Bug: Boolean subtract creates unrelated hole

Gunter

Location: In my public document "AnycubicMegaS" in tab "Cable Chain Restricted Copy 1" at boolean subtract step "Boo-CEndTaps".
Problem: In addition to the intended 2 metric threads a small odd-shaped blind hole appears in the target object that is nowhere near the tools.  If I suppress the nearer of the 2 taps the hole does not appear.  I put a text label "Bug" next to the hole.
URL: https://cad.onshape.com/documents/889dcf492394f6c435f4f00f/w/a6c375f368428d17c02411db/e/5534c8880c837aa6f8369a2b

Alex_Kempen

Your problem has to do with the offset option in your Boolean. The issue goes away if you don't use offset all, or you use a smaller offset. Internally, all the offset option in the boolean feature does is perform an offset face operation before performing the boolean operation; thus, you can try performing the offset operation manually yourself (using the offset face feature) to get a better idea of what's going wrong. I'd imagine your issue has something to do with offsetting weird curvature result in unexpectedly large, but ultimately correct, protrusions.

You can also try using a transform feature set to scale to increase the size of your part by some factor before you boolean it as well.

Gunter

Thank you.  Indeed the hole goes away if I don't offset, and I have identified the face the causes the hole when offset.
But that face is one that I need to offset, so I'm still stuck.
Scaling is not an option in this case because that would change the thread pitch.
What I'll try is revolving a chamfer around the ends of the tap; I assume there is something bad about how the thread runs out that sends an offset shard off to the side by more than the diameter of the tap.  Still, I'm not convinced that this is correct behavior on the part of OnShape.

Gunter

OK, when I twist the taps by 90⁰ I see a "dagger" slicing into the target at the other tap.  And twisting them by 270⁰ makes the dagger point away from the target so for the moment I'm OK, I can continue with my design.
But I contend that this is a bug, caused by an algorithm that isn't quite complete, that doesn't take into account a corner case which I found with my tap.  An "offset face" should never produce anything further away from the original geometry than the offset amount.