Counterbore
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In machining, 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 or fillister head 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.
The symbol is Unicode character U+2334 ⌴ COUNTERBORE.
Description
A counterbore hole is usually used when the head of a fastener, such as a hex head or socket head capscrew, is required to be flush with or below the level of a workpiece's surface.
For a spotface, material is removed from a surface to make it flat and smooth, usually for a
A counterbore is also used to create a perpendicular surface for a fastener head on a non-perpendicular surface. If this is not feasible then a
By comparison, a countersink makes a conical hole and is used to seat a flathead screw.
Machining
Counterbores are made with standard dimensions for a certain size of screw or are produced in sizes that are not related to any particular screw size. In either case, the tip of the counterbore has a reduced diameter section referred to as the pilot, a feature essential to assuring concentricity between the counterbore and the hole being counterbored. Counterbores matched to specific screw sizes generally have integral pilots that fit the clearance hole diameter associated with a particular screw size (e.g., .191 inches for a number 10 machine screw). Counterbores that are not related to a specific screw size are designed to accept a removable pilot, allowing any given counterbore size to be adapted to a variety of hole sizes. The pilot matters little when running the cutter in
The uppermost counterbore tools shown in the image are the same device. The smaller top item is an insert, the middle shows another three-fluted counterbore insert, assembled in the holder. The shank of this holder is a
See also
Notes
- ^ Degarmo, Black & Kohser 2003, p. 606.
References
- Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.