I am trying to create a simple part for use in solidworks flow.  The domain contains a metal fin inside a cube of surrounding air.  I want the solidworks model to include the fin and all of the air.  I have created the cube the includes both the fin and the air.  I have created the fin inside the cube by extruding a sketch.  Now I need to create the air, by "subtracting" the fin form the overall domain.  But the "insert > features > combine > subtract" option leaves me with the air space but deletes the metal fin.  Can someone explain to me how to end up with two bodies, of which one is just the fin and the other is just the surrounding air?  Thank you ...