Thank you
Grimmnar,
If I am not mistaken modified in 3D builder.
My love for scratch building / kitbashing has migrated into 3D also.
Let me make a suggestion.
Splitting is good in 3D Builder BUT I have found a better way for my needs .... create a cube ( desired size ) with a thickness of 0.01mm thickness .
Place it where you want to separate & subtract. It will not effect part as it is so thin.
Repair if required as 3D builder is moody sometimes
Hope this helps
Z