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[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.
verb (used with object), counterbored, counterboring.
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
Related forms
counterborer, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for counterbore
Historical Examples
  • If taken off at all it must be pulled, for the dowel-pins, key and counterbore are close fits.

    Steam Turbines Hubert E. Collins
  • For enlarging a hole for a certain distance the counterbore is employed.

  • A counterbore in the arbor hub provides clearance for the hub of the pulley which projects beyond the rim on one side.

    Turning and Boring Franklin D. Jones
  • For counterboring small holes or for facing the metal around their ends, the form of counterbore shown in Fig. 1746 is employed.

  • 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
  • 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

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