We bought this double headed power hammer from Metalcraft Tools in Tennessee. The upper hammer is held on a leather strap in a yoke which oscillates up and down powered by a rotating crank. The hammer head thus “flies” towards the workpiece creating 900 blows per minute. At impact, a small zone of the workpiece (a thin metal sheet) is abruptly compressed, and spreads sideways, thinning the sheet and creating a small increase in curvature. The effect of the process on the workpiece is similar to that of the English Wheel, but very much noisier! A set of alternative shaped hammers allows some control of the shape of the worked area.