Also another factor making a very slight inaccuracy in your 2-rail sweep method is that end tangents of a helix are not pointing in the same flat direction as the arc - a helix has a slight incline to its tangent direction and during the 2 rail sweep the profiles will follow a kind of average tangent between the 2 rails and so that will basically angle the profiles a little bit as the sweep traverses its path.

Basically if you want to have the most accuracy if you want a shape that is supposed to be a fragment of a cylinder it will be the most accurate to actually construct it literally that way by starting with a cylinder and then cutting it.

- Michael

Project an inclined line to the "cylindre" is not more simple than draw the first helix ?
Or maybe the projection of a line is not the same drawing than the helix!

Effectively there is not the same! :)

"scale" (climb up) definition: to climb up by means of a ladder.

Hi Pilou - no a projection of a line onto a cylinder is a different shape than a helix.

- Michael

Ok thanks for the help michael
:)

Another solution :)
Draw internal Helix
draw external Helix
Loft straight
Boolean diff
:)