Jigs, Fixtures and Gauge

Whenever a cutting operation is done on a job, it experiences different types of loads (forces) in different directions. These loads tend to move the job and cutting tool from its original position which results in a decrease in accuracy or dimensional defect of the prepared job. So, it is important that the job should be fixed at a position during the cutting operation and the cutting tool must be guided towards the cutting line, otherwise cutting operation will not be performed well. Generally, jigs and fixtures are used to fulfill these objectives.

In present days, mass production and batch production are being increasingly used in industry. So, the need for correct jigs and fixtures is also increased to give accuracy in

production. For accurate production, one more instrument is also very important, i.e. gauge. It is used to check the acceptability or rejection size of an object. In this chapter, we will study the jig, and fixture and will be taking brief ideas about gauges, their types, and other related aspects. First of all, we will start with the design of the jig and fixture.

Design Considerations of Jigs and Fixtures 

During the designing of jigs and fixtures, the following points should be kept in mind.

(i) Before designing the jig and fixture, measure all the loads that are acting on it at the time of working.

(ii) The jig and fixture should be light in weight and have strong holding capacity. 

(iii) The job should be guided to a single place.

(iv) Clamping of the job should be easy and strong.

(v) Sharp edges of the jig and fixture should be chamfered. 

(vi) The jig and fixture should be simple in construction so that the job can be gripped or loose easily. 

(vii) It should have strong construction so that with the negligence of semi-skilled workers, it should not be affected.

(viii) During the design of the jig, its locating point accuracy and the position of clamping should be kept in mind so that the job may not jump out due to loosening during vibration produced at the time of cutting by the tool.

(ix) Provision of clearance between jig and job should be made during designing. 

(x) All the safety measures should be considered during the design of jigs and fixtures. 

(xi) These are normally made up of cast iron, steel and non-ferrous metals.

Jig

A jig is a device, which is used to hold the job strongly in the proper position and at the same time guides the cutting tool. It is designed in such a way that it can hold many jobs at one time. It is generally used for drilling and boarding operations.

Depending on the job, various types of jigs are used. Bushes, which guide the tool, are generally made up of tool steel. Jigs, which are used for the drilling process are known as drill jigs.

Elements of a Jig

Generally, a jig consists of the following elements.

(i) Body 

It is the main part of the jig and helps in holding the other parts.

(ii) Locating Element 

These elements are used to provide the proper location for the job, which is fixed on the body of the jig.

(iii) Clamping Element 

These elements are used against the cutting tool to clamp the job during operation.

(iv) Guiding Element 

Guiding elements (generally bushes) are used to guide the cutting tool during operation.

(v) Miscellaneous Elements 

Screws, pins, locknuts, etc., are the miscellaneous elements, which are used for fastening the different parts of the jigs.

Types of Jigs

According to the material and structure, jigs are classified into the following types.

1. Template Jig

It is the simplest type of jig, which is made by drilling a hole in a plate at the required location. The size of the hole is the same as the hole size of the workpiece. This plate with a hole is known as a template.

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