Tolerance analysis for setup planning in computer-aided process planning



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Texas Tech University


Automated tolerance analysis is one of the most critical problems for computer aided process planning (CAPP) systems to be applied in the real manufacturing environment. The traditional way in CAPP uses only designed shape and nominal dimensions to generate an operation sequence arbitrarily, calculate operational dimensions and assign operational tolerances. Then, the dimensionai tolerance chain method is used to check the operational tolerances of the generated plan. The operational tolerances have to be made closer if the designed tolerance requirements are not met. Neither setup sequences nor datum elements are clearly specifíed. However, the economy of manufacture and the increase in the overall accuracy of many products can be greatly improved by the use of properly selected and specified datums for positioning purpose. How to generate feasible and economical operation sequences with specified datum elements and setups according to designed dimensional and geometric tolerances, how to evaluate altemative setup plans, and how to use computers to do it automatically have not been addressed. In this study, a graphical approach and a rulebase are developed and a neural network approach experiment is conducted for automated tolerance based analysis and selection of setups and datums for CAPP systems. The approaches and rules are based on a detailed analysis of the assembly tolerance analysis versus operational tolerance analysis, manually operated machining versus numerically controlled machining, and design datums versus machining datums. The knowledge and rules obtained in this study are used to train a back-propagation neural network to select datums and setups. The study is to improve the quality and economy of manufacturing by minimizing the effect of the cause of the variation of manufacturing process without controlling the cause itself performance measure of altemative plans is developed. The datums and setups are selected in such a way that the operation tolerances can be maximized to meet the design tolerance specifications. Because the manufacturing error sources have been considered and the effect of error sources is minimized, the selections will be more accurate and economical. The mlebase developed (A Manufacturing Datum And Setup Selection Rule Base For Rotational Parts-Technical Report) has been sent to the National Institute of Standard and Technology (NIST) depository of Process Planning Tested and is accessible by the process planning community all over the country.