THE MECH WAS WELDING A PLATE ON THE REAR OF A HAUL TRUCK. ANOTHER EE WAS TAPING THE PLATE INTO PLACE WHEN THE HAMMER SLIPPED STRIKING THE TOP OF THE MECHANIC'S HEAD. THE MECH FINISHED THE SH IFT WITH LITTLE COMPLAINT. THAT EVENING HE DECIDED TO SEEK MEDICAL ATTENTION. THE RESULT WAS A SLIGHT CONCUSSION. EE WAS OFF 8/6/02 AND RETURNED 8/7/02.