Abstract: In order to study the symmetry of the assembling parts more systematically, the concept of the image symmetry groups is first introduced into the study of the structural symmetry and its application to mechanical parts. The point symmetry groups of mechanical parts and their determination methods are discussed. Every part has two kinds of rotation angles when it is assembled. The sum of these two angles is the total rotation angle which affects the directional difficulty of the part. It is found that the total rotation angle of the point group of a part depends on its pure rotation subgroup and appears to be interval distribution. The orientation of symmetry design for assembling parts is summarized. The conceptual system of the part point symmetry groups and the corresponding determination method are put forward. By analyzing the graphs of the total rotation angle distribution interval of the part point group, the optimization of the structure of the part point group is proposed to improve the assembly of parts.

Key words: Design for assembly, Mechanical parts, Symmetry group, Total rotation angle