So notice the point structure on the top surface when starting. That "box" type structure is the indication of a separate surface type. A type of analytical surface (Michael can correct the terminology)...
So first I shrink the underlying surface, then I actually added a point structure, which converts it to a regular NURBS BREP, then I just flatten IT to use as the base....
Burr
[EDIT] Sorry. Somehow cursor didn't get recorded so you will have to look harder... fixed it for future [EDIT]
