What drill bit do I need?
Drill bits are cutting
tools used to create cylindrical holes, almost always of circular
cross-section. Bits are held in a tool called a drill,
which rotates them and provides torque and
axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes.
The shank is
the part of the drill bit grasped by the chuck of
The cutting edges of the drill bit are at one end, and the shank is at the
Drill bits come in standard sizes, described in the drill
bit sizes article. A
bit and tap size chart listsmetric and imperial sized
drill bits alongside the required screw tap sizes.
The term drill may
refer to either a drilling machine or a drill bit for use in a drilling
machine. In this article, for clarity, drill
bitor bit is
used throughout to refer to a bit for use in a drilling machine, and drill refers
always to a drilling machine.
Exceptionally, specially-shaped bits can cut holes of non-circular
cross-section; a square cross-section
Drill bit geometry has several aspects:
- The spiral (or
rate of twist) in the drill bit controls the rate of chip removal. A
fast spiral drill bit is used in high feed rate applications under low
spindle speeds, where removal of a large volume of swarf is
required. Low spiral drill bits are used in cutting applications where
high cutting speeds are traditionally used, and where the material has a
tendency to gall on the bit or otherwise clog the hole, such as aluminum
- The point
angle, or the angle formed at the tip of the bit, is determined by
the material the bit will be operating in. Harder materials require a
larger point angle, and softer materials require a sharper angle. The
correct point angle for the hardness of the material controls wandering,
chatter, hole shape, wear rate, and other characteristics.
- The lip
angle determines the
amount of support provided to the cutting edge. A greater lip angle will
cause the bit to cut more aggressively under the same amount of point
pressure as a bit with a smaller lip angle. Both conditions can cause
binding, wear, and eventual catastrophic failure of the tool. The proper
amount of lip clearance is determined by the point angle. A very acute
point angle has more web surface area presented to the work at any one
time, requiring an aggressive lip angle, where a flat bit is extremely
sensitive to small changes in lip angle due to the small surface area
supporting the cutting edges.
- The length of
a bit determines how long a hole can be drilled, and also determines the
stiffness of the bit and accuracy of the resultant hole. Twist drill
bits are available in standard lengths, referred to as Stub-length or
Screw-Machine-length (short), the extremely common Jobber-length (medium),
and Taper-length or Long-Series (long).
Most drill bits for consumer use have straight shanks. For heavy duty
drilling in industry, bits with tapered shanks
are sometimes used.
The diameter-to-length ratio of the drill bit is usually between 1:1 and
1:10. Much higher ratios are possible (e.g., "aircraft-length" twist bits,
drill bits, etc.), but the higher the ratio, the greater the
technical challenge of producing good work.
The best geometry to use depends upon the properties of the material being
drilled. The following table lists geometries recommended for some commonly
Lip relief angle
118 to 135
118 to 135
Many different materials are used for or on drill bits, depending on the
required application. Many hard materials, such as carbides, are much more
brittle than steel, and are far more subject to breaking, particularly if
the drill is not held at a very constant angle to the workpiece, e.g. when
- Soft low
carbon steel bits
are inexpensive, but do not hold an edge well and require frequent
sharpening. They are used only for drilling wood; even working with hardwoods rather
than softwoods can
noticeably shorten their lifespan.
- Bits made from high
carbon steel are more
durable than low-carbon steel bits due to the properties conferred by hardening
and tempering the
material. If they are overheated (e.g., by frictional heating while
driling) they lose their temper,
resulting in a soft cutting edge. These bits can be used on wood or
High speed steel (HSS)
is a form of tool
steel; HSS bits are hard, and much more resistant to heat
than high carbon steel. They can be used to drill metal, hardwood, and
most other materials at greater cutting speeds than carbon steel bits,
and have largely replaced carbon steels.
Cobalt steel alloys are
variations on high speed steel which contain more cobalt. They hold
their hardness at much higher temperatures, and are used to drill stainless
steel and other
hard materials. The main disadvantage of cobalt steels is that they are
more brittle than standard HSS.
