# Optimization Method of Assembly Tolerance Types Based on Degree of Freedom
## 2 Related Work
[34; 35] used ontology technology to describe various tolerance terminology concepts in the field of tolerance knowledge, as well as the attribute relationships between concepts, such as assembly constraint relationships and assembly feature space constraint relationships, and use rule language to describe the mapping relationship between tolerance types and assembly features, thereby achieving automatic generation of tolerance types of assembly parts. This type of method is easy to understand, but it recommends all shape tolerances and directional position tolerances that match assembly feature features (geometric features, space constraint relationship features), resulting in a large number of tolerance types. Designers also need to further screen and optimize based on their own design experience. When designers use the above methods to achieve the automatic generation of tolerance types, they need to make a preference for the recommended tolerance scheme at the end. Typically, the selection methods can be grouped into two main categories, one using alternative selection and the other using screening optimization. Alternative preference is applied to the following two methods for automatic generation of tolerance types: One is the TTRS-based method [24; 25], and the other is a method based on assembly positioning constraints [26; 27; 28; 29]. They first assume that the target feature surface elements must be positionally toleranced, and then consider whether to use directional tolerance instead of positional tolerance based on the spatial relationship between the reference-based elements and the target feature elements. Screening optimization is applied to tolerance design methods based on reasoning rules. The rule-based reasoning method has the obvious feature that all form and orientation position tolerances conforming to the assembly feature can be derived for the same assembly feature surface. This requires the designer to screen and optimize from the recommended tolerance types. At present, screening rules are generally divided into two categories: One is based on the type of tolerance domain and the visualization of tolerance symbols to optimize. For example, the cylindrical feature surface uses roundness shape tolerance instead of line profile tolerance, reflecting the visualization of tolerance symbols [31; 32; 33; 34]. For another example, if coaxiality tolerance is selected for two assembly feature elements, parallelism tolerance is excluded to reflect the comprehensiveness of the tolerance area. The other type is optimized based on the working characteristics of the mating part. For example, the non-reference datum feature surface of the rotating motion axis, the total run-out tolerance, and the control comprehensive tolerance are generally used to replace the position tolerance in other directions [9; 19]. Optimization of these tolerance types relies heavily on the design experience of the designer. In order to overcome the subjectivity and experience of designers and improve the scientific of decision making, [PERSON] et al. [36] proposed an optimal tolerance method based on the independent principle and entropy theory for rotating shaft parts. However, this method needs to form an independent principle according to certain rules and select a weight factor. This method is difficult to apply to the tolerance design of parts with complex structures. In summary, the alternative optimization tolerance scheme and the screening optimization tolerance both rely heavily on designers' experience, and in the screening process; there will be cases which the designers do not consider thoroughly, resulting in the selection of tolerance types that cannot meet the geometric tolerance functional requirements of mechanical products. Therefore, the optimization method of tolerance types based on DOF proposed in this paper not only meets the geometric tolerance requirements of assemblies, but also has good applicability and reduces the difficulty of designers' decision-making.