Boolean fail why?

Why I cant booelan this shapes?

wheel boolean.3dm (11.5 MB)

If you run Intersect you can identify when the intersections are discontinuous. These are likely your problem areas.

thanks I think a concept what I don’t understand very clear

Hi -

As Ryan said, run Intersect and then deselect all and run SelOpenCrv and check where those are open.

It looks like the face on the black object that is selected in this image is meant to coincide with the edge on the red polysurface - but it is off by 0.001, the document tolerance. This is confusing the math and Rhino puts part of the intersection on that edge whereas the rest is off and you get an open intersection curve.