Page 232 - DMGT553_RETAIL_STORE_MANAGEMENT

Unit 13: Different Fixtures Available for a Retail Store

13.2 Fixture Planning Notes

The fixture planning requires workpieces and machining information including geometry,
material, dimension and tolerance, processing plan and tool path. To hold a specific part, several
design constraints may be applied. Among them, four main constraints in the fixture planning
are as follows:
1. Geometrical constraints: Accurate locating of a workpiece should be ensured to meet
machining accuracy requirements of a workpiece.

2. Accessibility constraints: There should be no interference among fixture components,
workpieces, and machining tools during assembly and machining. In addition, it should
be easy to load and unload the workpiece.

3. Force constraints: The fixture should be strong enough to resist the forces and moments
produced by clamps and machining tools. A minimum clamp force should be specified for
the workpiece stability.
4. Deformation constraints: The stiffness of a fixture system should be sufficient to keep the
workpiece deformation within the design tolerance.
Commonly used fixture locating methods include:
(1) 3-2-1 point locating for prismatic workpieces, it uses three locators on the primary locating
surface, two locators on the secondary locating surface, and one locator on the tertiary
locating plane;
(2) one plane and two pins locating for general parts with two holes. It uses a primary
locating plane, a primary pin, and a secondary pin to restrict the freedom of a workpiece;
(3) V-block locating for external cylindrical parts. One wide V-block or two short V-pads may
be used to hold the workpiece. The clamping methods can be summarized as top clamping
and side clamping. It is usually used to restrict a workpiece’s movement to keep it stable
during processing. The clamping planning determines clamping surfaces and points on
the workpiece and clamping components, the magnitude of each clamping force, and the
clamping sequence when the stability of the workpiece becomes a concern. Some locating
and clamping variations may be applied for different part geometrical and processing
requirements. Fixture assembly planning determines the fixture assembly sequence and
assembly tools used. It can also verify the possible interference of fixture components and
workpieces in an assembly process, and the ease of workpieces loading and unloading.

Notes CAFP can be divided into four phases: problem description, fixture analysis, fixture
synthesis, and fixture verification. The problem description defines fixture design variables,
design constraints and design objectives. In the fixture analysis, a workpiece-fixture
interaction model is built in terms of geometry, kinematics, force, and deformation. The
analysis result is then used to select the locating, supporting and clamping surfaces and
points on the workpiece. The fixture synthesis determines details of the fixture configuration
including selecting fixture elements, placing the elements in suitable locations, and
generating fixture assembly plan. The fixture configuration is verified in respect to
geometrical interference, locating determinant, clamp stability and machining tolerance
in the fixture verification phase. If design objectives are not satisfied, the result will be
sent back to fixture analysis phase for further improvements.

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