On the lathe, turn a support to hold the disc on the bench grinder. Turn one end the same size as the hole through the middle of the disc.

Cut the support so the shoulder of the support is the same height as the tool rest on the bench grinder.

Unfortunately I never made an instructable on my lathe tool sharpening system which is what makes this step work. You could probably set up a holder of some sort to clamp down to your bench next to your bench grinder. All it needs to be able to do is be adjusted to grind more or less off the outside of the disk.

Once mounted, turn on the bench grinder and rotate the disc grinding off more and more until it is perfectly round.