Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes.
This article describes the types of drill bits in terms of the design of the cutter. The other end of the drill bit, the shank, is described in the drill bit shank article. Drill bits come in standard sizes, described in the drill bit sizes article. A comprehensive drill and tap size chart lists metric and imperial sized drills alongside the required screw tap sizes.
The term drill can refer to a drilling machine, or can refer to a drill bit for use in a drilling machine. In this article, for clarity, drill bit or bit is used throughout to refer to a bit for use in a drilling machine, and drill refers always to a drilling machine.
The twist drill bit was invented by Steven A. Morse of East Bridgewater, Massachusetts in 1861. He received for his invention on April 7 1863. The original method of manufacture was to cut two grooves in opposite sides of a round bar, then to twist the bar to produce the helical flutes. This gave the tool its name. Nowadays, the drill bit is usually made by rotating the bar while moving it past a grinding wheel to cut the flutes in the same manner as cutting helical gears.
Tools recognizable as twist drill bits are currently produced in diameters covering a range from 0.05 mm (0.002") to 100 mm (4"). Lengths up to about 1000 mm (39") are available for use in powered hand tools.
The geometry and sharpening of the cutting edges is crucial to the performance of the bit. Users often throw away small bits that become blunt, and replace them with new bits, because they are inexpensive and sharpening them well is difficult. For larger bits, special grinding jigs are available. A special tool grinder is available for sharpening or reshaping cutting surfaces on twist drills to optimize the drill 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 still drilled with a conventional bit of high speed steel.
The most common twist drill (the one sold in general hardware stores) has a point angle of 118 degrees. This is a suitable angle for a wide array of tasks, and will not cause the uninitiated operator undue stress by wandering or digging in. A more aggressive (sharper) angle, such as 90 degrees, is suited for very soft plastics and other materials. The bit will generally be self-starting and cut very quickly. A shallower angle, such as 150 degrees, is suited for drilling steels and other tougher materials. This style bit requires a starter hole, but will not bind or suffer premature wear when a proper feed rate is used.
Drills with no point angle are used in situations where a blind, flat-bottomed hole is required. These drills 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.
Drill bit geometry has several aspects:
Most drills for consumer use have straight shanks. For heavy duty drilling in industry, drills with tapered shanks are sometimes used.
Long series drills are extended length twist drills. They are not the best tool for drilling deep holes, as they require frequent withdrawal to clear the flutes of swarf and prevent drill breakages. Gun drills are the preferred drills for deep hole drilling.
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 apt, as the drill 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 is the latter task, while the former task is best done with a spotting drill (as explained in detail below). Nevertheless, because of the frequent lumping together of both the terminology and the tool use, suppliers may call center drills combined-drill-and-countersinks in order to make unambiguously clear what product is being ordered.
Centre drills 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 centers can 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 drills, 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) is a spotting drill, or a spot drill, as they are referred to in the U.S. The included angle of the spotting drill 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 drill's corners, which would cause premature failure of the drill and a loss of hole quality.
Most modern solid-carbide drills should not be used in conjunction with a spot drill or a center drill. They are specifically designed to start their own hole. Usually, spot drilling will cause premature failure of the carbide drill and a certain loss of hole quality. If it is deemed necessary to chamfer a hole with a spot or center drill when a carbide drill is used, it is best practice to do so after the hole is drilled.
Centre drills wander as easily as anything else in hand-held power drills—for such operations, so a center punch is often used to spot the planned hole centre prior to drilling a pilot hole. However, a centre drill works nearly as well as a spotting drill for most rigidly-clamped drilling operations, especially in softer metals such as aluminum and its alloys.
The small starting tip has a tendency to break, so it is economical and practical to make the drill bit double-ended.
A core drill bit (as pictured) is used to enlarge an existing hole. The existing hole may be the result of a core from a casting or a stamped (punched) hole.
The name of this bit may be somewhat confusing.
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 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 also has a tendency to grab on any protuberance (such as casting flash) which may occur in the product.
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 drills allow a machining operation to continue when the spindle either cannot be reversed or where the design of the machine makes it more efficient to run left-handed. With the increased use of the more versatile CNC machines their usage is less common than when specialised machines were required for machining tasks.
