S J Tate,  G E M Jared
SIMS, Cranfield University, Cranfield, Bedfordshire, MK43 OAL, UK.

K G Swift
 Department of Engineering, University of Hull, Hull, East Yorkshire, HU6 7RX, UK.


Assembly-oriented CAD has long been accepted as a necessary development from the current component-focused solid modelling systems. It is proposed that such an environment should incorporate assembly sequence generation and Design for Assembly (DFA) analyses to assist the designer, including some automatic inference to facilitate ease of use.  The key to enabling the various assembly analyses lies in interrogation of the CAD model and this poses some interesting challenges in the field of geometric reasoning.  Statistics from case studies show that, in particular, the identification of part symmetry and principal axes is fundamental to many of the required algorithms which have been identified.  This paper reviews methods for symmetry detection.  However, no pre-existing method suitable for this application is found and so a new technique is proposed which exploits the existence of loops within the CAD model.  This entails the comparison of loop areas to discover exact symmetry, partial symmetry and repeated features.  Implementation of this technique is described and in conclusion the benefits and problems associated with it are discussed.