EE was attempting to grind a burr off of the end of a 1/4"" x 1"" x 8"" piece of metal. When the EE went to feed the material into the floor grinder media, metal kicked & left hand was turned & pulled into grinder media lacerating left thumb. He immediately let go & pulled hand away. He was taken to hospital where they amputated thumb to 1st knuckle.