Hi @JWW, these are the default settings for the STL import:
the effect you see in your image is called smooth shading. If you look at the object with _FlatShade enabled, you’ll see what your printer will actually output. Don’t forget to set it back (disable it) or all shadeable objects will be displayed flatshaded. That is nothing bad about the mesh, read more here.
How did you create your object as STL? Are you using the _Export command or _Save or _SaveAs ? The reason i ask is that if you use _Export you have to explicitely select what is exported (1 object) while if you use _Save, all objects are saved, without having to select anything.
The parts in the inside cylinder which should not be there must exist in your Rhino file, otherwise they would not appear in your STL file.
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c.