Hello, fellow Rhinos, I am trying to cut a groove along the center of the ‘triangles’ composing this Star of David with a Flowed 1 Rail Surface, but for some reason the Boolean Difference isn’t working. Both are solid judging from successful volume analysis.
I first tried this with a triangle to make a sharp bottomed groove around a sharp triangle on the surfaces, but I kept getting overlapping ends. So I did an Arc Blend to round those corners, and the ellipse flowed along that pretty well, I don’t see any weird self-intersections inside the corners, yet still no Boolean Difference happening.
Any ideas? I uploaded the .3dm file
Job 183 Glenn Catania Star of David woven V1 010.3dm (5.6 MB)
Run the Intersect command and inspect the curve where that ratty patched in surface is.
There’s is a gap in the Intersection curve.
This is why the Boolean fails.
https://wiki.mcneel.com/rhino/booleanfaq
Thank you! I’ll do that. Ratty indeed 
I fixed that curve and produced new rails for ellipses to ride around. But I had to also add more cross-section curves at the corners to make it work nicely.
Why does the intersection curve even matter, when looking at the solid polysurfaces they obviously intersect?
Read the Boolean FAQ.
Rhino intersects to surfaces and uses the resulting curve to Split them. Then based on which Boolean command was called, and the polysurface normal direction, Rhino knows what to delete and what to Join.
If you can’t get a clean Intersection, Booleans will always fail.
That FAQ describes that.
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