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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Further,this graphically-oriented data can then be provided generativs manufacturing in the form of hardcopy drawings or work instruction displays. Geneative 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.
Routings which specify operations, operation sequences, work centers, standards, tooling and fixtures. For example, when one changes the design, it must be able to fall back on CAPP module to generate manufacturing process and cost estimates for these design changes.
Fabrication and assembly drawings to support manufacture as opposed to engineering drawings to define the part.
However, variant CAPP is based on a Group Technology GT coding and classification approach to identify a larger number of part attributes or parameters. While this improved productivity, it did not improve the quality of the planning of processes and it did not easily take into account the differences between parts in a family nor improvements in production processes.
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. 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.
The first step is the implementation of GT or FT classification and coding.
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. There are two major components of generative CAPP; a geometry based coding scheme and process knowledge in form of decision logic data. The results of the planning are:. Manufacturers have been pursuing an evolutionary path to improve and computerize process planning in the following five stages:.
The similiarities in design attributes and manufacturing methods are exploited for the purpose of formation of part families. Dynamic, generative CAPP also implies the genfrative for online display of the process plan on a workorder oriented basis to insure that the appropriate process plan was provided to the floor.
When comapred with manual experience-based process planning, CAPP offers following advantages; Systematic developemnt of accurate and consistent process plans Reduction of cost and lead generatove of process planning Reduced skill requirements of process planners Increased productivity of process planners Higher level application progams such as cost and manufacturing lead time estimation and work standards can be interfaced.
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.
Grouping parts into part families 3. 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. 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. Definition of coding scheme 2.
This approach would involve a user responding to a series of questions about a part that in essence capture the same information as in a GT or FT code. Tight integration with a manufacturing resource planning system is needed to track shop floor status and load data and assess alternate routings vis-a-vis cpp schedule. Prior to CAPP, manufacturers attempted to overcome the problems of manual process planning by basic classification of parts into families and developing somewhat generatiev process plans for parts families Stage I.
A second key to generative process planning is the available data related to the part to drive the planning. It includes selection of processes, machine tools, jigs or fixtures, tools, inspection equipments and sequencing operations. For example, if a primary work center for an operation s was overloaded, the generative planning process would evaluate work to be released involving that work center,alternate processes and the related routings. CAPP is a highly effective technology for discrete manufacturers with a significant number of products and process steps.
Process planning encompasses the activities and functions to prepare a detailed set of plans and instructions to produce a part.
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. The variant process planning approach can be realized as a four step process; 1.
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 genedative along with text instructions and warnings for each step. The planning begins with engineering drawings, specifications, parts or material lists and a forecast of demand. 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.
These attributes allow the system to select a baseline process plan for the part family and accomplish about ninety percent of the planning work.
Computer-Aided Process Planning
Process planning is very time-consuming and the results vary based on the person doing the benerative. The geometry based coding scheme defines all geometric features for process related surfaces together with feature dimensions, locations, tolerances and the surface finish desired on the features. In addition, there has been significant recent effort with generative process planning for assembly operations, including PCB assembly.
This is the function of CAPP. In order to produce such things as NC instructions for CAM equipment, basic decisions regarding equipment to be used,tooling and operation sequence need generatiive be made.
CAPP integrates and optimizes system performance into geenrative inter-organizational flow. Process knowledge in form of in the form of decision logic and data matches the part geometry requirements with the manufacturing capabilities using knowledge base.
Module G:Computer Aided Process Planning
Other capabilities of this stage are table-driven cost and standard estimating systems. In a detailed survey of twenty-two large and small companies using generative-type CAPP systems, the following estimated cost savings were achieved:. Similarly, in case of machine breakdown on the shop floor, CAPP must generate the alternative genfrative so that most economical solution can be adopted in the given situation.
In a detailed survey of twenty-two large and small companies using generative-type CAPP systems, the following estimated cost savings were achieved: When a genrrative part was introduced, the process plan for that family would be manually retrieved, marked-up and retyped.
Computer Aided Process Planning.
The planner will add the remaining ten percent of the effort modifying or fine-tuning the process plan. 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.
Rapid strides are being made to develop generative planning capabilities and incorporate CAPP into a computer-integrated manufacturing architecture. Reduced process planning and production leadtime; faster response to engineering changes Greater process plan consistency; access to up-to-date information in generxtive central database Improved cost estimating procedures and fewer calculation errors More complete and detailed process plans Improved production scheduling geenerative capacity utilization Improved ability to introduce new manufacturing technology and rapidly update process plans to utilize the improved technology SUMMARY CAPP is gemerative highly effective technology for discrete manufacturers with a significant number of products and process steps.
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. 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 multiple planners and manufacturing engineers Stage Generaive.
The majority of generative CAPP systems implemented to date have focused on process planning for fabrication of sheet metal parts and less complex machined parts. A typical CAPP frame-work is shown in figure