Computer-Aided Process Planning

Manufacturers have been pursuing an evolutionary path to develop and computerise process planning in the following five stages :

A. Stage 1       :           Manual classification, standardised process plans

B. Stage II      :           Computer maintained process plans

C. Stage III     :          Variant CAPP

D. Stage IV      :          Generative CAPP

E. Stage V        :          Dynamic, generative CAPP

Prior to CAPP, manufacturers attempted to overcome the difficulty of manual process planning by basic classification of parts into families and by developing somewhat standardised process plans for parts families (Stage I). While a new part was introduced, the process plan for the family would be manually retrieved, marked-up and retyped. While this enhanced productivity, it did not develop the quality of the planning of processes and it did not simply take into account the differences among neither parts in a family nor improvements in production processes.

Computer-aided process planning primarily evolved as a means to electronically store a process plan once it was formed, retrieve it, modify it for a new part and print the plan (Stage II). Other capabilities of this stage are table-driven cost and standard estimating systems.

This initial computer-aided approach evolved into what is now known as "variant" CAPP. Through, variant CAPP is depend on a Group Technology (GT) coding and classification approach to recognize a large number of part attributes or parameters. These attributes permit the system to choose a baseline process plan for the part family and accomplish around ninety percent of the planning work. The planner will add the remaining ten % of the effort changing or fine-tuning the process plan. The baseline procedure plans stored in the computer are manually entered by using a super planner concept that is, developing standardised plans depend on the accumulated experience and knowledge of multiple planners and manufacturing engineers (Stage III).

The next stage of evolution is toward generative CAPP (Stage IV). At this stage, process planning decision rules are made into the system. These decision rules shall operate based on a part's group technology or features technology coding to generate a process plan that shall require minimal manual interaction and modification (for example entry of dimensions).

While CAPP systems are moving more and more towards being generative, a pure generative system that may produce a complete process plan from part classification and other design data is a goal of the future. This kind of purely generative system (in Stage V) will involve the use of artificial intelligence type capabilities to generate process plans as well as be fully integrated in a CIM environment. In this stage a further step is dynamic, generative CAPP which would consider plant and machine capacities, work centre and equipment loads, tooling availability, and equipment status (for example maintenance downtime) in developing process plans.

The process plan developed with a CAPP system at Stage V would differ over time based on the resources and workload in the factory. For instance, if a primary work centre for an operation(s) was overloaded, the generative planning process will evaluate work to be released involving those alternate processes, work centre and the related routings. The primary work centre using an alternate routing that would have the least cost impact. Since finite scheduling is still a long way off.

Dynamic, generative CAPP also implies the requirement for online display of the process plan on a work order oriented basis to insure that the suitable process plan was provided to the floor. Tight integration along a manufacturing resource planning system is needed to track shop floor status and load data and assess alternate routing vis-à-vis the schedule. Finally, this stage of CAPP would feed shop floor equipment controllers directly or, in a less automated environment, display assembly drawings online in conjunction along process plans.