In this paper a model of the design process is proposed, based on manufacturable design transformations. The product model enables manufacturability checking while designing. The model is based on relations between geometrical entities, with a tolerance model as an inseparable part. When performing for instance assembly analysis, creating numerical code for a machining centre, or command code for a Co-ordinate Measuring Machine, the product description is interpreted for implications in those specific applications. The authors demonstrate a method to reason with tolerances in several stages in design and manufacturing. A typical example is used to illustrate the method.