For any project normally you must have an arborescence of different pieces for see more easily where there is something wrong...

Else you can try to make a boolean Diff between your object and a simple big box!

Hi Robert, you haven't done anything wrong. It can just be a difficult case for booleans to automatically determine separation when you place a cut that runs right along a lot of existing edges and geometry.

I've attached a file where I've split it into 2 using Edit > Separate and Construct > Planar.

- Michael

That works if you make like this... but curious only on this part!

THANK YOU! This is why I love MOI and these forums!