EMPLOYEE WAS REBUILDING A MAIN BEARING HYDRAULIC JACK. HE WAS HOLDING THE JACK BODY WITH HIS RIGHT HAND AND HITTING THE CYLINDER WITH A HAMMER IN HIS LEFT HAND. THE CYLINDER UNEXPECTEDLY DROPPED. HIS RIGHT HAND MIDDLE FINGER WAS IN THE LINE OF FIRE CAUSING THE TIP OF THE FINGER TO BE PINCHED BETWEEN THE JACK BODY AND CYLINDER.