The Good Old Days. I can only think of one serious accident on an old belt converted machine- and it had nothing to do with the drive or the machine. Operator error. It was a large water bath external cylindrical grinder. About a 4 foot wheel and 6 ft. or so maximum between centers. Had a mammoth 3 phase motor cobbled on it- probably 20 hp. I remember this episode well, because I was the junior guy in that shop, and one of the senior guys was setting up a run of parts- that I would be running. And I had never used this beast. I was nearby running some parts on a Bridgeport, watching him to learn. When he got done dressing the wheel and setting up the workpiece between the centers, I watched him hit the buttons to fire up the waterbath and then hit the wheel motor. Well, the wheel was gainst the workpiece. It pushed the work piece- a ~ 3 ft long 4 inch diameter partially square hydraulic landing gear part- right through the centers, hit a 1/4" thick steel guard over the works- pushing it into his shoulder, punching a pretty good sized wound in his chest. It was like an explosion it happened so fast. I ran over and shut the machine down. Ambulance ride, but he was fine. And I was tasked with picking up the pieces and running those parts.