The envelope surface of a swept volume is the set of points which are part of the frontier of the swept solid. The solid which generates the swept volume (i.e. the tool) is called generator. The envelope surface can be subdivided in two types of surfaces. The surfaces of the first type are a subset of the generator surface at the beginning and the end of the trajectory, while the surfaces of the second type are generated during the motion of the generator. For many industrial application only the second type of surfaces is relevant.