- Hi-moly tool steel is heat-treated at
1196 °C (2185 °F) and then nitro-carburize finished at 510 °C (950 °F)
to be measurably harder than high-speed steel. Nitro-carburized steel
withstands substantially higher drilling temperatures while maintaining
Tungsten carbide and
other carbides are
extremely hard, and can drill virtually all materials while holding an
edge longer than other bits. The material is expensive and much more
brittle than steels; consequently they are mainly used for drill bit
tips, small pieces of hard material fixed or brazed onto
the tip of a bit made of less hard metal. However, it is becoming common
in job shops to use solid carbide bits. In very small sizes it is
difficult to fit carbide tips; in some industries, most notably PCB manufacturing,
requiring many holes with diameters less than 1 mm, carbide bits are
- Polycrystalline diamond (PCD)
is among the hardest of all tool materials and is therefore extremely
resistant to wear. It consists of a layer of diamond particles,
typically about 0.5 mm (0.019") thick, bonded as a sintered mass to a
tungsten carbide support. Bits are fabricated using this material by
either brazing small segments to the tip of the tool to form the cutting
edges, or by sintering PCD
into a vein in the tungsten carbide "nib". The nib can later be brazed
to a carbide shaft; it can then be ground to complex geometries that
would otherwise cause braze failure in the smaller "segments". PCD bits
are typically used in the automotive, aerospace, and other industries to
drill abrasive aluminum alloys, carbon fiber reinforced plastics, and
other abrasive materials, and in applications where machine downtime to
replace or sharpen worn bits is exceptionally costly.
Black oxide is
an inexpensive black coating. A black oxide coating provides heat
resistance and lubricity, as well as corrosion resistance. Coating
increases the life of high-speed steel bits.
Titanium nitride (TiN) is
a very hard ceramic material that can be used to coat a high-speed steel
bit (usually a twist bit), extending the cutting life by three or more
times. However, when the bit is sharpened the new edge will not have the
benefits of the coating.
Titanium aluminum nitride (TiAlN) is
a similar coating that can extend tool life five or more times.
- Titanium carbon nitride (TiCN) is
another coating also superior to TiN.
Diamond powder is
used as an abrasive, most often for cutting tile, stone, and other very
hard materials. Large amounts of heat are generated by friction, and
diamond coated bits often have to be water cooled to prevent damage to
the bit or the workpiece.
Zirconium nitride has
been used as a drill bit coating for some tools under the Craftsman brand
General-purpose drill bits can be used in wood, metal, plastic, and most
The twist drill bit is the type produced in largest quantity today. It
comprises a cutting point at the tip of a cylindrical shaft with helical
flutes; the flutes act as an Archimedean
screwand lift swarf out of the hole.
The twist drill bit was invented by Steven A. Morse of East
Bridgewater, Massachusetts in
original method of manufacture was to cut two grooves in opposite sides of a
round bar, then to twist the bar (giving the tool its name) to produce the
helical flutes. Nowadays, the drill bit is usually made by rotating the bar
while moving it past a grindingwheel
to cut the flutes in
the same manner as cutting
Twist drill bits range in diameter from 0.002 to 3.5 in (0.051 to 89 mm) and
can be as long as 25.5 in (650 mm).
The geometry and sharpening of the cutting edges is crucial to the
performance of the bit. Small bits that become blunt are often discarded
because sharpening them correctly is difficult and they are inexpensive. For
larger bits, special grinding jigs are available. A special tool
grinder is available
for sharpening or reshaping cutting surfaces on twist drill bits in order to
optimize the bit for a particular material.
Manufacturers can produce special versions of the twist drill bit, varying
the geometry and the materials used, to suit particular machinery and
particular materials to be cut. Twist drill bits are available in the widest
choice of tooling materials. However, even for industrial users, most holes
are drilled with standard high
speed steel bits.
The most common twist drill bit (sold in general hardware stores) has a
point angle of 118 degrees, acceptable for use in wood, metal, plastic, and
most other materials, although it does not perform as well as using the
optimum angle for each material. In most materials it does not tend to
wander or dig in.
A more aggressive angle, such as 90 degrees, is suited for very soft
plastics and other materials; it would wear rapidly in hard materials. Such
a bit is generally self-starting and can cut very quickly. A shallower
angle, such as 150 degrees, is suited for drilling steels and other tougher
materials. This style of bit requires a starter hole, but does not bind or
suffer premature wear so long as a suitable feed rate is used.
Drill bits with no point angle are used in situations where a blind,
flat-bottomed hole is required. These bits are very sensitive to changes in
lip angle, and even a slight change can result in an inappropriately fast
cutting drill bit that will suffer premature wear.
