Thank you all for looking into this problem.
-phiro, I often use "Rebuild" and / or "ReconstructCurve", but, unless I'm mistaken, there will always be, even at best, a small difference between the original and the rebuild. And since my 3D spirals have to go through precise points, this is not suitable.
-Pilou, Je vais explorer le lien.
-Thank you in particular to bemfarmer for the links (funny, the last reference at the end of the super paper referring to the site that I cited here http://moi3d.com/forum/index.php?webtag=MOI&msg=10530.3). I will also be inspired by your solution.
-Thank you Michael: the method is simple and elegant (I will test it).

- Zooen

There is a peculiarity that I did not mention, in addition to the dumbbell shape with vertical symmetry: these are two identical mirrored spirals, which develop with irregular steps. So for now, and before I look into the problem, I decided to correct the spirals with "Rebuild", Refit with a tolerance of 0.01. the difference with the original is minimal! So I no longer have sharp angles.

This subject is related to this one:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10536.1

Good, have a good Christmas holiday, see you later!

- Zooen

where can I find the biconcave script?

Biconcave ici : http://moi3d.com/forum/index.php?webtag=MOI&msg=6718.1