Countersink

A countersink creates a conical beveled recess at the entrance of a hole to accept the tapered head of a flathead (countersunk) screw, allowing the fastener to sit flush with the surrounding surface without any protrusion. Unlike counterbore which creates a cylindrical recess, the countersink's conical geometry matches the 82-degree (or other standard) angle of the screw head, providing proper seating and load distribution.

Countersinks are specified by their included angle (82, 90, 100, or 120 degrees depending on screw type), depth, and diameter, with all dimensions called out on engineering drawings to ensure a clean, professional appearance. In CNC machining, countersinking is routinely performed as a secondary operation after drilling, either in the same setup or in a secondary setup on a different machine.

The relatively high spindle speeds and light feeds used for countersinking demand careful attention to prevent drill breakage or off-center recess creation on critical components.