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Full text of "Modern Mechanical Engineering Vol-I"

THE  ESSENTIALS  OF  ECONOMICAL  MACHINING    205

DIVISION IV
Jigs and Fixtures.—A jig is an appliance that guides and controls
the location of a tool relatively to the work. A fixture is one that locates and
secures the work being tooled. The one may be used without the other.
If both are employed, they may be entirely separate and distinct, or be com-
bined in one jig-fixture. Both are intimately associated with the standardiza-
tion and interchangeability of the parts of machines and mechanisms. They
eliminate the need for the tedious, separate lining-off and setting of single
pieces, and they lessen the errors that occur in machining.
The Jig.—The original of the present-day jigs in their myriad forms
was the drilling templet. The majority of jigs are employed still in the
work of drilling and boring. In these the bushes are the vital elements,
because on their accuracy the correctness of results depends. They are
made of steel, hardened and ground, and provision is made for their ready
renewal when they become worn by the friction of drills, reamers, and
boring-tools.
The simplest bushes are those which are a press fit in the jig. These are
only removed when worn out. A better and more accurate method is to
have a permanent lining bush to receive a removable one, the two being
fitted by grinding. It is well to make bushes with a collar, to prevent them
being pushed down too far in their holes. The edge of the bore where the
drill enters is slightly convex. Bushes are sometimes screwed in where they
must come into contact with the work. Lockings are employed to prevent
bushes from turning. A set-screw or a button is fitted to a slot in the collar,
or flats are made on collars of adjacent bushes. A bush may contain two or
more holes in close proximity. A simple bush is slightly longer than its bore.
A small bush will be of greater length, relatively, than one of large diameter.
All dimensions are usually standardized in shops where the system is a per-
manent one, and each size of bush has its own reference letter or figure.
Fixtures.—The employment of fixtures is the only alternative to the
practice of bolting articles directly to the tables of machine-tools. This is
a tedious process in the case of those of awkward shapes that require
packing, and have to be set by careful measurement. This often occupies
more time than the actual machining does, and distortion is liable to occur.
From the point of view of interchangeability it is hardly possible to set two
pieces precisely alike. The fixture is designed both to locate and to hold
the article, or often several, in the same exact position, so that each article
will be machined in the same way.
In good designs provisions are made to lessen the time occupied in
setting and in holding to a minimum. Often, as a result of high economies,
it becomes necessary to duplicate fixtures. One is unloaded and reloaded
while the other is on the machine-tool.
Jig-fixtures.—The highest developments are reached when the
fixture and jig are combined. The jig is generally hinged in some way to