ball peen hammer
Before the advent of pneumatic rivet guns, ball peen hammers were commonly used for riveting. First the flat head drove the nail through, then the round ball was used to "peen over" the other side of the rivet. The biggest danger while peening rivets is to strike the nail shaft straight on on as this can make the nail bend inside the hole. Then if the boards are stressed the bend might straighten and the boards separate – making for a structure that falls apart or leaks. The ball of the ball peen hammer tends to produce glancing blows that mash some of the metal away from the sides of the hole. This also hardens the metal so that it becomes as elastic as the surrounding material.
The word "peen" is sometimes spelled "pein" in the UK.
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