The CB counterbores are used extensively for recessing bolt and screw heads. Comes with a HSS slow spiral drill bit. Completely adjustable, with a hex key supplied. Use at drilling speeds only. Not suitable for use with metals. Ensure the flutes of drill are aligned with the flute of counterbore when setting-up. Use slow spiral type for replacement Twist Drills e.g. 501/532 to 501/012.