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A countersink (symbol: ) is a conical hole cut into a manufactured object, or the cutter used to cut such a hole. A common use is to allow the head of a countersunk bolt, screw or rivet, when placed in the hole, to sit flush with or below the surface of the surrounding material (by comparison, a
counterbore 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 ...
makes a flat-bottomed hole that might be used with a socket-head capscrew). A countersink may also be used to remove the burr left from a drilling or
tapping Tapping is a playing technique that can be used on any stringed instrument, but which is most commonly used on guitar. The technique involves a string being fretted and set into vibration as part of a single motion. This is in contrast to stand ...
operation thereby improving the finish of the product and removing any hazardous sharp edges. The basic geometry of a countersink (cutter) inherently can be applied to the plunging applications described above (axial feed only) and also to other milling applications (sideways traversal). Therefore, countersinks overlap in form, function, and sometimes name with chamfering endmills (
endmill An end mill is a type of milling cutter, a cutting tool used in industrial milling applications. It is distinguished from the drill bit in its application, geometry, and manufacture. While a drill bit can only cut in the axial direction, most m ...
s with angled tips). Regardless of the name given to the cutter, the surface being generated may be a conical chamfer (plunging applications) or a beveled corner for the intersection of two planes (traversing applications).


Types


Machining

A countersink may be used in many tools, such as drills,
drill press A drill is a tool used for making round holes or driving fasteners. It is fitted with a bit, either a drill or driverchuck. Hand-operated types are dramatically decreasing in popularity and cordless battery-powered ones proliferating due to i ...
es,
milling machine Milling is the process of machining using rotary cutters to remove material by advancing a cutter into a workpiece. This may be done by varying direction on one or several axes, cutter head speed, and pressure. Milling covers a wide variety of d ...
s, and
lathe A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to c ...
s.


Cross-hole countersink cutter

A cross-hole, "Weldon style" or "zero flute" countersink is a cone-shaped tool with a cutting edge provided by a hole that goes through the side of the cone. The intersection of the hole and cone form the cutting edge on the tool. The cone is not truly symmetrical as it is essential that the cone retreats away from the cutting edge as the tool rotates providing clearance. If this does not occur the cutting edge will lack clearance and rub rather than ''bite'' into the material. This clearance is referred to as
cutting relief Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force. Implements commonly used for cutting are the knife and saw, or in medicine and science the scalpel and m ...
. These tools are best used as deburring tools, where the burr from a previous machining operation needs to be removed for cosmetic and safety reasons, however they may be used in softer materials (such as wood or plastic) to create a countersunk hole for a screw.


Fluted countersink cutter

The
fluted Fluting may refer to: *Fluting (architecture) * Fluting (firearms) * Fluting (geology) * Fluting (glacial) *Fluting (paper) Arts, entertainment, and media *Fluting on the Hump ''Fluting on the Hump'' is the first album by avant-garde band Kin ...
countersink cutter is used to provide a heavy
chamfer A chamfer or is a transitional edge between two faces of an object. Sometimes defined as a form of bevel, it is often created at a 45° angle between two adjoining right-angled faces. Chamfers are frequently used in machining, carpentry, fu ...
in the entrance to a drilled hole. This may be required to allow the correct seating for a countersunk-head screw or to provide the lead in for a second machining operation such as
tapping Tapping is a playing technique that can be used on any stringed instrument, but which is most commonly used on guitar. The technique involves a string being fretted and set into vibration as part of a single motion. This is in contrast to stand ...
. Countersink cutters are manufactured with six common angles, which are 60°, 82°, 90°, 100°, 110°, or 120°, with the two most common of those being 82° and 90°. Countersunk-head screws that follow the
Unified Thread Standard The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a w ...
very often have an 82° angle, and screws that follow the
ISO standard The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...
very often have a 90° angle. Throughout the aerospace industry, countersunk fasteners typically have an angle of 100°.


Back countersink

A ''back countersink'', also known as an ''inserted countersink'', is a two piece countersink used on tough to reach areas. One component is a rod that is inserted into the existing hole in the workpieces; the other component is the cutter, which is attached to the rod, or extends out of it, after it is in position. This is comparable to other types of "back-" machining, such as back-spotfacing, back-boring, back-counterboring, back-milling, and back-deburring. The common theme is accomplishing machining operations on the far side of the workpiece from the spindle face, which obviates a "second operation" setup. This reduces setup time and frustration in several ways. Not only does it obviate the flipping over, cleaning, reclamping, etc., but it also can allow effortless high concentricity, parallelism, and squareness with the first setup's datum without the hassle of reestablishing it on another setup (via painstaking indicating).


Speeds, feeds, and avoiding chatter

It can often be difficult to avoid chatter when cutting with countersink cutters. As usual in machining, the shorter and more rigid the setup, the better. Better-quality fluted countersink cutters sometimes have the flutes (or at least one flute) at an irregular pitching. This variation in pitching reduces the chance of the cutting edges setting up a
harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
action and leaving an undulated surface. This surface ripple is also dependent on the surface speed of the cutting edges, material type, and applied pressure (or feed rate); once started it is hard to remove. Too light a feed tends to increase chatter risk. As in many other machining operations, an appropriate response to the chatter may be to decrease speed and increase feed. On a drill press, the slowest available spindle speed is usually best. With a variable-speed handheld power drill, the trigger is best squeezed lightly to yield a low spindle speed. Good chatter-free results can usually be had by countersinking by hand (as opposed to running the tool in a powered spindle). The slow speed and sensitive feed tend to prevent chatter. With a quarter-inch-hex shank, the countersink cutter can be held with a screwdriver handle of the indexable-bit type.


Form countersinking

Form countersinking, also known as ''dimpling'', is a countersink that is formed into
sheet metal Sheet metal is metal formed into thin, flat pieces, usually by an industrial process. Sheet metal is one of the fundamental forms used in metalworking, and it can be cut and bent into a variety of shapes. Thicknesses can vary significantly; ex ...
to increase the strength of a structure as the countersinks of multiple pieces nest together. There are two processes for producing formed countersinks: ''coin dimpling'' and ''modified radius dimpling''.. Such dimples in fairly thick sheet can even be tapped to yield a threaded
hardpoint A hardpoint is an attachment location on a structural frame designed to transfer force and carry an external or internal load. The term is usually used to refer to the mounting points (more formally known as a weapon station or station) on the ...
on the sheet without the bother and expense of welding a
nut Nut often refers to: * Nut (fruit), fruit composed of a hard shell and a seed, or a collective noun for dry and edible fruits or seeds * Nut (hardware), fastener used with a bolt Nut or Nuts may also refer to: Arts, entertainment, and media Co ...
to the sheet. This style of construction is often seen in modern household appliance design, because it allows the product to be lower-priced, and the quality can still be good as long as the sheet is thick enough.


See also

*
Counterbore 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 ...
* Speeds and feeds *
Spotface A spotface or spot face is a machined feature in which a certain region of the workpiece (a spot) is faced, providing a smooth, flat, accurately located surface. This is especially relevant on workpieces cast or forged, where the spotface's smoo ...


References


Bibliography

* {{Woodworking Metalworking cutting tools Metalworking terminology Woodworking hand tools