What is "counterbore"

Counterbore \Coun"ter*bore`\ (-b?r`), 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.

Counterbore \Coun`ter*bore"\ (koun`t?r-b?r"), v. t. To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.

n. 1 A cylindrical recess, typically machined around a hole to admit a screw so that it sits flush with a surface. 2 The tool with which a counterbore is machined. vb. (context transitive English) To create such a cylindrical recess.

n. a bit for enlarging the upper part of a hole [syn: countersink, countersink bit]

A counterbore (symbol: ⌴) is a cylindrical flat-bottomed hole that enlarges another coaxial hole, or the tool used to create that feature. A counterbore hole is typically used when a fastener, such as a socket head cap screw, is required to sit flush with or below the level of a workpiece's surface.

Whereas a counterbore is a flat-bottomed enlargement of a smaller coaxial hole, a countersink is a conical enlargement of such. A spotface often takes the form of a very shallow counterbore.

As mentioned above, the cutters that produce counterbores are often also called counterbores; sometimes, to avoid ambiguity, the term counterbore cutter is used instead.