a small question ; to extrude a closed curve on its exterior, is it possible?
@pou010, Offset and then Loft?
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indeed, thank you very much, I’m testing … I don’t have the same result as you (he fills the center for me)
Do you have the Closed Loft option checked?
the goal being to cut a cylinder of this shape
Can you upload a file (Rhino6 compatible) with your curves?
testfor6.3dm (92.3 KB)
OK, the issue is that you have groups of six open, duplicate curve segments for each curve, instead of two clean, closed curves!
First, you need to Ungroup everything and get rid of any duplicate curve segments. I had to Ungroup a couple of times. You seem to have produced groups of groups somehow. 
After that you need to Join the remaining 3 curve segments of the inner and outer curve into closed curves. Finally, you can proceed to Loft.
if I summarize correctly, it is the joins between the curves that are the problem? Thanks for the time spent.
No, rather the groups that seem to predefine a selection order in which the curves are matched and lofted, but which isn’t correct, and the duplicate geometries, which probably also mess with the curve segment matching! Wrong curve segments being paired by the loft operation produce the wild geometry in your screenshot above.
However, if you loft individual curve segments, you also end up with individual surfaces and not a single surface, which in your case is preferable, since you want to use it to trim a cylinder.
Vs.
You have to reply directly to a post of a user or tag him/her (e.g. @diff-arch), otherwise no notification is send and it’s hard to see if somebody has replied to you or not.
@diff-arch
thank you very much, very informative. I like this type of modeling unlike polygonal modeling.
You’re welcome! It’s called N.u.r.b.s. (non-uniform rational b-spline) or surface modelling. 
Yes, I like it too, especially for real world applications (architecture, prototyping, 3d printing, etc.), since it’s more precise than mesh modelling, but I like that too. It’s easier to go crazy with meshes and they are less heavy to handle computer-wise.
Hello - try this:
Make two cylinders - you can do this by making Circles in Top with 3Point option and the Near OSnap, then ExtrudeCrv the circles.
SelDup and Delete, then Join the remaining original curves to form two closed curves - you can delete or ignore the inner one.
Pull > Loose=Yes the curve to the outer cylinder, DeleteInput=Yes
Fin the curve from the outer surface.
Trim all the surfaces and Join.
-Pascal
