Comment by Animats

Ah. That's good to know. I wondered how they did that. I can see doing it with triangles. Smooth curves are far more difficult.

I'm not an expert at this but I've done CNC machining, used Autodesk Inventor, and coded 3D collision detection. CSG engines were considered mathematical nightmares, and I've used them, but never been inside one. The good ones can take a cylinder and a thread profile, project the thread profile along a spiral as a cutting tool, and make a bolt. Then you can chamfer the end of the bolt, and the ends of the thread are correct.