MILLWRIGHT AND COWORKERS WERE INSTALLING SCREEN DECK. TWO WORKERS TIGHTENED BOLTS, ONE ABOVE THE DECK WITH IMPACT WRENCH, THE OTHER BELOW THE DECK WITH PIPE WRENCH. COWORKER ACTUATED IMPACT WR ENCH PREMATURELY, AND NUT SPUN, LACERATING INJURED'S RIGHT THUMB. TWO SUTURES REQUIRED FOR WOUND.