Jig and fixtures

JIGS &FIXTURES

vishnu deth k jdepartment of mechanical engineering

St. joseph’s college of Engineering & TechnologyPalai

• Drill jigs Types 1.Leaf Jigs 2. Channel Jigs

3. Box Jigs4.Template Jigs5. Indexing Jigs

• Drill Bushings• Chip formation during drilling

• Type of Fixtures 1. Milling Fixtures

2. Lathe Fixtures3. Broaching Fixtures4.Boring Fixtures5. Slotting Fixtures6. Grinding Fixtures

• Economics of Fixtures

CONTENTS

Fixture & Jigs • Fixtures, being used in machine shop, are strong

and rigid mechanical devices which enable easy, quick and consistently accurate locating, supporting and clamping, blanks against cutting tool(s) and result faster and accurate machining with consistent quality, functional ability and interchangeability.

• Jig can be defined as a work piece holding and locating device that positions and guides or controls the cutting tool.

• Jig is a fixture with an additional feature of tool guidance.

• Drill jig is device to ensure a hole to be drilled, tapped or reamed in the work piece at proper place

Purpose of Jigs & Fixtures

• Reduces the effort for Marking ,Measuring and setting of work piece.

• Reduces the time• Increases the accuracy• It keeps a consistent quality• Low rejection rate, due to less defects• Increases the production capacity.• Semi skilled operators can do the work

CONSIDERATIONS WHILE DESIGNING JIGS & FIXTURES

• (a)Study of workpiece and finished component size and geometry.

• (b) Type and capacity of the machine, its extent of automation.

• (c) Provision of locating devices in the machine.• (d) Available clamping arrangements in the machine.• (e) Available indexing devices, their accuracy.• (f) Evaluation of variability in the performance results

of the machine.• (g) Rigidity and of the machine tool under

consideration.• (h) Study of ejecting devices, safety devices, etc.• (i) Required level of the accuracy in the work and

quality to be produced

Elements of Jig & Fixture

Drill Jigs

1.Leaf jig • They are small• Hinged leaf with bushes applies the clamping force• After the work piece located inside the , leaf is firmly

closed and locked.• Jig plate is clamped against the resting face using an

eye bolt.• It ensures that the bushing face is vertical to drilling.• The open slot in jig plate and swinging eye bolt

facilitates quick clamping & unclamping.

Leaf jig operation

1.1 Double Leaf Jig

2. Channel Jigs

• Used for drilling more than one surface • It is very expensive

3. Indexing jigs• When number of holes of same pitch and size, drilled in

a work piece,• Conventional drilling takes more time,• Need skilled labour• Increases the cost.

• (a) Indexing pin is first inserted into hole A. It is used as a stopper for drilling the first hole through the drill bush.

• (b) The pin is removed from hole A after drilling. Then workpiece is moved toward right till the centerline of drilled hole coincides with the centerline of bush B.

• (c) The indexing pin is inserted into the drilled hole in the workpiece through bush B.

• (d) In this way workpiece is indexed linearly to drill the next hole at 100 mm distance (i.e. pitch) from the hole drilled previously.

• (e) By repeating step c, we can make number of holes at 100 mm pitch in the workpiece.

3.1 Rotating index jig

• Rotary indexing helps in positioning a part accurately around the axis.

• It can be used conveniently for drilling equi-spaced

holes in cylindrical work pieces.• (a) Work piece is clamped and first hole is drilled

through bush A.• (b) Work piece is unclamped and rotated clockwise

to coincide the axis of drilled hole with the axis of indexing pin.

• (c) The indexing pin is inserted in the drilled hole to index the work piece for drilling the next hole.

• (d) The work piece is clamped again before drilling the next hole. In this way, the rotary indexing can be used to drill number of equi-spaced holes with

only two guide bushes.

4. Template jig

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

5. BOX JIG

• Shape is like a box • Hole is drilled and then reamed when the work piece is

held by jig. • The work piece is inserted in the jig. • The cam rod is taken out of the jig and the work piece

placed in the position inside the jig. • The cam rod is then replaced and rotated to its locking

position. • This holds the work piece firmly so that drilling

operation can be performed.

Drill Bushings

• Jigs uses drill bushes for guiding the drill, reamer and

other cutting tool to the work piece.• Generally, the length of bush should be twice the

diameter of the bushing hole. • The bushing hole should be very close to the diameter

of the drill but should not be so tight that drill will drag into the bushing.

• Materials used are• Hardened steel• Carbide• Bronze• Stainless steel

• Drill Bushings are classified into, three 1. Press fit bushes2. Renewable bushes3. Plate bush

Press fit bushes• These are the most common type of bushes and are

pressed interference fit in the jig plates.• The press fit bushes are further categorized into two

types: headed bushes and headless bushes.• Headed bushes are generally preferred over headless

bushes because collar provides the stops against the jig plate.

• It is found that chances of the bush getting loose in the jig plate and sliding axially with the drill are lesser in collared bushes.

• when spacing of bushes is close, headless bushes are preferred over headed one.

• In addition, as the top surface of the jig plate is required free from projecting collars, the headless bushes are used.

(i) Headed press fit bush (ii) Headless press fit

Renewable bushes

• The inside diameter of bush is subjected to severe wear due to continuous contact with hard cutting tool for continuous or large batch production.

• The guide bushes require periodic replacement.• The replacement is avoided by making outside

diameter precision location fit.• By providing flat on the collar the renewable bush

should be prevented from rotating and causing axial movement with the cutting tool.

• The flat arrests with the collar of the retainer shoulder screw to avoid rotation.

• Also bush flange below collar of shoulder screw avoids the bush getting lifted with cutting tool.

• The liner used in the jig plate provides hardened wear resistant mating surface to the renewable bush.

