The CB counterbores are extensively used for recessing bolt and screw heads. Supplied with a HSS slow spiral drill bit. Fully adjustable a hex key is supplied. TCT version is used for plywood particle boards and plastics (abrasive). Use a drilling speeds only. Not suitable for ferrous and non-ferrous metals. When setting-up ensure flutes of drill are aligned with flute of counterbore. For replacement Twist Drills use slow spiral type e.g 501/532 to 501/012.