Parametric modeling and configuration design methods are key technologies for mass customization in manufacturing. This paper describes the parametric modeling process of machine tool, and proposes a framework which parametrically models a machine tool assembly based on a design expert system. The concept of design unit, which is one level higher than functional feature, and the parametric modeling concept with functional features have been proposed. The domain knowledge in the knowledge-base has been mapped to the geometry of the CAD system. A design expert system to redesign assemblies of a machine tool has been implemented, because commercial CAD systems cannot handle the parametric design of assemblies. This system consists of a commercial expert system shell, a design knowledge-base, and a commercial CAD system with an API program. The API program interfaces the expert system with the CAD system through a GUI. (C) 2001 Elsevier Science Ltd. All rights reserved.