Measurement standards

Metrology and Quality Control

1. Measurement Standards
By
Prof N D Sadaphal
Assistant Professor

Sanjivani College of Engineering,
Kopargaon (Maharashtra State) 423601

Mechanical Engineering

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Mechanical
Engineering
COE, Kopargaon
Prof N D Sadaphal
Assistant Professor
Engineering Metrology and Quality Control
MeasurementStandards
UNIT-I
METROLOGY
AND MEASUREMENTS

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Metrology.
MetrologydefinesastheScienceofpure
measurement.Butinengineeringpurposes,
itinrestrictedtomeasurementsoflength
andanglesandotherqualitieswhichare
expressedinlinearorangularterms.
Definition of Standards:
•A standard is defined as “something that is set up
and established by an authority as rule of the
measure of quantity, weight, extent, value or
quality”.
•Rule which is universally accepted.

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Line and End standard measurements
•Line standard
Length is expressed as the distance between two lines.
•End standard
Length is expressed as the distance between two flat
parallel faces
•Wavelength standard
Wavelength of monochromatic light is used to measure
length.
•PrecisionDegreeofrepetitiveness.Ifaninstrument
isnotpreciseitwillgivedifferentresultsforthesame
dimensionfortherepeatedreadings.
•AccuracyThemaximumamountbywhichtheresult
differfromtruevalue(ie)Closenesstotruevalue
Terminology in Measurment

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•Accuracy
Accuracy is how close a measured value is to theactual
(true) value.
•Precision
Precision is how close the measured values areto each
other.
Examples of
Precision and
Accuracy:
Characteristics of Measuring Instruments
•Accuracy
•Precision
•Sensitivity-
The sensitivity denotes the smallest change in the measured variable
to which the instrument responds
•Resolution-
The least count of any instrument is taken as the resolution of the instrument.
•Stability-
It is the ability of an instrument to retain its performance throughout is
specified operating life.
•Range or span-
The minimum & maximum values of a quantity for which an instrument is
designed to measure is called its range or span.

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Gauge R&R
Repeatability and Reproducibility in measurement systems
Repeatability-
Theabilityofanoperatortoconsistentlyrepeatthesamemeasurementofthesamepart,usingthe
samegage,underthesameconditions.
Operator1measuresasinglepartwithGageA20times,andthenmeasuresthesamepartwithGageB.
ThesolidlineisthemeasurementsfromGageA.ThedashedlineisthemeasurementsfromGageB.Gage
Ahaslessvariation,soitismorerepeatablethanGageB.
Gauge R&R
Repeatability and Reproducibility in measurement systems
Reproducibility
Theabilityofagage,usedbymultipleoperators,toconsistentlyreproducethesamemeasurementofthe
samepart,underthesameconditions.
Operators1,2,and3measurethesamepart20timeswiththesamegage.
ThethreelinesarethemeasurementsfromOperator1,2,and3.Thevariationinaveragemeasurements
betweenAppraisers1and2ismuchlessthanthevariationbetweenAppraisers1and3.Therefore,the
gauge'sreproducibilityistoolow.

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What/why is a gage R&R study?
•A gage R&R study helps you investigate:
Whether your measurement system variability is small compared with the
process variability.
How much variability in the measurement system is caused by differences
between operators.
Whether your measurement system is capable of discriminating (
good
judgement
)between different parts.
For example, several operators measure the diameter of screws to ensure that
they meet specifications. A gage R&R study indicates whether the inspectors
are consistentin their measurements of the same part (repeatability) and
whether the variation between inspectors is consistent (reproducibility).
. Classification of measuring Instruments.
According to the functions:
•Lengthmeasuring instrument
•Anglemeasuring instrument
•Instrument for checking deviation from geometrical
forms
•Instrument for determining the quality of surface
finish.

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Linear measuring instruments
•Straight edge (Steel rule)
•Outside caliper
•Inside caliper
•Vernier caliper
•Outside micrometer
•Inside micrometer
•Vernier height gauge
•Vernier depth gauge
•Dial gauges
Angular measurements
•Measuring the angle of Taper.
1. Bevel Protractor
2. Tool Makers microscope
3. Sine bar
4. Auto Collimator
5. Sine Centre

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Measuringtoolsandinstruments
Direct (contact) measurement
(e.g. micrometer or caliper)
Indirect(non-contact)measurement
(advancedmethodssuchasoptical,
ultrasonic,laser,etc.)
h Calipers
h Gauges and Gauge Blocks
h Sine Bar
h Special-purpose tools
hRules
hVernierCalipers
hVernierGauges
hMicrometers
hProtractors
hDialIndicators
1
Measuringtoolsandinstruments
Graduated
(eitherlinearorangular
graduationsincorporated
intomeasuringsystemof
thetool
)
Non-graduated
(gauges or adjustable
tools which compare
the measurements)
Imperial steel rule with various lengths
having graduations on each side
Same rule with relatively larger
graduations
Metric steel rule with various lengths
having graduations on each side
resolution?
How to read a rule:
h A = 12 mm (12
th
graduation)
h B = 22 mm (22
nd
graduation)
hC=31.5mm(between
hD=40.5mm(between
31
st
40
th
and 32
nd
)
and 41
st
)
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GraduatedLinearMeasurement-Rules

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3
GraduatedLinearMeasurement-VernierCalipers
Direct reading of an internal length
using digital Vernier caliper Direct reading of an external length
using digital vernier caliper
Verniercaliperwithadialindicator
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GraduatedLinearMeasurement-VernierCalipers

