13157_Jigs and Fixtures

7/28/2019 13157_Jigs and Fixtures

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Manufacturing Technology

MEC205Jigs and Fixture


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Jigs and Fixtures Jigs and fixtures are the production devices

that are used for the accurate production of repeated parts essentially for mass

production. Location Clamping Support Resistance to Cutting forces Safety


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Fixture

A fixture is a means through whicha part is securely fastened to the machinetool table to accurately locate, support and

hold the part during the machiningoperation.

Jig A jig is a special class of fixture,

which in addition to provide all the functionsas above, also guides the cutting tool duringmachining.

This is generally used for theoperations such as drilling, boring, reaming,tapping, counter boring, etc.


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Jigs Jigs are commonly used for making parts that

contain holes.


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Fixtures

Fixtures are widely used in large batch production toensure the easy setup and achieving the desiredaccuracy.


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Important parameters for Jig andFixture Design

Functional Surfaces Location Surfaces

Support Surfaces Clamping surfaces


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Principles of locationConfiguration is the major factor in determining

how a work piece will be located

Configuration of work piece is determined bycombination of flat, circular and irregular surface.

Basic reference for location is a flat planegenerally a m/c table


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Locating from plane surface(3-2-1) It is necessary to reduce the contact area of locating

surface. A minimum of three points (locators) mustbe used to locate the work piece


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Locating from plane surface The locating points should be placed as far as

possible to minimize the effect of inaccuracies in thework piece and locators.


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Locating from plane surface


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Locating from Circular surface It is necessary to locate the axis of the circular work piece.


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Locating from Circular surface One commonly used method is referred as conical location

and employed when locating is done from the hole.


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Locating from Circular surface V method primarily to locate round work piece with convex

circular surfaces.


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Locating from Circular surface Variation in the work size will cause the axis of round work

piece to shift


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Locating from Circular surface A method of locating from a circular surface so common that

it is often overlooked is the use of Chucks.


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Locating from irregular surface


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Locating from irregular surface Locating the remaining two surfaces


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Locating from irregular surface Locating the remaining two surfaces


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Locating Methods and Devices A round pin or Button is used firmly support or hold the work piece inposition

The Main Difference in the pin and button is length.

Buttons are generally shorter than pins and are used for vertical

location.Pins are usually used for horizontal location.

Larger sizes are some time called plugs.

Pin and Button Locators


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Lock- Screw pins are sometime used where pin is

subjected to excessive wear


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Diamond pins


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Rest Pads and Plates: Used to support and locate workvertically in a manner similar to rest buttons, but they are

used with larger and heavier work pieces


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Because of the larger bearing surfaces, rest plates aresometimes grooved to reduce bearing area and stillprovide support rigidity.


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Nest or Cavity Location


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Locating Methods and Chip Controls


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Clamping


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The Basic Principles of Clamping

Clamping forces should direct towards the nest or locatingsurfaces.

They should be arranged in such a way that the thrust of the cutting tool is away from the clamp

Clamping forces should be applied over a heavy part of theworkpiece whenever possible.

Clamping mechanism should be designed in such a mannerthat they cannot be applied in any way except the correct

one (Fool Proof) .


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Types of Clamps

Clamps can be classified in seven basic groupsStrap

Cam

Latch

Screw

Toggle

Wedge

Rack and pinion


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Types of Clamps

Strap Clamps- The Simplest and probably most commonlyused clamp.

A two piece spherical washer

is some times used to provideproper seating of the nut,washer and bar when theworkpiece height varies.


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Types of Clamps

Strap Clamps- The Simplest and probably most commonlyused clamp.


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Types of Clamps Strap Clamps- The Fulcrum of the cam should be located as close as

possible to the work piece in order to obtain maximum mechanicaladvantage

Other types of clamps based on lever principle are toe clamp(dog point clamp)


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Screw ClampsScrew clamps incorporate a screw thread to clamp a work piece. They

have the advantage of exerting adequate force besides resistingloosening tendencies set up by vibration.

Screw clamps vary from a simple set screw to complicated clampingassembly actuated by a screw clamps


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Screw ClampsQuick acting screw clamp

Swing Clamp


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Latch ClampsThis type of construction is limited to relatively light work, as it is

difficult to secure rigidly.


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Wedge Clamps


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Toggle Clamps :- Depends upon the movement of rigid links for theirmovement. These clamps have been used extensively to hold sheet metal partsin position while they are being welded or otherwise fastened and they provide

relatively heavy clamping pressure, are quick acting, and give ample clearancefor loading and unloading.


