Introduction to JIGS AND FIXTURES

Introduction
The successful running of any mass production depends
upon the interchangeability to facilitate easy assembly
and reduction of unit cost. Mass production methods
demand a fast and easy method of positioning work for
accurate operations on it.
Jigs and fixtures are production tools used to
accurately manufacture duplicate and interchangeable
parts. Jigs and fixtures are specially designed so that large
numbers of components can be machined or assembled
identically, and to ensure interchangeability of
components.

JIGS
It is a work holding device that holds, supports and locates the workpiece
and guides the cutting tool for a specific operation. Jigs are usually fitted
with hardened steel bushings for guiding or other cutting tools. a jig is a
type of tool used to control the location and/or motion of another tool. A
jig's primary purpose is to provide repeatability, accuracy, and
interchangeability in the manufacturing of products. A device that does
both functions (holding the work and guiding a tool) is called a jig.
An example of a jig is when a key is duplicated, the original is used as a jig
so the new key can have the same path as the old one.
BORING JIG

FIXTURES
It is a work holding device that holds, supports and locates the workpiece
for a specific operation but does not guide the cutting tool. It provides
only a reference surface or a device. What makes a fixture unique is that
each one is built to fit a particular part or shape. The main purpose of a
fixture is to locate and in some cases hold a workpiece during either a
machining operation or some other industrial process. A jig differs from a
fixture in that a it guides the tool to its correct position in addition to
locating and supporting the workpiece.
Examples: Vises, chucks
A VISE-JAW FIXTURE

