Hi Olga, I see... - basically the problem here is that sweep will get confused if you use a scaling rail with a sharp corner in it.
Really it is intended that the scaling rail should be a completely smooth curve, that prevents the sweep from doing any sudden jumps in shape as it is stretched out to meet the scaling rail.
In these cases the scaling rail corner happens near the bottom of your ring, that is why there is some messy areas down in that spot.
I can see that the scaling rail gives you the kind of shape that you were wanting, but it is basically not going to work right for this particular kind of thing, it is just too difficult for the bottom tip of the sweep to be formed correctly using this method.
I'd really recommend sweep more for rings where you want a single totally smooth surface. If the body should have edges like you want for this style, then it is easiest to get a clean final result by not using sweep but instead do an extrusion and then boolean away the side parts to taper it, that's the kind of style that is shown in that post that Pilou linked to above.
Here is an overview of that using your curves:
I started by discarding a few pieces and drawing a line between the curved pieces like this:
Then you can select that profile with the curves and extrude it (use Both sides option):
Now select that extrusion and use Construct / Boolean / Diff and use the lines as the cutting objects:
That will cut the object into 3 pieces (shown slightly separated here for illustration), just delete the outer 2 pieces and the inside piece is the one you keep:
The surfaces created in this way will be much cleaner and should fillet without any difficulty - sweeping has the bad side effect of collapsing down to a single point at the tip, that type of collapsing tends to be hard to control right in that tip area.
Another way to do the boolean method is to have 2 fully closed profiles:
Then you can select them both, and run Construct / Boolean / Merge:
It will basically bundle all those previous steps up for you automatically.
- Michael