TWO EE'S WERE WORKING ON SCREENING PLANT TO REPLACE THE TEFLON SHEET ON THE RIP RAP OUTLET. AFTER REPLACING PART, EE HAD HIS HAND PROPPED ON BRACE. CO-WORKER CRANKED UP PLANT. WHEN IT WENT INTO MOTION, A ROCK THAT WAS ON CONVEYOR BELT BOUNCED OFF HITTING EE'S LEFT HAND MIDDLE FINGER. ALL OPERATION CAME TO A HALT WHILE EE WAS TRANSFERRED TO THE HOSPITAL.