
Z12.5 5 8 15 28 54 100
120 180 T 18254063100160250400630100016002500

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H1/22.5 4 6 6 9 20 20
Hp/21.75 1.75 2.5 2.5 2.5 4 4
Y1 3 4 6 0 0 0 0
Z1 3 6 9 18 32 60 110
180 250
T 20294672115185290460720115018502900
H1/23.5 5 7 7 10 23 23
Hp/22.25 2.25 3.5 3.5 3.5 5 5
Y1 3 5 6 7 0 0 0 0
Z1 4 7 122124404580100170210
a1 1 2 3 4 4 7 101525 45 70 110
250 315
T 23325281130210320520810130021003200
H1/24 6 8 8 11.5 26 26
Hp/23 3 4 4 4 6 6
Y1 3 6 7 9 0 0 0 0
Z1 5 8 142427455090110190240
a11.53 4 6 6 9 152035 55 90 140
315 400
T 25365789140230360570890140023003600
H1/24.5 6.5 9 9 12.5 28.5 28.5
Hp/23.5 3.5 4.5 4.5 4.5 6.5 6.5
Y1 4 6 8 9 0 0 0 0
Z1 6 10 1628325065100125210280
a12.54 6 7 7 11153045 70110180
400 500
T 27406397155250400630970155025004000
H1/25 7.5 10 10 13.5 31.5 31.5
Hp/24 4 5 5 5 7.5 7.5
Y1 4 7 911 0 0 0 0
Z1 7 11 1832375570110145240320
a1 3 5 7 9 9 14203555 90140220
FORMULAE FOR GAUGE DIMENSIONS
GAUGES
FOR
GAUGE
SIZE
NORMAL SIZE
UP TO 180 mm. ABOVE 180 mm.
GAUGES REE GAUGE GAUGES REE GAUGE
BASIC
SIZE
MFG.
TOL.
BASIC
SIZE
MFG.
TOL.
BASIC
SIZE
MFG.
TOL.
BASIC
SIZE
MFG.
TOL.

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INSIDE
MEASUR-
EMENTS
NO GO G
2
HS
OR
2
H
±
±
NOT
PROVIDED
*
2
H
OR
2
HS
G ±-a
NOT PROVIDED
GO
(NEW)
K + Z
2
H
± K + Z
2
HS
OR
2
H
±
±
WEAR
LIMIT
K – Y - K–Y + a -
OUTSIDE
MEASUR-
EMENTS
WEAR
LIMIT
G + Y1 - G + Y1
2
HP
± G+Y1-a1 - G+Y1-a1 2
HP
±
GO
(NEW)
G - Z1
2
H
1
± G - Z1
2
HP
± G - Z1
2
HP
± G - Z1
2
HP
±
NO GO K
2
H
1
± K
2
HP
± K + a1
2
H
1
± K + a1
2
HP
±
2
H
*
SHOULD ONLY BE USED WHEN SPHERICAL GAUGES ARE NOT USED.

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FOR VALUES IN MICRONS REFER TABLE

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SUMMARY:
Manufactured parts must be checked to determine whether they are according to the
specifications or not, and also to control their dimensions. The measure dimensions are
compared with the standard specified dimensions to decide whether the components are
acceptable or not. In mass production, where large numbers of similar components are
produced, to measure the dimensions of each part will be a time consuming and costly
process. Therefore, gauges can check conformance of the part with tolerance specification.
Gauges are scale less inspection tools at rigid design, which are used to check the
dimensions of manufactured parts. They also check the form and relative positions of the
surfaces of parts. Gauges are easy to employ and can be used in many cases by unskilled
operators. Gauges are differing from measuring instruments. Gauges find wide application
in engineering particularly for mass production.
QUESTIONS:
1. Explain clearly what selective assembly means. Give one practical example.
2. What are the various types of plug gauges? Sketch any four of them and state their
specific applications.
3. What are snap gauges? Sketch and describe an adjustable snap gauge.
4. Distinguish between measuring instrument and a gauge.
5. What are the essential considerations in selecting the material for gauges? List some of
the materials commonly and explain the manufacture of gauges.
6. State and explain the “Taylor’s Principle of Gauge Design”.
7. Explain the following in connection with the gauge design:
(i) Gauge maker’s Tolerance(ii) Wear allowance
8. Differentiate between ‘Workshop Gauges’ and ‘Inspection Gauges’.
9. Explain Taylor’s principal & justified wear allowance is provided on Go gauges.
10.Explain about sub zero treatment of gauges.
11.Define a gauge & state the purpose of gauge.
12.Sketch and explain the use of limit gauges in mass production.
13.Explain the following gauges.
a) Plug gauge b) Ring gauge c) Snap gauge d) Receiving gauge

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Introduction
Purpose of jigs and fixtures
1) Automobile Industries
2) Air craft industries
3) Fixtures in numerically controlled machine tools
4) Fixtures for flexible manufacturing systems
5) Other application of jigs and fixtures (Ex. Plastic, textile, consumer products
Industries, etc.)
Advantages of Jigs & Fixtures -
1)Productivity: By way of eliminating the human effort in marking positioning and
frequent checking there is a considerable reduction in machine tool time and human
fatigue by using Jigs & Fixtures which ultimately contribute to the increase in productivity.
2)Interchangeability: Jigs and Fixtures facilitate the production of similar components of
uniform quality. So much so contribute to the interchangeability. Waiting for assembly is
totally avoided. Selective assembly of components is completely eliminated. They help
in the maintenance of uniform assembly and unified assembly schedules.
3)Skill Reduction: Skill of the individual is taken over by ji gs and fixtures by simplifying
the locating and clamping techniques. There is no need for skillful setting of work or tool.
Any average worker can be trained in the skills of using jigs and fixtures. The
replacement of unskilled/semi skilled workmen, in place of skilled workmen results in
considerable saving in labour cost.
CHAPTER OUTLINE
A2.1 Introduction of Jigs & Fixtures
TOPIC OUTLINE
a) Purpose of Jigs & Fixtures
b) Advantages of Jigs & Fixtures
c) Disadvantages of Jigs & Fixtures

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4)Cost Reduction : Higher production, reduction in wastage, reduction in labour cost,
easy assembly reduction in inspection costs etc. affects in substantial reduction in costs
by using jigs and fixtures.
c) Disadvantages of Jigs & Fixtures
1) Initial cost is high
2) Maintenance of the tool is a problem
3) More premium paid for Jig & Fixture.

