I had an Italian mill about the same size as a Bridgeport, and the key was sheared off; in spite of the fact that it still worked fine, I replaced the key, which was like a flat head screw with a teat on the threaded end; it was a goodly amount of trouble to do, as the quill had to be disassembled to get to the spindle --- bottom line, it sheared off not long after and we continued to use it for about 25 or so years after without any inconvenience whatever; as long as the spindle is rotating CW, the drawbar is self tightening if there is any slippage of the adaptor used, if using CCW, use care not to put a lot of torque into the cut. The quest to replace the key was secondary to having to replace the quill due to some idiots trying to use it to broach a keyway, they broke a couple of teeth off the rack, tried to weld it up and made a major mess of it; I bought a new quill and a nice mill was the result; brand name was Induma, model 1 - S.