I used the origin assistant and later also the manual set x/y/z/b inside the run assistant.
The problem is not the defining of the part. I want just to define a different start point. Not the tailstock center.
I think it is equal to this: When setting the work origin - #2 by albutch
I configured the piece now 5cm longer and set the origin to a virtual center point 5cm in front of the piece. And now it works. But it is a dirty hack and no solution.