The selection of tolerances for assemblies is one of the most difficult problems and still remains unsolved in the area of Computer Aided Design. Research has been underway to use computers to select tolerance for assemblies. The main problems faced in this research are: (1) it is difficult to obtain relationships between the parameters of the solid primitives and the dimensioned quantities of parts for tolerance calculations; (2) current solid model based CAD systems lack mechanisms to support conventional dimension and tolerance data; and (3) it is computationally expensive to obtain information from a solid model for tolerance calculations.; The research described in this dissertation proposes a new way of thinking about computer aided tolerancing for assembly. In this method, data structures and calculations for tolerancing are fully compatible, yet separate from, the traditional data structures in a CAD-based solid model. It can greatly simplify the computer modeling of complex tolerance problems by eliminating unnecessary information and details.; In this research, an assembly is modeled by a hierarchical frame data structure. An assembly relation is specified by the mating relations through the mating surfaces of each component. Features on each component are defined by feature parameters. Tolerance calculations are applied on the assembly data structure and feature parameters. An assumed boundary representation system (BREP) is only used for the purpose of geometrical generation. In this model, assembly relations, topology, and geometry are completely separate.; A computer procedure for the automatic dimensional loop generation is developed. The procedure uses mating relations of the assembly model to establish dimension loops. A tree search based algorithm is developed to select discrete tolerances for component features in length fits. A knowledge-based model is developed to specify tolerances for cylindrical fits.; On the basis of the proposed model, a prototype system is developed. The prototype does automatic tolerance allocation and process selection for products with an axial symmetric assembly. The validation has been tested by successfully selecting tolerances and processes for several sample problems.
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