Abstract: A hammer device for attachment to a hand power tool for longitudinally folding the overlapping edge of a sheet metal panel. It comprises a body member having a central bore, one end of which is provided with means for connection to a hand tool which powers a reciprocating drive. Connected to the diametral sides of the body member are stud members, the distal ends of which longitudinally extend beyond the other end of the body member and are adapted to support an anvil plate member. The hammer member comprises a longitudinal shaft member having a hammer pad extending radially therefrom. The shaft is reciprocatingly and rotatably movable in the bore of the body member. The drive means of the power tool reciprocatingly drives the hammer pad against the anvil member while the hammer pad is free to pivot back and forth.