THE PRINCIPLES OF JIG AND FIXTURE DESIGN

 THE PRINCIPLES OF JIG AND FIXTURE DESIGN

1 Location

a) Ensure that the workpiece is given the desired constraint.

b) Position the locators so that swarf will not cause mal-alignment.

c) Make the location points adjustable if a rough casting or forging is being

machined.

d) Introduce foolproofing devices, such as fouling pins and projections, to

prevent incorrect positioning of the workpiece.

e) Make all location points visible to the operator from his working position.

f) Make the location progressive (i.e. locate on one locator and then on to the

other).

g) Ensure that redundant location is not present (i.e. two location points to

control one constraint).

2 Clamping

a) Position the clamps to give best resistance to the cutting forces.

b) Position the clamps so that they do not cause deformation of the work

piece.

c) Design the clamps so that they are not deformed by the clamping forces.

d) If possible, make the clamps integral with the fixture body.

e) Make all clamping and location motions easy and natural to perform.

3 Clearance

a) Allow ample clearance to allow for variation of workpiece size.

b) Allow ample clearance for the operator's hands.

c) Ensure that there is ample swarf clearance and cutting-fluid clearance.

d) Allow clearance so that the work piece can be removed after machining,

when burrs may be present.

4 Stability And Rigidity

a) Provide four feet so that uneven seating will be obvious, and ensure that the

forces caused by the mass of the work piece and the cutting action act

within an area enclosed by a line joining the seating points.

b) Make the equipment as rigid as is necessary for the operation to be

performed.

c) Provide means of positioning and bolting the equipment to the machine

table or spindle if required.

5 Handling

a) Make the equipment as light as possible, particularly if it is to be moved

about by the operator for loading, etc.

b) Consider the shape of the equipment so that it can be handled easily; ensure

that there are no sharp corners or awkward projections.

c) If possible, provide lifting handles or lifting hooks.

6 General

a) Keep the design simple in order to minimise the cost and to avoid break down

caused by over-complication.

b) Utilise standard and 'bought out' parts as much as possible.

c) Ensure that the workpiece can be loaded into and removed from the

equipment. When designing jigs and fixtures, it is usual to draw the

component, and to design the equipment around it; it is a common error to

produce a design that does not allow for the passage of the workpiece.

The basis of jig and fixture design is the location and the clamping of the

workpiece, and Chapters 2 and 3 deal with the basic principles involved, and the

established methods of location and clamping are described.

In the chapters that follow, the essential features of the more common equipment

are stated, and the various types of jigs and fixtures are illustrated. The examples given are

only representative, and endless variations are possible.

7 Design Study

In the early stages of the study of this subject the student can learn much by

designing jigs and tools on the lines of the examples shown in the text, but as soon as the

basic principles are understood, design should be developed by a careful analysis of the

problem, allowing the final design to be gradually developed; the folly of commencing a

design with preconceived ideas cannot be over-emphasised. Chapter 13 describes the

design technique, and also compares the various methods of construction that are

available.

8 Gauges

Limit gauges are used so that certain dimensions can be checked to ensure learned

that they are within the limits of size stipulated by the designer; the disadvantage

associated with limit gauging is that no indication of the actual Size is given, but the

advantage is that the work pieces can be checked by semi-skilled personnel. The

correctness of work produced by automatics and similar machines is often checked by

actual measurement in conjunction with statistic methods to avoid having to measure

every piece produced; the machine can be given 'running adjustments' to prevent scrap.

Chapter 9 illustrates the basic types of limit gauges, many of which are the subject of

British Standards. Receiver gauges as used to check castings for completeness, and to

ensure that mating components produced in different places will assemble together

correctly. Position gauges are special fixture like gauges that enable the relative positions

of many features to be checked.