Exporting STL Files

 

Generating STL files is usually a fairly simple process. Virtually all modern CAD systems now include STL output as a standard feature and, for the most part, the files created are suitable for rapid prototyping. But the extra step of converting CAD models into STL format still presents a barrier to Rapid Prototyping & Manufacturing Services novices or occasional users. Imagine how annoying it would be if you had to select parameters and convert word-processing documents or spreadsheets into a special file format before you could print them.

Making STL files is further complicated by the fact that every CAD system uses different terms and parameters for defining the STL file’s resolution, requiring users to interpret such mystifying terms as chord height, absolute facet deviation, angle control, and adjacency tolerance.

Such terms are needlessly confusing. The STL file format is simple. It’s not necessary to understand precisely what all the CAD system parameters mean in order to create useful files.

An STL file is simply a mesh of triangles wrapped around a CAD model. CAD system settings specify how closely the STL mesh conforms to the actual surface geometry of a part. A mesh with triangles that are too large will create a small STL file, but the prototype made from it will have visible facets. A mesh with triangles approximately the size of the layers used by the Rapid Prototyping & Manufacturing Services systems (typically about 0.003 inches or 0.075 mm) will produce a prototype with the best fidelity. A mesh with even smaller triangles will increase the size of the STL file and take longer to process, but it won’t increase prototype accuracy or resolution.

Coarse Mesh

Fine Mesh

If an STL file is set with too loose a tolerance, facets will show on the prototype (left). Ideally, the triangle size should be close to the layer thickness used by the Rapid Prototyping system.

It would make sense for the Rapid Prototyping machine manufacturers to work with the CAD software folks to develop a set of standard drivers that would eliminate the need for users to select STL output parameters. Whenever the idea has been suggested to 3D Systems or Stratasys, however, executives have brushed it off, saying they will get around to it sooner or later.

Service-bureau recommendations:

We have surveyed rapid prototyping services and find that they generally agree on the STL settings for various CAD systems that will produce the best rapid prototypes. Regardless of the parameters that any CAD system uses to define model resolution, service bureaus say the chord height of an STL file (the maximum distance a point on a triangle can deviate from the true surface of the part) should be between 0.001 and 0.003 inches (0.025 and 0.075 mm). Whenever possible, STL files should be in binary format. (In the past, some CAD systems did such a poor job of STL creation that developers included an ASCII STL option as a debugging tool. ASCII STL files tend to be huge, but they can be edited with a text editor.)

Some CAD systems will generate error messages during STL conversion saying that some part geometry is outside of the positive X, Y, Z quadrant or is in negative space. These messages can be ignored, as the STL file will be located in the Rapid Prototyping machine’s build space and oriented for optimal surface finish and build speed by the system software.

If you’re using a solid-modeling CAD system listed below, the following settings should produce good STL files. If you’re using a surface modeler or another system not listed, we’d suggest discussing file preparation with your service provider or in-house prototyping shop. Surface-modeling CAD systems describe part geometry using mathematical patches with no thickness. In order to create good STL files, all of these surfaces must be joined so that there are no gaps or overlaps. This stitching or sewing, as it is frequently called, can be a tedious process, and it’s easy to miss small flaws that can crash a Rapid Prototyping system.