Jigs and Fixtures

Introduction

Fixtures : being used in machine shop, are strong

and rigid mechanical devices which enable easy,

quick and consistently accurate locating, supporting

and clamping of blanks against cutting tools and

result in faster and accurate machining with

consistent quality, functional ability and

interchangeability.

Jig : It is a fixture with an additional feature of tool

guidance.

Purpose of using Fixtures and Jigs

To eliminate marking, punching, positioning,

alignments etc.

Easy, quick and consistently accurate locating,

supporting and clamping the blank in alignment of

the cutting tool.

Guidance to the cutting tool like drill, reamer etc.

Increase in productivity and maintain product quality

consistently.

To reduce operator’s labour and skill – requirement.

To reduce measurement and its cost.

Enhancing technological capacity of the machine

tools.

Reduction of overall machining cost and also

increase in interchangeability.

Economic viability of jig and fixture for batch production.

Major elements of jig and fixtures.

Design Considerations for Jigs and Fixtures

Easy, quick and consistently accurate locating of the

blank in the jig or fixture in reference to the cutting

tool.

Providing strong, rigid and stable support to the

blank.

Quick, strong and rigid clamping of the blank in the jig

or fixture without interrupting any other operations.

Tool guidance for slender cutting tools like drills and

reamers.

Easy and quick loading and unloading the job to and

from the jig or fixture.

Use of minimum number of parts for making the jig or

fixture.

Use of standard parts as much as possible.

reasonable amount of flexibility or adjustability, if

feasible, to accommodate slight variation in the job -

dimensions.

Easy, quick and accurate indexing system if required.

prevention of jamming of chips, i.e. wide chips-space

and easy chip disposal.

easy and safe handling and moving the jig or fixture on

the machine table, i.e., their shape, size, weight and

sharp edges and corners

• easy and quick removal and replacement of small parts

• manufacturability i.e. ease of manufacture

• durability and maintainability

• service life and overall expenses

Principles or rules of locating in jigs and fixtures

Possible degrees of freedom of a solid body.

Methods of Location (3-2-1 Principle)

- A workpiece will be completely confined when banked against:- 3 points in one plane- 2 points in another plane- 1 point in a third plane- All three planes are perpendicular to each

other.

- Buttons should be as far apart as possible because Greater the spread, lesser alignment error.

12 degrees of freedom

3 pins arrest 5 dof

5 pins arrest 8 dof

6 pins arrest 9 dof

Arresting all degrees of freedom of a blank in a fixture.

7 dof arrested by V locator with stop pin

Workholder with multiple V locators

basic principles or rules need to be followed while planning for locating blanks in fixtures.

One or more surfaces (preferably machined) and / or

drilled / bored hole(s) are to be taken for reference.

The reference surfaces should be significant and

important feature(s) based on which most of the

dimensions are laid down.

Locating should be easy, quick and accurate.

In case of locating by pin, the pins and their mounting

and contact points should be strong, rigid and hard.

A minimum of three point must be

used to locate a horizontal flat surface.

The locating pins should be as far apart

as feasible.

Vee block and cones should be used for self-locating

solid and hollow cylindrical jobs.

For locating large jobs by rough bottom surface one of

the three pins may be replaced by a pivoted arm. The

pivoted arm provides wo contact points.

Locating by Vee block and cone

Clamping of workpiece in fixtures

While designing for clamping the following factors essentially need to be considered :

1. Clamping need to be strong and rigid enough to hold

the blank firmly during machining.

2. Clamping should be easy, quick and consistently

adequate.

3. Clamping should be such that it is not affected by

vibration, chatter or heavy pressure.

4. The way of clamping and unclamping should not hinder

loading and unloading the blank in the jig or fixture.

5. The clamp and clamping force must not damage or

deform the workpiece.

6. Clamping operation should be very simple and quick

acting when the jig or fixture is to be used more

frequently and for large volume of work.

7. Clamps, which move by slide or slip or tend to do so

during applying clamping forces, should be avoided.

8. Clamping system should comprise of less number of

parts for ease of design, operation and maintenance

9. The wearing parts should be hard or hardened and also be

easily replaceable.

10. Clamping force should act on heavy part(s) and against

supporting and locating surfaces.

11. Clamping force should be away from the machining

thrust forces.

