Other Types Of Bushes

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2 Other Types Of Bushes Circuit board drill bushes: Designed to accommodate larger shank for making drill on circuit board Chip breaker bushes: Designed with chip breaking notch. Reduces friction and heat build up due to machining process, also reduces wear at drill exit. Direct Coolant bushes: Coolant will circulate on passage of the bushes. It also washes chips during drilling operation.

3 Clearance between work piece and bushing There should be proper clearing to permit removal of chips. C= Clearance between work piece and chip = 0.5D for metals like cast iron that produce small chip = D to 1.5 D for metals like steel tat produce long chip

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5 Bushing Clearance End of bushing should not touch the work Normally 1 to 1.5 times the tool diameter is sufficient for the clearance, but if you want precision clearance should be decreased If end of bush is modified,sufficient bearing area for the drill must be maintained inside the bushing. So, it is best to use straight bushing having same inside diameter instead of counter bored bushing. Proper bush clearance is necessary for overall function since chip causes bush to wear rapidly if bushing are unnecessarily close, also precision is lost if they are placed too far away.

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7 2. Burr Clearance In any drilling, two burrs are produced, the primary and the secondary burrs. Primary is made on the side opposite the drill bushing Secondary is made at the point where drill enters the work Hence these burrs must be considered and sufficient clearance must be given.

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9 Another problem is when holes are too close In this case, we can use bushings of thin walls Another alternative is to grind flats on adjacent bushes Also holes can be drilled and reamed and bushing can be alternated from 1 hole to another

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11 Common defects in jig design Variations in the of rough work pieces Variation in material Intrinsic defects in machine tools Wear Deflection Thermal expansion Dirt and chips Human deficiencies in judgment, perception and skill

12 Variation in rough work piece For rough work piece, it becomes necessary to select proper locating devices. As the stock to be removed varies which results in variation of cutting force, the machining allowance should be added Variation in material The variation in material result in the variation of forces and power requirement and also they result in variation of thermal expansion wear and deflection.,the machining allowance should be added

13 Defect in tools As tools have tolerances, depending on the accuracy required, it is customary to allow 10% tolerances of work pieces on jigs, fixtures and gauges. Wear It also increases variations. So wear resistant materials should be used for parts of jigs and fixtures Deflection Forces causing deflection may occur from handling, clamping or cutting action. For this purpose, clamps should be never applied on the over hanging section of the work piece. Thermal expansion The temperature dissipation should be minimized by adopting sharp tools of proper shape, including provision for heat dissipation from tools and machine and employing controlled room temperature.

14 Dirt chips and burrs These directly interfere with work piece location, positioning and gauging. For this, sharp corners should be avoided between locating surfaces as they catch dust are hard to clear. Human deficiencies Jigs and fixtures should be designed to provide means of achieving desired accuracy with lest skill and effort on the part of operator

15 Elements of Jig Jig body Jig feet Jig bushing Jig plate or bush Locators Clamp Fool proof element

16 Jig body The jig body supports work piece and has locating and clamping elements in it. It is provided with four jig feet and rests on the machine table. Jig feet A jig feet which is not bolted to the machine table is provided with four jig feet. They usually 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. They can be replaced at lower cost.

17 Jig plate or bush plate Usually the jig 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. Locators Locators help work piece to rest in proper position in a jig. The locators are usually detachable type. Clamps Clamps are used for holding the work piece rigidly against all disturbing forces. They also keep the work piece firmly in contact with locating pins of surfaces.

18 Fool proof element This element prevent work piece from being loaded wrongly into the jig. The elements may be simple toulings pegs, cross pieces or pins.

19 Construction of jigs Jigs are constructed by one of the following methods Built up construction Welded construction Cast construction

20 Built up construction All the parts can be completely machined before assembly and worn parts can be easily be replaced. Jigs of this type are held together by socket head cap screws and dowels. The dowels are used to hold the part in alignment. A minimum of one screw and two dowels must be used to fasten each component to the jig body.

21 Welded construction It is often used in constructing a drill jig. Welded jig must be stress relieved before machining because the stresses set up during welding will warp the jig. It may be faster to weld the components together and then machine them to finished size.

22 Cast construction Cast construction can be used for several jigs, when every jig is made from a standard. Cast construction of a jig may be made to the required shape, size and finish.

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24 Materials for jig element Element Materials Heat treatment Jig body Cast iron or MS Stress relieved for welded construction Locator Silver steel Hardened and ground Clamps MS Carburized and hardened Supporting elements MS ` Hardened and ground Guiding elements MS Case hardened Bushes Steel Hardened and ground Bush plates Mild steel Stress relieved

25 Types of jigs Template jig Plate jig Channel jig Leaf jig Pot jig Turnover jig or table jig Box jig or Tumble jig Turnion jig Indexing jig Post jig Sandwich jig Nut cracker jig Pumping or universal Latch jig Vise jig Solid jig Angle plate jig Angular post jig

26 Template jig Simplest and basic jig used in production Have same profile as that of work piece Do not have clamp and depend on pins, nests or part shapes to reference them to part. Have a disadvantage that they are not that fool proof as other type of jigs. Types of template jigs 1.Flat 2.Circular 3.Nesting

27 Flat plate locators are used to locate holes on flat surfaces. This template is usually located with pins referenced from the edge or other holes.

28 Circular plate template jigs are used for cylindrical work pieces. They are usually located on a cylindrical work and is used to drill holes on any circular work

29 Nesting template jigs use a cavity nest or a pin nest to locate the work piece. They can accommodate any shape of work, but the only restriction is the complexity of the cavity.