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
• 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
• 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
• (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
• (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
• They are small
• Hinged leaf with bushes applies the clamping force
• After the work piece located inside the , leaf is firmly closed
• 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.
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
• 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
• (e) By repeating step c, we can make number of holes at
100 mm pitch in the workpiece.
• 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
• (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
• Least expensive
• Shape is like a box
• Hole is drilled and then reamed when the work piece is held by
• 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
• This holds the work piece firmly so that drilling operation can
• 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
• Materials used are
• Hardened steel
• Stainless steel
• Drill Bushings are classified into, three
1. Press fit bushes
2. Renewable bushes
3. 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
• 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
• The flat arrests with the collar of the retainer shoulder screw to
• 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.
• In plate bushes drilled hole themselves act as a bush.
• As shoulder is not provided, holes can be placed as close as
• 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 chips
2. Segmental chips
• Continuous chips are long and stringy and tend to take the
form of long coils. These chips are produced while drilling
• 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
• 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
(c) Chips may be removed by compressed air. Guards should be
provided to prevent scattering of chips throughout the working
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
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
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 FIXTURES
1. 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
• 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
• 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
• 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
• 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
• 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.
• 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
• The simplicity of this fixture makes it useful for most
• Its adaptability makes it popular
• Indexing fixtures are very similar to indexing
• These fixtures are used for machining parts
that must have machined details evenly