12. Clamping method should be fool proof and safe.

13. lamping must be reliable but also inexpensive

Methods of Clamping

Clamping method and system are

basically of two categories :

1. General type without much consideration on speed of clamping operations.

2. Quick acting types.

General clamping methods of common use

Screw operated strap clamps

The clamping end of the strap is pressed against a spring which enables quick unclamping.

Clamping from side for unobstructed through machining (like milling, planing and broaching) of the top surface.

Clamping from side for free machining of the top surface.

Clamping by swing plates

Such clamping are simple and relatively quick in operation but is suitable for jobs of relatively smaller size, simpler shape and requiring lesser clamping forces.

Other conventional clamping methods include :

Vices like drilling and milling vices.

Magnetic chucks.

Chucks and collets for lathe work.

Quick clamping methods and systems

Cam clamping

Quick clamping by cam is very effective and

very simple in operation.

The cam and screw type clamping system is

used for clamping through some interior parts

where other simple system will not have access.

Cam Clamping

screw and cam clamping from

distance

Quick multiple locating and clamping of cylindrical

jobs.

Quick multiple clamping by pivoted clamps in series and

parallel is capable to simultaneously clamp number of rods

even with slight diameter variation.

Quick clamping by hydraulic and pneumatic force for

strong and light clamping respectively.

Light but quick clamping by bayonet type clamp.

Quick acting screw (bayonet type) clamping.

Use of quick acting nut for rapid clamping

Drill – jig bushing

Necessity

Slender and cantelever type cutting tools, mainly drills,

usually suffer from run – out due to possible errors in the

drill, sockets and drilling machine spindle and finally in

the overall alignment.

Such run out causes over sizing, out of roundness and

surface roughening of the drilled holes.

Such run out aggravates further with the increase in drill

speed (rpm) and the thrust force, specially if the drill is not

geometrically symmetrical.

This often leads to, in addition to poor product quality,

breakage of the drill by bending and / or buckling.

To reduce such problems, bushes are used in the jigs to

guide the drill bits.

The factors to be considered while designing for jig – bushing

The bushes, used to guide and properly locate drills,

reamers etc. are generally made of carbon or alloy steel

and made wear resistive by hardening to RC 60 and above.

Often bushes are also made from grey cast iron for

antifriction and protection of the tools.

The hardened jig bushes are finished outside by grinding

and inside by grinding and lapping if high precision is

insisted.

The bush’s length should be sufficient (≥ twice drill

diameter) and its diameter should be slightly larger than

the drill diameter

Design and construction should enable easy and quick

proper fitting and removal or replacement of the bushes

Bushes should not come out from its seat along with the

drill during its return.

Types of jig bushes

Depending upon nature of fitting, quick mounting and

replacement, job requirement etc., jig bushes are

classified into several types.

• Press fitted type

• Slip type

• Screwed type

Press fitted thin sleeve type bushes are generally used

for shorter runs and are not renewable.

Renewable type slip bushes are used with liner.

screw bushes, though renewable may be used without

or with liner.

Bushes may be

Without head

With head

With a flange being screwed on the bracket

Frequently replaceable bushes are provided with some

locking system.

Locking of frequently

replaceable bushes

Some special jig bushings are often designed and used as and when required.

two close holes – in one bush

two very close holes – using eccentric bush

two close holes – by two adjacent modified bushes

Types of Jigs

Open jig (Plate jig or drill template)

It is the simplest type of drill jig.

It consists of a plate with holes to guide the drills,

and may have locating pins that locate the

workpiece on the jig.

Jig body

Holds the various parts of a jig assembly.

Cap screws and dowel pins

Hold fabricated parts together.

Components of Drill Jig

Locating devices

Pins, pads, and recesses are used to locate the

workpiece on the jig.

Clamping devices

Locking pins

Inserted after the first hole is

drilled to lock or hold the

workpiece securely to the

jig plate while subsequent holes

are being drilled.

Template Jig

Template jigs are normally used for accuracy rather

than speed.

This type of jig fits over, on, or into the work and is

not usually clamped.

Templates are the least expensive and simplest type of

jig to use.

They may or may not have bushings. When bushings

are not used, the whole jig plate is normally hardened.

Plate jigs are similar to templates. The only difference

is that plate jigs have built-in clamps to hold the work.

These jigs can also be made with or without bushings,

depending on the number of parts to be made.

