1) I read a post on a Fidonet guns & shooting group (think prehistoric forums, or a Usenet newsgroup) about a guy who had a rifle barrel blank in a lathe and one end wasn't secured properly when he turned it on. The barrel bent and the loose end flogged the guy pretty good. He was off work for several months, but mostly just broke a number of bones and made a full recovery. He made a post about it in the same group later and explained exactly what he did wrong.
2) My mom's dog loved to play tug-of-war with an old sock, and also loved playing with a tennis ball. So one day I put the tennis ball into the sock and tossed it to the dog. She took the sock in her mouth about halfway along the length and shook it hard like she did when we'd play tug-of-war. The ball went "whap-whap-whap-whap" agains the side of her head until she dropped it and ran away.
So I figure a rubber ball on a rope attached to a machine tool is probably a good balance.