ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
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typically can only direct a tool to move along linear and circular paths. However, geometric profiles of<br />
CAD-generated parts are usually defined by third- or higher-order curves and surfaces. Hence, it is<br />
necessary to fit lines and circular arcs, or their combination, to these CAD-defined curves to produce<br />
the CNC tool paths to some desired tolerance. To check and verify that the complex surfaces are generated<br />
to the desired accuracy, computer-controlled coordinate measurement machines (CMMs) are used. A<br />
logical approach is to use the same CAD-represented part to generate the NC program for the measurement<br />
of the part by the CMM. Setup, processing and data analysis requirements are quite different from<br />
those for machining operations. These are discussed in the last section.<br />
2.2 Automation of Process Planning for Computer-Generated<br />
NC Programs<br />
Process planning provides the necessary link between design and manufacture. Various computerautomated<br />
or computer-aided process planning (CAPP) systems have been developed and reported. 4–7<br />
The recent trend is towards integrating CAPP with CAD and production planning and control activities. 6–7<br />
Some systems aim to cater to the processing of a large variety of parts. Others are product- or geometryspecific,<br />
catering to a few specific families of parts, such as rotational parts. The applications of CAPP<br />
systems in industrial environment are somewhat limited and their full potentials have yet to be realized. 7<br />
In general, a fully automated (generative) process planning system performs satisfactorily for well-defined<br />
families of parts with established process plans. 5 Even for a family of parts with pre-determined sequence<br />
of operations and finite variation in geometry, much development effort is still required to ensure that<br />
all possible permutations and combinations are considered and an efficient plan generated. The following<br />
two sections present CAPP for part families of more specific geometry where the parts are processed by<br />
machining and all processing requirements are completed on a single CNC machine:<br />
• CAPP for NC turning8,9<br />
• CAPP for NC milling of spherical parts: space frame nodes10<br />
The outputs obtained from a CAPP system are used for job scheduling, NC programming and other<br />
manufacturing tasks. Prior to the generation of the NC programs, it is necessary to decide on the machining<br />
operations, select the cutting tools, and determine the cutter paths and cutting conditions. For a given machine<br />
tool, the activities involved in a typical machining operation planning include the following steps:<br />
• Interpretation of part geometry or feature extraction<br />
• Determination of cutting operations and boundaries<br />
• Selection of setups and cl<strong>amp</strong>ings<br />
• Selection of tool types<br />
• Determination of cut distribution and cut paths<br />
• Determination of cutting parameters<br />
• Determination of operation sequence and routing<br />
CAPP for Turning<br />
One approach to automating the decision making is to apply artificial intelligence (AI) techniques, such<br />
as expert system techniques. Using expert system techniques to provide a machining knowledge-based<br />
model, an integrated machining system (IMS) has been developed for the manufacture of rotational<br />
parts. 9 It aims to integrate and automate the feature extraction, operation planning, machinability data<br />
selection and NC program generation.<br />
The architecture of this system is illustrated in Figure 2.2. This PC-based system comprises a user<br />
interface for interaction, a feature recognizer, a knowledge base of facts and rules. A software tool called<br />
GOLDWORKS (III) with COMMON LISP as the programming platform is employed in the knowledge<br />
building. With this frame-based knowledge building tool, the information being manipulated is repre-<br />
© 2001 by CRC Press LLC