countersinking

Countersinking is a machining and woodworking process that creates a conical recess in a material to accommodate the head of a countersunk screw or bolt, allowing the fastener to sit flush with or below the surrounding surface. The operation is performed with a countersink bit, which forms a conical seat by removing material around the hole. Many countersink bits have one or more pilot sections to start the hole and one or more flutes to cut the conical surface; common included angles for the cone are 82°, 90°, and 100°, with 60° being used in some woodworking applications.

In practice, a pilot hole is usually drilled first in metal or hard plastics, and the countersink bit is then used to create the seating crater to the desired depth. Depth is controlled with a drill press stop, a depth gauge, or by observing a marked depth. There are also combined drill-countersink tools that cut the pilot and the cone in a single operation.

Countersinking is distinct from a spotface, which creates a flat bearing surface around an existing hole rather than seating a screw head in a cone. It is also related to chamfering, but the latter generally bevels edges rather than forming a precise conical seat for a fastener.

Applications include flush-mounted fasteners in cabinetry, furniture, electronics, and metal assemblies. When performed improperly, countersinking can weaken material around the hole or result in an undersized seat, so proper hole size, angle selection, and depth control are important.