A type of hammer used for forming metal, especially "peening over" rivets.
A ball peen hammer is similar to the more familiar claw hammer used to drive nails. One side of a ball peen hammer head has a regular flat head for just that purpose. But instead of a claw for pulling out nails, the other side has a round steel ball.
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You may have found a ball peen hammer in your grandfather's toolbox while searching for a hammer to drive nails. You may have wondered, "What the heck is this thing for?" More likely, you simply started driving nails with it, cursing when you bent a nail and had no claw to pull it out with.
Hammers are used to hit things. But the form of the hammer you use depends upon what you want to hit. For driving nails, a claw hammer is best. But for setting rivets manually, the best choice is a ball peen hammer.
Riveting involves driving a soft metal nail through a hole in the boards (or metal plates) you are fastening together, cutting off excess material on one side, then "peening over" the nail shaft sticking out on the other side, striking it to form a mushroom shape. A rivet makes a nice tight permanent fastener if you do it correctly.
The greatest danger while peening a rivet is in hitting the nail shaft straight on. This will most likely make the nail bend inside the hole, "crippling" the rivet -- if stress is placed on the boards, the bend wil straighten out and the boards will separate. This makes for a leaky boat, or a bridge that will fall apart.
The flat head of a ball peen hammer is used to drive the nail through; the round ball on the other side of the head is used to peen over the other side of the rivet. The ball makes it less likely to strike the nail shaft directly; the glancing blows produced by the ball mash a little bit of the metal away from the sides of the hole.
Since the advent of pneumatic rivet guns, the use of ball peen hammers for riveting bridges or steel ships together is virtually nil. But they are still used for traditional handcrafts, such as wooden boat building or reproducitons of medieval armor.
Ball peen hammers come in different sizes, depending upon the type of nails you are turning into rivets. A six-ounce ball peen hammer is ideal for making small wooden boats. Model boat builders have tiny ball peen hammers for making rivets out of copper wire (and unimaginable patience). My father had a big 2-pound ball peen hammer; why, I'll never know. We (mis) used it for breaking up concrete.