COUNTERBORE

counterbore, countersink, countersink bit

(noun) a bit for enlarging the upper part of a hole

Source: WordNet® 3.1

Etymology

Noun

counterbore (plural counterbores)

A cylindrical recess, typically machined around a hole to admit a screw so that it sits flush with a surface.

The tool with which a counterbore is machined.

Verb

counterbore (third-person singular simple present counterbores, present participle counterboring, simple past and past participle counterbored)

(transitive) To create such a cylindrical recess.

Source: Wiktionary

Coun"ter*bore` (-br`), n.

1. A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.

2. A kind of pin drill with the cutting edge or edges normal to the axis; -- used for enlarging a hole, or for forming a flat-bottomed recess at its mouth.

Coun`ter*bore" (koun`tr-br"), v. t.

Definition: To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.

Source: Webster’s Unabridged Dictionary 1913 Edition