Hi Dimitri

@you: the result is not the desired one, the fill is curved in its shape, while the thing I needed is to have a totally flat fill


A flat fill means to have a ring with two sharp edges touching and ...scratching your finger :)
You want this? I think no...maybe you have planned to fillet the flat fill but Moi's filleter is weak in complex situations like this one.
I did a try using Michael's technique and mine,starting with a minimum radius 0.01....so nothing ;)
Instead i did some blends G2 with a low bulge to have sections little bit filleted but also flat ,extract them using isocurves command.

Second solution that allows you to fillet your fill is to draw a line and adding a point in the middle,so move a little the point to get a little arc





This will be the section(red one) of the sweep,we got the rails trimmed like in earlier pictures





Now i'm able to fillet






Why flat fill don't fillet instead a "bulged" fill do? Maybe because the filleter" have more space to eat" :) let's Michael do and explanation



We have something to check surface quality inside Moi:Dino's lighting !
Here i used onyx light,you can see little artifacts on the fill surface







You can have an alternative to flat snap Michael's showed you(best choice):project your curve on a flat plane








M