This paper reports on the development of XPLANE, a generative computer aided process planning system for part manufacturing. Described is its position and functioning as a part of a more extended computer aided manufacturing system that includes a link to CAD systems, as well as systems for computer aided design and selection of jigs and fixtures, NC part-program generation, tool management and capacity planning. The XPLANE system automatically selects tools, machining operations and their sequence starting from a full part description created by a boundary-representation solid modeller. Apart from the description of the CAM environment the paper focuses on some of the most important specifications of XPLANE, the knowledge driven expert system, i.e. the knowledge representation, the inference engine, the explain facility and the knowledge-base editor.