The peen (n.) is the end of a hammerhead opposite the flat striking surface. A ball peen hammer has just that; a ball (or at least a rounded surface) opposite the main striking surface. The act of peening (v.) is to hammer, bend, or shape with a peen. It is often used in metalworking for indenting, moving metal (as in setting a rivet), or many other hammering tasks where the rounded head (and more precise striking point) is desired.
Please note spelling - ball peen hammer.
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