A Computer Aided Tolerancing Tool based on Kinematic Analogies

A computer aided tolerancing tool is presented that assists the designer in functional tolerance specification. The theoretical concepts for subsequent tolerance analysis are also provided. The computer aided tolerancing tool is part of a feature based object oriented (re)-design support system, called FROOM. FROOM's assembly modelling capabilities provide basic information for functional tolerance specification. Assembly constraints are satisfied by means of degrees of freedom (DOF) analysis. This method is based on the use of kinematic analogies. The rotations and translations (macro--DOF's) that components are allowed to have, are inferred using this technique. The tolerance representation in FROOM is based on the TTRS method, by Clment et al., which is also based on kinematic analogies. In this method, the small displacements that are allowed in the tolerance zone can be described by a tolerance torsor or transformation matrix. Using the tolerance torsor or transformation matrix, tolerances are described as constraints. The small displacements that are still allowed by means of the torsor are referred to as micro-DOF's. For tolerance analysis, the torsor approach offers a mathematically correct description of tolerance zones, although a lot of equations are generated. These are reduced by applying a kind of degrees of freedom analysis considering both the macro-DOF's and the micro DOF's (tolerances)