Long series drill bits are
unusually long twist drill bits. However, they are not the best tool for
routinely drilling deep holes, as they require frequent withdrawal to clear
the flutes of swarf and to prevent breakage of the bit. Instead, gun
drill bits are
preferred for deep hole drilling.
A step drill bit is
a drill bit that has the tip ground down to a different diameter. The
transition between this ground diameter and the original diameter is either
straight, to form a counterbore, or angled, to form a countersink. The
advantage to this style is that both diameters have the same flute
characteristics, which keeps the bit from clogging when drilling in softer
materials, such as aluminum; in contrast, a drill bit with a slip-on collar
does not have the same benefit. Most of these bits are custom-made for each
application, which makes them more expensive.
A unibit (often
called a step
drill bit) is a roughly conical bit
with a stair-step profile. Due
to its design, a single bit can be used for drilling a wide range of hole
sizes. Some bits come to a point and are thus self-starting. The larger-size
bits have blunt tips and are used for hole enlarging.
Unibits are commonly used on sheet metal and
in general construction. One drill bit can drill the entire range of holes
necessary on a countertop, speeding up installation of fixtures. They are
most commonly used on softer materials, such as plywood, particle board,
drywall, acrylic, and laminate. They can be used on very thin sheet metal,
but metals tend to cause premature bit wear and dulling.
Unibits are ideal for use in electrical work where thin steel, aluminum or
plastic boxes and chassis are encountered. The short length of the unibit
and ability to vary the diameter of the finished hole is an advantage in
chassis or front panel work. The finished hole can often be made quite
smooth and burr-free, especially in plastic.
An additional use of unibits is deburring holes left by other bits, as the
sharp increase to the next step size allows the cutting edge to scrape burrs
off the entry surface of the workpiece. However, the straight flute is poor
at chip ejection, and can cause a burr to be formed on the exit side of the
hole, more so than a spiral twist drill bit turning at high speed.
The unibit was invented by Harry C. Oakes and patented in
was sold only by the Unibit Corporation in the 1980s until the patent
expired, and was later sold by other companies.
Hole saws take the form of a short open cylinder with saw-teeth on the open
edge, used for making relatively large holes in thin material. They remove
material only from the edge of the hole, cutting out an intact disc of
material, unlike many drills which remove all material in the interior of
the hole. They can be used to make large holes in wood, sheet metal and
Center drill bits are used
in metalworking to
provide a starting hole for a larger-sized drill bit or to make a conical
indentation in the end of a workpiece in which to mount a lathe
center. In either use, the name seems appropriate, as the bit is
either establishing the center of
a hole or making a conical hole for a lathe center.
However, the true purpose of a center drill bit is the latter task, while
the former task is best done with a spotting
drill bit (as explained in
detail below). Nevertheless, because of the frequent lumping together of
both the terminology and the tool use, suppliers may call center drill bits combined-drill-and-countersinks in
order to make it unambiguously clear what product is being ordered. They are
numbered from 00 to 10 (smallest to largest).
Center drill bits are meant to create a conical hole for "between centers"
manufacturing processes (typically lathe or cylindrical-grinder work). That
is, they provide a location for a (live, dead, or driven) center to locate
the part about an axis. A workpiece machined between
centerscan be safely removed from one process (perhaps turning in a
lathe) and set up in a later process (perhaps a grinding operation)
with what is often a negligible loss in the co-axiality of features.
Traditional twist drill bits may tend to wander when started on an
unprepared surface. Once a bit wanders off-course it is difficult to bring
it back on center. A center drill bit frequently provides a reasonable
starting point as it is short and therefore has a reduced tendency to wander
when drilling is started.
While the above is a common use of center drill bits, it is a technically
incorrect practice and should not be considered for production use. The
correct tool to start a traditionally-drilled hole (a hole drilled by a
high-speed steel (HSS) twist drill bit) is a spotting
drill bit (or a spot
drill bit, as they are referenced in the U.S.). The included angle of
the spotting drill bit should be the same as, or greater than, the
conventional drill bit so that the drill bit will then start without undue
stress on the bit's corners, which would cause premature failure of the bit
and a loss of hole quality.
