IRWIN Tools Black Oxide Countersink Drill Bit, 5-Piece Set (1877791) Made from industrial grade High-Speed Steel for increased durability. Black oxide finish adds corrosion resistance. Five-flute design reduces vibration for smoother drilling and extended bit life. Precision-ground 82-degree tip drills countersink holes into most machinable metals, as well as wood and plastic. 1/4-inch round shank fits all handheld and stationary drills.