About 25 years ago I went to buy a 389 Pontiac engine from an ad. They owner had his arm in a sling and I thought he had broken/twisted something and didn't think of it much until everything was loaded and ready to go. I asked what happened and he explained that he was using a grinder that turned about 5,000 rpm when the 2,500 rpm rated, job-supplied disc exploded, severing the nerves and tendons in his left arm. The damage was permanent-the sling was forever. I'd been using grinders without guards for years and never checked the rpm rating of the discs until then. I haven't forgotten that.