03-02-2012, 04:13 PM
Types and Functions of Jigs and Fixtures
JIGS AND FIXTURES
Jigs and fixtures are production-workholding devices
used to manufacture duplicate parts accurately. The
correct relationship and alignment between the cutter,
or other tool, and the workpiece must be maintained.
To do this, a jig or fixture is designed and built to hold,
support, and locate every part to ensure that each is
drilled or machined within the specified limits.
Jigs and fixtures are so closely related that the
terms are sometimes confused or used interchangeably.
The difference is in the way the tool is guided to the
workpiece.
A jig is a special device that holds, supports, or is
placed on a part to be machined. It is a production
tool made so that it not only locates and holds the
workpiece but also guides the cutting tool as the operation
is performed. Jigs are usually fitted with hardened
steel bushings for guiding drills or other cutting
tools (Figure 2–1A).
As a rule, small jigs are not fastened to the drill
press table. If, however, holes above .25 inch in diameter
are to be drilled, it is usually necessary to fasten
the jig to the table securely.
A fixture is a production tool that locates, holds,
and supports the work securely so the required
machining operations can be performed. Set blocks
and feeler or thickness gauges are used with fixtures
to reference the cutter to the workpiece (Figure 2–1B).
A fixture should be securely fastened to the table of
the machine upon which the work is done. Though
largely used on milling machines, fixtures are also
designed to hold work for various operations on most
of the standard machine tools.
Fixtures vary in design from relatively simple
tools to expensive, complicated devices. Fixtures also
help to simplify metalworking operations performed
on special equipment.
CLASSES OF JIGS
Jigs may be divided into two general classes: boring
jigs and drill jigs. Boring jigs are used to bore holes
that either are too large to drill or must be made an
odd size (Figure 2–2). Drill jigs are used to drill,
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Types and Functions of Jigs and Fixtures
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ream, tap, chamfer, counterbore, countersink, reverse
spotface, or reverse countersink (Figure 2–3). The
basic jig is almost the same for either machining operation.
The only difference is in the size of the bushings
used.
TYPES OF JIGS
Drill jigs may be divided into two general types, open
and closed. Open jigs are for simple operations where
work is done on only one side of the part. Closed, or
box, jigs are used for parts that must be machined on
more than one side. The names used to identify these
jigs refer to how the tool is built.
Template jigs are normally used for accuracy rather
than speed. This type of jig fits over, on, or into the work
and is not usually clamped (Figure 2–4). Templates are
the least expensive and simplest type of jig to use. They
may or may not have bushings. When bushings are not
used, the whole jig plate is normally hardened.
TYPES OF FIXTURES
The names used to describe the various types of fixtures
are determined mainly by how the tool is built.
Jigs and fixtures are made basically the same way as
far as locators and positioners are concerned. The
main construction difference is mass. Because of the
increased tool forces, fixtures are built stronger and
heavier than a jig would be for the same part.
Plate fixtures are the simplest form of fixture
(Figure 2–17). The basic fixture is made from a flat
plate that has a variety of clamps and locators to hold
and locate the part. The simplicity of this fixture
makes it useful for most machining operations. Its
adaptability makes it popular.
The angle-plate fixture is a variation of the plate
fixture (Figure 2–18). With this tool, the part is normally
machined at a right angle to its locator. While
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Figure 2–11 Channel jig.
Figure 2–10 Box or tumble jig.
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most angle-plate fixtures are made at 90 degrees,
there are times when other angles are needed. In
these cases, a modified angle-plate fixture can be
used (Figure 2–19).
Vise-jaw fixtures are used for machining small
parts (Figure 2–20). With this type of tool, the standard
vise jaws are replaced with jaws that are formed
to fit the part. Vise-jaw fixtures are the least expen-
