counterbore

[ koun-ter-bawr, -bohr; koun-ter-bawr, -bohr ]

/ ˈkaʊn tərˌbɔr, -ˌboʊr; ˈkaʊn tərˈbɔr, -ˈboʊr /

a tool for enlarging a drilled hole for a portion of its length, as to permit sinking a screw head.

the portion of the hole so enlarged.

to enlarge (a drilled hole) along part of the length of the hole by drilling with a bit having a greater diameter.

Origin of counterbore

First recorded in 1605–15; counter- + bore²

coun·ter·bor·er, noun

The jaw end is counterbored out with a 45-degree counterbore to form a bearing for the forward end of the micrometer screw.

Measuring Tools |Unknown
If taken off at all it must be pulled, for the dowel-pins, key and counterbore are close fits.

Steam Turbines |Hubert E. Collins
That it may always do this and yet leave sufficient clearance, the counterbore has been devised.

Farm Engines and How to Run Them |James H. Stephenson
For counterboring small holes or for facing the metal around their ends, the form of counterbore shown in Fig. 1746 is employed.

Modern Machine-Shop Practice, Volumes I and II |Joshua Rose

For enlarging a hole for a certain distance the counterbore is employed.

Modern Machine-Shop Practice, Volumes I and II |Joshua Rose