Abstract: There is disclosed a method of producing holes or recesses in a workpiece of fiber reinforced materials using a rotary tool, preferably a rotary end mill, comprising a rear shaft portion and an integral forward cutting portion having a central axis. The cutting portion is a sintered body having at least a pair of longitudinally extending flutes and a corresponding pair of cutting edges made of diamond or cubic boron nitride at the circumference adjacent each flute so as to define the diameter of said tool. The tool is rotated about an axis and fed along said axis until the initial penetration of the tool is made whereby the tool supported after the initial penetration so that its central axis rotates eccentrically to the feed axis of the tool by a distance sufficient to drill a hole of a desired diameter greater than the diameter of the tool. The tool is fed in said direction until the hole has been completed.