## 1 Introduction  
Computer-aided tolerance design is the key content of the digitalisation of the intelligent manufacturing of products [1; 2], in which the realisation of tolerance specification automatic generation (TSAG) is one of the key links. With the efforts of many scholars, the research on achieving the computer-aided design of tolerance specification has also made important progress: [PERSON] et al. [3] constructed a TSAG algorithm based on a multi-colour set method; [PERSON] et al. [4] and [PERSON] et al. [5] developed a TSAG method based on a hierarchical tolerance information representation model; and [PERSON] et al. [6] and [PERSON] et al. [7] proposed the adjacency matrix method to represent constraint relations of assemblies. On this basis, these methods are combined with computer technology to achieve the automatic reasoning of tolerance types for assembly parts. However, the above methods of automatically generating tolerance types have shortcomings. When reasoning about the tolerance types of the same assembly element (AE) of a part, it will reason about all the shape, orientation and position tolerances that correspond to the characteristics of the AE. However, in the actual detailed design of the tolerance specification scheme for the AE, further manual screening is required based on the geometric functional requirements and working characteristics of the product [4; 5; 6; 7; 8]. This situation can increase uncertainty and decision-making difficulties in product development. In addition, the design of complex products may be carried out using commercial CAD systems with different kernels. In the automatic generation of assembly tolerance types, it is necessary to implement a data exchange of tolerance specification semantic information between heterogeneous CAD systems [3; 9; 10; 11]. However, such tolerance specification information is difficult to express on a computer, making it difficult to efficiently generate and filter tolerance types for assembled parts in heterogeneous CAD systems [11; 12]. In order to achieve the data exchange of tolerance specification information between heterogeneous CAD systems and achieve the automatic selection of tolerance types, the following two tasks need to be carried out well. One is to select appropriate tolerance type screening methods. The second is to choose an appropriate product data representation language to meet the requirements of the tolerance information representation in computers and exchange between heterogeneous CAD systems [2; 11; 12]. In the process of designing assembly tolerance (AT) specifications, the use tolerance-zone DOFs can effectively achieve the screening and optimisation of common types and meet the geometric functional requirements of the assembly [13; 14]. In the literature [14], our team has analysed the influence of the tolerance-zone DOFs of assembly feature surfaces (AFSs) on the geometric functional tolerance of the assembly in the case of a single datum in a typical assembly and obtained the control parameter DOFs; based on this, we have established the process of manually screening the tolerance types with the control parameter DOFs. In order to reduce the uncertainty of manually screening tolerance types and improve design efficiency, it is necessary to adopt suitable means to achieve the automatic screening of tolerance types. And, in terms of tolerance information data exchange between heterogeneous CAD systems, using OWL (web ontology language) to achieve knowledge representation and tolerance semantic information has a good application effect in tolerance specification design [11; 12; 15] and inspection [16]. Therefore, this paper proposes an assembly geometry tolerance type screening method based on tolerance-zone DOFs tolerance-band-free ontology, using tolerance-band freedom to achieve the screening of tolerance types and at the same time using ontology technology to achieve the screening of tolerance types of automatic reasoning.