Many applications such as computer aided process planning require an interpretation of the complex geometry of a mechanical part in terms of simpler local shapes such as machining features. Decomposition of non-polyhedral parts is difficult as compared to that of polyhedrons, but mechanical parts are seldom polyhedral. A decomposition method that makes use of primitives for planar and cylindrical faces of parts is presented in this paper. A semi-automatic method for mapping the resulting shapes to library-features is also presented. The proposed method for decomposition and mapping is simple, intuitive and easy to implement using standard geometric and solid modeling operators.