HSS Countersink 20.5mm with 1/4" Shank Countersink cutters are used  to create a chamfer around the rim of a pilot hole enabling the head of a countersunk screw to sit flush. A 90 degree countersink  is usually used for  cleaning holes, enlarging holes or countersinking holes . For use in a wide range of materials such as aluminium, metal, plastic, steel, wood and brass. PTI HSS countersinks provide a smooth finish on a variety of materials. 90° flutes for use with machines and portable tools Reduced clogging Improve clearance Chatter free operation - less mechanical vibration Cost effective and efficient Made from HSS – this type of steel can withstand increases in temperature without losing its temper (hardness), allowing the steel to cut faster than high carbon steel