Hi Philip,

I often use Colin's method when building the lower part of the shank on a signet ring.
Having the same number of control points in the profile curves helps to steer the sweep in the right direction.

I took another approach to build a shank that still has the flats on the sides at the base, but without the sharp creases.

I filleted 2 corners on the bottom profile curve just a little and then drew an arc
on the outer side of the shank. Then all the curves were used for a network surface.
I did a 0.2 mm fillet, but you may want to use a smaller fillet..it will still work.

When modeling a signet ring in two halves such as this, the lower shank must blend well into the top of the ring, so that must be considered
when drawing the outer arc. You can sometimes use the "cont" arc to make a good transition between the top of the ring and the shank
Hope this makes sense.

Jesse