EE & A CO-WORKER WERE REBUILDING A SIZING FEED PUMP. THEY HAD REMOVED THE PUMP CASING & EE WAS REMOVING THE IMPELLER. HE WAS USING THE CRANE CONTROLLER WITH HIS RIGHT HAND & WAS HOLDING IMPELL ER W/LEFT HAND. WHEN IMPELLER SPUN FREE, IT SWUNG OUTWARD & EE'S LEFT THUMB WAS CAUGHT BETWEEN IMPELLER & FEED SPOOL PIECE, LACERATING IT. HE WAS TAKEN TO ER FOR TREATMENT & STITCHES WERE REQ