A Method to Generate Exact Contour Files for Solid Freeform Fabrication

Existing methods to create contour files generate a polygonal approximation of the contours instead of an exact representation. This paper presents a method to generate exact contours from Constructive Solid Geometry (CSG) representations. The method preserves the accuracy of the contour files provided the primitives used to generate the CSG tree are polygonal or quadric objects. Due to the inclusion of quadric objects into the primitive set an additional effort to solve for the intersection points between two quadric curves is required. The paper also presents a method to convert piecewise quadratic contours to toggle point files for raster scanning solid freeform fabrication processes. IntrOduction Computer software plays a major role in Solid Freeform Fabrication (SFF) techniques. The main aim of the computer software is to generate contour files at different cross sections of a given part geometry. These contour files can be rasterized to generate laser toggle point files. Present methods for generating contour files use faceted representations of the part geometry. The contour files generated by these methods are polygonal approximations of the actual contours. This paper presents a method to generate exact contours, instead of polygonal approximations, by using a higher order geometric description of the part. Description of the Method Part Geometry Representation The form of the geometric description of mechanical parts to be produced by SFF significantly affects the accuracy of the final part. The current state-of-the-art for most SFF technologies, consists of tessellating the surfaces of the geometric model into a mesh of non-overlapping triangular facets. The resulting geometry is transmitted in a standard file format, the so-called STL file format, established by 3D Systems, Inc. [1]. This format has been adopted by many CAD vendors, is readily available, and is considered adequate for most visualization applications. However, for producing accurate patterns and functional parts, the adequacy of the STL fonnat is unclear [3]. There is a trade-off between the accuracy and the size of the geometric description. Highly non-linear surfaces, such as those found on turbine blades, manifolds, etc., must be tessellated into a large number of small facets, resulting in very large data files, and the accuracy of such descriptions is still suspect. For this reason richer geometric descriptions must be introduced to the SFF community.