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