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SUMMARY :
Jigs and fixtures are the special purpose tools used to facilitate production of similar
components with greater accuracy and productivity. With the development of technology the
application of jigs and fixtures is made even in batch production, where quality and reliability
are more important than the cost.
The industrial applications of jigs and fixtures is gaining importance in various
industries like Machine Tools, automobile, aircraft, Textile, Plastics, etc.
With the use of jigs and fixtures the present and future production processes can be
made more versatile with greater productivity.
QUESTIONS :
1) Explain advantages & disadvantages in using jigs & fixtures?
2) Explain purpose of jigs & fixtures?
3) What do you understand by the term jigs & fixtures?
4) What is the difference between jig & fixture?

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A5.1 Introduction
Jigs and fixtures are special purpose tools used in mass production. They provide a
means of manufacturing interchangeable parts as they establish a relationship with
predetermined tolerance between the work and the tool. This apart they eliminate the
necessity of a special set up for each individual part. Jigs are the devices that hold, locate
and guide the cutting tool on to the job. They are classified into boring and drilling jigs, which
are further classified as template jigs, plate jigs, box jigs etc. Fixtures are devices that hold
and locate a work piece for a specific operation but do not guide the cutting tool. They are
identified by the operation they perform such as assembly fixtures, welding fixtures, milling
fixtures etc. Some aspects rel ating to the use, classification and identification of jigs and
fixtures are discussed in this unit.
CHAPTER OUTLINE
A5.1 Introduction
A5.2 Function of jig & fixture
A5.3 Factor characteristic in jig design
A5.4 Jig support
A5.5 Jig bodies & rigidity
A5.6 Classification of jig
A5.7 Types of jig & their description
A5.8 Maintenance, safety & storage
TOPIC OUTLINE
A5.3a Tool design
A5.3b Design procedure
A5.3c Determining dimensions
A5.3d Initial Jig design
A5.6a Angle Plate jig
A5.6b Leaf jig
A5.6c Template jig
A5.6d Table jig
A5.6e Sandwich jig
A5.6f Box jig
A5.6g Channel jig
A5.6h Plate jig
A5.6i Trunning jig
A5.6j Multistation jig
A5.6k Indexing jig
A5.6l Universal jig
A5.6m Boaring Jig

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A5.2 Functions of Jigs & Fixtures
Jig is a special device used in production to hold, support, locate the work piece
besides guiding the cutting tool as the operation is performed.
A fixture, on the other hand, is a production tool that locates, holds and supports the
work securely to accomplish the machining operation. Fixtures also help to simplify metal
working operations performed on special equipment.
Essential difference between jig and a fixture is that the former incorporates bushes
that guide the tools employed whilst the latter holds the component, with the cutters working
independently of it.
A5.3 Factors Characteristic to Jig Design
The factors of design previously discussed are applicable to both jigs and fixtures,
since the function of a jig differs from that of fixture, there are some practices, which are
characteristic to the design of jigs and others which apply to fixtures.
A5.3a Tool Design
Tool design is the process of designing and developing the tools, methods, and
techniques necessary to improve manufacturing efficiency and productivity. It gives industry
the machines and special tooling needed for today’s high speed, high volume production. It
does this at a level of quality and economy that will insure that the cost of the product is
competitive. Since no single tool or process can serve all forms of manufacturing, tool
design is an ever changing, growing process of creative problem solving.
A5.3b Design Procedures
Once the tool designer decides that a template jig is the best choice for a particular
job, the design process begins. Following the planning processes outlined, the tool designer
assembles and evaluates allthe necessary data.
The part is a flat disc, 2.56 inches in diameter and 75 inch thick, with a 1.000 inch
hole in its center.
·The material specified is 1020 steel.
·The only operation required of the jig is to drill two holes .19 inch in diameter
1.770 inches apart.
·The blank part received for drilling is faced, drilled, and reamed to the specific
dimensions.

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A5.3c Determining Dimensions
When the preliminary sketch is completed, sizes and proportions for various parts of
the tool, and some dimensions are added. The exact dimensions are usually computed
when the final tool drawing is made.
A5.3d Initial Jig Design
After calculating the locator and bushing values, the designer is ready to plan the rest
of the tool. This first step in this initial design is rough sketching the part. Since the butt plate
is a flat disc, only two views need to be sketched, Figure.
Starting with the top view, the designer sketches in the rough outline of the jig plate,
Figure. To avoid confusion, draw the part in red and the tool sketch in black. This allows for
easy identification of the part at all times and prevents confusion where several lines lie
close together. Once thejig plate is sketched in, the designer adds the dimensions.
Onto the front view of the part, the tool designer sketches in the front view of the jig
plate. In this view, the designer must decide how long the locator pin should be. To avoid
sticking andjamming, make the locator no longer than one half the part thickness.
In this case, the pin should be .38 inch long. To maintain the proper alignment
between the holes, a pin must be placed in the first hole after it is drilled. The designer
should specify a standard jig pin to keep the tool cost as low as possible.
·Before the design is started, the tool drawing must be studied to obtain all necessary
information about the part.
·Careful calculations must be made to determine the exact size and location of the
locators and bushings.
·Each design should start as a sketch.
-Sketching helps to formulate the design.
-Sketches help reduce problems and show relationships.
-Once the design is set and the problems solved, the final design drawingis prepared.
A5.4 Jig support

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Common practice in jig design is the used of supporting feet for the jig. On cast iron
jig bodies the feet may be cast as parts of the body, or they may be a detachable type.
Detachable jig feet, usually made of hardened steel for wear resistance, may be purchased
as standard jig accessories. In some cases a hard alloy is welded to the jig feet for wear
resistance. Jig feet should be placed so as to give adequate and level support. As a rule, a
jig has four feel so that, a chip gets caught under one of the feet, the jig would rock. A three
legged jig would not rock with a chip under one of the feet. This might result in inaccurate
drilling.
Supports should be provided under points where the cutting tools exert pressure, so that
the jig and the work will not tip or spring under pressure.
A5.5 Jig Bodies & Rigidity
The design of the body of a jig or a fixture depends upon the locators, the clamps, or
other fastening devices and accessories already planned for the jig. Bodies and body
constructions are taken from standards.
The completed sketch includes a suitable method of latching the leaf in place in a
level position. The welded body design also includes satisfactory feet for this drill jig. Other
features are added which will provide a proper functioning tool.
A5.6 Classification of Jig
Jigs are broadly divides into two classes :
1. Drill Jigs
2. Boring Jigs
Jigs
Template
jig
Sandwitch
jig
Box jig
Leaf jig
Trunnion
jig
Multistation
jig
Plate jig
Angle
plate jig
Channel
jig
Indexing
jig
Pump jig
Combination
jig

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1. Drill jigs are used to drill, ream, tap, chamfer, counter bore, counter sink, reverse
spotface or reverse counter sink as shown in fig.
2. Boring jigs are used to bore holes which are either too large to drill or must be made on
off size.

