Countersink Reaming

a method of finishing the surfaces of previously drilled holes made by swaging or cold stamping and casting. It is used for finishing cylindrical openings, the depressions receiving the head or neck of bolts and screws and the butt ends of bosses receiving washers or thrust rings. It is also an intermediate operation after drilling and before reaming. It increases precision and decreases surface roughness, since the countersink has more cutting edges than a drill. The hole made by a countersink has a more precise axial direction than a drilled hole and is therefore commonly used for finishing and semifinishing work. Countersinking is performed on drilling machines, turret lathes, and boring machines.