jig and fixture are important for Mechanical engineer.

Jig and fixture are used for Mass Productions.

1. JIG AND FIXTURE Prepared By: DHIREN P. PATEL Lecturer Mech. Engg . Dept

2. Content: Introduction Purpose of using Jig and Fixture Elements of Jig Types of bushing Requirement of Jig Types of Jig 2

3. Introduction Jig can be defined as a workpiece holding and locating device that positions and guides or controls the cutting tool . Drill jig is device to ensure a hole to be drilled, tapped or reamed in the workpiece at proper place. Drill jig consists of a clamping device to hold the part in the position under hardened steel bushings. Drill passes through the steel bushings during drilling operation. The drill is guided by these bushings. Generally workpiece is held by jig and the jig is arranged in such a way that the workpiece can be quickly loaded and unloaded. 3

4. The basic purpose of developing and using suitable jig and fixture for batch production in machine shops are: To eliminate marking, punching, positioning, alignments etc. Easy, quick and consistently accurate locating, supporting and clamping the blank in alignment of the cutting tool. Guidance to the cutting tool like drill, reamer etc. Increase in productivity and maintain product quality consistently To reduce operator and labours skill requirement To reduce measurement and its cost. Enhancing technological capacity of machine tool. Reduction of overall machining cost and also increases in interchangeability. 4

5. Design Consideration in Jig. The main frame of jig must be strong enough so that the deflection of jig is as minimum as possible. This deflection of jig is caused due to the forces of cutting, clamping of the workpiece or clamping to the machine table. The mainframe of the jig should have the mass to prevent vibration and chatter. Frames should be built from simple sections so that frames can be fastened with screws or welded, whenever necessary. Those parts of the frame that remain intact with the jig may be welded. The parts needing frequent changing may be held with the screws. Where the body of jig or fixture has complex shape, it may be cast from good grade of cast iron. Clamping should be fast enough and require least amount of effort. Clamps should be arranged so that they are readily available and may be easily removed. Clamps should be supported with springs so that clamps are held against the bolt head wherever possible. 5

6. Continue….. If the clamp is to swing off the work, it should be permitted to swing as far as it is necessary for removal of the workpiece . All locators, clamps should be easily visible to the operator and easily accessible for cleaning, positioning or tightening. Provision should be made for easy disposal of chip so that storage of chips doesn’t interfere with the operation and that their removal during the operation doesn’t interfere with the cutting process. All clamps and support points that need to be adjusted with a wrench should be of same size. All clamps and adjustable support points should be capable of being operated from the fronts of the jig. 6

7. Continue….. Workpiece should be stable when it is placed in jig. If the workpiece is rough, three fixed support points should be used. If workpiece is smooth, more than three fixed support points may be used. Support point should be placed as farthest as possible from each other. The three support points should circumscribe the centre of gravity of the workpiece . The surface area of contact of support should be as small as possible without causing damage to the workpiece . This damage is due to the clamping or work forces. Support points and other parts are designed in such a way that they can be easily replaced on failure. 7

8. Elements of Jig: Jig Body Jig Feet Jig Bushing Jig plate of Bush plate Locators Clamps Fool proof element 8

9. Elements of Jig: Jig Body The jig body supports the workpiece and has locating and clamping elements in it. It is provided with four jig feets and rests on the machine tool. Jig Feet A jig feet which is not bolted to machine table is provided with four jig feet, instead of the entire bottom surface lying flat on the machine table. Jig feet are either cast with the jig body or detachable or welded to the base. They are usually hardened and ground to have flat bottom. Jig Bushing For guiding drills, reamers and boring bars, hardened steel jig bushes are employed which are fixed in the jig plates. The bushes can be replaced when worn-out with less cost than the entire jig plate. 9

10. Continue….. Jig Plate or Bush plate Usually the jig plate carries the jig bush for guiding the tools. The jig plate may be rigidly constructed as a single unit of jig or it may be of leaf or latch type. In this type the jig plate must be clamped with the jig frame. Locators Locators help a workpiece to rest in proper position in a jig. Depending on the type of workpiece , various types of locators are used for the locating the component in the jig with reference to the tools. The locators are usually detachable type, fixed to the jig frame, So whenever wear occurs the locators can be easily replaced with new ones. Clamps Clamps are used for holding the workpiece rigidly against all disturbing forces. They also keep the workpiece firmly in contact with locating pins of surface. Sufficiently thickness of the section should be provided to withstand clamping forces. 10

