Fred R "spam writes:

Are you starting with a thin disk and machining the edge or face

Yup. In particular, I am using medallions with artistic designs -
think of it as a coin.

Most of the good advice here has presumed the parting off. If you are
actually starting with thin disks you can stick them to a face plate
with double-stick tape or superglue. Hit superglued disks with hot air
or a gentle propane torch flame to release.

Ah. Good to know, but the opposite side may not be 100% flat.
So superglue might not hold it well.

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