Use of Vectorial Tolerances for Direct Representation and Analysis in CAD-systems

The paper deals with the representation of vectorial tolerances in the parametric CAD-model using surface-type specific attribute containers, which are attached to the desired nominal surfaces. Since CAD-systems can only evaluate linear, one–dimensional tolerance chains, currently additional CAx-systems are used for tolerance representation, analysis and synthesis. In this paper the representation of vectorial tolerance parameters in the CAD-model is explained. A major motivation for their integration into the CAD-model is the similarity of the vectorial tolerance representation to the B-Rep (Boundary representation) description in current CAD-systems. Furthermore, most engineers think in the standardized tolerance system. So for the user input, a user-controlled translation between the ISO and vectorial tolerances is necessary. The presented approach was realized in a software prototype using the CAD programming interface of SolidWorks.A main focus of the paper is the handling of partially closed tolerance-loops. Most of the CAT-systems only handle open tolerance chains. In contrast, a real system, even if the overall tolerance chain can be considered as an open chain, consists of partially closed tolerance-loops just in the joints or base frames. The developed approach is based on the statically determined placement of a rigid body on three points, where the resulting line of action of the force - even for several parts – goes through the support triangle. Assuming ideal rigid but deviation-affected components the new placement-triangle can be determined. This is a departure from the classical component mates in 3D-CAD that assume geometrically ideal elements. The concept is demonstrated in the article by examples.