I use the old cheap but reliable rotary disc. I however modified mine with bronze bushings that fit well and stabilizes the side motion on the disc. I have done alot of the .043 rings at 4.600 and it works fine. Todays rings are too tough, too thin, and too big around for a flat file in a vice like we did 45 yrs ago.
I still use the manual rotary disc with a diamond disc. When I get to some of the hard top rings I have cheated and removed the handle, and put my cordless drill on the shaft. I don't use the drill on second rings because they grind away to fast. OOPS!!