[jjdon]

I don't cut coins, but I've greatly enjoyed reading this thread from an engineering/inventor perspective. Great fun and it was good to see Rube Goldberg again, too. A thought: The biggest issue people seem to be having is with coins coming loose or being marred or both. I think Kurt's original design is pretty good, myself, but for several things. It's aluminum, the edge is thin, and the center hole is too small. The small hole means much force to clamp it, the thin edge means that the (flimsy by nature) aluminum will flex under pressure. Brass, bronze or even steel will be much more rigid, inherently.
Then: You have a metal-to-metal clamp. You can squeeze the nickel as hard as you want and it won't go anyplace, but either the clamp will flex or it will mar the coin or both. Aluminum will flex, steel will mar the coin. One solution is to make the top surface wide aluminum (like 1") so it can't flex. Another is to make it all out of something softer than the coin, like Delrin, but again it would have to be wide to prevent flexing. If you bear down on that first picture enough, that aluminum rim will literally move.
IMO the ideal design would be a steel clamp with either "O" rings or a felt ring set into a matching groove (1/2 in each clamp part, of course)- the strength of steel (could be brass, I guess) with a buffer and also a material that provides a grip under pressure. Strength AND grip AND safety.
But that's a $250 clamp....

Well, I just enjoy the mental challenge - ignore me if you like...