They may also be used as an aid in the removal of common right-hand screws. Since the rotation of the drill bit is such as it would loosen the screw, using it to drill into the damaged screw head will usually remove the screw, providing the bit "grabs" the damaged material successfully.
Another type of left-hand bit is an extraction tool used expressly for removing broken or seized screws, other than by drilling. It has a highly tapered thread structure on it, and is inserted into a drilled hole (of the recommended size) in the damaged screw. If a left hand drill bit is used initially, and the act of drilling the hole does not release the screw, this tool may remove it. In use, the extractor is rotated and the action of the taper and spiral digs into the damaged material causing it to lock tightly and hopefully applies enough pressure to remove the screw. The tool has a tendency to continue winding in while being turned and this may cause the extractor to expand the screw in the hole causing it to bind further, leading to failure of the process. These bits are made of very hard, but brittle, steel, which means they can break off inside the screw if too much force is applied, making the removal much more difficult. Because of this an alternative extractor has four parallel edges, which tends not to self-tighten. Alternatively, the hole can be drilled with successively larger bits until it can be tapped.
Typically indexable drills are used in holes that are no deeper than about 5 times the drill diameter. They are capable of quite high axial loads and cut very fast.
Spade drills are capable of cutting to a depth of about 10 times the drill diameter. Cut diameters are typically in the range of about 3/4" to 3".
Used almost exclusively for deep hole drilling of medium to large diameter holes (about 3/4" up to about 4" diameter). An ejector drill uses a specially designed carbide cutter at the point. The drill 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 drill.
The lip and spur drill bit is a variation of the twist drill which is optimized for drilling in wood. It is also called the brad point bit or dowelling bit.
Conventional twist drill bits do tend to wander when presented to a flat workpiece. For metalwork, this is countered by drilling a pilot hole with a spotting drill. In wood, there is another possible solution, that used in the lip and spur drill. The centre of the drill bit is given not the straight chisel of the twist drill, 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 shears the edges of the hole cleanly. Wood drilled across the grain has long strands of wood fibre. 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 maximises the chance that the fibres 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 drills 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 vibrates.
In metal, the lip and spur drill is confined to drilling only the thinnest and softest sheet metals in a drill press. The drills 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 drills tend to bind in metal; given a workpiece of sufficient thinness, they have a tendency to punch through and leave the drill'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").
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 drills, they are not practical to use as hand tools.
The bit has a centre point which guides it during the cut (and incidentally spoils the otherwise flat bottom of the hole). The cylindrical cutter around the perimeter shears the wood fibres at the edge of the bore, and also helps guide the bit into the wood precisely. The tool in the image has a total of two cutting edges in this cylinder. Sawtooth Forstner bits are available, with many more cutting edges in the cylinder. These cut faster, but produce a more ragged hole.
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 must be pulled out periodically to do this.
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.
A step bit, step drill, speed bit, or Unibit is a roughly conical bit with a stair-step profile. Due to their 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. They are now available in fractional inch and metric sizes.
Step bits are most commonly used in general construction and plumbing. One drillbit can drill the entire range of holes necessary on a countertop, speeding up installation of fixtures. They are most commonly used on softer materials - plywood, particle board, drywall, acrylic, laminate, etc. They can be used on very thin sheetmetal, but metals tend to cause premature drill wear and dulling. A metal hole saw is more appropriate for large-hole applications in thicker metals.
An additional use of step bits 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 turning at high speed.
The step bit was invented by Harry C. Oakes of Wyoming, New York in 1971. He received for it on 11 September 1973. Introduced by Unibit Corporation in the 1980s (formerly a subsidiary of Petersen Manufacturing Company and now part of Irwin Industrial Tools), step bits have been copied by other manufacturers since the patent expired.
The centre of the bit is a tapered screw thread. This screws into the wood as the drill 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 fibres of the wood, as in the lip and spur drill. 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 drill must be periodically withdrawn to clear the chips.
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 fibres.
Center bits are made of relatively soft steel, and can be sharpened with a file.
The drill bit shown was made sometime before 1950, and still worked to drill holes in 2005. It drills a hole 19 mm (3/4 inch) in diameter.