Most modern solid-carbide bits should not be used in conjunction with a spot
drill bit or a center drill bit, as the solid-carbide bits are specifically
designed to start their own hole. Usually, spot drilling will cause
premature failure of the solid-carbide bit and a certain loss of hole
quality. If it is deemed necessary to chamfer a
hole with a spot or center drill bit when a solid-carbide drill bit is used,
it is best practice to do so after the hole is drilled.
Center drill bits wander as easily as anything else in hand-held power
drills—so for such operations, a center
punch is often used to
spot the planned hole center prior to drilling apilot
hole. However, a center drill bit works nearly as well as a
spotting drill bit for most rigidly-clamped drilling operations, especially
in softer metals such as aluminum and
The small starting tip has a tendency to break, so it is economical and
practical to make the drill bit double-ended.
The term core drill bit is
used for two quite different tools.
- Enlarging holes
A bit used to enlarge an existing hole, as pictured, is called a core drill
bit. The existing hole may be the result of a core from
a casting or
a stamped (punched) hole. The name comes from its first use, for drilling
out the hole left by a foundry
core, a cylinder placed in a mould for a casting that leaves an
irregular hole in the product. This core drill bit is solid.
These core drill bits are similar in appearance to reamers as
they have no cutting point or means of starting a hole. They have 3 or 4
flutes which enhances the finish of the hole and ensures the bit cuts
evenly. Core drill bits differ from reamers in the amount of material they
are intended to remove. A reamer is only intended to enlarge a hole a slight
amount which, depending on the reamers size, may be anything from 0.1
millimeter to perhaps a millimeter. A core drill bit may be used to double
the size of a hole.
Using an ordinary two-flute twist drill bit to enlarge the hole resulting
from a casting core will not produce a clean result, the result will
possibly be out of round, off center and generally of poor finish. The two
fluted drill bit also has a tendency to grab on any protuberance (such as flash)
which may occur in the product.
- Extracting core
A hollow cylindrical bit which will cut a hole with an annular cross-section
and leave the inner cylinder of material (the "core") intact, often removing
it, is also called a core drill bit. Unlike other drills, the purpose is
often to retrieve the core rather than simply to make a hole. A diamond core
drill bit is intended to cut an annular hole in the workpiece. Large bits of
similar shape are used for geological work, where a deep hole is drilled in
sediment or ice and the drill bit, which now contains an intact core of the
material drilled with a diameter of several centimeters, is retrieved to
allow study of the strata.
Main article: Countersink
A countersink is a conical hole cut into a manufactured object; a
countersink bit (sometimes called simply countersink) is the cutter used to
cut such a hole. A common use is to allow the head of a bolt or screw, with
a shape exactly matching the countersunk hole, to sit flush with or below
the surface of the surrounding material. (By comparison, a counterbore makes
a flat-bottomed hole that might be used with a hex-headed capscrew.) A
countersink may also be used to remove the burr left from a drilling or
Used almost exclusively for deep hole drilling of medium to large diameter
holes (about 3/4" up to about 4" diameter). An ejector drill bit uses a
specially designed carbide cutter at the point. The bit body is essentially
a tube within a tube. Flushing water travels down between the two tubes.
Chip removal is back through the center of the bit.
Gun drills are straight fluted drills which allow cutting fluid (either
compressed air or a suitable liquid) to be injected through the drill's
hollow body to the cutting face.
Indexable drill bits are primarily used in CNC and
other high precision or production equipment, and are the most expensive
type of drill bit, costing the most per diameter and length. Like indexable
lathe tools and
milling cutters, they use replaceable carbide, HSS or ceramic inserts as a
cutting face to alleviate the need for a tool grinder. One insert is
responsible for the outer radius of the cut, and another insert is
responsible for the inner radius. The tool itself handles the point
deformity, as it is a low-wear task. The bit is hardened and coated against
wear far more than the average drill bit, as the shank is non-consumable.
Almost all indexable drill bits have multiple coolant channels for prolonged
tool life under heavy usage. They are also readily available in odd
configurations, such as straight flute, fast spiral, multiflute, and a
variety of cutting face geometries.
Typically indexable drill bits are used in holes that are no deeper than
about 5 times the bit diameter. They are capable of quite high axial loads
and cut very fast.
Left-hand bits are almost always twist bits and are predominantly used in
the repetition engineering
industry on screw machines or drilling heads. Left-handed drill bits allow a
machining operation to continue where either the spindle cannot be reversed
or the design of the machine makes it more efficient to run left-handed.
