| Set up of a geometry model, that is, the description of drawing data is the basis of a CAD system. It decides the performance of a CAD system to a great extent. So it is necessary to learn the concept of geometry model to deeply understand and develop a CAD system. Commonly used geometry models are wireframe model, surface model and solid model. The first one only defines uncon-tinuous information about wireframes, containing no information about surfaces and solids. Developed from wireframe model, the surface model lacks information about solids, although it adds information of surfaces. So a solid model has been developed and widely used in CAD systems. Constructive Solid Geometry (CSG) is one of the modelling methods of solid model. In this paper, some important techniques about CSG, such as choose of basic drafting elements and their main parameters are introduced. The process of solid modelling is shown by taking a ball bearing as an example. |