Here is the simplest example I could find on how bad Rhino’s mesh intersect is.
The image is from V5 and it is even worse in V6.
Try to project the line onto the mesh. I really hope that this can help you figure out why Rhino fails so badly when a mesh is split through vertices.
File tolerance is 0.001 and if you MOVE the line 0.0001 unit to the side (1/10th of the tolerance) then it projects just fine, so I think the tolerance isn’t correctly taken into consideration.
Note that if you use the moved curve to TRIM with then that goes fine too, but it leaves the mesh with some nasty 0.0001 wide polygons, that should NOT be there as the file tolerance is only 0.001.
I really hope you investigate this stuff as good mesh trimming tools are very much needed.
Mesh intersect bug.3dm (27.3 KB)
PS! When opening the file in V6 it takes a long time for the perspective, rendered view to update for some reason.