My jig was a good fix for some rings, but I found the perfect solution in a two year old post of Kurt's.
http://www.delviesplastics.com sells cast acrylic sheets. The small sizes are 12"x12" and run $5.50 ea for 1/16" and 1/8" thick. They are perfectly flat and parallel. They come in thicknesses up to 3/8". 1/4" and 3\8" thicknesses are $8.50 each. I ordered white in 1/16". Ivory in 1/8" and 1/4", and Black in 1/8, 1/4 and 3/8". To get perfect rings, all you need is a 3/4" hole saw from McMaster-Carr for under $5.00 and an arbor with drill bit, also under $5.00. I got the arbor with the 1/4" drill bit and will redrill with a 21/64" drill bit to slide over the 5/16" tenon without being loose. the arbor takes a hex bit (1/4" incl) My machinist friend is going to modify the 21/64 bit to work in the arbor so I won't have to redrill after that. Perfect acrylic rings using a hole saw.
The brass and copper rings are nothing but brass and copper washers from McMaster-Carr. Every one I've used has been perfectly flat and parallel. I just use sandpaper on a marble tile to rough them up for gluing. I haven't had to flatten or true them at all. I get the 3/4" diameter with 5/16" hole. The brass comes in different thicknesses. They're basically ready to glue up as is, I just sand to provide more "bite" for the glue.
My wooden rings are made from thin stock lumber from Woodcraft. It comes in thicknesses from 1/16 to 1/2. It has also proven to be flat and parallel and comes in all kinds of exotic or domestic hardwoods. 3"x24" size. Very affordable and no work. Just cut out with the hole saw and glue up.
My rings have become the easiest part of pipe making since I learned all these shortcuts. If you like rings, give these ideas a shot. I have no metal lathe, and my need for one is rapidly disappearing.
Wallace