Not quite a few extruded aluminum alloys are used for parts of building materials today. We develop an expert system for blanking process planning for such three dimensional materials by feature elimination. We think that blanking is the procedure adding' features to a raw material and process planning is the procedure eliminating features from a product. We attempt to implement this feature eliminating procedure on computers. A shape model of a product consists of two parts; a basic-extruded-material and features. A basic extruded material model represents shape of a raw material. It is a plain surface model by polygonal faces. Feature models are surface models, too. But, they describe negative volume representing blanked parts of the product. The expert system attempts to eliminate features by eliminating faces of features on each face of basic material model. At first, the system chooses one face of basic material model, and collects features' faces on it. Then, it searches database, we call 'feature eliminating case base, ' at each feature, and finds the blanking procedure which can eliminate, or make it. Repeating the above from the product to its raw material, the system gets process plans of blanking for the product.