of the cutter as to which special conditions discussed below apply according to one aspect of the invention all regions r i and r i 1 adjacent each other in the ordered cycle are on different cutters and overlap each other along axis 36 as a result of the overlap the chip spaces in the rock left by teeth of successive regions will similarly overlap along the vertical axis of the hole being bored preventing circumferential ridges from forming between tooth regions according to another aspect of the invention regions r i taken in order in the cycle have alternately high and low effective tooth densities to counteract the tendency of teeth of one region to track the chip spaces left by teeth of the previous region this prevents gear cutting by causing the teeth of one region to remove material between the chip spaces left by teeth of the previous regions since it is undesirable for one region to have exactly twice or any other integer multiple the number of teeth as in an adjacent region an integer multiple relationship would not prevent chip space tracking and since it is also desirable to have as many teeth as possible in the low density regions the preferred arrangement is for adjacent high and low density regions to have respective numbers of.