The process of shaping sheet metal involves several steps of shaping and bending the metal until the desired shape becomes apparent. After several instances of sinking or raising–two of the techniques used to shape sheet metal–the metal itself will hold the general shape of the desired finished product, but these processes cause several ridges and dents in the metal. Therefore, the metal must be planished using a planishing hammer; this process smoothes out the metal and gives the final product a solid shape and finish.
A planishing hammer is a flat or slightly curved hammer that is used in conjunction with a planishing stake. A planishing stake is a solid metal pole that must be affixed to the ground to provide stability. Its top end is formed into a ball or other smooth, curved surface. The metal product is placed on the planishing stake, and the planishing hammer is struck against the metal product to work out ridges and other imperfections. A series of soft blows work best, as hitting the product too hard will cause dimpling.
The planishing hammer was used most often in medieval times to fashion suits of armor for knights. Today, the process of using a planishing hammer and planishing stake is somewhat outdated, though it is still used. Today, most planishing hammers are motorized and/or pneumatic, which speeds up the process considerably. They can shape, form, smooth, or stretch sheet metal, aluminum, copper, brass, and other metals. Common applications for the planishing hammer today are fashioning airplane parts, car body parts, motorcycle gas tanks and fenders, and other custom settings in which metal shaping is necessary.
Another type of planishing hammer is operated by a foot pedal. This is a non-motorized, free-standing planishing hammer controlled by a foot pedal that allows the user to maintain the use of both hands to guide the metal through the hammering process, as compared to the medieval process which necessitated the person planishing to hold the metal with one hand and the planishing hammer with the other. Some modern planishing hammers can smooth out steel up to 18 gauge, and planishing hammers can be useful in smoothing out welds and panel crowning.
Small anvils are used in place of the planishing stake on modern planishing hammers. The anvils come in a variety of sizes, the most common of which are one inch (2.54 cm), two inch (5.08 cm), and three inch (7.62 cm). Planishing hammers today vary in price anywhere from $100 USD to $800-900 USD.