I'll second answer #12, Chris Worsley.I have one, with the pin in the center and have been guessing about it fot years. It does fit an old drill bit I have. I did use it to make some interesting copper conchos against a urethane block Abusive comment hidden. (Show it anyway.)
I have one, with the pin in the center and have been guessing about it fot years. It does fit an old drill bit I have. I did use it to make some interesting copper conchos against a urethane block