With the increased use of the more versatile CNC machines,
their use is less common than when specialized machines were required for
Screw extractors are
essentially left-hand bits of specialized shape, used to remove common
right-hand screws whose
heads are broken or too damaged to allow a screwdriver tip to engage, making
use of a screwdriver impossible. The drill bit is pressed against the
damaged head and rotated counter-clockwise and
will tend to jam in the damaged head and then turn the screw
counter-clockwise, unscrewing it.
A spade drill bit for metal is a two part bit with a tool holder and an
insertable tip, called an insert. The inserts come in various sizes that
range from 7⁄16 to
2.5 inches (11 to 63 mm). The tool holder usually has a coolant passage
running through it. They
are capable of cutting to a depth of about 10 times the bit diameter. This
type of drill bit can also be used to make stepped holes.
Straight fluted drill bits do not have a helical twist like twist drill bits
do. They are used when drilling copper or brass because
they have less of a tendency to "dig in" or grab the material.
A trepan, sometimes called a BTA drill bit (after the Boring and Trepanning
Association), is a drill bit that cuts an annulus and leaves a center core.
Trepans usually have multiple carbide inserts and rely on water to cool the
cutting tips and to flush chips out of the hole. Trepans are often used to
cut large diameters and deep holes. Typical bit diameters are 6" to 14" and
hole depth from 12" up to 71 feet.
The lip and spur drill bit is
a variation of the twist drill bit which is optimized for drilling in wood.
It is also called the brad
point bit or dowelling bit.
Conventional twist drill bits tend to wander when presented to a flat
workpiece. For metalwork, this is countered by drilling a pilot hole with a
spotting drill bit. In wood, the lip and spur drill bit is another solution:
The centre of the drill bit is given not the straight chisel of the twist
drill bit, but a spur with a sharp point and four sharp corners to cut the
wood. The sharp point of the spur simply pushes into the soft wood to keep
the drill bit in line.
Metals are typically isotropic,
and an ordinary twist drill bit shears the edges of the hole cleanly. Wood
drilled across the grain has long strands of wood fiber. These long strands
tend to pull out of the wood hole, rather than being cleanly cut at the hole
edge. The lip and spur drill bit has the outside corner of the cutting edges
leading, so that it cuts the periphery of the hole before the inner parts of
the cutting edges plane off the base of the hole. By cutting the periphery
first, the lip maximizes the chance that the fibers can be cut cleanly,
rather than having them pull messily out of the timber.
Lip and spur drill bits are also effective in soft plastic. Conventional
twist drill bits in a hand drill, where the hole axis is not maintained
throughout the operation, have a tendency to smear the edges of the hole
through side friction as the drill bit vibrates.
In metal, the lip and spur drill bit is confined to drilling only the
thinnest and softest sheet
metals in a drill
press. The bits have an extremely fast cutting tool geometry: no
point angle and a large (considering the flat cutting edge) lip angle causes
the edges to take a very aggressive cut with relatively little point
pressure. This means these bits tend to bind in metal; given a workpiece of
sufficient thinness, they have a tendency to punch through and leave the
bit's cross-sectional geometry behind.
Lip and spur drill bits are ordinarily available in diameters from 3 mm
(1/8") to 16 mm (5/8").
Spade bits are used for rough boring in wood. They tend to cause splintering
when they emerge from the work piece. They are flat, with a centering point
and two cutters. The cutters often are equipped with spurs in an attempt to
ensure a cleaner hole. With their small shank diameters relative to their
boring diameters, spade bit shanks often have flats forged or ground into
them to prevent slipping in drill chucks. Some bits are equipped with long
shanks and have a small hole drilled through the flat part, allowing them to
be used much like a bell-hanger
bit. Intended for high speed use, they are used with electric
hand drills. Spade bits are also sometimes referred to as "paddle bits."
Spade drill bits are ordinarily available in diameters from 6 mm (1/4") to
36 mm (1 1/2").
