cylinder offset fail

 From:  wimverbe
5291.1 
I try to offset a cylinder with a rounded hole, so that the hole is convergating towards the center of the cylinder. but the sides of the hole are not regular, and a strange shaped object appears in the hole.
any clues how to solve this? I could of course make the convergating shape first and boolean substract it from a cylinder with a wall, but that is quite a workaround I think.

tnx
wim