ComputerAided_Design_Engineering_amp_Manufactur.pdf

Other Fixturing Techniques
There are several other techniques proposed for workholding systems. Examples of these include the multileaf
vice, petal collet, and stackable washer fixtures. 1 These have received less attention due to the lack of
robustness and the small range of workpieces which they can accommodate. Generally, these techniques
do not provide the flexibility, accuracy, and integration capability required in a world-class manufacturing
environment. The most suitable approaches for flexible and computer-integrated manufacturing environments
are deemed to be modular and reconfigurable fixturing, together with programmable clamps.
3.4 Reconfigurable Fixture Modules
In this chapter, attention is mainly focused on reconfigurable workholding. However, most of the planning
and integration techniques can readily be applied to other fixturing approaches such as modular or
programmable. In fact, for any fixturing system to be acceptable in a manufacturing environment, it
must be capable of integration with the CAD/CAM systems and also automation devices such as robots
and machine tools. 20 The reconfigurable workholding approach employs a number of adjustable fixture
modules to locate and constrain the workpiece in the desired location. For the purpose of establishing
the planning and analysis strategies, four types of fixture modules have been developed. 21 These modules
include vertical support, horizontal support, horizontal clamp, and vertical clamp, as shown in Figure 3.5.
These modules are mainly single axis and have been developed for fixturing workpieces for robotic
assembly; however, the concept can be extended for the design of multi-axis modules for panel type
workpieces (see Figure 3.6).
The fixture setup procedure involves a robot retrieving a number of vertical supports from the fixture
magazine and placing these on a fixture platform such as an electromagnetic chuck. The robot proceeds
to adjust the height of these modules, which support the workpiece at the required height. The electromagnetic
chuck is used to secure the fixture modules once they are placed at the assembly station before any
adjustments are to take place. Next, a number of horizontal supports are placed on the chuck where they
would be in contact with the workpiece reference faces or locating tabs. The robot then places the
workpiece within this fixture layout (referred to as the “initial” fixture layout); a number of horizontal
clamps and vertical clamps are then placed on the chuck. The robot adjusts the height of these modules
and activates the clamping mechanisms on them, and thereby fixturing the workpiece into the desired
position and orientation (pose). The robot may now perform the subsequent assembly operations on
the workpiece. It must be noted that other fixture modules have also been developed by other researchers
for operations such as assembly and drilling. 22–23
3.5 Fixture Task Planning and Analysis
An important objective of any flexible workholding system is to provide for rapid design of fixture
configuration and analysis, and automated generation of fixture setup programs. 13,17,24 The fixture planning
and analysis system must reside within the overall process planning of the manufacturing system.
An approach is now presented describing an interactive computer-aided fixture planning and analysis
(CAFPA) system for reconfigurable fixtures. In the proposed approach, the CAFPA consists of six
submodules comprising pre-processor, planning, analysis, verification, interference checking, and postprocessor
(Figure 3.7). The following sections describe the individual submodules and their functionalities.
Pre-Processor Phase
The first operation in the pre-processor segment is to access the CAD data base and retrieve the model
data of the workpiece. The boundary representation (B-Rep) scheme provides an excellent structure for
manipulation of appropriate data and has a format as shown in Figure 3.8. There are several CAD systems
© 2001 by CRC Press LLC