Spoon bits consist of a grooved shank with a point shaped somewhat like the
bowl of a spoon, with the cutting edge on the end. The more common type is
like a gouge bit that ends in a slight point. This is helpful for starting
the hole, as it has a center that will not wander or walk. These bits are
used by chair-makers for boring or reaming holes in the seats and arms of
chairs. Their design is ancient, going back to Roman times. Spoon bits have
even been found in Viking excavations. Modern spoon bits are made of
hand-forged carbon steel, carefully heat-treated and then hand ground to a
Spoon bits are the traditional boring tools used with a brace. They should
never be used with a power drill of any kind. Their key advantage over
regular brace bits and power drill bits is that the angle of the hole can be
adjusted. This is very important in chair making, because all the angles are
usually eyeballed. Another advantage is that they do not have a lead screw,
so they can be drilled successfully in a chair leg pretty much without
having the lead screw peek out the other side.
When reaming a pre-bored straight-sided hole, the spoon bit is inserted into
the hole and rotated in a clockwise direction with a carpenters' brace until
the desired taper is achieved. When boring into solid wood, the bit should
be started in the vertical position; after a "dish" has been created and the
bit has begun to "bite" into the wood, the angle of boring can be changed by
tilting the brace a bit out of the vertical. Holes can be drilled precisely,
cleanly and quickly in any wood, at any angle of incidence, with total
control of direction and the ability to change that direction at will.
Parallel spoon bits are used primarily for boring holes in the seat of a Windsor
chair to take the back
spindles, or similar round-tenon work when assembling furniture frames ingreen
The spoon bit may be honed by using a slipstone on the inside of the cutting
edge; the outside edge should never be touched.
Forstner bits, named after their inventor, Benjamin
Forstner, bore precise, flat-bottomed holes in wood, in any
orientation with respect to the wood grain. They can cut on the edge of a
block of wood, and can cut overlapping holes. Because of the flat bottom to
the hole, they are useful for drilling through veneer already glued to add
an inlay. They require great force to push them into the material, so are
normally used in drill presses or lathes rather than in portable drills.
Unlike most other types of drill bits, they are not practical to use as hand
The bit includes a center point which guides it throughout the cut (and
incidentally spoils the otherwise flat bottom of the hole). The cylindrical
cutter around the perimeter shears the wood fibers at the edge of the bore,
and also helps guide the bit into the material more precisely. The tool in
the image has a total of two cutting edges in this cylinder. Forstner bits
have radial cutting edges to plane off the material at the bottom of the
hole. The bit in the image has two radial edges. Other designs may have
more. Forstner bits have no mechanism to clear chips from the hole, and
therefore must be pulled out periodically.
Sawtooth bits are also available, which include many more cutting edges to
the cylinder. These cut faster, but produce a more ragged hole. They have
advantages over Forstner bits when boring into end
Bits are commonly available in sizes from 8 mm (5/16") to 50 mm (2")
diameter. Sawtooth bits are available up to 100 mm (4") diameter.
Originally the Forstner bit was very successful with gunsmiths because of
its ability to drill an exceedingly smooth-sided hole.
The center bit is optimized for drilling in wood with a hand
brace. Many different designs have been produced.
The center of the bit is a tapered screw thread. This screws into the wood
as the bit is turned, and pulls the bit into the wood. There is no need for
any force to push the bit into the workpiece, only the torque to turn the
bit. This is ideal for a bit for a hand tool. The radial cutting edges
remove a slice of wood of thickness equal to the pitch of the central screw
for each rotation of the bit. To pull the bit from the hole, either the
female thread in the wood workpiece must be stripped, or the rotation of the
bit must be reversed.
The edge of the bit has a sharpened spur to cut the fibers of the wood, as
in the lip and spur drill bit. A radial cutting edge planes the wood from
the base of the hole. In this version, there is minimal or no spiral to
remove chips from the hole. The bit must be periodically withdrawn to clear
Some versions have two spurs. Some have two radial cutting edges.
Center bits do not cut well in the end grain of wood. The central screw
tends to pull out, or to split the wood along the grain, and the radial
edges have trouble cutting through the long wood fibers.
Center bits are made of relatively soft steel, and can be sharpened with a
The cutting principles of the auger bit are the same as those of the center
bit above. The auger adds a long deep spiral flute for effective chip
Two styles of auger bit are commonly used in hand braces: the Jennings or
Jennings-pattern bit has a self-feeding screw tip, two spurs and two radial
cutting edges. This bit has a double flute starting from the cutting edges,
and extending several inches up the shank of the bit, for waste removal.
