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I have an STL that has a flat surface which should contact the build place when 3D printing. The model has an unknown rotation. How can I rotate the 3D model to be flush with the build plate?

I am very comfortable with OpenSCAD, and I can use it to rotate the STL if I know the correct angle. Is there a tool that will let me analyze a single face (triangle) of a model to determine it's normal? If I can even get the coordinates from 3 points from the desired face I can calculate the normal and use that to calculate the rotation vector.

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  • $\begingroup$ I made a little bit of progress: Meshmixer's "Mesh Query" analysis tool allows me to select individual triangles. The "Triangle ID" it shows is a 0-based index to a "facet" of the STL file in ASCII mode. It looks like the normal of the 3 vertexes is already computed for me. I am going to have to do this several times, so if there is a better way I am open to suggestions. $\endgroup$ Feb 6, 2018 at 5:01
  • $\begingroup$ I needed to get something started on the printer so I just imported an STL into OpenSCAD and started guessing values for the rotation vector. Since I had a large flat surface, I was able to use orthogonal view and the static camera angles to align the model to each axis. It took about 6 passes using 2 decimals of precision to get it to the point that the slicer treated the bottom as a single layer. That took 5-10 minutes per STL. Doing this 6 times was pretty manageable. Much more and I definitely would have probably done the math and scripted the process. $\endgroup$ Feb 7, 2018 at 16:40

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Some tools like Cura or Repetier Host have slicers that analyses and tell you if it is ok to print or not. Both of them allow rotations.

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    $\begingroup$ Cura's rotate tool has a "lay flat" function! I just had to manually get the rotation in the general direct and it was able to rotate it the rest of the way. Thanks! $\endgroup$ Feb 7, 2018 at 16:56

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