I think you have a point where the two items are too close to symetry?
If you do the boolean in the reverse order it works ok for me anyway.
Brian

Hi JTB, thanks for posting the example. Yeah that is actually does not leave too skinny of bits, so that stuff I posted earlier doesn't really apply here.

It seems to be getting confused with the coincident surfaces in this case, if you thicken the cutting one by scale1d so that it doesn't have side faces that are coincident with the other one, then it will work.

I'll put it on my list to investigate to see if I can improve it.

- Michael

OK,thanks!