EMPLOYEE AND HIS PARTNER WERE IN THE PROCESS OF CHANGING A WORN WEAR CAP FROM PARTITION #4 SHANK ON THE SHOVEL BUCKET. EMPLOYEE WAS POUNDING ON WEAR CAP WITH AN 8 LB SLEDGE WHEN A SMALL PIECE OF THE WEAR CAP BROKE OFF AND IMBEDDED IN HIS RIGHT THUMB. DOCTOR REMOVED METAL AND CLOSED WOUND.