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 century.
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 sources today.
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 tip of the gimlet bit acts as a tapered screw, to draw the bit into the wood and to begin forcing aside the wood fibres, without necessarily cutting them. The cutting action occurs at the side of the broadest part of the cutter. Most drills cut the base of the hole. The gimlet bit cuts the side of the hole.
The gimlet bit in the photos was made sometime before 1950.
The hinge sinker bit is an example of a custom drill design for a specific application. Many European kitchen cabinets are made from particle board or medium-density fibreboard (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. Softer steel cutting edges soon wear. A tungsten carbide cutter is needed, and making that in the form of a Forstner bit is impractical. So, this special drill is commonly used. It has cutting edges of tungsten carbide brazed to a steel body. A centre 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 drills are used at relatively low speeds.
Masonry bits typically are used with a hammer drill. The bit is both rotated and hammered into the workpiece. The hammering breaks up the masonry at the drill bit tip. The flutes of the drill bit body carry away the dust. Rotating the bit brings the cutting edges onto a fresh portion of the hole bottom with every hammer blow.
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.
Holesaws take the form of a small circular saw with the teeth parallel to the axis of the drill. They can be used on wood, metal and other materials. See main article at Hole saw.
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 with their solid carbide construction, they still have a short lifespan. PCB bits 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 equipment.
Due to the high speed these bits are used at (30,000–100,000 RPM or higher is common), their small size, and the brittleness of the material, even the slightest wobble of an operator's hand will shatter one, as will accidental contact with almost any object. Due to their delicate nature, these bits cannot be used in a hand drill, and even most moderately expensive drill presses will have too low a speed and too much chuck wobble to use these bits without breaking them.
Installer bits are available in various materials and styles for drilling wood, masonry and metal.
A variant of the installer bit has a very long flexible shaft, up to 72 inches (1.8 m) long in the US, with a small twist bit at the end. The shaft is made of spring steel instead of hardened steel, so it can be flexed while drilling without breaking. This unique design allows the bit to be curved inside walls, for example to drill through studs from a light switch box 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.
The flexible variant of the installer bit does not appear to be routinely available in the EU.
Different drill bits are used, depending on the material being drilled for the well. There are three main categories: soft, medium and hard formation bits. Soft formation rock bits are used in unconsolidated sands, clays, soft limestone, red beds and shale, etc. Medium formation bits are used in calcites, dolomites, lime stones, and hard shale, while hard formation bits are used in hard shale, calcites, mudstones, cherty limestone and hard and abrasive formations.
Many different materials are used for or on drill bits, depending on the required application.
high Carbon steel bits are made from high carbon steel and are an improvement on plain steel due to the hardening and tempering capabilities of the material. These bits can be used on wood or metal, however they have a low tolerance to excessive heat which causes them to lose their temper, resulting in a soft cutting edge.
High speed steel (HSS) is a form of tool steel where the bits are much more resistant to the effect of heat. They can be used to drill in metal, hardwood, and most other materials at greater cutting speeds than carbon steel bits and have largely replaced them in commercial applications.
Cobalt steel alloys are variations on high speed steel which have more cobalt in them. Their main advantage is that they hold their hardness at much higher temperatures, so they 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.
Polycrystalline diamond (PCD) is among the hardest of all tool materials and is therefore extremely wear resistant. The material 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 and ground to complex geometries that 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, or in applications where machine downtime is undesirable.
Black oxide is an inexpensive black coating. A black oxide coating provides heat resistance and lubricity, as well as corrosion resistance. These result in a longer drill life than the typical uncoated high-speed steel drill.
Titanium nitride (TiN) is a very hard ceramic material, and when used to coat a high-speed steel bit (usually twist bits), can extend the cutting life by three or more times. A titanium nitride bit cannot properly be sharpened, as the new edge will not have the coating, and will not have any of the benefits the coating provided.
Titanium aluminum nitride (TiAN) is another coating frequently used. It is considered superior to TiN and can extend tool life five or more times.
Titanium carbon nitride (TiCN) is another coating and is 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, and diamond coated bits often have to be water cooled to prevent damage to the bit or the workpiece.