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JIGS AND FIXTURES 14

14.1 Introduction

The requirements of mass production and interchangeable assembly in industry, demand that

the components be machined to identical form and size. For this purpose, devices known as

jigs and fixtures are used for holding, locating the work and guiding the tools while machining.

The use of jigs and fixtures makes possible rapid as well as accurate manufacturing; at the

same time, reducing the production cost.

A jig is a device, which holds and supports the work and also guides the path of the

cutting tool, as the operation is performed. Jigs are used extensively for operations such as

drilling, reaming, tapping and counter-boring. The jig need not be secured to the machine.

A fixture is a device which locates and holds the work securely in a definite position. It

is usually secured to the table of the machine and it does not guide the cutting tool. The cutting

tool is either moved into position for the operation or the table is moved under the cutting tool.

Fixtures are used while performing milling, turning, honing, broaching, grinding and welding

operations.

14.2 Presentation of Work Piece

The drawing of a jig or fixture includes a work piece in position. However, the work piece

should be easily discernible from the jig/fixture. To accomplish this, the work piece is drawn in

a chain dotted line, preferably in colour which will be easily distinguished.

The work piece in a fixture/jig drawing is considered transparent. As a result, locators,

studs or other parts of fixture/jig passing through the work piece are drawn in full lines instead

of dotted, as shown in Fig. 14.1. Similarly, the parts of the fixture/jig placed behind the work

piece are also drawn.

The work piece is drawn mainly to facilitate design of the jig/fixture. The designer can

thus position the locators, clamps and bushes quickly and correctly with respect to the work

piece. It also facilitates tool room and manufacturing engineers to read the jig/fixture drawing

and easily understand the functions and requirements of each part.

14.3 Jig Components

14.3.1 Jig Body

The jig body is a frame which holds various parts of a jig. It may be a single integral part of

welded construction or an assembled one. The body must be rigid in construction and at the

same time light in weight for easy handing.