CAPP Computer Aided Process Planning

1. Introduction

 1.1 System Organization Given the engineering design of an item which has to be manufactured, process planning is the act of generating an ordered sequence of the manufacturing operations necessary to produce that part within the available manufacturing facility. For maximum efficiency, the developed process plan should produce the part at the lowest cost consistent with acceptable quality standards and using established procedures. In achieving these aims, it is essential that process plans for parts which are basically similar should be standardized. Traditionally, process planning has been regarded as a manual operation, usually carried out by a manufacturing engineer (process planner, process engineer, etc.) who is often a qualified and experienced machinist or tool maker. The success rate achieved by an individual planner is largely dependent upon individual skill and aptitude for the planning task, knowledge of manufacturing processes, equipment, materials and methods in general, and those available in a production facility in particular. T he rapid evolution of increasingly sophisticated production methods, the ever increasing trend towards the use of flexible manufacturing systems, and the need for improved manufacturing efficiency has necessitated a requirement for improved process planning. The use of computers to achieve this is now well established, and new computerized methods are continually being sought.  

1.1.1 Traditional Process Planning

 Traditional process planning methods usually allocate planning jobs to the next available planner. Each task is therefore treated as a new process plan with usually limited attention being given to process plans for similar parts already in existence. As a result, each process plan will reflect primarily the skills and experience of the individual planner concerned; hence, it is extremely difficult to maintain consistency and discipline in the planning process. Process planning also requires the use of many complex disciplines including sequencing, machine selection, time and motion study, programming, and material flow to name some of the most common. All of these skills are needed in an increasingly complex manufacturing environment and are making it even more difficult to produce efficient process plans. Several studies have been conducted to investigate the efficiency of manual process planning methods. One, conducted by Cincinnati Milacron, reports the results of a study into the process planning for the production of a family of spur gears. For a sample of 425 relatively simple spur gears, there were 377 different process plans (operation sequence and machine groups), 54 different types of machines required, and 15 different materials utilized. The plans gave 252 unique combinations of machine tools for successive operations. The inefficiency arising from this situation must be obvious to anybody with manufacturing experience. A perfect example of the problem concerns the production of the simple part shown in Figure 1, together with the processes proposed by four different process planners. The four different methods of producing the 18 mm hole are readily apparent and obviously reflect the past experience of the individual planners concerned. Attempts to standardize and rationalize the process planning task revolve around the maintenance of card index systems which relate to previous plans. Such systems quickly become outdated or are, at best, very cumbersome to use. The use of a computer for storing such data was an obvious first step and the first 

1.1.2 Classification and Coding

 The number of different part numbers which flowed through most manufacturing facilities were such that they made the initial card index approach only a very marginal improvement on the manual systems which they superseded. The development of standardized process plans required the identification of part families where such families can be distinguished by the sequence of manufacturing operations required. Several part families may share common machine tool routings but a unique production part family has the same, or nearly the same, list of operations. To define part families, it is necessary to have some form of parts classification and coding system. Classification and coding systems are often considered to be a part of the Group Technology (GT) production concept. However, classification and coding systems have advantages quite separate and distinct from their use with GT. For example, a well designed classification and coding system can provide significant advantages in the basic engineering design process by reducing the number of drawings and different part numbers which commonly occur over a period of time through the duplication of part designs which perform the same or similar function. 

When used with GT, the benefits of classification and coding can be summarized thusly:

 

• The formation of part families and machine groups .

• Quick retrieval of designs, drawings and production plans.

 • Design rationalization and reduction of design costs.

 • Secure reliable workpiece statistics.

 • Accurate estimation of machine tool requirements, rationalized machine loading, and optimized capital expenditure.

 • Rationalization of tooling setups and the reduction of set-up times and overall production times.

 • Rationalization and improvement of tool design and the reduction of tool design time and cost as well as tool fabrication time and cost.

 • Rationalization of production planning procedures and scheduling.

 • Accurate cost accounting and cost estimating.

 • Better utilization of machine tools, work holding devices, and manpower. 

• Improvement of Numerical Control (N/C) programming and the more effective use of N/C machines.

Many of these benefits can be achieved without using them in conjunction with the G T cell concept. There are a large number of proprietary classification and coding systems available ranging from the "look and see" approach to large numerically coded systems. The most well known systems include among others, the OPITZ system developed in Germany, the MICLASS system developed in Holland, and the BRISCH BIRN system developed in Sweden. In practice, most proprietary systems have been found to be inadequate for particular company applications and most successful classification and coding systems have been developed by adapting a basic proprietary system to suit the needs of the particular facility concerned. A basic problem in using any classification and coding system is the human problem of applying it to particular parts; i.e., different people using the same coding system will arrive at different codes for the same part. Most applications nowadays therefore, use computer assistance in order to obtain standardized coding. On e such aid is the DCLASS system developed at Brigham Young University, which can be utilized in conjunction with any classification and coding system to obtain consistent results. This system is an interactive system which enables a planner seated at a CRT to obtain a classification code by responding to a series of system produced queries. T he basic starting point of a CAPP system is the implementation of a good classification and coding system. However, work put into classification and coding has many other useful applications in the manufacturing and design environments, such that implementation costs can be easily recouped from efficiency improvements.