EE & COWORKER WERE TIGHTENING LOCKNUTS ON THE PUSH BOLTS FOR THE COAL MILL ROLLER ADJUSTMENTS. EE WAS HOLDING A LARGE HAMMER WRENCH ON LOCKNUT WHILE COWORKER HIT IT W/SLEDGEHAMMER. THE WRENCH IS OPEN END TYPE, 40"" LONG. IT WAS BEING HELD BY EE'S LT HAND SO IT WOULDN'T COME OFF THE NUT. WHEN THE SLEDGEHAMMER HIT THE WRENCH IT PIVOTED SIDEWAYS & PINCHED EE'S FINGER AGAINST MILL BODY.