The conventional milling operation calls for the use of an upright milling machine and a horizontal milling machine. The upright milling machine is provided with an upright milling cutter to facilitate the longitudinal cutting of a metal workpiece. Upon completion of the process of the longitudinal cutting of the workpiece, the workpiece is transferred to the horizontal milling machine on which the horizontal cutting of the workpiece is done by a horizontal milling cutter of the horizontal milling machine. It is therefore readily apparent that the conventional milling machines are inefficient and uneconomical.