EE HAD OPENED A 2' BY 4' SECTION OF HANGED GRATING TO ACCESS #3 TRAY OVERFLOW HEADER VENT VALVES. AS HE WAS REPLACING THE GRATING HIS LEFT HAND MIDDLE FINGER GOT CAUGHT BETWEEN THE GRATING. A FRACTURE WAS DIAGNOSED ON MAY 6, 2002. NO LOST TIME, RESTRICTED DUTY ONLY.