Plate jigs are sometimes made with legs to raise the jig

off the table for large work. This style is called a table

jig.

Plate Jig

Plate jig

Table jig

Sandwich jigs are a form of plate jig with a back plate.

This type of jig is ideal for thin or soft parts that could

bend or warp in another style of jig.

The use of bushings is determined by the number of

parts to be made.

Sandwich Jig

Sandwich jig

Angle-plate jigs are used to hold parts that are

machined at right angles to their mounting locators.

Pulleys, collars, and gears are some of the parts that

use this type of jig.

A variation is the modified angle-plate jig, which is

used for machining angles other than 90 degrees.

Both of these examples have clearance problems with

the cutting tool.

Angle-Plate Jig

Angle-plate jig Modified Angle-plate jig

As the drill exits the product being drilled, it has little

or no room for the drill point to clear the product

completely, produce a round hole all the way through

the part wall, and avoid drilling the part locator.

This is most noticeable, where an angled hole requires

additional clearance to the relieved portion of the part

locator.

Additional clearance here would allow the drill to

complete the hole and avoid drilling the relieved

portion of the locator.

Box Jig or Tumble Jig

Box jigs, or tumble jigs, usually totally surround the

part.

This style of jig allows the part to be completely

machined on every surface without the need to

reposition the work in the jig.

Channel Jig

Channel jigs are the simplest form of box jig.

The work is held between two sides and machined

from the third side.

In some cases, where jig feet are used, the work can

be machined on three sides.

Leaf Jig

Leaf jigs are small box jigs with a hinged leaf to allow

for easier loading and unloading.

The main differences between leaf jigs and box jigs are

size and part location.

Leaf jigs are normally smaller than box jigs and are

sometimes made so that they do not completely

surround the part.

They are usually equipped with a handle for easier

movement.

Indexing Jig

Indexing jigs are used to accurately space holes or

other machined areas around a part.

To do this, the jig uses either the part itself or a

reference plate and a plunger.

Larger indexing jigs are called rotary jigs.

Types of Fixtures

Jigs and fixtures are made basically the same way as

far as locators and positioners are concerned.

The main construction difference is mass.

Because of the increased tool forces, fixtures are built

stronger and heavier than a jig would be for the same

part.

Classification of Fixtures

Fixtures are normally classified by the type of machine

on which they are used.

Fixtures can also be identified by a sub classification.

For example, if a fixture is designed to be used on a

milling machine, it is called a milling fixture.

If the task it is intended to perform is straddle milling, it

is called a straddle milling fixture.

The same principle applies to a lathe fixture that is

designed to machine radii. It is called a lathe-radius

fixture.

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.

Angle-plate Fixture

The angle-plate fixture is a variation of the plate

fixture.

With this tool, the part is normally machined at a right

angle to its locator.

While most angle-plate fixtures are made at 90

degrees, there are times when other angles are needed.

In these cases, a modified angle-plate fixture can be

used.

Modified Angle-plate

fixture

Angle-plate fixture

Vice-jaw Fixture

Vise-jaw fixtures are used for machining small parts.

With this type of tool, the standard vise jaws are

replaced with jaws that are formed to fit the part.

Vise-jaw fixtures are the least expensive type of

fixture to make.

Their use is limited only by the sizes of the vises

available.

Indexing Fixture

Indexing fixtures are very similar to indexing jigs.

These fixtures are used for machining parts that must

have machined details evenly spaced.

Parts machined with an indexing fixture

Multistation Fixtures

Multistation fixtures are used primarily for high

speed, high-volume production runs, where the

machining cycle must be continuous.

Duplex fixtures are the simplest form of multistation

fixture, using only two stations.

This form allows the loading and unloading

operations to be performed while the machining

operation is in progress.

For example, once the machining operation is

complete at station 1, the tool is revolved and the

cycle is repeated at station 2.

At the same time, the part is unloaded at station 1 and

a fresh part is loaded.

Duplex Fixture

Profiling Fixtures

Profiling fixtures are used to guide tools for

machining contours that the machine cannot normally

follow.

These contours can be either internal or external.

Since the fixture continuously contacts the tool, an

incorrectly cut shape is almost impossible.

The operation in Fig. shows how the cam is accurately

cut by maintaining contact between the fixture and the

bearing on the milling cutter.

This bearing is an important part of the tool and must

always be used.