To form undercut grooves in a work piece, especially to retain turbine rotor blades, in a first step, the grooves are cut such that the flanks of the grooves have a preliminarily desired shape; then, in a second step, a bell-type countersink is moved radially to cut the desired shape of the flanks of the groove. Countersink cutters for the second step include a plurality of groups of cutting bits or inserts secured to posts on a shell, or bell-type countersink, in accordance with the profile to be cut.