Abstract: A method of machining an axially parallel groove in the periphery of a shaft includes fixedly mounting the shaft in the main spindle of a CNC turning machine, moving a rotating milling tool (whose diameter decreases in a direction from its free end and whose maximum diameter is less than the desired width of the groove) radially and axially of the shaft so as to form a first portion of the groove and to finish one of the lateral surfaces which bound the groove, extracting the tool and changing the angular position of the shaft through an angle which equals or approximates the taper of the tool), and again moving the tool axially of the shaft to complete the machining of the groove and to simulaneously form the other lateral surface. The tool can be reground a substantial number of times so that its useful life is a multiple of that of cylindrical milling tools having a diameter which matches the desired width of the groove.