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CAM Clamps Properly designed cam clamp provide an effective and rapid

means of clamping but they are not a positive clampingmethod and may loosen under vibrating forces The two main types of cam clamps are eccentric and spiral. An eccentric clamp is easier to make but does not lock as well

as spiral cam


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CAM Clamps Various cam clamps


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Rack and pinion clamp

Hydraulic and pneumatic clamping - Many of the previouslymentioned clamps may be actuated by hydraulic or pneumaticmethods.

The advantage are faster clamping, uniform and equalized

clamping pressures, and less operator fatigue. Pneumatically operated clamps differ from hydraulically

operated ones in size of the cylinder, which is smaller withhydraulics because of the higher pressures.


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Hydraulic and pneumatic clamps


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Template jigs

No clamping arrangement Plates with bushing to guide the tool Directly placed on the part Simple Least expensive


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Template jigs

Disadvantages Not as foolproof as other types Orientation of the hole pattern to workpiece

datums may not be as accurate as other types They are usually not practical when locating

datums are dimensioned


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Plate jigs

The plate jig is so named because a plate containing bushingsforms the main structural member.

It is of open construction , sometimes called open jig Slip bushings may be used with liner bushings to allow

operations other than drilling. The disadvantage of plate jigs are that only one surface can be

drilled at one loading and drilling forces are generally directedtowards clamping device

It is therefore necessary that clamping device be rigid enoughto withstand drilling forces


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Universal jigs

Also called as Pump jigs Universal that can be used for any given part

by adding the necessary locators and bushes

The moving parts are completely protectedfrom chips.


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Leaf jigs

Hinged leaf with bushes which also applies the clampingforce

Leaf jigs can be loaded unloaded quickly and are suitable forcomplicated workpiece with irregular contours.

The disadvantage of leaf jigs is that chips may accumulateinside and cause trouble unless provision is made for them.

Play in the leaf may affect accuracy When bushings are loacted in the hinge plate , the hole

pattern is foxed. The only chance of error is in the location of hole relative to

the part outline.


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Channel and tumble jigs(BOX)

This type recieves its name from the sahpewhich , in general, resembles a box.

For drilling in more than one surface(TumbleJig)

Some time box jigs are constructed in achannel shaped trough and clamped by meansof screw. This type is limited to jobs havingworkpieces of simple symmetricalshape(Channel Jig)


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Indexing jigs

To drill holes in a pattern Indexing arrangement


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Jig Design Guidelines

Drill Jigs should be of light construction, consistent withrigidity to facilitate handling, especially when jigs have to beturned over so that holes can be drilled from more than oneside.

All unnecessary metal should be cored out of the jig body. If possible, eliminate spanners by the use of levers. Consideration should be given at the design stage to the use

of standardised jig components.


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Jig Design Guidelines

A jig which is not bolted to the machine table shouldbe provided with feet, preferably four, opposite allsurfaces containing guide bushings, so that it will'rock' if not standing square on the table and so warnthe operator.

Clearance holes or burr slots should be provided inthe jig to allow for the burr formed when the drill

breaks through the Make all component clamping devices as quick acting

as possible.


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Jig Design Guidelines Design the jig fool-proof by the use of foul pins and similar

devices, that is arrange it so that the component, tools orbushes cannot be inserted except in the correct way.

Make some locating points adjustable when the component is

a rough casting and may be out of alignment Locate clamps so that they will be in the best position to resist

the pressure of the cutting tool when at work. If possible, make all clamps integral parts of the jig and avoid

the use of loose parts. Avoid complicated clamping and locating arrangements which

are liable to wear or need constant attention. All sharp edges should be removed from the various detail

parts of the jig.


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Provide handles or other devices wherever these will makethe handling of the jig more convenient.

Make, if possible, all locating points visible to the operatorwhen placing the component in position in the jig so that the

component can be seen to be correctly located. The operatorshould also be able to have an unobstructed view of theclamps.

Springs should be used whenever possible to elevate the

clamps clear of the component whilst being loaded orunloaded.

Locating and supporting surfaces should, whenever possible,be renewable. Such surfaces should be of hard material.


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Basic Design Steps

1. Method of locating the part identify thestandard components required for locatingpurpose.

2. Design the clamping method. Make a properchoice of clamps C-washer, swing washer,nut, strap clamp, toggle clamp, etc.

3. Design any supports required4. Design the jig bushes required.

5. Design the jig body.


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Design Study Drill Jig

1. Method of locating the part The central holewhich helps in locating as well as indexing for

the hole.2. Clamping can be done with a nut and a C-

washer

3. Indexing will have to be done with a plungertype retracting.


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Documents

13157_Jigs and Fixtures