How do jigs and fixtures differ
JIGS FIXTURES
1. It is a work holding device that holds,
supports and locates the workpiece and
guides the cutting tool for a specific
operation
1. It is a work holding device that holds,
supports and locates the workpiece for a
specific operation but does not guide the
cutting tool
2. Jigs are not clamped to the drill press
table unless large diameters to be drilled
and there is a necessity to move the jig to
bring one each bush directly under the drill.
2. Fixtures should be securely clamped to
the table of the machine upon which the
work is done.
3. The jigs are special tools particularly in
drilling, reaming, tapping and boring
operation.
3. Fixtures are specific tools used
particularly in milling machine, shapers and
slotting machine.
4. Gauge blocks are not necessary. 4. Gauge blocks may be provided for
effective handling.
5. Lighter in construction. 5. Heavier in construction.
Advantages of Jigs and Fixtures
PRODUCTIVITY:
Jigs and fixtures increases the productivity by eliminating the
individual marking, positioning and frequent checking. The
operation time is also reduced due to increase in speed, feed
and depth of cut because of high clamping rigidity.
INTERCHANGEABILITY AND QUALITY:
Jigs and fixtures facilitate the production of articles in large
quantities with high degree of accuracy, uniform quality and
interchangeability at a competitive cost .
 SKILL REDUCTION:
There is no need for skillful setting of work on tool. Jigs and
fixtures makes possible to employ unskilled or semi skilled
machine operator to make savings in labour cost.
 COST REDUCTION:
Higher production, reduction in scrap, easy assembly and
savings in labour cost results in ultimate reduction in unit
cost.
Fundamental principles of Jigs
and Fixtures design
LOCATING POINTS: Good facilities should be provided for
locating the work. The article to be machined must be easily
inserted and quickly taken out from the jig so that no time is
wasted in placing the workpiece in position to perform
operations. The position of workpiece should be accurate with
respect to tool guiding in the jig or setting elements in fixture.
FOOL PROOF: The design of jigs and fixtures should be such
that it would not permit the workpiece or the tool to inserted in
any position other than the correct one.
REDUCTION OF IDLE TIME: Design of Jigs and Fixtures should be
such that the process, loading, clamping and unloading time of the
workpiece takes minimum as far as possible.
• WEIGHT OF JIGS AND FIXTURES: It should be easy to handle,
smaller in size and low cost in regard to amount of material used
without sacrificing rigidity and stiffness.
• JIGS PROVIDED WITH FEET: Jigs sometimes are provided with feet
so that it can be placed on the table of the machine.
• MATERIALS FOR JIGS AND FIXTURES: Usually made of hardened
materials to avoid frequent damage and to resist wear. Example-
MS, Cast iron, Diesteel, CS, HSS.
CLAMPING DEVICE:
It should be as simple as possible without sacrificing
effectiveness. The strength of clamp should be such that not
only to hold the workpiece firmly in place but also to take the
strain of the cutting tool without springing when designing
the jigs and fixtures.
Essential features of Jigs and Fixtures
Reduction of idle time – Should enable easy clamping and
unloading such that idle time is minimum
Cleanliness of machining process – Design must be such that not
much time is wasted in cleaning of scarfs, burrs, chips etc.
Replaceable part or standardization – The locating and supporting
surfaces as far as possible should be replaceable, should be
standardized so that their interchangeable manufacture is possible
Provision for coolant – Provision should be there so that the tool is
cooled and the swarfs and chips are washed away
 Hardened surfaces – All locating and supporting surfaces
should be hardened materials as far as conditions permit so
that they are not quickly worn out and accuracy is retained
for a long time
 Inserts and pads – Should always be riveted to those faces of
the clamps which will come in contact with finished surfaces
of the workpiece so that they are not spoilt
 Fool-proofing – Pins and other devices of simple nature
incorporated in such a position that they will always spoil the
placement of the component or hinder the fitting of the
cutting tool until the latter are in correct pos
 Economic soundness – Equipment should be economically
sound, cost of design and manufacture should be in
proportion to the quantity and price of producer
 Easy manipulation – It should be as light in weight as possible
and easy to handle so that workman is not subjected to
fatigue, should be provided with adequate lift aids
 Initial location – Should be ensured that workpiece is not
located on more than 3 points in anyone plane test to avoid
rocking, spring loading should be done
 Position of clamps – Clamping should occur directly above
the points supporting the workpiece to avoid distortion and
springing
 Clearance – Sufficient amount of clearance should be provided
around the work so that operator’s hands can easily enter the
body for placing the workpiece and any variations of work can be
accommodated
 Ejecting devices – Proper ejecting devices should be incorporated
in the body to push the workpiece out after operation
 Rigidity and stability – It should remain perfectly rigid and stable
during operation. Provision should be made for proper positioning
and rigidly holding the jigs and fixtures
 Safety – The design should assure perfect safety of the operator
General rules for designing
Compare the cost of production of work with present
tools with the expected cost of production, using the
tool to be made and see that the cost of buildings is not
in excess of expected gain.
Decide upon locating points and outline clamping
arrangement
Make all clamping and binding devices as quick acting as
possible
Make the jig fool proof
Make some locating points adjustable
Avoid complicated clamping arrangements
Round all corners
Provide handles wherever these will make handling easy
Provide abundant clearance
Provide holes on escapes for chips
Locate clamps so that they will be in best position to
resist the pressure of the cutting tool when at work
Place all clamps as nearly as possible opposite some
bearing point of the work to avoid springing action
Before using in the shop, test all jigs as soon as made
MATERIALS USED
 Jigs and Fixtures are made of variety of materials, some of
which can be hardened to resist wear.
 Materials generally used:
 High speed Steel: Cutting tools like drills, reamers and milling
cutters.
 Die steels: Used for press tools, contain 1% carbon, 0.5 to 1%
tungsten and less quantities of silicon and manganese.
 Carbon steels: Used for standard cutting tools.
 Collet steels: Spring steels containing 1% carbon, 0.5%
manganese and less of silicon.
5. Non shrinking tool steels:
High carbon or high chromium
Very little distortion during heat treatment.
Used widely for fine, intricate press tools.
6. Nickel chrome steels: Used for gears.
7. High tensile steels: Used for fasteners like high tensile
screws
8. Mild steel:
Used in most part of Jigs and Fixtures
Cheapest material
Contains less than 0.3% carbon
9. Cast Iron:
Used for odd shapes to some machining and
laborious fabrication
CI usage requires a pattern for casting
Contains more than 2% carbon
Has self lubricating properties
Can withstand vibrations and suitable for base
10. Nylon and Fiber: Used for soft lining for clamps to
damage to workpiece due to clamping pressure
11. Phospher bronze:
used for nuts as have high tensile strength
Used for nuts of the lead screw
Factors to be considered for design of
Jigs and Fixtures
1. Component-
Design to be studied carefully
Ensure work is performed in a proper sequence
Maximum operations should be performed on a machine in
single setting
2. Capacity of the machine-
Careful consideration to be performed on type and capacity
of machine.
3. Production requirements-
Design to be made on basis of actual production
requirements. Then comes decision on manual and automatic
tooling arrangements.
4. Location-
• Location should ensure equal distribution of forces
throughout all sequence of operation.
• Location should be hard resistant, wear resistant and
high degree of accuracy.
• Movement of workpiece should be restricted.
• Should be fool proofed to avoid improper locations of
the workpiece.
• Should facilitate easy and quick loading of workpiece.
• Redundant locators should be avoided.
• Sharp corners must be avoided.
• At least one datum surface should be establised.
5. Loading and Unloading arrangements-
There should be adequate clearance for loading and
unloading. Hence process becomes quick and easy.
Size variation must be accepted.
It should be hardened material and non sticky.
6. Clamping arrangements-
Quick acting clamps must be used as far as possible.
The clamping should not cause any deformation to the
workpiece
It should always be arranged directly above points supporting
the work.
Power driven clamps are favoured as they are quick acting,
controllable, reliable and operated without causing any
fatigue to the operators.
Features of clamps:
Clamping pressure should be low
Should not cause distortion
Simple and fool proof
Movement of clamp should be minimum
Case hardened to prevent wear
Sufficiently robust to avoid bending
7. Clearance between Jig and Component-
To accommodate various sizes if work
Chips to pass out of the opening between them
8. Ejectors-
To remove work from close fitting locators.
Speeds up unloading of the part from the tool and hence
production rate.
9. Base and Body construction-
Methods used: Machining, Forging and machining, Casting,
Fabricating, Welding.
10. Tool guiding and cutter setting-
By adjusting the machine or using cutter setting block, the
cutter is set relative to the work in a fixture. The drill bushes
fitted on jig plates guides the tools.
11. Rigidity and vibration-
Must possess enough rigidity and robustness.
Should not vibrate as it may lead to unwanted movement of
workpiece and tools.
12. Safety-
Operation should be assured full safety.
13. Cost-
Should be simple as possible.
Cost incurred should be optimum.
14. Materials generally used-
Sl. No Part Name Material
1 Jig body CI
2 Stud MS
3 Drill/Bush Gun metal
4 Pin MS
5 Nut MS