I've also made unusually thick aluminum rings out of flat stock. I had one type of aircraft grade stuff that buffed out so well, that based on looks alone It was hard to tell the difference in It and stainless. There are several ways to accomplish the same thing, and It's been a while, but I think what I did was start with a thicker piece then My actual finished ring thickness, I cut a circle out with My milling machine and a large hole saw. Stuck the piece in the jaws of the lathe, Drilled and bored the ID, then faced each side to the final thickness. In the end It produced a really nice looking ring.