A temporary employee was teaching an employee how to clean rollers in the pouring and debridge area of the facility. When they were done cleaning the roller, the employee placed it back into the lathe and used a chuck key to tighten it into place. The temporary employee instructed the other employee to turn on the lathe. As the lathe was being turned on, the temporary employee saw the key was still in the chuck and went to remove it. The employee's finger got caught between the key and the guard resulting in amputation of the left index fingertip.