Hi Marco,
not sure if this is the result your looking for but this was produced by using half of the helix (outer diameter of ring).
I extruding it to max depth then trimmed as Michael as shown.
I then shelled it and filleted.
It was necessary to trim the solid where it was going to join in the middle as this was not flat due to the shelling.
I then copied the half revolved it 180 and joined.
Hope this helps.
Barry