Cross hole countersink

A cross hole deburrer is a cone shaped tool with a cutting edge provided by a hole that goes through the side of the cone. The intersection of the hole and cone form the cutting edge.
The cone is not a truely conical as it essential that the cone retreats away from the cutting edge as the tool rotates. If this does not occur the cutting edge will 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 drilling operation needs to be removal for appearance and safety reasons.

Fluted Countersink

The fluted countersink, shown at left, is used to provide a heavy chamfer in the enterance 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 machning operation such as tapping. Countersinks are manufactured with two common angles, 90° and 82°. This difference provides a choice in seating angles for the mating part.
The better quality fluted countersinks will have the flutes (or at the least one) 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 extremely hard to remove.