Computer Aided Process Planning

COMPUTER AIDED   PROCESS  PLANNING

There is much interest by manufacturing firms in automating the task of process planning using computer-aided process planning (CAPP) systems. The shop trained people who are familiar with the details of machining and other processes are gradually retiring, and these people will be unavailable in the future to do process planning. An alternative way of accomplishing this function is needed, and CAPP systems are providing this alternative.

CAPP is usually considered to be part of computer-aided manufacturing (CAM). However, this lends to imply that CAM is a standalone system. In fact, a synergy results when CAM is combined with computer-aided design to create a CAD/CAM system. In such a system, CAPP becomes the direct connection between design and manufacturing. The benefits derived from computer -automated process planning include the following:

   Process rationalization and standardization. Automated process planning leads to more logical and consistent process plans than when process planning is done completely manually. Standard plans tend to result in lower manufacturing costs and higher product quality.

   Increased productivity of process planners. The systematic approach and the availability of standard process plans in the data files permit more work to be accomplished by the process planners.

   Reduced lead time for process planning. Process planners working with a CAPP system can provide route sheets in a shorter lead time compared to manual preparation.

   Improved legibility. Computer-prepared route sheets are neater and easier to read than manually prepared route sheets,

Incorporation o other application programs The CAPP program can be interfaced With other application programs, such as cost estimating and work standards.

Computer-aided process planning systems are designed around two approaches. These approaches are called: (1) retrieval CAPP systems and (2) generative CAPP systems. Some CAPP systems combine the two approaches in what is known as semi-generative CAPP.

            Retrieval CAPP Systems

A retrieval CAPP system, also called a variant CAPP system, is based on the principles of group technology (GT) and parts classification and coding (Chapter 15), In this type of CAPP, a standard process plan (route sheet) is stored in computer files for each part code number. The standard route sheets are based on current part routings in use in the factory or on an ideal process plan that has been prepared for each family. It should be noted that the development of the data base of these process plans requires substantial effort.

A retrieval CAPP system operates as illustrated in Figure 25.3. Before the system can be used for process planning, a significant amount of information must be compiled and entered into the CAPP data files. This is what Chang et al., refer to as the "preparatory phase:' It consists of the following steps: (1) selecting an appropriate classification and coding scheme for the company, (2) forming part families for the parts produced hy the company; and (3) preparing standard process plans for the part families. It should be mentioned that steps (2) and (3) continue as new parts are designed and added to the company's design data base.

After the preparatory phase has been completed, the system is ready for use, For a new component for which the process plan is to be determined. the first step is to derive the GT code number for the part. With this code number, a search is made of the part family, file to determine if a standard route sheet exists for/he given part code. If the file contains a process plan for the part it is retrieved (hence. the word "retrieval" for this CAPP system) and displayed for the user. The standard process plan is examined to determine whether any modifications are necessary. It might be that although the new part has the same code number. there are minor differences in the processes required to make it. The user edits the standard plan accordingly. This capacity to alter an existing process plan is what gives the retrieval system its alternative name: variant CAPP system.

If the file does not contain a standard process plan for the given code number, the user may search the computer file for a similar or related code number for which a standard route sheer does exist. Either by editing an existing process plan, or by starting from scratch, the user prepares the route sheet for the new part. This route sheet becomes the standard process plan for the new part code number

The process planning session concludes with the process plan formatter, which prints alit the route sheet in the proper format. The formatter may call other application programs into use: for example, to determine machining conditions for the various machine tool operations in the sequence. to calculate standard times (or the operations (e.g., for direct labor incentives). or to compute cost estimates for the operations.

One of the commercially available retrieval CAPP systems is MultiCapp, from OIR the Organization for Industrial Research. It is an online computer system that permits the user to create new plans. or retrieve and edit existing process plans, as we have explained above. An example at a route sheet representing the output from the MultiCapp system is shown in Figure 25.4.

            Generative  CAPP Systems

Generative CAPP systems represent an alternative approach to automated process planning. Instead of retrieving and editing an existing plan contained in a computer data base, a generative system creates the process plan based on logical procedures similar to the procedures a human planner would use. In a fully generative CAPP system, the process sequence is planned without human assistance and without a set of predefined standard plans.

The problem of designing a generative CAPP system is usually considered part of the field of expert systems, a branch of artificial intelligence. An expert system is a computer program that is capable of solving complex problems that normally require a human with years of education and experience. Process planning fits within the scope of this definition.

There are several ingredients required in a fully generative process planning system. First. the technical knowledge of manufacturing and the logic used by successful process planners must be captured and coded into a computer program. In an expert system applied to process planning, the knowledge and logic of the human process planners is incorporated into a so called "knowledge base."The generative CAPP system then uses that knowledge base to solve process planning problems {i.e., create route sheets).

The second ingredient in generative process planning is a computer-compatible description of the part to be produced. This description contains all of the pertinent data and information  needed to plan the process sequence, Two possible ways of providing this description are: (l) the geometric model of the part that is developed on a CAD system dur10g product design and (2) a GT code number of the part that defines the part features in significant detail.

Figure 25.4 Route sheet prepared by the MultiCapp System (courtesy of OIR, the Organization for Industrial Research).

The third ingredient in a generative CAPP system is the capability to apply the process knowledge and planning logic contained in the knowledge base to a given part description. In other words, the CAPP system uses its knowledge base to solve a specific problem-planning the process for a new part. This problem-solving procedure is referred to as the "inference engine’ in the terminology of expert systems. By using its knowledge base and inference engine, the CAPP system synthesizes a new process plan from scratch for each new part it is presented.