Hi Michael
thanks for your very quick answer
so i made the surface with loft and got a solid.
Fine
After that I copied it, scaled the copy with 0.9875 and set it in the 1. form.
I tried to make a difference (after selecting both) from these two forms but i failed.
Why??

Update
Now I Know Why: I was too stupid
ok works fine
Thanx again Michael
what a little rest can do....