EMPLOYEE WAS REPLACING THE BALL TRAY SCREEN, ATTACHING IT TO THE BALL TRAY OF #3 ROTER SCREENER, SITTING IN THE UPRIGHT POSITION. EMPLOYEE LAID THE SCREEN ON THE TRAY WHILE ANOTHER EMPLOYEE WAS SCREWING THE SCREEN ONTO THE TRAY, EMPLOYEE ALLOWED SCREEN TO FALL ABOUT 1/2 INCH AND CUT HIS FINGER TIPS.