A post pounding machine was being used to pound a post on site. The operator lifted the hammer top high and the hammer came out of the mast. The mast assembly missed an in-plate that would have prevented the hammer from separating from the mast when lifted above a certain height. The hammer struck the employee while he was setting up another post for placement. The hammer fell onto his hard hat and grazed his lip/chin area. The impact caused significant lacerations to the head and face.