This is a high quality HSS-E countersink ideal for a range of materials including steel and aluminium. A countersink is a tool that creates a conical impression onto the surface of material. This conical impression shares the same name as the tool. A countersink is used to seat a countersunk bolt or fastener of some kind. The main reason to use a countersunk bolt or fastener is to create a flat surface so that nothing can get cuaght on it. It is also widely used to lightly deburr holes or chamfer them .