Abstract
In order to reduce the uncertainty in the selection of geometric tolerance items, a qualitative method for top-down transfer of geometric tolerance items is proposed. The assembly joint which is composed of two mating surfaces with functional requirements or structural constraints is acted as the basis of geometric tolerance items transmission. According to the structural characteristics, the assembly joints are divided into meta-assembly joints and composite assembly joints, and the priority rules for assembly joints are proposed. The transfer path of part-level geometric tolerance items is established according to the functional requirements and structural constraints among parts. On this basis, by adding information about the composition and constraint types of assembly joints between parts and the position constraint relationship of the general structure surface in the part, the transfer path of part-level geometric tolerance items is extended to the transfer path of geometric feature surface-level. With the development of product design, the initial functional requirements will be transformed into structural constraints between parts and geometric feature surfaces, and the structural transformation model of functional requirements is constructed. The generation specifications of geometric tolerance items based on structural constraints and the transfer specifications of datums are established. And based on the above specifications, the mapping relationship between functional requirements, structural constraints, and geometric tolerance items is defined. The synchronous transmission of geometric tolerance items along with the product design process are realized which provides an effective analysis tool for the top-down design of geometric tolerance items. Finally, the effectiveness of the method is verified by taking the transmission parts and connection parts in the crankshaft-piston mechanism as an example.
Highlights
With the continuous development of high-precision machinery and equipment, geometric tolerance design has become a key link that must be considered in the product design stage
Armillotta [5] proposed a method of generating tolerance specifications based on product data. This method realized the reasoning of geometric tolerance items based on assembly requirements; Jiang et al [6] presented a geometric tolerance information inference based on polychromatic set theory and its normative verification method, which improves the accuracy and efficiency of geometric tolerance design; By using the crawler graph to identify the position of the functional surface, a 3D manufacturing tolerance synthesis algorithm based on technology and topology-related surface rules was adopted by Jaballi [7]; In order to analyze error and its stack-up of mechanical product effectively, Zhang et al [8] presented an integrated modeling method of unified tolerance representation based on Key Features (KFs) and Graph Theory in order to deal with these types of tolerances simultaneously
The number of recommended assembly tolerance types generated can be further reduced on the basis of this; Start from the basic spatial relationship between geometric elements, the description logic of geometric tolerance, the judgment algorithm and the automatic generation algorithm of the tolerance type were proposed by Qin et al [9]; To realize the automatic generation of the geometric tolerance area in the CAD system, a method based on description logic was proposed by the research group in paper 10 and paper 11; Zhong et al [12,13] used description logic to construct a meta-model of assembly tolerance items, and automatically generated tolerance items based on the ontology
Summary
With the continuous development of high-precision machinery and equipment, geometric tolerance design has become a key link that must be considered in the product design stage. The positioning rule method is mostly based on the functional requirements of the assembly joints between the parts to select geometric items, the structural correlation between the general structure surfaces in the parts was ignored; The theoretical rules have problems with the accuracy and adaptability of the rules; Case-based inference may be subject to variation in the analogy process, resulting in errors in recommended tolerance items. To solve the above problems, a top-down transfer and design method of geometric tolerance items based on assembly joints was proposed based on the initial functional requirements. The assembly joint is the basic carrier of assembly connection and geometric tolerance item transmission, which expresses the spatial shape and positioning relationship and the matching characteristics between matching parts.
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