Abstract: To provide for self-centering of boring tools, particularly of larger diameter boring tools having replaceable rotatable cutter bits, the cutter bit closest to the central axis of the tool holder or tool shank, i.e. the radially innermost bit is formed with auxiliary cutting surfaces adjacent the corners of a square or parallelogram-shaped bit, for example by chamfering the corner, with a relief surface therebehind, and positioning the auxiliary cutting surface to intersect the central axis of the boring tool so that, in operation, a small conical tip will be left which is continuously reduced in size and cut; and wherein the cutter bits are positioned on the tool bodies to present cutting edges at the end face which overlap with respect to workpiece removal, so that each cutter bit will independently cut into the workpiece, at least the inner cutter bit being guided in its cutting circle by the radially outer corner fitting against the surface cut in the workpiece.