Study On Generation Of Assembly Dimension Chain And Tolerance Analysis System Based On MDT

Based on the "Study on Computer Aided Tolerancing System" (National Nature Science Fund Project, No: 59705022), the technology of the automatic generation of assembly dimension chain and tolerance analysis based on MDT is presented. The prototype system for computer aided tolerance analysis is developed and proved by examples.In the first chapter, State-of-the-arts of computer aided tolerancing (CAT) system at home and abroad are summarized. The technology of the automatic generation of assembly dimension chain is also analyzed. Main contents and general structure scheme of this dissertation are present.In the second chapter, the constraints mechanism of MDT is studied. Dimension constraints, geometric constraints and assembly constraints of MDT are analyzed. Combined with new feature type defined by ISO/TC 213 and based on the feature of spacial movement-pair, seven features are analyzed. Variational geometric constraints (VGC) are classified as self-referenced VGC, cross-referenced VGC and mating VGC. Seven kinds of self-referenced VGC, and six kinds lower-pair mating VGC are classified.In the third chapter, the relationship of mechanical assembly and information request is studied. Several common assembly information models are analyzed. The concept and feature of object are imported. Top-down assembly database way is adopted and the assembly information model is improved with the concern of the character of the CAT. The concept of assembly database faced to dimension chain is put forward and realized. At last, the definition of the part class, assembly class and constraint class of the assemble database are given.In the fourth chapter, the automatic generation theory of the dimension chain based on assemble drawing is put forward. First, the closing loop is specified in the assembly drawing by the user. Then system can get the assembly constraints from the drawing and find all the parts joined in this assemble relation. The parts' related dimension information is also obtained. The dimension chain is generated later.In the fifth chapter, the dimension distributing models of the buildup loop and closing loop are studied. Putting forward that closing loop can be divided into normal distribution model and beta distribution model. At the end, tolerance analysis is studied based on the beta distribution model.In the sixth chapter, the prototype system for three-dimensional tolerance design based on MDT is studied.In the seventh chapter, all of the work in this dissertation is summed up. The future research of computer-aided tolerancing is prospected.