Boolean split fails as two surfaces chosen that abutt at edge

slicing through my spinner to use a third of it, in view aft, draw a line from spinner centre, and copy rotate it 120 deg,

I create two planar surfaces from these two exact same lines that meet at spinner centre (view aft), extruded at same time, so two surfaces result abutting along one edge like a butterflies wings effect.
run BooleanSplit, select spinner, it asks for surfaces, select both surfaces, boolean fails.

Join two surfaces (Join command) try again and boolean succeeds.

As it asks for surfaces and not a single surface, why did the two surfaces need joining together ?

…and why does boolean split not just split but clone the item before splitting ?


They didn’t. Then it asks for “Surfaces” that means you can input one or more surfaces as splitting objects. the idea being that you can split one or more objects with one or more surfaces or polysurfaces in the same operation.

Your mistake was selecting two concurrent unjoined surfaces, which would fail because you cant split an object in exactly the same place with two different surfaces. I’m actually surprised that it let you do it with two joined surfaces. That may have caused your “cloning” as well.