THE INCIDENT OCCURRED AS EE WAS USING AN IMPACT WRENCH TO TIGHTEN THE BOTTOM ROLLER CAP BOLTS ON A CAT D 11 ROLLER FRAME. TO ACCOMPLISH THIS TASK, EE WAS LYING ON THE GROUND ALONG SIDE THE ROL LER FRAME, WITH HIS HEAD DIRECTLY BELOW THE OPENING TO THE GUIDE ASSEMBLY HOUSING. THE VIBRATION FROM THE IMPACT WRENCH CAUSED THE GUIDE ASSEMBLY TO SLIDE OUT OF ITS HOUSING STRIKING IN FACE.