Plate bush

• In plate bushes drilled hole themselves act as a bush. • As shoulder is not provided, holes can be placed as

close as possible.• Liner bushing is also called master bushing.

• These are with head or without head.• This bushing eliminates the need for locking device.

Chip formation – Drilling

• During drilling operation, two types of chips formed.1. Continuous chips2. Segmental chips• Continuous chips are long and stringy and tend to

take the form of long coils. These chips are produced while drilling ductile steel.

• Segmental chips are produced while drilling cast iron. Segmental type of chip is easy to remove from the drill jig.

• Enough space should be provided between work and bottom of drill bushing.

• It will allow chips to pass between the workpiece and bushing plate instead of through the drill bushing.

• This eliminates storing of chip in drill flutes. • This arrangement allows more coolant to reach the

cutting edge of drill and helps to prevent wear of drill bushings due to abrasion of chip.

• If the chips are continuous, they may tangle between the work piece and bushing and become very difficult to remove.

• This is avoided by locating the bushing closer to work so that bushing will force the long stringy chip through the bushing where any entanglement can be easily removed by the operator.

• Chips are removed from jig in three ways :(a) Coolant helps to clean chips whenever they formed.(b) Chips may be removed manually with the help of brush or hook.(c) Chips may be removed by compressed air. Guards should be provided to prevent scattering of chips throughout the working area.

TYPES OF FIXTURES• Design Consideration 1. The main frame of fixture must be strong enough so

that deflection of the fixture is as minimum as possible.

2. Frames may be built from simple sections so that frames may be fastened with screws or welded whenever necessary.

3. Clamping should be fast enough and require least amount of effort.

4. Clamps should be arranged so that they are readily available and may be easily removed.

5. Clamps should be supported with springs so that clamps are held against the bolt head wherever possible.

6. 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

7.All locator’s clamps should be easily visible to the operator and easily accessible for cleaning, positioning or tightening.8.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.

9.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 fixture.

10.Workpiece should be stable when it is placed in fixture. If the work piece is rough, three fixed support points should be used. If work piece is smooth, more than three fixed support points may be used. Support point should be placed as farthest as possible from each other.11.The three support points should circumscribe the centre of gravity of the work piece.12. The surface area of contact of support should be as small as possible without causing damage to the work piece. This damage is due to the clamping or work forces.13. Support points and other parts are designed in such a way that they may be easily replaced if they break.

TYPES OF FIXTURES1. VISE FIXTURE

Simple vise jaw for holding round work piece.

• Stop pin is used to prevent the bending of work piece

by the application of clamping force.

• Here guide pins are used to secure alignment.• To hold workpiece firmly in all directions, wedge type

jaws are useful.• It is used for rough castings and forgings, because it

permits considerable variations in dimensions.

2. MILLING FIXTURES

• This holds the part in correct relation to the milling cutter.

• Fixture is attached to milling machine table. Milling fixture consists of the base, clamps, rest blocks or nest, locating points and gauging surfaces.

• The base of milling fixture consists of a base plate. • A base plate has a flat and accurate under surface and

forms main body on which various components are mounted.

• This surface aligns with the surface of the mill table and forms the reference plane with respect to the mill feed movement.

• It may be constructed of steel plate or cast iron, depending upon the size and complexity of the part.

• The slots are provided in the base for clamping the fixture to the mill table.

• The base plate also has keyways along with length of the base for two keys.

• These keys are used to align the fixture on the milling machine table. The keys are pressed into the keyway at both ends of fixture and held there by socket head caps screw

• Cutting forces may change as the cutter enters or leaves the work piece and throw an extra load on clamps.

• Clamps should not be loosened by vibrations, which are caused by interrupted cutting by the mill cutter at the beginning and at the end of the cut.

• Clamp should be located opposite to bearing surfaces and locating points.

• These should be designed in a way so that these can be easily operated by the operator.

3. FACING FIXTURE

• Milling machines are extensively used for facing seating and mating flat surfaces.

• The work piece is positioned by three adjustable spherical ended pads .

• These pads are adjusted to suit the variation in the size of work piece and lock in the position by check nuts.

• Two self adjusting supports are pushed upward by light spring.

• These springs are used to make sure that the supports are positively in contact with the work piece.

• Clamping screw is used to lock supports. • On tightening the edge clamp, the work piece is

pushed against the fixed jaw

• This jaw is keyed in the fixture body to provide solid support to work piece against the heavy thrust developed in the operation.

• The cutter should be fed to the work piece in such a manner that the milling thrust should be directed towards the solid support of fixed jaws.

• The setting can be set in the path of cutter to set it before starting of facing operation.

• Four clamping slots are provided to take care of the heavy forces developed during the operation.

4.0 Grinding Fixture

• The standard magnetic tables are used to rest workpiece such that resting surface will be parallel to the surface to be ground.

• However, for light workpiece with lesser resting area, the resting area tends to tilt and fly off the magnetic table due to high speed of grinding wheel and due to high feed, also.

• Hence, it is necessary to provide additional support by nesting the workpiece.

• This can be done by placing the solid plates around the workpiece

• The nest plates are held firmly by the magnetic force of table with more weight and more resting area.

• The nest plates surround the workpiece from outside and arrest its movement in the horizontal plane.

• Thus, this arrangement will help in preventing it from flying off and tilting due to high speed and feed in grinding.

5.Plate Fixture

• Plate fixtures are the simplest form of fixture.• The basic fixture is made from a flat plate that

has a variety of clamps and locators to hold and locate the part.

• The simplicity of this fixture makes it useful for most machining operations.

• Its adaptability makes it popular

Indexing fixture

• Indexing fixtures are very similar to indexing jigs.

• These fixtures are used for machining parts that must have machined details evenly spaced.