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The basic jig is the same for all the machining operations but with the only difference in the
size of the busing used. Drill jigs may be divided into general types open and closed. Open
jigs are for simple operations where work is done on only one side of the part. Closed or box
jigs on the other hand are used for parts which need to be machined on more than one side.
The names used to identify the jigs refer to the way the tool is built. Following are some of
the common used jobs.
a. Angle Plate jig
b. Leaf jig
c. Template jig
d. Table jig
e. Sandwich jig
f.Box jig
g. Channel jig
h. Plate jig
i.Trunning jig
j.Multistation jig
k. Indexing jig
l.Universal jig
a) Angle Plate Jigs
These jigs are used to hold parts which are machined at right angled to their mounting
locators. Modified angle plate jigs are used for machining angles other than 90 degrees.

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b) Leaf Jigs
These are small box jigs with a hinged leaf to allow for easier loading and unloading. The
difference between the leaf and box jig is the size and part location. Leaf jigs are normally
smaller than box jigs and are made some times so that they do not surround the part totally.
Leaf jigs are usually provided with a handle for easier movement.

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c) Template Jigs
Normally used for accuracyrather than speed. This type of jig is not usually clamped but
fits over, on, or into the work. Templates are the least expensive and simplest type of jigs to
use. They may or may not have bushings. When bushings are not used, the whole jig plate
is normally hardened.
d) Table Jig
The table jig, figure, is basically a plain plate jig with legs. Its main purpose is holding
irregular or nonsymmetrical workpieces that cannot be held in other plate jig forms. With this
jig, the part is referenced by the surface being machined rather than the opposite side. This
surface relationship can be seen in figure. Table jigs can accommodate almost any shape
workpiece. Their only limitatio ns are the size of the part and the availability of clamping
surfaces. One other important point to consider is the tool thrust. The part is clamped
between the jig plate and the clamping device. Therefore, the tool thrust is directed toward
the clamps rather than the solid parts of the jig. Clamping devices must be selected to resist
this thrust.

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e) Sandwich Jigs
These are plate jigs with a back plate. These types of jigs are ideal for thin of soft
parts which could bend or wrap in another type of jigs.

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f) Box or tumble Jigs
Usually they surround the total part. This type of jig allows the part to be completely
machined on every surface without repositioning the work in the jig.
g) Channel Jig
It is the simplest form of box jig. The work is held between two sides and machined from
the third. In some cases, where jig feet are used the work can be machined on three sides.
h) Plate Jigs

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These are similar to templates but will have built in clamps to hold the work. These jigs
can also be made with or without bushings, depending on the number of parts to be made.
Plate jigs are sometimes made with legs to raise the jig off the table for large work and are
called table jigs.
i) Trunning Jigs
These are the forms of rotary jigs used for very large or odd shaped parts.
j) Multistation Jigs

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These are provided with simultaneous location and machining of several parts.
While one part is drilled another can be reamed, and yet another can be counter bored. The
final station can be used for unloading the finished part and loading the fresh parts. This jig
commonly used on multispindle machines.
k) Indexing Jigs
These are used to space the holes or other machined areas accurately around a part. To
accomplish this jigs need to use either the part it self or a reference plate and a plunger.
Rotary jigs are the larger versions of indexing jigs.

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l) Universal or Pump Jigs
These are commercially made jigs for very fast loading and unloading. They are
produced as basic units and are adopted to specific jobs. The salient features of these jigs
are:
i) Rigidity

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ii)Low height
iii)Ample chip clearance
iv)Ease of operation
v) One jig can be used for different workpieces by changing or removing the top plate.
The moving parts are compl etely protected from chips. The working parts consist of a
handle connected to a cam or rack which moves either a bushing plate or a nest vertically in
order to clamp a work piece. If the top plate is removable with a handle they are called pump
jigs.
m) Boring jigs
Boring jigs must be fastened to the bed or table of the machine. This req uirement
makes necessary a very rigidity designed body which will prevent springing. Often a
combination of a drilling and boring jig is required. The design of boring jigs is similar in
many respects to that of drill jigs.
Bored holes are often reamed as a final operation, the reamers being guided by slip
bushings which are inserted in the jig following the boring operations. Both the boring bar
and the reamer may be piloted by two bushings, one at each end of the hole. This is known
as front and backpiloting. It minimizes the springing of the cutting tool and assures
alignment and greater accuracy.

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A5.7 Types of jig and their description
1. Template Jig
A plate having holes at the desired positions serves as a template
which is fixed on the component to be drilled
2. Plate Jig or Table Jig
Is an improvement of the template Jig by incorportiporting drill
bushes on the template
2a. Modified Angle plate
Jig
A Jig for drilling holes at an angle
3. Channel Jig Is a simple type of Jig having a channel like cross –section.
4. Diameter Jig Is used to drill radial holes on a cylindrical or spherical workpieces
5. Leaf Jig The leaf Jig has a leaf or a plate hinged on the jig body
6. Ring Jig The ring Jig is employed to drill holes on circular flanged parts
7. Box Jig
The box jig is of box like construction within which the component
is located by the buttons.
8. The vise as a drill Jig
By providing attachments for holding drill bushings, the machine
vises may be used as drill jigs
9. Indexing drill Jigs
Indexing jigs are used for circular hole patterns in which the part is
indexed sucessively to the different positions under a single
bushing

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A5.8 Maintenance, Safety and Storage of Jig
Provision for Maintenance
Has provision been made for lubricating the tool mechanisms?
Have all wearing parts been hardened?
Are these parts easily made and replaced?
Have correct materials and heat treatment been specified?
Has provision been made for easy removal or pressed in parts?
Can vulnerable parts be removed and replaced quickly without disturbing the set up of the
fixture on the machine?
Manufacture and Maintenance Cost
Is the cost of the tool been properly related to the quantity and accuracy of the part to be
produced?
Is it too expensive for low volume production?
Are the production requirements high enough to warrant a better class of tool?
Standards
Havestandard, or readily purchasable, parts been specified wherever practicable?
Have all parts been designed for manufacture from stock size materials with a minimum
amount of machining?
If the material is not carried in stock, is the right kind and size readily available?
Manufacturing Facilities
Can the tool be made with the available tool making labor and equipment?
Are the tool dimensional tolerance as wide as possible?
Has the design included suitable datum surfaces for tool making operations?
Is it easy to set up the fixture for grinding locators or supports which have to be sized in
assembly?
If so, there plenty of clearance for the grinding wheel and spindle?
Has provision been made for easy alignment and starting of pressed in parts which need
accurate location?
Have blind holes been avoided wherever possible?
Would it help the heat treater to drill a small hole in each part, which normally has no holes in
it, so that it may be suspended it in a salt bath by a wire threaded through the hole?

