• WordNet 3.6
    • n counterbore a bit for enlarging the upper part of a hole
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Webster's Revised Unabridged Dictionary
    • Counterbore A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.
    • Counterbore 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.
    • v. t Counterbore To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.
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Century Dictionary and Cyclopedia
    • counterbore To bore out (a hole) to a larger diameter for a given depth.
    • n counterbore A tool-holder or tool-bar for a drilling-machine. It is passed through a hole in the work and brings the cutting tool against the farther or opposite (connter) end of the work.
    • n counterbore A tool for boring out a portion of a hole to a larger diameter. Such a tool is usually made with a tip on the end to fit the drilled hole, so that the enlarged section will be concentric with and parallel to the drilled hole.
    • n counterbore An enlarged section of a drilled hole, made for the purpose of receiving the cylindrical head of a screw or some similar object.
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In news:

Counterboring is a machining operation that will enlarge an existing hole's diameter to a specified depth.
Traditionally, counterboring is done with either a special counterboring tool that has a pilot, or it is done with an end mill.
Eight more operations – including drilling, boring, counterboring, countersinking and tapping holes at various angles – remove an additional 60 pounds, resulting in a finished component that weighs half as much as the raw stock.
Mortises grooves and channels and counterbores and recesses cleanly.
This gets more complicated with different kinds of holes, such as countersunk, tapped, and counterbored.