Possible bug trying to trim a surface

I’m trying to trim a surface with a boundary but the result is not clean and create a kind of bug

trim.3dm (15.0 MB)

No, this is not a bug, it’s a bad surface definition. Both bad trims are the result of the same problem.

First, Explode your extrusion to make it into separate surfaces and look at the surface where the problem is occurring. If you turn on control points, you will see there are some extra ones that shouldn’t be there - it is a degree 1 x 1 surface, but there are some extra knots in it which are actually microscopic kinks - it’s about 0.01cm out of planar.

I suspect that the surfaces were made from projecting a part of your trim curves to a Z parallel plane? Because they are also slightly out of planar on that surface portion. When I replaced that one surface with a vertical plane and joined it to the rest, the original curves will no longer split the new surface - they are too far away from it. So I pulled those to the surface and joined all, then it splits…

Trimmed-msh.3dm (326.3 KB)


First, you have to extract the flat surfaces and use the “Rebuild” command to make them Degree 1 with 2 points in each direction. Then, you have to use “Explode” to detach the straight lines and join only those that will be used as a cutting object for the flat surfaces. That will make them again Degree 1. Once you do that, the flat surfaces will be split properly.