Comparison of Variant and Generative Process planning methods and their Computer Aided Process Planning Presented By- Pratik Patel. Approaches to Computer Aided Process Planning (CAPP). Variant Process Planning, Advantages and Disadvantages. Generative Process. The next stage of evolution is toward generative CAPP (Stage IV). At this stage, process planning decision rules are built into the system. These decision rules.

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Process knowledge in form of in the form of decision logic and data matches the part geometry requirements with the manufacturing capabilities cap knowledge base. A further step in this stage is dynamic, generative CAPP which would consider plant and machine capacities, tooling availability, work center and equipment loads, and equipment status e.

However, variant CAPP is based on a Group Technology GT coding and classification approach to identify a larger number of part attributes or parameters. Significant benefits can result from the implementation of CAPP.

In a detailed survey of twenty-two large and small companies using generative-type CAPP systems, the following estimated cost savings were achieved: Process plans which typically provide more detailed,step-by-step work instructions including dimensions related generayive individual operations, machining parameters, set-up instructions, and quality assurance checkpoints. A number of methods have been developed for part family formation using coding and classification schemes of group technology GTsimiliarity-coefficient based algorithms and mathematical programming models.

A typical CAPP frame-work is shown in figure The tools that are widely used in development of this database are flow-charts, decision tables, decision trees, iterative algorithms, concept of unit machined surfaces, pattern recognition techniques and artificial intelligence techniques such as expert system shells.

Dynamic, generative CAPP also implies the need for online display of the process plan on a workorder oriented basis to insure that the appropriate process plan was provided to the floor.

Computer-Aided Process Planning

As the design process is supported by many computer-aided tools, computer-aided process planning CAPP has evolved to simplify and improve process planning and achieve more effective use of manufacturing resources.

For example, when one changes the design, it must be able to fall back on CAPP module to generate manufacturing process xapp cost estimates for these design changes. The system logic involved in establishing a variant process planning system is relatively straight forward — it is one of matching a code with a pre-established process plan maintained in the system.


Grouping parts into part families 3. The initial challenge is in developing the GT classification and coding structure for the part families and in manually developing a standard baseline process plan for each part family. Similarly, in case of machine breakdown on the shop floor, CAPP must generate the alternative actions so that most economical solution can be adopted genetative the given situation.

Fabrication and assembly drawings ca;p support manufacture as opposed to engineering drawings to define the part. When a new part was introduced, the process plan for that family would be manually retrieved, marked-up and retyped.

Since finite scheduling systems are still in their infancy, this additional dimension to production generatige is still a long way off. The nature of the parts will affect the complexity of the decision rules for generative planning and ultimately the degree of success in implementing the generative CAPP system. The first step is the implementation of GT or FT classification and coding.

Process planning encompasses the activities and functions to prepare a detailed set of plans and instructions to produce a part. The assembly is shown on the screen and as a employee steps through the assembly process with a footswitch, the components to be inserted or assembled are shown on the CRT graphically along with text instructions and warnings for each step.

Prior to CAPP, manufacturers attempted to overcome the problems of manual process planning by basic classification of parts into families and developing somewhat standardized process plans for parts families Stage I. Manufacturers have been pursuing an evolutionary path to improve and computerize process planning in the following five stages:.

The decision rules would result in process plans that would reduce the overloading on the primary work center by using an alternate routing that would have the least cost impact. CAPP integrates and optimizes system performance into the inter-organizational flow.

The baseline process plans stored in the computer are manually entered using a super planner concept,that is, developing standardized plans based on the accumulated experience and knowledge of acpp planners and manufacturing engineers Stage III.

For example, details such as rough and finished states of the parts and process capability of machine tools to transform these parts to the desired states are provided. Definition of coding scheme 2.


Computer-Aided Process Planning

For example, if a primary work center for an operation s was overloaded, the generative planning process cap; evaluate work to be released involving that work center,alternate processes and the related routings. In the generative CAPP, process plans are generated by means of decision logic, formulas, technology algorithms and geometry based data to perform uniquely many processing decisions for converting part from raw material to finished state.

Retrieval and modification of standard process plan A number of variant process planning schemes have been developed and are in generatlve. This type of purely generative system in Stage V will involve the use of artificial intelligence type capabilities to produce process plans as well as be fully integrated in a CIM environment.

Other capabilities of this stage are table-driven cost and standard estimating systems. In a genegative survey of twenty-two large and small companies using generative-type CAPP systems, the following estimated cost savings were achieved:.

This suggests a system in which design information is processed by the process planning system to generate manufacturing process details. Further,this graphically-oriented data can then be provided to manufacturing in the form of hardcopy drawings or work instruction displays.

While CAPP systems are moving more and more towards being generative, a pure generative system that can produce a complete process plan from part classification and other design data is a goal of the future.

Module G:Computer Aided Process Planning

The geometry based coding scheme defines all geometric features gdnerative process related surfaces together with feature dimensions, locations, tolerances and the surface finish desired on the features. Process planning translates design information into the process steps and instructions to generatvie and effectively manufacture products.

This type of system uses work instruction displays at factory workstations to display process plans graphically and guide employees through assembly step by step.

The variant process planning approach can be realized as a four step process; 1.