Abstract: A method of and apparatus for electrolytically forming relatively narrow grooves which may extend either part way or entirely through a workpiece, in which excessive groove width is avoided by reducing electrical current density between major sides of the electrode disk and side surfaces of the groove as the groove is being formed. To reduce the density of electrical current between the side surfaces of the groove and the major sides of the electrode disk, the electrode disk is provided with openings which extend between the major sides of the disk, and are positioned radially inwardly from the periphery of the disk. The openings reduce the side surface area of the disk available for the establishment of current flow to thereby tend to minimize the extent to which the sides of the groove are electrolytically eroded during formation of the groove.