Abstract: Because of the principle and method of modeling, the surface in the 3D model often does not have a clear engineering semantics, which may produce ambiguity in tagging the dimension and other information on the 3D model of the project. This problem is not well solved by commercial CAD software. Based on OpenCASCADE 3D modeling kernel, this paper studies the methods to solve the above problems, and puts forward the concepts of modeling surface and process surface. By recognizing the mathematical attributes of the modeling surface and combining the engineering semantics in mechanical engineering, the modeling surface is bound to form a process surface that conforms to the engineering semantics. The information of 3D annotation information of the model is regarded as the attribute of the process surface, which increases the intelligence of the system's 3D dimensioning, and also makes it possible to extract the product semantics information automatically in subsequent product virtual assembly, processing and manufacturing as well as in product service.

Key words: MBD, 3D annotation, modeling surface, process surface, OpenCASCADE