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