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Safety
1. Does the fixture design protect the operator from coolant spray or flying chips?
2. Is the designed tool safe to operate with?
Handling and Storage
Lifting Aids
Have lifting lugs, eyebolts, or chain slots been provided for slinging heavy tools?
Have lifting handles been attached to all awkward or heavy loose parts of the fixture?
Loose Parts
If loose parts such as spacing pieces, wrenches, or locating pins are unavoidable, can they
be attached to the fixture with keeper screws or light chains toprevent loss in storage?
Fragile Parts
Is there any fragile part of the jig which needs a protective cover in storage?
Is the tool so delicate or highly finished as to require a special case, cover, or box to protect
it in storage?
Identification
Has the tool, and all loose items belonging to it, been marked clearly with identification
numbers or symbols?
Storage Aids
Can the tool be stowed safety without danger of tipping over?
Is a special storage stand or rack desirable for safe and convenient storage?

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SUMMARY:
The use of jigs is extending and developing very fast. The quality, type and
complexity of jigs used depend upon the type of job and its method of production. Jigs are
used for drilling, reaming, tapping, counter boring operations etc., and are broadly divided
into two types names, boring and drilling jigs. Drilling jigs are further divided into open and
closed jigs.
QUESTIONS:
1. How are the jigs normally classified?
2. What is an open jig?
3. What are the box jigs used for?
4. What class of jig would normally be used to tap holes?
5. Suggest a jig to be used for holding a part for machining angle other than 90 degrees.
6. What are the principles to be followed in designing of jigs?
7. Explain design procedure for jig.
8. How will you classify jigs?
9. Explain any one jig with figure.
10.What are the purposes of using bushes in jigs?
11.What are the rules for selecting clamp of work piece in jigs?
12.Why should a jig have four feet and not three? Explain the reason.
13.What are the main types of jigs? Discuss three with the help of suitable sketches.
14.Sketch the various types of jig feet.
15.What are the checks to be made for jigs for (a) provisionfor maintenance, (b)
manufacturing & maintenance cost, (c) handling, (d) loading & unloading, (e)
storage, (f) human factors

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A4.1 Introduction
In nature two extremely similar (identical) things are difficult to obtain. If at all we
came across exactly similar things, it must be only by chance. This fact holds good for
production of component parts in engineering also. No production process is good enough
to produce all items of products exactly alike.
Every production process involves a combination of three elements viz, men
machines and materials. Each of these elements has some inherent or natural variation as
well as some unnatural variations. The natural variations are due to chance causes, which
are difficult to trace and control. The unnatural variations are due to assignable causewhich
can be easily traced, controlled and reduced to economic minimum. These variables result
in the variation of size of components.
CHAPTER OUTLINE
A4.1 Introduction
A4.2 Advantages of Limit & fit
A4.3 Tolerance
A4.4 Limits
A4.5 Fits
A4.6 Types & assembly
A4.7 Allowance
A4.8 Deviation
A4.9 Minimum & maximum metal
conditions
TOPIC OUTLINE
A4.3a Tolerance & parts
A4.3b Tolerance zone
A4.3c Grades of Tolerance
A4.3d Unilateral tolerance
A4.3e Bilateral tolerance
A4.4a Limit of size
A4.4b Maximum limit of size
A4.4c Minimum limit of size
A4.5a Types of fits
A4.6a Trial & error assembly
A4.6b Interchangeable assembly
A4.6c Selective assembly
A4.7a Difference between
tolerance & allowance
A4.8a Actual deviation
A4.8b Upper deviation
A4.8c Lower deviation
A4.8d Fundamental deviation

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For example, suppose a drilling operation is to be performed on castings. The first
source of variation is the material itself (some castings may be harder than the others, some
of them may have blow holes, cracks etc.). If the operations are done on a mass production
by number of workers on different machines, the second source of variation is the machine.
The condition of machines may differ. The third source of variation, man, is the most variable
of them all. There may be differences in skill, experience of the workers doing the same job.
The same person may act in different ways in different psychological conditions and adds to
variability in the quality characteristics of the product.
If the process is under control, i.e., all the assignable causes of variation are
controlled or eliminated, the variations in sizes of similar components will be within
reasonable limits.
Generally, in engineering the article manufactured consists of assembly of number of
components. Thus a component manufactured is required to fit or match with some other
mating component. The correct and prolonged functioning of most manufacture d articles
depends on the correct size relationships between the various components of the assembly.
This means that the component parts must fit in a certain desired way, e.g., if a shaft is to
rotate in a hole, there must be enough clearance between theshaft and the hole to allow an
oil film to be maintained, for lubrication. Similarly, if the shaft is to be held tightly in the hole
there must be enough degree of tightness (interference) between them to ensure that the
forces of elastic compression gri p them tightly and do not allow any relative movement
between them. However, interference must not be excessive, as it may result in splitting of
component containing the hole.
Ideally any such condition could be obtained by specifying a definite size for the hole
and for the shaft, but unfortunately this is not possible due to inevitable inaccuracy of
manufacturing methods.
A4.2Advantages of Limits and Fits
1. It is not possible to make any part precisely to a given dimension, due to variability of
elements of production processes.
2. Even if by chance the part is made exactly to a given dimension, it is impossible to
measure it accurately enough to prove it.
3. If attempts are made to achieve perfect size the cost of production will increase
tremendously.

