The development of a face-based attributed graph structure for modeling geometric tolerances is described here. The structure, designated as the DTF graph, provides an integrated view of the dimensioning scheme, dimensions, tolerances, features, and datums. In the current version of the DTF model, all ANSI Y14.5M tolerance classes are supported, except profile tolerances. Edge related tolerances (straightness, circularity) are supported by derived face intersections. Other tolerances, and datum reference frames, are supported as face attributes or attributes of the DTF graph. The tolerance model is compatible with commonly used hybrid CSG-Brep solid modelers and has the property of uniqueness for any dimensioning scheme. Applications of the DTF Graph include: detection of over and under-constrained dimensions, automatic re-dimensioning if the designer changes the dimensioning scheme, and automatic discovery of dimension-tolerance stack-up loops.

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