EMPLOYEE WAS HELPING TO HOLD UP A JACKHAMMER DRIVING 2"" BOLTS THROUGH THE SAE MILL LINER. HE HAD POSITIONED HIS RIGHT THUMB IN FRONT OF THE JACKHAMMER BOLT; WHEN THE HAMMERING BEGAN, THE BOLT GAVE WAY SUDDENLY CAUSING THE HAMMER STEEL TO ENTER THE BOLTHOLE AND CATCH HIS THUMB BETWEEN THE MACHINE AND THE MILL LINER. INJURY RESULTED IN FRACTURE TO TIP OF RIGHT THUMB; SUTURES OF NAIL