Hi Felix,
> The only reason I can think of is that "spiral" as a self intersecting part.
Yes that is the reason - in general self intersecting surfaces will cause problems with the booleans, they don't have a clear positive and negative side to them and that creates ambiguity.
In your case here the particularly messy part is right near the center point of the spiral - that seems to be an area where the surface is not just intersecting other loops of itself but also kind of folded right back on top of itself and just generally messy:
Zoom in there to see the sudden tight bend where it is folded over itself:
That kind of sudden tight fold over top of itself will prevent surface/surface intersections from being able to get calculated cleanly.
- Michael
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