I have an easier way to cut the rings though, if you have access to a router. If you don't have a router table, make a temporary "router table" out of plywood and a couple of 2x4s big enough to set across a two sawhorses. Mount the router upside down with the bit sticking up through a hole in the table.
Drill a small hole in the center of the plywood you're using for the rings and drill a matching small hole in table. The second hole should be set the radius of the rings away from the router bit. Drill another hole in the plywood to use as a starting point for the router at the edge of the circle. Then screw the plywood to the table. Spin the plywood against the router bit. Be sure you account for the width of the router bit in your calculations. Measure to the far side of the router bit for inside diameters and the near side for outside diameters.
I tried different sizes of screws and found less error with smaller ones. I ended up with a #6. Smaller router bits work better than big ones. I found a 1/4" straight carbide bit to be the best that I tried. You also get better results by making two or three passes with the router, rather than trying to cut the whole 3/4" at one shot.
And please be careful. The router needs to be securely attached to the table and your fingers need to stay *far* away from the bit.
Bob Bond