Counterbore

A counterbore is a cylindrical recess created concentric to an existing hole, designed specifically to allow the oversized head of a socket cap screw, button head cap screw, or bolt to sit completely flush with or slightly below the part surface. The counterbore diameter must be sized to match the specific screw head being used (typically 0.015 to 0.025 inches larger than the head diameter), and the depth must accommodate the full head height plus any assembly clearance.

Counterbore operations are routinely performed in CNC machining centers immediately after drilling the clearance hole, often in the same machine setup with the same fixture. This operation is essential for producing clean, assembled products where fastener heads cannot protrude, such as equipment covers, fixtures, and aesthetic assemblies where appearance matters.

Engineering drawings specify counterbore dimensions (diameter and depth) using standard tolerances, and programmers ensure proper work offsets and tool selection to hit these targets consistently.