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4. The components will assemble together at random. During assembly, individual fitting is
not necessary. This result in reduction in cost of production because the elimination of
fitting reduces the time required to build the product.
5. Components can be manufacture in large batches or lots and all treated alike.
6. Machine tools which have been developed for quantity production enable the
components to be manufactured more rapidly using cheaper labour.
7. Repair of existing machines or products is simplified because component parts can be
easily replaced.
A4.3 Tolerance
Tolerance on a dimension is the difference between the high and low limits of size. It is
got to be allowed in order to cover the reasonable imperfection in workmanship and the
inevitable inaccuracy of manufacturing processes, and varies with different grades of work.
It can be unilateral or bilateral.
Fig. illustrates the concept of limits of size and tolerances.
A4.3a Tolerance of Parts
Due to the inevitable inaccuracy of manufacturing methods, it is not possible to make
a part precisely to a given dimension and may only be made to lie between two limits,
maximum and minimum, the difference between which is the permissible tolerance.
Forthe sake of convenience a basic size is a ascribed to the part and each of the two
limits is defined by its deviation from the size (the magnitude and sign of the deviation is
obtained by subtracting the basic size from the limit in question).
Tolerance is equal to the algebraic difference between the upper and lower
deviations and has an absolute value without sign. In the context of this terminology for
limits and fits, the difference between the maximum limit of size and minimum limit of size is
called the tolerance.

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A4.3b Tolerance Zone
In a graphical representation of tolerance, the zone bounded by the two limits of size
of the part is called the tolerance zone. It is defined by its magnitude (i.e. tolerance) and by
its position in relation to the zero line.
A4.3c Grades of tolerance
In a standardised system of limits and fits, group of tolerance are considered as
corresponding to the same level of accuracy for all basic sizes. It designated by the letters IT
followed by a number, e.g., IT01…01 16.
A4.3d Unilateral Tolerance
In this system, the dimension of a part is allowed to vary only on one side of the basis
size i.e., tolerance lies wholly on one side of the basic size either above or below it.
Unilateral system is preferred in interchangeable manufacture, especially when
precision fits are required, because:
(i)It is easy and simpler to determine deviations.
(ii)Another advantage of this system is that Go gauge ends can be standardized as
the holes of different tolerance grades have the same lower limit and all the
shafts have same upper limit.
(iii)This form of tolerance greatly assists the operator, when machining of mating
parts. The operator machines to the upper limit of shaft (lower limit for hole)

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knowing fully well that he still has some margin left for machining before the parts
are rejected.
A4.3e Bilateral Tolerance
In this system, the dimension of the part is allowed to vary on both the sides of the
basic size i.e., the limits of tolerance lie on either side of the basic size; but may not be
necessarily equally disposed about it.
In this system it is not possible to retain the same fit when tolerance is varied ad the
basic size of one or both of the mating parts is to be varied. This system is used in mass
production where machine setting is done for the basic size.
A4.4 Limits
These are two extreme permissible sizes for any dimension (high and low).
A4.4a Limits of Size
The two extreme permissible sizes between which the actual size is contained.
A4.4b Maximum Limit of Size
The greater of the two is called the maximum limit.
A4.4c Minimum Limit of Size
The smaller one of the two limits of size is called the minimum limit.

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A4.5 Fits
When two parts are to be assembled, the relation resulting from the difference between
their sizes before assembly is called a fit. Depending upon the actual limits of hole or shaft,
the fit may be clearance fit, or a transition fit, or a n interference fit.
Tolerance is considered on one side only. Actually it is representative of total tolerance.
It is done for sake of clarity and simplification. In schematic representation of tolerances, no
regard is given to show shafts and holes fully, but only their magnitude and relative position
w.r.t. basic size are highlighted.
The relationship existing between two parts, shaft and hole, which are to be assembled,
with respect to the difference in their sizes before assembly is called fit.
A4.5a Types of Fits (Classification of fits)
On the basis of positive, zero and negative values of Clearance, there are three basic
types of fits :
i) Clearance Fit
ii)Transition Fit
iii)Interference Fit
Fits
Clearance Fit Transition Fit Interference Fit
1) Slide fit
2) Easy slide fit
3) Running fit
4) Slack running fit
5) Loose running fit
1) Push fit
2) Wringing fit
1) Force fit
2) Tight fit
3) Shrink fit

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i) Clearance Fit
In this type of fit, the largest permitted shaft diameter is smaller than the diameter of the
smallest hole, so that the shaft can rotate or slide though with different degrees of freedom
according to the purpose of the mating members.
In this type of fit shaft is always smaller than the hole i.e., the largest permissible
shaft diameter is smaller than the diameter of the smallest hole. So that the shaft can rotate
or slide through with different degrees of freedom according to the purpose of mating part.
Clearance fit exists when the shaft and the hole are at their maximum metal
conditions. The tolerance zone of the hole is above that of the shaft as shown in figure.
Clearance
This is difference between the size of the hole and shaft, before assembly, when this
difference is positive.
Maximum clearance
In the case of clearance or transition fit, it refers to the difference between the maximum size
of hole and the minimum size of shaft.
Minimum clearance
In a clearance fit, it refers to the difference between the minimum size of hole and the
maximum size of shaft.
1) Slide fit
This type of fit has a very small clearance, the minimum clearance being zero. Sliding
fits are employed when the mating parts are required to move slowly in relation to each other

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e.g., tailstock spindle of lathe, feed movement of the spindle quill in a drilling machine, sliding
change gears in quick change gear box of a centre lathe etc.
2) Easy Slide Fit
This type of fit provides for a small guaranteed clearance. It serves to ensure alignment
between the shaft and hole. It is applicable for slow and non regular motion, for example,
spindle of lathe and dividing heads, piston and slide valves, spigots etc.
3) Running fit
Running fit is obtained when there is an appreciable clearance between the mating parts.
The clearance provides a sufficient space for a lubrication film between mating friction
surfaces. It is employed for rotation at moderate speed, e.g., gear box bearings, shaft
pulleys, crank shafts in their bearings etc.
4) Slack running fit
It is obtained when there is a considerable clearance between the mating parts. This
type of fit may be required as compensation for mounting errors e.g., arm shaft of I.C.
engine, shaft of certifigual pump etc.
5) Loose running fit
Loose running fit is employed for rotation at very high speed, e.g., idle pulley on their
shaft such as that used in quick return mechanism of a planer.
ii)Transition Fit
In a fit of this type the diameter of the largest allowable hole is greater than that of the
smallest shaft, but the smallest hole is smaller than the largest shaft, so that small positive or
negative clearance between the shaft and hole members are employable. Location fits e.g.,
spigot in mating holes, coupling rings and recesses are the examples of transition fit.
Transition fit lies mid way between clearance and interference fit. In this type the size
limits of mating parts (shaft and hole) are so selected that either clearance or indifference
may occur depending upon the actual sizes of the parts. Push fit and wringing fit are the
examples of this type of fit.

