A countersink is a tapered hole drilled with a wide outer portion. A common usage is to allow the head of a countersunk bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material. A countersink may also be used to remove the burr left from a drilling or tapping operation thereby improving the finish of the product and removing any hazardous sharp edges.

Cross hole countersink cutter

Side and end view of 

a cross hole deburrer

A cross hole countersink is a cone-shaped tool with a cutting edge provided by a hole that goes through the side of the cone (see picture at right). The intersection of the hole and cone form the cutting edge on the tool. The cone is not truly symmetrical as it is essential that the cone retreats away from the cutting edge as the tool rotates. If this does not occur the cutting edge will lack clearance and rub rather than bite into the material. This clearance is referred to as cutting relief.

These tools are best used as deburring tools, where the burr from a previous machining operation needs to be removed for cosmetic and safety reasons, however they may be used in softer materials (such as wood or plastic) to create a countersunk hole for a screw.

Fluted countersink cutter

Side and end view of a 

4 fluted countersink

The fluted countersink cutter is used to provide a heavy chamfer in the entrance to a drilled hole. This may be required to allow the correct seating for a countersunk head screw, to provide the lead in for a second machining operation such as tapping. Countersink cutters are manufactured with two common angles, 90 and 82. This difference provides a choice in seating angles for the mating part.

Better quality fluted countersink cutters will have the flutes (or at least one flute) at an irregular pitching. This variation in pitching reduces the chance of the cutting edges setting up a harmonic action and leaving an undulated surface. This surface ripple is also dependent on the surface speed of the cutting edges, material type and applied pressure; once started it is hard to remove.