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RAPID MODELS - STL FILE FORMAT The STL file format has become recognized as the standard for RP machines. Note that STL files are produced by CAD systems that are based on surface and/or solid modeling only. 2-Dimensional and Wireframe CAD systems do not output STL files. Fig 1. Under tessellated Fig 2. Good Fig 3. Over tessellated STL employs a method of representing surfaces of a CAD model as a closed volume mesh comprised of triangles. The quantity and size of these triangles dictate how accurately the surface mesh represents the actual product. The following example illustrates this concept using a triangulated mesh to describe a common circle: Desired Shape Low Quality Mesh Medium Quality Mesh High Quality Mesh As the number of triangles increases and the relative triangle size decreases, the shape begins to more accurately represent the outline of a circle. This is often referred to as the "facet resolution." It is an important concept because parts constructed on RP equipment may require additional finishing or may not measure to accurate dimensions if the facet resolution is too low. CAD programs often contain optional settings that will give the designer some level of control over the quality of the STL file output. The most common settings or variables pertain to "Chord Height" and "Angle Control." Chord Height is a term that numerically describes the maximum distance between the actual part surface and the tessellated surface of the STL file that describes the part. The following illustration represents the concept of chord height: The smaller the chord height, the more triangles required to describe a particular section of part geometry, and thus, better facet resolution. Angle control is another common setting which influences the tessellation of curves with relatively small radii in comparison to the overall size of the CAD model. Output of a high-resolution STL file does come with a trade-off. File size often becomes extremely large with low chord height and angle control settings. Parts that have an extremely large file size usually contain over 150,000 triangles or are in an ASCII format. Files such as these can cause problems when setting up a RP machine. It is suggested to generate all .STL files in binary format using a chord height minimum of .0005 of an inch (.0127mm). Consult your CAD documentation when optimizing STL output for facet resolution and file size. The images to the left show examples of different quality STL files.

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