Mechanic used a 12-lb hammer to strike flat end of a 68"" pipe that EE was holding to push out plate that wedged between the mill's arm and wall. As the mechanic hammered the pipe, the pipe slipped off the plate and EE tried to steady the pipe by placing EE's hand on the flat end of the pipe just as the mechanic swung the hammer striking EE's hand lacerating L middle finger.