## 6 Discussion  
In this paper, a semantic representation model of tolerance information based on tolerance-zone DOFs and a semantic model of AT meta-ontology based on tolerance-zone DOFs were proposed. On this basis, a tolerance specification design method combining tolerance-zone DOFs and ontology was constructed, which is able to automatically filter assembly geometric tolerance types. This section compares the method proposed in this paper with the two studies on ontology-based tolerance design and tolerance type screening described in Section 2, as follows. From the tolerance specification design point of view, our proposed method combines tolerance-zone DOFs with the hierarchical assembly tolerance information representation model, which improves the assembly tolerance information representation model, realises the screening of tolerance types and meets the geometric functional requirements of the product. From the automatic screening of tolerance types, we constructed a semantic representation model of tolerance information containing tolerance-band freedom. On this basis, the semantic model of assembly tolerance meta-ontology based on tolerance-band freedom ontology was constructed, which can be well integrated with OWL-based ontology technology combined technology integration. For this reason, this proposed approach greatly supports semantic interoperability in assembly tolerance specification design, and the field of tolerance specification design using heterogeneous CAD systems is expanded. In contrast, the automatic tolerance generation method based on assembly positioning  
Figure 16: Tolerance marking for part Part\({}_{3}\). \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**ASF 1**} & \multirow{2}{*}{**\
\begin{tabular}{c} **\(\mathsf{In}\)** \\ **Plan** \\ \end{tabular}} & \multicolumn{2}{c}{\(\mathsf{In}\)\(\mathsf{Any}\)} & \multirow{2}{*}{\(\mathsf{O}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} & \multirow{2}{*}{\(\mathsf{\diagup}\)} \\  & & & & & & & & & & & \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Selected \\ tolerances \\ \end{tabular} } & \(\begin{array}{c}\mathsf{P}_{3}\mathsf{S}_{1}\\ \mathsf{P}_{3}\mathsf{S}_{2}\\ \mathsf{P}_{3}\mathsf{S}_{3}\\ \end{array}\) & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} & \(\begin{array}{c}\mathsf{\diagup}\) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Tolerance after screening of part Part\({}_{3}\). constraints proposed in the literature [27; 28; 29], which firstly recommends positional degree tolerance and, on this basis, adds directional tolerance or replaces the positional degree with directional tolerance according to the type of constraints on the assembly feature faces, does not facilitate automatic screening in combination with OWL ontology technology. From the results of the case validation, the improved tolerance specification design method outlined in this paper achieved the effect of using the tolerance-band degrees of freedom to screen the tolerance types and achieved the purpose of automatically generating tolerance types and a small number of recommended tolerances for the elements of the assembly features of the workpiece.