18-04-2014, 02:52 PM
JIGS AND FIXTURES
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INTRODUCTION
The jigs and fixtures are the economical ways to produce a component in mass. So jigs
and fixtures are used and serve as one of the most important facility of mass production
system. These are special work holding and tool guiding device. Quality of the
performance of a process largely influenced by the quality of jigs and fixtures used for
this purpose. 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 the cases hold a workpiece
during an operation. A jig differs from a fixture in the sense that it guides the tool to its
correct position or towards its correct movement during an operation in addition to
locating and supporting the workpiece.
An example of jig is when a key is duplicated, the original key is used as base for the
path reader which guides the movement of tool to make its duplicate key. The path
reader of a CWC machine here works as a jig and the original is called template.
Sometimes the template and jig both are the name of same part of a manufacturing
system.
MEANING OF LOCATION
It is very important to understand the meaning of location before understanding abut the
jigs and fixtures. The location refers to the establishment of a desired relationship
between the workpiece and the jigs or fixture correctness of location directly influences
the accuracy of the finished product. The jigs and fixtures are desired so that all
undesirable movements of the workpiece can be restricted. Determination of the locating
points and clamping of the workpiece serve to restrict movements of the component in
any direction, while setting it in a particular pre-decided position relative to the jig.
Before deciding the locating points it is advisable to find out the all possible degrees of
freedom of the workpiece. Then some of the degrees of freedom or all of them are
restrained by making suitable arrangements. These arrangements are called locators.
PRINCIPLES OF LOCATIONS
The principle of location is being discussed here with the help of a most popular
example which is available in any of the book covering jigs and fixtures. It is important
that one should understand the problem first.
Any rectangular body many have three axis along x-axis, y-axis and z-axis. It can more
along any of these axes or any of its movement can be released to these three axes. At
the same time the body can also rotate about these axes too. So total degree of freedom
of the body along which it can move is six. For processing the body it is required to
restrain all the degree of freedom (DOF) by arranging suitable locating points and then
clamping it in a fixed and required position. The basic principle used to locate the points
is desirable below.