This pattern of bit was developed by Russell Jennings in the mid-19th
The Irwin or
solid-center auger bit is similar, the only difference being that one of the
cutting edges has only a "vestigal flute" supporting it, which extends only
about 1/2" (12 mm) up the shank before ending. The other flute continues
full-length up the shank for waste removal. The Irwin bit may afford greater
space for waste removal, greater strength (because the design allows for a
center shank of increased size within the flutes, as compared to the Jenning
bits), or smaller manufacturing costs. This style of bit was invented in
1884, and the rights sold to Charles Irwin who patented and marketed this
pattern the following year.
Both styles of auger bits were manufactured by several companies throughout
the early- and mid-20th century, and are still available new from select
The diameter of auger bits for hand braces is commonly expressed by a single
number, indicating the size in 16ths of an inch. For example, #4 is 4/16 or
1/4" (6 mm), #6 is 6/16 or 3/8" (9 mm), #9 is 9/16" (14 mm), and #16 is
16/16 or 1" (25 mm). Sets commonly consist of #4-16 or #4-10 bits.
The bit shown in the picture is a modern design for use in portable power
tools, made in the UK in about 1995. It has a single spur, a single radial
cutting edge and a single flute. Similar auger bits are made with diameters
from 6 mm (3/16") to 30 mm (1-3/16"). Augers up to 600 mm (2 feet) long are
available, where the chip-clearing capability is especially valuable for
drilling deep holes.
The gimlet bit is a very old design. The bit is the same style as that used
in the gimlet,
a self-contained tool for boring small holes in wood by hand. Since about
1850, gimlets have had a variety of cutter designs, but some are still
produced with the original version. The gimlet bit is intended to be used in
a hand brace for drilling into wood. It is the usual style of bit for use in
a brace for holes below about 7 mm (1/4") diameter.
The tip of the gimlet bit acts as a tapered screw, to draw the bit into the
wood and to begin forcing aside the wood fibers, without necessarily cutting
them. The cutting action occurs at the side of the broadest part of the
cutter. Most drill bits cut the base of the hole. The gimlet bit cuts the
side of the hole.
The hinge sinker bit is an example of a custom drill bit design for a
specific application. Many European kitchen cabinets are made fromparticle
board or medium-density
fiberboard (MDF) with
a laminated plastic veneer.
Those types of pressed
wood boards are not
very strong, and the screws of butt hinges tend
to pull out. A specialist hinge has been developed which uses the walls of a
30 mm (1-3/16") diameter hole, bored in the particle board, for support.
This is a very common and relatively successful construction method.
A Forstner bit could bore the mounting hole for the hinge, but particle
board and MDF are very abrasive materials, and steel cutting edges soon
wear. A tungsten
carbide cutter is
needed, but making a tungsten carbide Forstner bit is impractical, so this
special drill bit is commonly used. It has cutting edges of tungsten carbide
brazed to a steel body; a center spur keeps the bit from wandering.
An adjustable wood bit has a small center pilot bit with an adjustable,
sliding cutting edge mounted above it, usually containing a single sharp
point at the outside, with a set
screw to lock the
cutter in position. When the cutting edge is centered on the bit, the hole
drilled will be small, and when the cutting edge is slid outwards, a larger
hole is drilled. This allows a single drill bit to drill a wide variety of
holes, and can take the place of a large, heavy set of different size bits,
as well as providing uncommon bit sizes. A ruler or vernier
scale is usually
provided to allow precise adjustment of the bit size.
These bits are available both in a version similar to an auger bit or brace
bit, designed for low speed, high torque use with a brace or other hand
drill (pictured to the right), or as a high speed, low torque bit meant for
a power drill. While the shape of the cutting edges is different, and one
uses screw threads and the other a twist bit for the pilot, the method of
adjusting them remains the same.
The diamond masonry mortar bit is a hybrid drill bit, designed to work as a
combination router and drill bit. It consists of a steel shell, with the
diamonds embedded in metal segments attached to the cutting edge. These
drill bits are used at relatively low speeds.
The masonry bit shown here is a variation of the twist drill bit. The bulk
of the tool is a relatively soft steel, and is machined with a mill rather
than ground. An insert of tungsten
carbide is brazed into
the steel to provide the cutting edges.