11. Continue….. Fool Proofing element This elements prevents the work from being loaded wrongly into jig. The elements may be simple fouling pegs , cross pieces or pins. 11

12. Types of Bushing: Press fit Bushing: Press fit bushing are the most common type of bushings and are pressed interference fit in the bushing plate also referred as a jig plates. These bushings are used in batch production where the bushing often outlasts the life of the jig. Press fit bushings are also used for as liners for renewable and slip bushings. 12

13. Continue….. Slip Bushing : When a hole in the workpiece requires two operations such as drilling and reaming, it is necessary to use two different guide bushing for the different tools. The hole is first drilled using a bushing having a bore suitable for the drill. After drilling, the drill bushing is removed and a reaming bushing is used to guide the reamer. In mass production, the changeover of these bushings should be effected quickly. This is accomplished by provision of slip bushes. There are number of different types of slip bushing in use. In the most common type, the bushing is provided with a flat on the head similar to the renewable bushing and a circular cutout in the flange to facilitate quick assembly and removal. 13

14. Continue….. Renewable type Bushing: For continuous or large batch production, the inside diameter of the bushing is subjected to severe wear due to continuous contact with hard cutting tool. The guide bushing requires periodic replacement. The replacement is simplified by making the outside diameter precision location fit(h6). The bushing can then be assembled manually without any press. The renewable bushing must be prevented from rotating and moving axially with the cutting tool. This is accomplished by provision of a flat on the collar. The flat arrests with the collar of the retainer shoulder screw to prevent rotation. The bushing flange below the collar of the shoulder screw prevents the bushing from getting lifted with the cutting tool. 14

15. Continue….. Threaded Bushings : The bushing used for clamping the workpiece are threaded on the outside. There should be another plain guiding diameter for accurate location of the bushing (Figure). The collar of the linear bushing is usually placed on the opposite side of the axial thrust of the screw. The linear bushing should be prevented from rotation by a grub screw or a flat on the collar( Figure). The flat mates with a machined step on the jig plate. 15

16. Continue….. Special Bushing: Some workpiece or operations require unusual types of bushing. In many instances, these involves simple modifications in the standard bushing. A twist drill tends to slide down inclines and curves . These causes bending and breakage of the drill. The problem can be countered by altering the shape of the drill bushing to provide better support and resistance against bending. This generally involves matching the end of the bushing with the profile of the workpiece . 16

17. Requirements of a Drill Jig Quick and accurate location of the workpiece Easy loading and unloading of the workpiece Prevention of wrong Loading Prevention of bending or movement of the workpiece during drilling operation Ample chip clearance Facility of swarf removal and cleaning Light weight to minimise operator fatigue Prevention of loss of part by using chains Clearance for overshoot of the drill 17

18. Types of Jig: Plate type jigs mainly consists of a single bushing plate with a provision for location and the clamping of workpiece. Fig shows a plate type of jig for the shown workpiece. The workpiece profile is located by six location pins and clamped by two knurles screw against the location pins. Plate Jig. Sometimes, it is economical to make the jig channel shaped in order to simplify location and clamping. 18

19. Template Jig This is the simplest type of jig; It is simply a plate made to the shape and size of the work piece; with the require number of holes made it. It is placed on the work piece and the hole will be made by the drill; which will be guided through the holes in the template plate should be hardened to avoid its frequent replacement this type of jig is suitable if only a few part are to be made. 19

20. Box jig : When the holes are to drill more than one plane of the work piece, the jig has to be provided with equivalent number of bush plates. For positioning jig on the machine table feet have to be provided opposite each drilling bush plate. One side of the jig will be provided with a swinging leaf for loading and unloading the work piece, such a jig would take the form of a box. 20

21. Sandwich Jig 21

22. Channel Jig 22

23. Leaf Jig 23

24. Angle plate Jig 24

25. Modified Angle plate Jig 25

26. Indexing Jig 26 When number of holes with the same size and pitch are to be drilled in a workpiece, then conventional drilling takes more time to drill each hole individually and requires skilled worker for complete production run. Also one cannot get required accuracy. This automatically increases the cost. This arrangement is shown in Figure. Such situation is avoided by using indexing jig. Indexing jig not only saves the time and the cost but also increases accuracy. And unskilled worker may operate the indexing jig.

