Description and Terminology
– A counterbore is a cylindrical flat-bottomed hole that enlarges another coaxial hole.
– It is used when a fastener needs to sit flush with or below the surface of a workpiece.
– A countersink is a conical enlargement of a hole.
– A spotface is a shallow counterbore often used for specific purposes.
– The tool used to create a counterbore is also called a counterbore cutter.
Machining
– Counterbores can be made with standard dimensions for specific screw sizes or in sizes not related to any particular screw size.
– Counterbores with integral pilots ensure concentricity between the counterbore and the hole being counterbored.
– Counterbores without integral pilots can accept removable pilots for versatility.
– The pilot is less important in milling setups with assured rigidity and hole center location.
– Counterbore tools can have different shank types, such as Morse taper or drill chuck.
Uses
– Counterbores are used when a fastener head needs to be flush with or below the surface.
– Spotfacing is a type of counterbore used to make a surface flat and smooth.
– Counterbores can create perpendicular surfaces for fastener heads on non-perpendicular surfaces.
– Self-aligning nuts may be required when creating perpendicular surfaces is not feasible.
– Countersinks are used to seat flathead screws and create conical holes.
Standards
– Standards exist for counterbores’ sizes, especially for fastener head seating areas.
– These standards can vary between corporations and standards organizations.
– Boeing Design Manual BDM-1327 and ASME Y14.5-2009 have different definitions for spotfaces.
– Boeing Design Manual BDM-1327 defines the nominal diameter of the spotfaced surface.
– ASME Y14.5-2009 defines the spotface as equal to the flat seat diameter.
Related Concepts
– Countersinks are related to counterbores and are used for different purposes.
– Countersinks create conical holes to seat flathead screws.
– Countersinks and counterbores both involve enlarging holes but have different shapes.
– Counterbores and countersinks can be used together in certain applications.
– Understanding the differences between counterbores and countersinks is important in machining. Source: https://en.wikipedia.org/wiki/Counterbore
This article needs additional citations for verification. (December 2009) |
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.
