Hi,

Newbie here - I have been trying to subtract a solid from a bigger one as per the usual Bool Diff command. However, this merges the two solids into one instead! I am baffled and cannot figure out how things might have gone wrong. Any clues anyone?

Please post .3dm
- Brian

Here!

One of the door halves behaves normally, the other one not so much.

I you move Left or Right any doors you have this...so your single door(s) is not alone!

Clean all first for have only 2 objects solids when you select something for avoid some overlapings during the Boolean union!

Hi MA,

> I have been trying to subtract a solid from a bigger one as per the usual Bool
> Diff command. However, this merges the two solids into one instead! I am
> baffled and cannot figure out how things might have gone wrong. Any clues anyone?

When this happens it means that MoI has not been able to determine which direction is the outside direction of the solid. The most common cause is having curves that criss-cross back over themselves making a self-intersecting area. Such self intersections can interfere with solid volume calculations.

Thanks for posting the file, I'll take a look and see if I can figure out where the problem is at, my first area of focus would be looking for self intersections in the illustration geometry.

- Michael

Hi MA, I haven't found the specific area that is problematic yet but in the process of removing the one bad booleaned piece that was left on it, it now seems to be doing ok. Something like it doesn't happen to fire a ray for inside/outside testing through a problematic area anymore. I've attached the result of the boolean so you can keep going with working on it.

- Michael

Hi MA, I didn't find any self-intersections in the illustration area, but there are several areas like this with very very skinny remnants from previous booleans just kind of barely not matching up to other pieces:





Those little surface fragments are skinny enough (like only 0.0003 units across) that they probably appear to the volume ray fire mechanism the same as a self-intersection. So if a rayfire happens to try and cross over that kind of skinny fragment it can get confused.

That's my best guess right now.

One other thing you can try if you run into this situation with boolean difference behaving like union instead is to select your main object and use Edit > Separate and then follow that by an Edit > Join. That's going to make it recalculate the outside/inside directions. If I do that on your original model it seems to be ok with the boolean difference after that.

- Michael

Excellent help! Thank you.