Definition
A machining operation that enlarges the upper portion of an existing hole to a specified diameter and depth, producing a flat-bottomed recess that allows the head of a bolt or socket-head screw to sit flush with or below the surface of the part.
Plain English
Widening the top of a drilled hole so the head of a bolt or screw can sit down inside the part instead of sticking up above the surface.
Context Anchor
Seen in aircraft maintenance, sheet metal work, and repair instructions where holes are prepared for specific fasteners or hardware.
Derivation
From 'counter' (opposite or against) plus 'boring' (cutting a round hole). The cutter works back against the existing hole to widen its upper portion.
Why Pilots Care
Improper counterboring can weaken a part, leave hardware seated incorrectly, or create an uneven surface where the repair depends on exact fit.
Intuition Check
Counterboring is not the same as countersinking. Counterboring makes a straight-sided, flat-bottomed recess; countersinking makes an angled recess for a tapered screw head.
Example Sentence 1
The mechanic counterbored the hole so the bolt head would sit flush with the surface of the bracket.
Example Sentence 2
After counterboring the panel, the installer could torque the screws without creating a raised lip that would disturb airflow.