Rapid Prototyping (RP)/Layered Manufacturing (LM) machines typically use a Stereolithography (STL) file as a basis to manufacture parts. However, the conversion of the part CAD file to STL results in the distortion of the part geometry, particularly if the part consists of freeform curved surfaces. Existing algorithms and software tend to reduce this distortion globally, which increases the size and memory requirements of the STL file. This paper presents a new approach for reducing the CAD to STL translation error locally, using chordal error as the criteria. The algorithm presented here compares the STL file to the design surface of the part, expressed as a NURBS surface, and computes the chordal error for multiple points on the STL facets. The point within each STL facet having the largest chordal error is modified to coincide with its corresponding point on the design surface. This replaces the original facet of the STL file with three new facets with significantly lower chordal error than that of the original facet. This Vertex Translation Algorithm (VTA), reduces the chordal error in areas with high curvature and areas having tighter profile tolerance specifications and provides the user the flexibility to selectively modify the STL file according to the tolerance requirements. The algorithm has been validated with the help of a test case.

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