counterbore

A counterbore is a cylindrical hole that enlarges a portion of a hole to a larger diameter with a flat bottom. The purpose is to create a recess that seats the head of a fastener or a nut so the fastener sits flush with or below the surrounding surface. Counterbores are contrasted with countersinks, which widen a hole with a conical, angled surface. In a counterbored hole, the bore has a uniform cylindrical profile to the specified depth.

Manufacturing a counterbore uses a counterbore bit, which combines a pilot and a cutting part of a larger diameter. The operation can follow an initial drilling of a pilot hole, or the counterbore bit may guide itself with the pilot. Depth is controlled with a stop or gauged measurement to avoid penetrating too far.

Applications and considerations: Counterbores are common in metalworking and mechanical assemblies to recess flat-headed machine screws or bolts, or to recess nuts, so the fastener head does not protrude. The counterbore diameter is sized to accommodate the fastener head, and the depth is chosen so the head seats flush with the surface. The through-hole diameter remains for the shank of the fastener or for alignment purposes.

In practice, the term counterbore is often used alongside related operations such as countersinking and spotfacing. While countersinks create conical seats, counterbores produce flat-bottomed recesses, both serving to provide a finished, flush fastening surface.