Strange interactions when subtracting solids



I’ve been trying for the past day to make a simple object with a curved surfaces, but I can’t seem to figure it out.

I drew two simple shapes, checked OK with Solid Inspector.
When I tried to subtract one shape from the other, it works, but it modifies geometry at the opposite side of my shape, which should be unaffected by this operation. Specifically, it deletes a visible angle and adds a small face. See below pictures before/after subtract operation. Any idea why?


I’ve tried looking on the forums, but never found the exact solution (or did not understand it).

Thank you so much for your help!!



Can you upload or share a link to your model? Without it we’re just guessing.


Yes of course
Please find it attached
thanks a lot for your help!
problem.skp (61.6 KB)


After playing around with your model, I have come to the conclusion that this is an effect of SketchUp’s close vertices tolerance. The original edge on the near side of your cross passes very close to the arc that defines the edge of the arm (within 0.2mm). I think that during the post-operation cleanup, the arc is “capturing” the edge and for some reason SketchUp is also softening it. With hidden lines turned on, I see the following:

The first two images are the top and bottom of the near edge before the subtract, and the second and third are after. See how the vertical edge has been replaced by two that go to the next vertices on the arc?

The really strange thing is that this happens even if I use the “Dave Method” of scaling up before doing the subtract. But, stranger still, everything works fine if I scale up and then use Intersect Faces with… instead of the solid tools subtract! We’ve always been told that the solid tools just automate what you can do manually with Intersect Faces, but that now seems to be false!

This leads me to suspect that the solid tools operates on the unscaled Definition regardless of how you may have scaled instances, and therefore is affected by small geometry issues even if you scale. It’s a matter of semantics whether that is a bug (incorrect result) or not (operating as designed - even though the design is flawed). I’d call it a bug, myself.


A workaround for this issue…


For small faces, I find SU doesn’t like solid subtractions when the solids don’t overlap (not sure if this is a real
issue - just a habit I got into). So I extend the subtracting solids past the solid to be retained. I also mirrored the subtracting solid (exploded and made one solid) to delete both sides at once - it worked! (did not create an extra face on either side, and no softening).


work in meters and pretend they are mm, SU will work better and doesn’t mind…



Thank you so much for your help, very helpful and very appreciated!!!