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In this type of fit the tolerance zones of the hole and shaft overlap completely or in
part.
iii)Interference Fit
In this type of fit, the minimum permitted diameter of the shaft is larger than the
maximum allowable diameter of the hole. In this case the shaft and the hole members are
intended to be attached permanently and used as a solid component but according to the
application of this combinations, this type of fit can be varied. Thus, if in use one of the two
members is subjected to wear, it should be possible to drive or force the two members apart
for replacement purposes. Examples of this type of fit are be aring bushes which are in an
interference fit in their housing e.g. a small end in the connecting rod of an engine.
In this type of fit the minimum permissible diameter of the shaft is larger than the
maximum allowable diameter of the hole. Thus the shaft and the hole members are
intended to be attached permanently and used as a solid component.
Elastic strains developed on the mating surfaces during the process of assembly prevent
relative movement of the mating parts. For example, steel tyres on railway car wheels,

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gears on intermediate shafts of trucks, bearing in the gear of a lathe head stock, drill bush in
jig plate, cylinder linear in block, steel rings on a wooden bullockcart wheels etc.
Interference
This is the arithmetical difference between the sizes of the hole and shaft before
assembly, when the difference is negative.
Minimum Interference
It is the difference between the maximum size of hole and the minimum size of shaft in an
interference fit prior to assembly.
Maximum Interference
In an interference or a transition fit it is the difference between the minimum size of hole and
the maximum size of shaft prior to assembly.
Basic size of a Fit
It is that basic size which is common to the two parts of a fit.
Variation of Fit
This is the arithmetical sum of the tolerance of the two mating parts of fit.
1) Force fit
Force fit are employed when the mating parts are not required to be disassembledduring
their total service life. In this case the interference is quite appreciable and, therefore,
assembly is obtained only when high pressure is applied. This fit, thus, offers a permanent
type of assembly, e.g., gears on the shaft of a concrete mixture, forging machine etc.
2) Tight fit
It provides less interference than force fit. Tight fits are employed for mating parts that
may be replaced while overhauling of the machine, for example, stepped pulleys on the drive
shaft of a conveyor, cylindrical grinding machine etc.
3) Heavy force and Shrink fit

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It refers to maximum negative allowance. Hence considerable force is necessary for the
assembly. The fitting of the frame on the rim can also be obtained first by heating the frame
and then rapidly cooling it in its position.
Hole basis system
In this system, the hole is kept constant and the shaft diameter is varied to give the
various types of fits. The basic size of the hole is taken as the low limit of size of the hole.
The high limit of size of the hole and the two limits of size for the shaft are then selected to
give the desired fit.
It is clear, therefore, that in this system, the actual size of a hole that is within the
tolerance limits is always more than the basic size; it can equal the basic size as a particular
case but can never be less. In the ‘Basic Hole System’, the holes get the letter ‘H’ and shafts
get different letters to decide the position of tolerance zone to obtain a desired fit.
Shaft basis system:
Here, the shaft is kept constant and the size of hole is varied to give the various fits.
The basic size of the shaft is taken as one of the limits of size (maximum limits) for the shaft.
The other shaft limit of size and the two limits of size for the hole are then selected to give the
desired fit.

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TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES
JIGS, FIXTURES & GAUGES
107
It is clear, therefore, that in this system, the actual size of a shaft that iswithin the
tolerance limits is always less than the basic size. As a particular case, it can equal the basic
size but can never be larger. In the ‘Basic Shaft System’, the shaft gets the letter ‘h’ and
holes get different letters to decide the position of tolerance zone to obtain a desired fit.
From a manufacturing point of view, it is preferable to use the “hole basis” system,
because it is economical. This is because a great many holes are produced by standard
fixes size tools, such as, twist drills,reamers, core drills, tapes, broaches, etc. The
advantages of using fixed size tools is that the machine need not be set up to obtain the
proper size of the hole, setting up operations can consequently be made quicker and
cheaper. Subsequently, the shaftsizes are more readily variable about the nominal size by
means of turning or grinding operation. It is easier and more convenient to manufacture
shafts of varying sizes than holes of varying sizes, as given above. The hole basis system is
preferred, because it lessons the range of cutting and measuring tools for machining of
holes, which are more expensive that tools to machine shafts. Also, the control of the size
and shape of holes is more complicated and less accurate than the control of shafts.
Applications : Machine and engine building, locomotive, construction.
The shaft basis system is more advantageous in certain cases, for example, this
system can be efficiently applied for long shafts machined to the same size over their full
lengths (smooth drawn shafts, shafts ground on centreless grinding machines etc.), if the
shaft is to mate with at least two parts having holes that require different types of fit.
Examples of “shaft basis” system are : the mating of a piston pin with both the piston and the
connecting rod, and the outer rings of antifriction bearings with various bores in housings,
electric motors, power transmission and products made from bright drawn bars.
It has been found in practice that a number of different fits of each basic type of fit are
required which can provide different degrees of tightness or freedom between the mating
parts.

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TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES
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The most commonly used fits of clearance type are : (1) Slide fit, (2) Easy slide,
(3) running fit, (4) slack running fit and (5) loose running fit.
1) Wringing fit
A wringing fit provides either zero interference or a clearance. These are used where
parts can be replaced without difficulty during minor repairs.
2) Push fit
The fit provides small clearance. It is employed for parts th at must be dis-assembled
during operation of a machine for example, change gears, slip bushing etc.
A4.6 Types of Assemblies
There are three ways by which the mating parts can be made to fit together in the
desired manner. These are:
4.6a Trial and Error
4.6b Interchangeable Assembly
4.6c Selective Assembly
A4.6a Trial and Error
When a small number of similar assemblies are to be made by the same operator the
necessary fit can be obtained by trial and error. This technique simply requires one part to
be made to its nominal size as accurately as possible, the other part is then machined with a
small amount at a time by trial and error until they fit in the required manner. This method
may be used for “one off jobs”, tool room worketc. where both parts will be replaced at once.
A4.6b Interchangeable Assembly
When a large number of components are to be produced then it will not be economical to
produce both the mating components by the same operator. In addition to economy it is also
essential to produce the components within the minimum possible time. This is only
possible by mass production system. In mass production system there is a division of
labour. The components are produced in one or more batches by different operatorson
different machines. Under such conditions in order to assemble the mating components with

