Jigs and Fixtures 201
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d:\N-Design\Des14-1.pm5
D
A
B C
A – Workpiece drawn in
chain dotted lines and
considered transparent
B – Location pins situated
below workpiece
C – Locator situated within
workpiece bore
D – Fixture drawing
Fig.14.1 Presentation of workpiece
14.3.2 Locating Devices
The location should prevent linear and rotary motion of the work piece, along and around
three major axes X, Y, and Z. Depending upon the shape of the work piece, the locating devices
may be either internal or external in nature. Cylindrical, diamond and conical pin locators are
the common internal locators; V-grooves, button stops, supports and dowel pins are used as
external locators. Location system should prevent wrong loading of the work piece in a fool
proof manner.
14.3.3 Clamping Devices
Clamping elements hold the work piece firmly engaged with locating elements during the
operation. The clamping system should be strong enough to withstand forces developed during
the operation and at the same time it should not dent or damage the work piece. Speed of
operation and operator fatigue also are important in selecting the clamping devices.
Clamps are broadly divided based on their construction and principle of operation as:
screw clamps, strap clamps, pivoted clamps, hinged clamps, swinging clamps, quick action
clamps, multiple clamps and power clamps.
14.3.4 Bushings
Drill jigs use bushes to guide drills, reamers and other cutting tools, to the work piece. These
are made of carbon steels with 0.85 to 1% carbon and 0.5 to 0.9% manganese, and hardened to
RC 60 to 64 to minimize wear due to contact with hard, rotating tools. Bushes are generally
finished by grinding inside and outside diameters within 0.01 mm concentricity. The inside
diameter is ground to precision running fit (F7) with the drill/reamer to be guided, whereas
the outside diameter is made press fit (p 6), precision location fit (h6) or precision running fit
(f6) depending upon the function and application of the bush.
14.3.4.1 Press Fit Bush
Of the various types of bushes used, the press fit bushes are the most common type and are
pressed (interference fit) in the jig plates (Fig. 14.2a). These bushes are used in batch production
where the bushes often outlast the life of the jig. Press fit bushes are also used as liners for
renewable and slip bushes.
14.3.4.2 Renewable Bushes
For continuous or large batch production, the inside diameter of bush is subjected to severe
wear due to continuous contact with the hard cutting tool. The guide bushes require periodic
