FEATURE-BASED MODELS FOR CAD/CAM AND THEIR LIMITS

Contemporary CAD/CAM systems do not manage a complete product definition. Geometrical models are almost completely defined, but important information such as tolerances or features should provide a more powerful foundation for future CAD/CAM systems. This paper will concentrate on form features. It is possible to extract features from an existing solid model. This extraction can be human-assisted or automatically deducted from a B-rep or CSG model. The algorithms are rather complex and none of them is perfect. They are also model-dependent. Another way to define features is to create them during design, using boolean operations (hole, pocket, chamfer, etc.). This requires a solid model in which features are recognized. This model can be CSG or B-rep and it leads to feature-based modelling. We shall survey the different approaches (extraction or feature-based modelling). Then we will concentrate on the requirements for a feature-based model and try to show which kind of problems can or cannot be solved following this approach. Some difficult and important problems are not easily solved using features. Assembly will be used as an example to derive requirements for a feature-based modelling system, even though it takes into account only a part (larger than the solid models) of the information necessary for a complete CAD/CAM model.