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TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES
JIGS, FIXTURES & GAUGES
109
a desired fit, a strict control is exercised and the parts are manufactured with specified
tolerance limits.
When a system of this kind is used any one componentselected at random will
assemble correctly with any other mating component that too, selected at random, the
system is called interchangeable assembly. The manufacture of components under such
conditions is called interchangeable manufacture. Production on an interchangeable basis
results in increased output with a corresponding reduction in manufacturing cost.
Example : Suppose a clearance fit is required between the mating parts with hole,
specified as
mm00.0
04.0
25
-
+
And shaft
mm04.0
02.0
25
-
-
In this case the maximum permissible size of the hole will be = 25.04mm and the
minimum permissible size = 25.00mm. The dimensions of the number of holes produced will
lie between these two limits. Similarly, the maximum permissible shaft size = 24.94 mm and
the minimum permissible size of shaft = 24.96. The dimensions of all the shafts produced
will lie between these two limits. Therefore, even if we select any hole at random and
similarly any shaft at random with these permissible tolerances they will assemble with each
other and give the desired clearance fit.
Interchangeable assembly requires precise machines or processes whose process
capability is equal to or less than the manufacturing toleran ce allowed for that part. Only
then every component produced will be within desired tolerance and capable of mating with
any other mating component to give the required fit.
¨Advantages of Interchangeability
(a) The operator is not required to waste his skill in fitting the mating components by trial
and error and thus assembly time is reduced considerably.
(b) There is an increased output with reduced production cost.
(c) There is a division of labour, the operator has to perform same limited operations
again and again thus he becomes specialized in that particular work, which helps to
improve quality and reduce the time for operations.
(d) It facilitates production of mating components at different places, by different
operators.
(e) The replacement of worn out or defective parts and repair becomes very easy.
(f) The cost of maintenance and shutdown period is also reduced to minimum.

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110
A4.6c Selective Assembly
It is sometimes found that it is not economical to manufacture parts to the required high
degree of accuracy so as to make them interchangeable. The consumer not only wants
quality and precision trouble free products but also he wants them at economical prices.
Often special cases of accuracy and uniformity arise which might not be satisfied by
certainty at the fits given under fully interchangeable system. For example, if a part of its low
limit is assembled with the mating part at high limit, the fit so obtained may not fully satisfy
the functional requirements of the assembly. Complete interchangeability in the above
cases can be obtained at some extra cost in inspection and material handling by using
selective assembly whereby parts are manufactured to rather wider tolerances.
In selective assembly the components produced are classified into groups according to
their sizes by automatic gauging. This is done for both mating parts, holes and shafts, and
only matched groups of mating parts are assembled. It results in complete protection
against defective assemblies and reduces matching costs since the parts may be produced
with wider tolerances.
A practical example of this system is the assembly of pistons with cylinder bores. Let the
bore size be 50 mm and the clearance required for the assembly is 0.12 mm on the
diameter. Let the tolerance on bore and the piston each = 0.04mm. Then,
Dimension of bore diameter is 50
±0.02
mm.
Dimension of piston shirt is 49.44
±0.02
mm
By grading and marking the bores and the pistons they may be selectively assembled to
give the clearance of 0.12 mm as given below:
Cylinder bore 49.94 50.00 50.02
Piston 49.49 49.44 49.90

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111
A4.7 Allowance
An international difference between the hole dimension and shaft dimension for any type
of fit is called the allowance. Maximum allowance is obtained by subtracting the minimum
shaft size from the largest hole size and the minimum allowance is the differe nce between
the largest shaft and the smallest hole size. Thus allowance is positive for clearance fit and
negative for interference fit.
Allowance : The difference between the maximum shaft size and minimum hole is known
as allowance. In a clearance fit, this is the minimum clearance and is positive allowance. In
an interference fit, it is the maximum interference and is negative allowance. Positive and
negative allowances are shown in Figures respectively.
A4.7a Difference between tolerance & allowance
Tolerance Allowance
1. It is the permissible variation in It is the prescribed difference between the

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112
dimension of a part (either a hole or a
shaft).
dimensions of two mating parts (hole and
shaft).
2. It is the difference between higher and
lower limits of a dimension of a part.
It is the intentional difference between the
lower limit of hole and higher limit of shaft.
3.The tolerance is provided on a dimension
of a part as it is not possible to make a
part to exact specified dimension.
Allowance is to be provided on the
dimension of mating parts to obtain desired
type of fit.
4. It has absolute value without sign.Allowance may be positive (clearance) or
negative (interference).
A4.8 Deviations
The algebraic difference between a size (actual, maximum etc.) and the corresponding
basic size.
4.8a Actual Deviation
The algebraic difference between the actual size and the corresponding basic size.
4.8b Upper Deviation
The algebraic difference between the maximum of size and the corresponding basic size. It
is a positive quantity when the maximum limit of size is greater than the basic size and a
negative quantity when the maximum limit of size is less than the basic size. It is designated
by ESfor a hole and es for a shaft.
4.8c Lower Deviation

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The algebraic difference between the minimum limit of size and the corresponding basic
size. It is a positive quantity when the minimum limit of size is greater than the basic size and
a negative quantity when the minimum limit of size is less than the basic size. It is
designated by EI for a hole el for a shaft.
Deviation is defined as the algebraic difference between a size (act ual, maximum
etc.) and the corresponding basic size.
Upper deviation is the algebraic difference between the maximum limit of size (of
either hole or shaft) and the corresponding basic size. It is designated by letters ES for hole
and es for shaft. It is a positive quantity when the maximum limit of size is greater than the
basic size and a negative quantity when the maximum limit of size is less than the basic size.
Lower deviation is the algebraic difference between maximum limit of size and the
corresponding basic size. It is a positive quantity when the minimum limit of size is greater
than the basic size and a negative quantity when the minimum limit of size is less than the
basic size. It is designated by EI for a hole and ei for a shaft.
4.8d Fundamental Deviation
It is that one of the two deviations which is conventionally chosen to define the
position of the tolerance zone in relation to the zero line.
Fundamental deviation is that one of the two deviations which is conventionally
chosen to define the position of the tolerance zone in relation to the zero line. This may be
upper or lower deviation which is closet to the zero line. It fixes the positionof zero line.
Basic shaft is a shaft whose upper deviation is zero, e.g., shaft ‘h’.
Basic hole is one whose lower deviation is zero, e.g., hole ‘H’.
For shafts ‘a’ to ‘h’, the deviation is below the zero line and for shafts ‘j’ to ‘zc’ it is
above thezero line. For holes ‘A’ to ‘G’, lower deviation is above the zero line and for ‘j’ to
‘zc’, it is below the zero line. Upper deviation for shaft is denoted by es and lower deviation
by ei. For holes the corresponding deviations are denoted by ES and EI repectively. In the
specifications, formulae are given to determine the fundamental deviation. For example, for
shafts, the fundamental deviation (upper deviation es or lower deviation ei) is determined by

