While installing a metal channel post, an employee secured the post in a vertical position with his hands while a co-worker struck the top of the post with a sledge hammer. A post cap was placed on top of the post to prevent damage to the post and to protect workers. When the employee determined the channel post was driven deep enough, he told his co-worker and proceeded to remove the post cap with his left hand when the head of the hammer struck the employee's hand and crushed it against the post cap, resulting in broken bones.