Tolerancing is an essential part of a product design process. It is traditionally performed after the product structure has been determined. The product precision requirements may not be fully utilized for the decision making for selecting the best structure in the early stage of product design — the conceptual design phase. A growth design method has been proposed for product structure design and concurrent tolerancing, in which the product form and tolerance can be generated in the same way as an organism grows. The basic feature of biology, cell division, has been borrowed as the design guiding principle. Product conceptual model has been built as the basis of form and tolerance growth design. The relationship of product function, structure and tolerance was analysed to explain on what and how they act in product design process. Tolerance mathematic model expressed by screw parameters and expression model based on Technologically and Topologically Related Surface (TTRS) are described to execute the tolerance allocation. The automation selection of Datum reference frame (DRF) can facilitate the generation of tolerance. Finally, a prototype form design and tolerancing system has been implemented and applied to a dedicated fixture design to verify the validity of the growth design.

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