BOUNDARY EVALUATION FOR SOLID FREEFORM FABRICATION

The de facto standard for geometric data exchange for Solid Freeform Fabrication (SFF) technologies is based on faceted approximation of the source geometry. This approach simplifies the subsequent geometric processing operations required for fabricating the part. However, the faceting algorithms used by many commercial CAD systems are not robust, resulting in faceted approximations that do not represent valid solids. This paper presents a method to process Constructive Solid Geometry (CSG) representations for layer-based SFF processes. The method is based on the philosophy that the geometric processor should be intimately tied to SFF technology, resulting in a CSG boundary evaluator for SFF. The paper focuses on the algorithms for slicing CSG representations of mechanical parts. The general approach is to slice the primitives in the CSG tree, then to apply set operations to the resulting contours. For the common quadric primitives, exact slice contours are generated. For higher-order primitives, such as the torus, a method for approximating the contours is presented. The algorithm for applying regularized set operations is described briefly. This method results in a significant improvement in geometric processing for SFF by providing a more accurate and compact format for geometric data exchange, by preserving the accuracy of lower-order surfaces, and by providing a rational means for controlling the accuracy of approximations of higher-order surfaces.