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114
means of the formulae given in the table 4.2 o n page 343. The other deviations may be
derived directly using the absolute value of the tolerance IT by means of the algebraic
relationship.
ei = es –IT
es = ei + IT
The deviation given in the table is that corresponding in principle to the limitclose to
the zero line, in other words the upper deviation es for shafts a to h and lower deviation ei for
shafts j to zc.
For holes also the deviations are derived from those of the corresponding shafts as
follows. The general rule is that hole limits are identical with the shaft limits of the same
symbol (letter and grade) but disposed on the other side of the zero line, i.e., EI upper
deviation of hole)=es of the shaft of same letter symbol but of opposite sign.
Hole Basis System and Shaft Basis Sys tem are defined in Figure for clearance fit,
transition fit and interference fit. In hole basis system different clearance and interferences
are obtained by associating various shafts with a single hole whose lower deviation is zero
(H hole). This is standard practice also as it is very convenient to make correct holes of fixed
sizes. Since holes are produced by drilling, reaming, etc. their size is not easily adjustable
because size of such tools are standard. On the other hand, size of shaft produced by
turning and grinding can be easily varied. In shaft basis system upper deviation of shaft is
zero, and different fits are obtained by varying the limits on the holes.

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For any basic size there are 25 different holes. These are obtained by providing a
series of holes which are progressively oversize and a series of holes which are
progressively undersize. The difference from basic size of the various holes is given by the
fundamentaldeviation and it is these differences in size which give the fit required. The 25
holes are designated by capital letters: A, B, C, D, E, F, G, H, JS, J, K, M N, P, R, S, T, U, V,
X, Y, Z, ZA, ZB, ZC.
Each of the 25 holes has a choice of 14 tolerances which are designated : IT 01, IT0,
IT 1, IT 2 up to and including IT 16. The tolerance grade decides the accuracy of
manufacture. The seven finest grades (IT 01 to IT 05) cover sizes up to 500 mm and the
eleven coarsest grades up to 3150 mm. The tolerance in each grade depends on the size of
shaft/hole.
Similarly for shafts, for any given size there are 25 different shafts designated by
small letters from a to zc. Also each shaft has 14 grades of tolerance grades which are
designated as for the holes.
The setting of tolerance values is not by itself sufficient to define particular limit, the
position of the tolerance zone relative to the basic size of the feature must also be specified.
This is done by establishing fundamental deviations which are differences between the basic
size and the nearest limit of tolerance. These fundamental deviations are obtained from
empirical formulae given in Table. These are designated by capital letters for holes and
small letters for shafts.
The fundamental deviations for holes A to H correspond exactly in value with those
for shafts a to h but are in opposite direction. Hole A and shaft a have the largest
fundamental deviations, hole being positive and shaft being negative, and the fundamental
deviations for both H and h are zero. Thus the first eight designations represent a clearance
fit system. The remaining groups JS to ZC (holes) and js and zc (shafts) do not correspond
in their deviations in quite the same way, they are intended for use in interference and
transition fits. The above facts are valid irrespective of any basic size.
The values of the fundamental deviations are functions not of the basic size but of the
range of sizes in which the basic size falls.
Maximum and Minimum Metal Conditions
Maximum m etal condition (MMC) corresponds to condition when a part has
maximum amount of metal, i.e. corresponding to high tolerance of shaft and low tolerance of
the hole. Similarly minimum metal condition corresponds to minimum size of shaft and

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JIGS, FIXTURES & GAUGES
116
maximum size of hole. MMC has special importance with regard to geometrical tolerancing
as it critically affects the interchangeability of manufactured parts, which are to be
assembled together.
SUMMARY:
With all the advancement in the machine tool technology, it is not possible to achieve
dimensional perfection due to the following reasons : Temperature changes, tool wear,
deflections and vibrations of the machine and the work and human error. Even if the
dimension is to be maintained within a very close degree of accuracy, lot of time will be
consumed resulting in increased cost of manufacture. In mass production where the work
has to be done in a set competitive time, greater variations will result. This fact is recognized
and certain variations are allowed in the size of the machine elements or parts. This system
of manufacture in which the dimensions of a part lie within some specified limits leads to
“interchangeable manufacture”. Interchangea ble part manufacture is a major feature of
modern serial and mass production. The parts which go in to assembly have been produced
with all their dimensions within their specified limits, need not be made in the same shop or
even in the same company. This system of interchangeable part manufacture, that is, the
system of limits, fits & tolerance results in the lot of advantages.
QUESTIONS:
1. Define the following terms:
(i) Limits (ii) fit
(iii) Tolerance

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117
2. Why it is necessary to give tolerance on engineering dimension? Give an example of
both a unilateral and bilateral tolerance.
3. Explain the unilateral and bilateral systems of writing tolerances with suitable examples.
Which system is preferred in interchangeable manufacture? Why?
4. Draw the conventional diagram of limits and fits and explain the terms:
(i) Basic size (ii) Upper deviation
(ii) Lower deviation(iii) Fundamental deviation
(v) Zero line.
5. With the help of neat sketches state the essential conditions for
(i) Clearance fit (ii) Interference fit
6. Define fits. Describe the various types of fits in brief.
7. Explain clearly the following type of fits and how they can be achieved:
(a) Push fit (b) Wringing fit (c) Force fit (d) Sharing fit
8. Define the terms
(i) Allowance (ii) Limits
(ii) Tolerance (iii) Fit.
9. Differentiate between Tolerance and Allowance.
10.Describe briefly the systems of obtaining different types of fits, with suitable sketches.
11.Differentiate between ‘Hole basis system’ and ‘Shaft basis system’ of fits.
12.Describe briefly the principal features of the Indian Standard System of limits and fits (IS
–91 g and 2409).
13.Explain briefly the difference between the Interchangeable manufacture and selectiv e
assembly.
14.Study the given drawing. The dimensions stated show the HIGH and the LOW LIMIT of
the detail. Which of the following figures is the value of TOLERANCE for the detail?
35.2
35.4

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118
a) 40.6 mm ……………..
b)0.2 mm ……………
c) 35.6 mm …………..
d) 35.2 mm …………..

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Jig, fixture & guages theory