I had a used spinner backing plate that was already drilled for nutplates, (though they had been removed and used on some other project) and when I went to install the spinner dome (no holes in it) I was faced with having to hit those existing nutplate holes dead nuts, but without being able to simply back drill as stuff was in the way. I tried re indexing the already drilled back plate on the hub dowels, but that didn't work,So I made this tool, which operates on the same principle as a strap hole duplicator. The thick nylon piece went underneath on the inside of the backing plate, and was moved around until the AN-3 clevis pin clicked into the 3/16" nutplate hole, then the aluminum top part, which had a corresponding 3/16" hole (both drilled on the drill press,only after the "tool" was assembled to insure automatic lining up) served as a guide to let me drill partially through the spinner, then the tool was removed and the drilling was completed. All this while the dome was precisely located (don't want another wobbler) and while clamped to the back plate with edge clamps. Everything fit up perfect, no wallowed out holes at all.