Hi Lara,

re:
> I tried an other way...but the result was exactly the same. I made a offset of the two big circles
> and moved it for 1 degree together. Sweep along the segment.

I'm sorry but I still don't understand what the problem is.

So you've got this:


Looking at these edges by themselves:


In the Top view, the distance here on-screen:



Will be different than the distance here on-screen:



So yes that is true but it's just how projection of a rotated shape works.

If you have a regular truncated cone like this:



If you rotate it:




Then view the rotated cone in a top view projection it will look like this:






That's normal and it's just how geometry works with projection.

If you wanted the projection to look like this:



That would need a slanted cone rather than a regular cone:



- Michael