Countersink
A countersink is a type of hole used in machinery consisting of a conical shape usually cut into a part or piece of machinery. Typically, a countersink hole allows a screw or pin to be inserted with the tip of the screw or pin sitting flush with the surface of the surrounding material, or just below it. Serving as a functional surface, a countersink hole provides an area on which a fastener bears. Due to this functionality, the tolerance of both the angle and diameter of a countersink is crucial to its operation. Countersinks are usually small in diameter and the angles are specific, tending to be 30, 82, 90,100, or 130 degrees.