Problem with Subtract on a revolved surface

Subtract is giving me some crazy results on a revolved surface.

I have created a "ring’ by revolving a circle through 360 degrees. I have then created 2 semi circles which I revolve through 20 degrees- looking to “subtract” them and get a cut out in the ring. when I do this WOW, the result is crazy. Nothing like it should be…

Help, very frustrated.

Can you share a screenshot? From this, it’s hard to imagine what is happenning.