27. Trunion Jig 27

28. Multi-station Jig 28

29. Latch Jig 29

30. Turnover Jig 30

31. Fixture 31

32. What is fixture?????? 32 A fixture is a device for locating, holding and supporting a workpiece during a manufacturing operation. Fixtures are essential elements of production process as they are required in most of automated manufacturing, inspection, and assembly operations. Fixture must correctly locate a workpiece in a given orientation with respect to a cutting tool or a measuring device, or with respect to another component, as for instance in assembly or welding. Such location must be invariant in the sense that the devices must clamp and secure the workpiece in that location for the particular processing operation.

33. What is fixture?????? 33 There are many standard work holding devices such as jaw chucks, machine vices, drill chucks, collets etc. which are widely used in workshops and are usually kept in stock for general applications. Fixtures are normally designed for definite operations to process a specific workpiece and are designed and manufactured individually. Jigs are similar to fixtures, but they not only locate and hold the part but also guide the cutting tools in drilling and boring operations. These work holding device are collectively known as jigs and fixtures.

34. What is fixture?????? 34 Fixtures are designed specifically for an operation and so these are named on the base of the operation to be carried out with their help. Fixtures are used to hold the workpiece properly to carry out the operations. Different types of fixtures are listed below. (a) Turning fixtures (b) Milling fixtures (c) Fixture for grinding (d) Fixture for broaching (e) Fixture for boring/drilling (f) Tapping fixture (g) Fixture for welding (h) Assembling fixture

35. Milling Fixtures 35 Fixtures used to perform different types of milling operations are called milling fixtures. The fixture is probably located on the table of the machine and secured in position by means of bolts and nuts. The workpiece located on the base of fixture and clamped.

36. Milling Fixtures 36 The fixtures for milling operations are designed and described on the basis of milling operations, milling techniques and clamping power techniques. (a) On the basis of types of operations the milling fixture may be of different types are listed below : These are straddle milling fixture, face milling, slot milling, plain milling, side milling, form milling and gang milling fixtures. (b) On the basis on technique of milling machining, milling fixtures are named as single piece milling fixture, string milling, reciprocal milling, index milling and abreast milling fixture. (c) On the basis of clamping power and clamping method milling fixtures are named as fixture with mechanical clamping, hydraulic clamping, pneumatic clamping, automatic clamping and vice jaw clamping fixture.

37. Milling Fixtures 37

38. Essentials of Milling Fixtures 38 Strength: Milling Fixtures should be heavy and robust to withstand heavy forces and severe vibrations developed in milling Thrust: Heavy thrust caused in milling should be transmitted to strong solid structures. The thrust should not be directed towards a clamp or a movable part. Cutter Setting: Suitable cutter setting pieces should be provided for easy and accurate tool setting. It is customary to keep 0.4 to 0.5 mm gap between the cutter and setting block. This gap facilitates cuttersetting with the help of a feeler gauge.

39. Essentials of Milling Fixtures 39 Machine Location Tennons : Milling fixtures are generally fixed with locating tennons (tongues). These are fixed in slots machined on the resting face of the base. Tennons are made close sliding fit in the machine T-slots. Tennons align the milling fixture with the longitudinal stroke of the machine. Tenons should be as far apart as possible, to produce maximum accuracy. Motion Economy: Clamping time can be reduced by securing more than one workpiece with a single clamp. Rigid Clamping: The fixtures must be clamped firmly to the machine table in order to prevent sliding due to cutter thrust and vibrations Metal Filing Disposal: Unlike drilling, milling chips are discontinuous. In case metal fillings get accumulated in closed pockets, big chip removal gates should be provided for cleaning the fixture.

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