The generation of planning and control data for the machining of mechanical components is a complex task which requires the application of skill and experience. With current interest in applying expert system (ES) techniques to problems in the area of CAD/CAM, this particular aspect of process planning appears to be well suited to a knowledge-based (KB) approach. However, the knowledge used to solve such problems involves the interpretation of three-dimensional (3-D) geometry. This paper outlines the development of a process planning system using knowledge-based techniques with derivation of component geometric information from a three-dimensional solid modeller. The knowledge-based components of the system are centred on a production rule based expert system, coexisting with a nonlinear knowledge-based planning system. These are in turn coupled to a solid modeller via a geometric interface. The knowledge resident within the system operates on the geometry of the component to produce the planning and control data. The system is targetted at planning the manufacture of components with 2½-D geometries, using a three-axis machining centre, from rectilinear blanks.