A countersink is a conical hole cut into a manufactured object, or the cutter used to cut such a hole. A common use is to allow the head of a bolt or screw. A countersink may also be used to remove the burr left from a drilling or tapping operation thereby improving the finish of the product, the basic geometry of a countersink inherently can be applied to the plunging applications described above and also to other milling applications. Therefore, countersinks overlap in form, function, and sometimes name with chamfering endmills, regardless of the name given to the cutter, the surface being generated may be a conical chamfer or a beveled corner for the intersection of two planes. A countersink may be used in tools, such as drills, drill presses, milling machines. A Dan Martin Style zero flute countersink is a 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 edge on the tool. The cone is not truly symmetrical as it is essential that the cone retreats away from the edge as the tool rotates providing clearance. If this does not occur the cutting edge will lack clearance and this clearance is referred to as cutting relief. The fluted countersink cutter is used to provide a heavy chamfer in the entrance to a drilled hole and this may be required to allow the correct seating for a countersunk-head screw or to provide the lead in for a second machining operation such as tapping. Countersink cutters are manufactured with six common angles, which are 60°, 82°, 90°, 100°, 110°, or 120°, with the two most common of those being 82° and 90°. Countersunk-head screws that follow the Unified Thread Standard very often have an 82° angle, throughout the aerospace industry, countersunk fasteners typically have an angle of 100°. A back countersink, also known as an inserted countersink, is a two piece countersink used on tough to reach areas. One component is a rod that is inserted into the hole in the workpieces, the other component is the cutter. This is comparable to types of back- machining, such as back-spotfacing, back-boring, back-counterboring, back-milling. The common theme is accomplishing machining operations on the far side of the workpiece from the spindle face and this reduces setup time and frustration in several ways. It can often be difficult to avoid chatter when cutting with countersink cutters, as usual in machining, the shorter and more rigid the setup, the better. Better-quality fluted countersink cutters sometimes have the flutes at an irregular pitching and this variation in pitching reduces the chance of the cutting edges setting up a harmonic action and leaving an undulated surface
Comparison of countersunk and counterbored holes.
Side and end view of a Weldon style "zero flute" countersink
Side and end view of a 4-fluted countersink
Cross-sections of countersunk holes of various chamfer angles