all normals got pointed to the outside.
But I am still not out of the woods - even though the normals are OK now. I attached a 3DM file baked from my current GH layout. The part is made using 4 basic steps: (1) capping the outside surface which is made by joining 2 surfaces, each lofted from 11 polyline curves, (2) capping the inside surface made by lofting 11 circles, (3) whacking 5 units of height off the bottom of (2) by doing SDiff with a 5 unit high capped cylinder, and (4) using SDiff to subtract (3) from (1)
Step 3 is necessary to allow for a solid bottom thickness of 5 units. All the surfaces are NURBS, none have been converted to meshes.
I would have thought that there would be no problems printing this part because it is made from only solid surfaces. The 3DM file has no naked edges - which is what I expected. But my generated STL file is 23.19 MB in size and the 3D Builder program says it has errors. The 3DB program "fixes" the errors, but when it does so it closes the top of the part, so that function is useless. 3DB will also simplify the STL file and create a resulting file that is much smaller, but this also has problems that would result in a failed 3D print.
I totally realize that my problem may lie completely outside GH & Rhino and I don't mean to add clutter up this board with extraneous posts. It's just that I have not had issues like this until I tried the idea of joining 2 lofted surfaces that have reverse twists. I have made many parts before that included reverse twists - but not lofted surfaces. Here is just one example: http://www.thingiverse.com/thing:1606442…
Added by Birk Binnard at 3:22pm on September 14, 2016