Manufacturing Data and Knowledge

Machining methods (also referred to as machining practices) provide CAPP with the knowledge, expertise, and procedures that a human process planner uses. These methods may be based on sound scientific principles, experimental results, experience, or preferences established within a particular machining context. They also may be generic and applicable over a wide range of machining problems or specific to a single one. The challenges in using machining methods within CAPP fall into the following categories: • Identification and retrieval • Implementation • Maintenance • Customization Identification and retrieval are concerned with understanding how a human process planner applies experience and techniques to make decisions when generating process plans: What decisions are being made? What characteristics of the situation are being recognized by the planner that trigger these decisions? The main challenge here stems from the fact that human planners do not necessarily follow a consistent strategy in applying methods. The process often requires complex trade-offs of information from several sources. When one of these sources is experience, the basis of the applied method can be difficult to verify. Thus, identification and retrieval of methods are not just a bookkeeping task. Rather, it requires the cultivation of an attitude toward process planning based on a sound methodology for applying machining methods. Methods implementation requires an approach that is general enough to capture information from very different sources while at the same time is simple enough to provide a maintainable, noncorruptible environment. Rule-based expert systems have been the most commonly adopted implementation strategy among CAPP system developers. Because the need to update or add new methods always exists as more information becomes available or as new methods are applied to more applications, maintenance of the knowledge base becomes a key concern. As changes are made, the integrity of the information needs to be preserved. One problem occurs when new methods are added that conflict with old ones. The system needs to include a strategy for resolving such conflicts. One approach that has been used extensively with expert systems is to place the onus on a knowledgable engineer to avert such problems. However, as the size of the knowledge base grows, the cost of employing dedicated personnel for this task becomes prohibitive. Finally, creating off-the-shelf CAPP systems with the methods included is a difficult if not impossible task. This is because it is unlikely that the system developer can capture all the desired methods from all potential users during system development. Thus, while a system may come with some generic, widely accepted methods, it must include a facility to allow new methods customized to each context to be added to the system.

THE MECHANICAL
SYSTEMS
DESIGN
HANDBOOK
Modeling, Measurement,
and Control
OSITA D. I. NWOKAH
YILDIRIM HURMUZLU
Southern Methodist University
Dallas, Texas
CRC PRESS
Boca Raton London New York Washington, D.C.