Masonry bits typically are used with a hammer
drill, which hammers the bit into the material being drilled as
it rotates; the hammering breaks up the masonry at the drill bit tip, and
the rotating flutes carry away the dust. Rotating the bit also brings the
cutting edges onto a fresh portion of the hole bottom with every hammer
blow. Hammer drill bits often use special shank shapes such as the SDS type,
which allows the bit to slide within the chuck when hammering, without the
whole heavy chuck executing the hammering motion.
Masonry bits of the style shown are commonly available in diameters from
5 mm to 40 mm. For larger diameters, core bits are used. Masonry bits up to
1000 mm (39") long can be used with hand-portable power tools, and are very
effective for installing wiring and plumbing in existing buildings.
drill bit, similar in appearance and function to a hole punch or
chisel, is used as a hand powered drill in conjunction with a hammer to
drill into stone and masonry.
A star drill bit's cutting edge consists of several blades joined at the
center to form a star pattern.
Glass bits have a spade-shaped carbide point. They generate high
temperatures and have a very short life. Holes are generally drilled at low
speed with a succession of increasing bit sizes. Diamond drill bits can also
be used to cut holes in glass, and last much longer.
A great many holes with small diameters of about 1 mm or less must be
drilled in printed
circuit boards (PCBs)
used by electronic
equipment with through-hole components.
Most PCBs are made of highly abrasive fiberglass,
which quickly wears steel bits, especially given the hundreds or thousands
of holes on most circuit boards. To solve this problem, solidtungsten
bits, which drill quickly through the board while providing a moderately
long life, are almost always used. Carbide PCB bits are estimated to outlast
high speed steel bits by a factor of ten or more. Other options sometimes
used are diamond or diamond-coated bits.
In industry, virtually all drilling is done by automated
machines, and the bits are often automatically replaced by the
equipment as they wear, as even solid carbide bits do not last long in
constant use. PCB bits, of narrow diameter, typically mount in a collet rather
than a chuck,
and come with standard-size shanks, often with pre-installed stops to set
them at an exact depth every time when being automatically chucked by the
Very high rotational speeds—30,000 to 100,000 RPM or
even higher—are used; this translates to a reasonably fast linear speed of
the cutting tip in these very small diameters. The high speed, small
diameter, and the brittleness of the material, make the bits very subject to
breaking, particularly if the angle of the bit to the workpiece changes at
all, or the bit contacts any object. Drilling by hand is not practicable,
and many general-purpose drilling machines designed for larger bits rotate
too slowly and wobble too much to use carbide bits effectively.
Resharpened and easily available PCB drills have historically been used in
many prototyping and home PCB labs, using a high-speed rotary tool for
small-diameter bits (such as a Moto-Tool by Dremel) in a stiff drill-press
jig. If used for other materials these tiny bits must be evaluated for
equivalent cutting speed vs material resistance to the cut (hardness), as
the bit's rake
angle and expected
feed per revolution are optimised for high-speed automated use on fiberglass
- Fishing bit
Installer bits, also known as bell-hanger bits
or fishing bits,
are a type of twist drill bit for use with a hand-portable power tool. The
key distinguishing feature of an installer bit is a transverse hole drilled
through the web of the bit near the tip. Once the bit has penetrated a wall,
a wire can be threaded through the hole and the bit pulled back out, pulling
the wire with it. The wire can then be used to pull a cable or pipe back
through the wall. This is especially helpful where the wall has a large
cavity, where threading a fish
tape could be
difficult. Some installer bits have a transverse hole drilled at the shank
end as well. Once a hole has been drilled, the wire can be threaded through
the shank end, the bit released from the chuck, and all pulled forward
through the drilled hole. Sinclair Smith of Brooklyn,
New York was issued U.S.
Patent 597,750 for
this invention on January 25, 1898.
Installer bits are available in various materials and styles for drilling
wood, masonry and metal.
- Flexible shaft bit
Another, different, bit also called an installer bit has a very long
flexible shaft, typically up to 72 inches (1.8 m) long, with a small twist
bit at the end. The shaft is made of spring
steel instead of
so it can be flexed while drilling without breaking. This allows the bit to
be curved inside walls, for example to drill through studs from
switchbox without needing to remove any material from the wall.
These bits usually come with a set of special tools to aim and flex the bit
to reach the desired location and angle, although the problem of seeing
where the operator is drilling still remains.
This flexible installer bit is used in the USA, but does not appear to be
routinely available in Europe.