Power planishing hammers shape and smooth sheet metal workpieces by striking them with forming dies. Also known as power hammers, they hold material between two dies and repeatedly strike the top die (hammer) against the bottom die. They accept die sets of various sizes and shapes to form metal to the desired surface profile. Planishing hammers are commonly used to shape and finish domed workpieces and to remove dents or wrinkles from damaged workpieces. Corded units plug into an outlet and operate in areas without a compressed air supply. Air-powered units generally strike faster than corded units. They contain few moving parts, so they typically run cool and have a long lifespan.