Fluted countersink cutter

The fluted countersink cutter is used to provide a heavy chamfer in the entrance to a drilled hole. This may be required to allow the correct seating for a countersunk-head screw or to provide the lead in for a second machining operation such as tapping. Countersink cutters are manufactured with six common angles, which are 60°, 82°, 90°, 100°, 110°, or 120°, with the two most common of those being 82° and 90°. Countersunk-head screws that follow the Unified Thread Standard very often have an 82° angle, and screws that follow the ISO standard very often have a 90° angle. Throughout the aerospace industry, countersunk fasteners typically have an angle of 100°. The ideal countersink angle for holes tapped with 60° threads, when no countersunk fastener head will sit in the countersunk area, is often 60°; but often another angle is used if that is the cutter that is at hand, and the difference usually doesn't matter.