EMPLOYEE WAS REMOVING THE NIPPER CYLINDER ATTACHMENT BUCKET ON A D-10 DOZER WITH AIR IMPACT TORQUE MULTIPLIER WRENCH. THE IMPACT WRENCH ENGAGED DUE TO A LOOSE PLANETARY GEARING HOUSING AND PIN CHED THE EMPLOYEES LEFT THUMB BETWEEN THE BRACKET AND THE WRENCH SOCKET. THE EMPLOYEE RECEIVED 6 SUTURES AT THE E.R. ROOM & CAME BACK TO WORK WITHOUT ANY RESTRICTIONS.