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Designedforuseintoolrooms,workshops,inspectiondepartmentstomeasureormarkoffvertical
heightsandlocatingcenterdistances.
StandardHeightgauge DialHeightGauge DigitalHeightGauge
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GraduatedLinearMeasurement-VernierHeightGauges
Designedforuseintoolrooms,workshops,inspectiondepartmentstomeasuredepthsofholes,slots,
recesses,andsoon.
StandardDepthGauge DialDepthGauge DigitalDepthGauge
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GraduatedLinearMeasurement-VernierDepthGauges

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THIMBLEREADING
VERNIERREADING
MetricMicrometer
SLEEVE (BARREL) READING
Metric Vernier
Micrometer
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GraduatedLinearMeasurement-OutsideMicrometers
V-anvil Micrometer (measuring odd-fluted taps, milling
cutters, reamers, and checking out of roundness)
Dial-indicatingMicrometer
Screw Thread Micrometer
(measuring pitch diameter
of screw threads)
Direct-reading
Micrometer
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GraduatedLinearMeasurement-OutsideMicrometers

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StandardInsideMicrometers
DigitalInsideMicrometers
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GraduatedLinearMeasurement-InsideMicrometers
h Standard calipers have a fine adjustment screw and a quick-adjusting spring nut.
h Accuracy obtained with these tools depends mostly on the inherent skill of users.
h The measurements are carefully transferred to a graduated measuring tool.
Caliper for inside
measurement
Caliper for outside
measurement
Caliper used
as a divider
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Non-GraduatedLinearMeasurement-Calipers

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Screw Pitch Gauges (consisting of a metal case containing
many separate leaves. Each leaf has teeth corresponding to
a definite pitch. By matching the teeth with the thread on
work, the correct pitch can be read directly from the leaf)
Tap and Drill Gauges (consisting
of a flat rectangular steel plate with
holes accurately drilled and
identified according to their size)
Radius Gauges (available as individual leaves and each
leaf is marked with its radius. They are designed to check
both convex and concave radii)
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Non-GraduatedLinearMeasurement-SpecialPurposeGauges
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Non-Graduated Linear Measurement -Rectangular Gauge Blocks
Slip Gauge Box

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(a)
(b)
Simple
Protractor
(measuring
angles from
0 to 180º) Universal Bevel Protractor (main
scale consists of 4 portions of 90º)
Measuring acute (a) and
obtuse (b) angles
How to read an angle on a bevel protractor:
Main div. = 1º = 60´
Vernier div. = 1/12
th
of main div. ≈ 0.0833º = 5´
h The highest figure: 50 * (main div.) = 50º
h The matching figure: 4 * (vernier div.) ≈ 0.333º = 20´
h The final reading is: ≈ 50.333º or 50º 20´
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GraduatedAngularMeasurement-Protractors
*
Limitations of Sine Bar:
Maximum angle 45°
Sine Center -60°
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Non-GraduatedAngularMeasurement-SineBar

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Calibration
•Calibrationisoneoftheprimaryprocessesusedto maintain
instrumentaccuracy.
•Calibrationistheprocessofconfiguringaninstrumenttoprovidea
resultforasamplewithinanacceptablerange.
•MeasurementofAccuracy.
•Establishmenttherelationofaninstrument’saccuracytothe
internationalstandard.
Success of Calibration
•Consistency of results obtained
Need of Calibration
•Quality control & quality assurance in production.
•To meet requirement of ISO
•To comply with requirement of global market.
•To promote international recognition.
Benefits of Calibration
•Fulfils requirement of ISO 9000, ISO 14000.
•As a proof that the instrument is working properly.
•Confidence in using instrument.
•Reduce rejection, failure rate.
•Improved product & service quality leading to satisfied
customer.
•Cost saving, safety.

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Types and sources of ERRORS
Systematic Errors
•Systematic errors are regularly repetitive and can be
eliminated.
•They results from improper condition or procedure of
experiment .
•These error can be controlled & reduced if properly
analyze, so called as Controllable errors.
Errors may be of four kinds;
1. Instrumental :
For example, a poorly calibrated
instrument such as a thermometer that reads 102°C
when immersed in boiling water and 2°C when
immersed in ice water at atmospheric pressure. Such a
thermometer would result in measured values that are
consistently too high.
2. Observational: For example, parallax in reading a
meter scale.
3. Environmental: Variation in atmospheric
condition i.e. temperature, pressure etc. at place of
measurement.
4. Stylus pressure:
Variation in Force applied by anvils of micrometer
on component to be measured results in different
reading.

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Random Errors
•Randomerrorsinexperimentalmeasurementsarecausedby unknown
andunpredictablechangesintheexperiment.Thesechangesmayoccurin
themeasuringinstrumentsorintheenvironmentalconditions.
•Sourcesofrandomerrorscannotalwaysbeidentified.Possiblesourcesof
randomerrorsaresmallvariationsinthepositionofsettingstandardsand
workpiece,slightdisplacementofleverjointsinthemeasuringjointsin
themeasuringinstrument.
•Examples of causes of random errors are:
1.electronic noise in the circuit of an electrical instrument,
2.Irregular changes in the heat loss rate from a solar collector due to
changes in the wind.
•Theseerrorcannotbeeliminated.
1.Observational:Forexample,errorsinjudgmentofanobserverwhen
readingthescaleofameasuringdevicetothesmallestdivision.
2.Environmental:Forexample,unpredictablefluctuationsinlinevoltage,
temperature,ormechanicalvibrationsofequipment.
Parallax Error :
•Parallaxisadisplacement
ordifferenceintheapparent
positionofanobjectviewed
alongtwodifferentlinesofsight,
andismeasuredbytheangleor
semi-angleofinclination
betweenthosetwolines.

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Comparators
•Classification of comparators
1.Mechanical
2.Electrical and Electronics comparators
3.Optical comparators
4.Pneumatic comparators
5.Electro-Mech. Comparators.