A countersink 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 or screw, when placed in the hole, to
sit flush with or below the surface of the surrounding material.
A countersink may also be used to remove the burr left from a drilling or tapping operation
thereby improving the finish of the product and removing any hazardous sharp edges.
The basic geometry of a countersink inherently can be applied to the plunging applications
described above and also to other milling applications.
Therefore countersinks overlap in form, function, and sometimes name with chamfering endmills.
Regardless of the name given to the cutter, the surface being generated may be a conical
chamfer or a beveled corner for the intersection of two planes.
Types Machining
A countersink may be used in many tools, such as drills, drill presses, milling machines,
and lathes.
Cross-hole countersink cutter
A Weldon Style "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.
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 to create a countersunk hole for a screw.
Fluted countersink cutter
The fluted countersink cutter is used to provide a heavy chamfer 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.
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 very often have an 82° angle,
and screws that follow the ISO standard 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.
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 at an irregular
pitching.
This variation in pitching reduces the chance of the cutting edges setting up a harmonic
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; 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.
Very good chatter-free results can usually be had by countersinking by hand.
The slow speed and sensitive feed tend to obviate 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 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 on
the sheet without the bother and expense of welding a nut 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 Chamfer
Counterbore Speeds and feeds
References
Bibliography Reithmaier, Lawrence W., Standard aircraft
handbook for mechanics and technicians, McGraw-Hill Professional, ISBN 978-0-07-134836-2.