EMPLOYEE WAS HELPING MILLWRIGHT TAKE UP NO.1 CRUSHER USING A 1-1/2' WRENCH TO LOOSEN 3"" NUTS ON CRUSHER. EMPLOYEE TRIED TO LOOSEN THE NUT TWO TIMES WITHOUT MOVING NUT. ON THIRD TIME NUT RELEAS ED CAUSING LEFT THUMB TO BE PINCHED BETWEEN WRENCH AND 8"" PIPE.