What does a counterbore do to a hole?

A counterbore is a type of machining operation that involves creating a larger diameter at the top of a hole, but only for a specific portion of the depth. This process allows for a flat-bottomed recess that can accommodate a fastener's head or to provide a larger diameter at the entrance of the hole for other purposes, such as providing clearance for a bolt or screw.

The correct choice reflects this definition, as counterboring does not go through the entire depth of the original hole; instead, it enlarges just the upper portion to create a seating area for a fastener. This is particularly useful in applications where a smooth surface is desired, or to ensure that the fastener is flush with or below the surface.

The option that suggests the hole is enlarged for its full depth would imply that the entire hole is modified to a larger diameter, which is not accurate for counterboring. Other options, such as creating a through hole or drilling a completely new hole, do not correctly describe the counterboring process, which specifically targets the upper section of an existing hole rather than altering its entirety.