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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.
=Manufacturing Tools=
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.
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.
Metal Drills
Twist drill
The twist drill bit is the type produced in largest quantity today. It can be used to create holes in metal, plastic, wood and stone.The twist drill bit was invented by Steven A. Morse* of East Bridgewater, Massachusetts in 1861. He received for his invention on 7 April 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 is usually made by rotating the bar while moving it past a grinding wheel to cut the flutes.
Tools recognisable as twist drill bits are currently produced in diameters covering the range at least from 0.05 mm to 100 mm. Lengths up to about 1000 mm 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.
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.
Hobbyists are most familiar with straight-shank twist drills. For heavy duty drilling in industry, drills with tapered shank are 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, however, used carefully they are functional. Gun drills are the preferred drills for deep hole drilling.
Gun drill
see Gun drillCenter drill
Center drill bits are used in metalworking to provide a starting hole for a larger sized drill bit, or a conical indentation in the end of a workpiece to mount a lathe center . These centers are used when turning or grinding workpieces. 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) without losing any concentricity.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 provides a good starting point as it is short and therefore has a reduced tendency to wander when drilling is started.
While the above is common, it is incorrect practice. Centre drills are meant to create a centre for lathe work only. The correct tool to start a hole is a spotting drill, because the included angle of the spotting drill is the same as a conventional drill bit so the drill bit will then start without chatter. Centre drills wander as easily as anything else in hand-held power drills - for such operations, a centre punch should be used to spot the planned hole centre prior to drilling a pilot hole. That said, a centre drill works nearly as well as a spotting drill for most rigidly-clamped drilling operations, especially in softer metals such as aluminium and its alloys.
The small starting tip has a tendency to break, and it is economical and practical to make the drill bit double ended.
Core drill
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.
- A diamond core drill bit cuts a cylindrical core, cutting an annulus in the workpiece. The diamond core bit is cylindrical.
- A core drill bit is named because its first use was in 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.
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.
Step bit
- This section currently under construction.
A step bit, step drill, or Unibit is a roughly conical bit with a stair-step profile. They are used for drilling in sheet metal up to the thickness of one of the steps. 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.
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.
Left-hand bit
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 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 or breakage of the extractor. Because of this an alternative extractor has four parallel edges, which tends not to self tighten.
Countersink bit
see countersinkWood Drills
Lip and spur drill
The lip and spur drill bit is a variation of the twist drill which is optimised 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.
Metal has no long-distance structure, 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 and sheet metal. Conventional twist drills can, in some kinds of plastic, smear the edges of the hole, perhaps through local heating. When used on thin sheet metal the lips cut a disc from the sheet rather than tearing and deforming the edge of the hole.
Lip and spur drill bits are ordinarily available in diameters from 3 to 16 mm.
Spade bit
Spade bits are used for rough boring in wood. They tend to cause splintering when they emerge from the workpiece. 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. Having small shank diameters relative to their boring diameters, spade bits 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.Forstner bit
Forstner bits, named after their inventor, Benjamin Forstner (25 March 1834 – 27 February 1897), 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 fixed, 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. They are impractical to use other than in power tools.The bit has a centre point which locates the drill for the start of the cut (and incidentally spoils the flat bottom of the hole). The cylindrical cutter around the perimeter shears the wood fibres at the edge of the bore, and also guides 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 less clean 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 eject chips from the hole, and must be pulled out periodically to clear them.
Bits are commonly available in sizes from 8 mm to 50 mm diameter. Sawtooth bits are available up to 100 mm diameter.
Brace drill bit
The brace drill bit is optimised for drilling in wood with a hand brace. Many different designs have been produced.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 no helix 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.
Brace drill 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.
Brace drill 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 works to drill holes in 2005. It drills a hole of diameter 3/4 inch.
Auger bit
The cutting principles of the auger bit are the same as those of the brace drill bit above. The auger adds a long deep helix for effective chip removal.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-start thread for its helix. Similar auger bits are made with diameters from 6 mm to 30 mm. Augers up to 600 mm long are available, where the chip-clearing capability is especially valuable for drilling deep holes.
Gimlet bit
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 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 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.
Hinge sinker bit
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 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.
Adjustable Wood Bit
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.
Other Materials
Diamond core bit
see Diamond core drillMasonry drill
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. 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 long can be used with hand-portable power tools, and are very effective for installing wiring and plumbing in existing buildings.
Holesaw
Can be used on wood or metal see Hole sawDrill saw bit
- This section currently under construction.
Rather than flutes, the sides of the bit have a profile that act as saw teeth, allowing the bit to cut sideways as well as down, similar to a router. While these can be used in a hand drill, they are often used in a specially made tool (RotoZip being a common brand) that turns at much higher RPM.
PCB through-hole drill
Printed circuit boards are usually made of fiberglass, which due to being highly abrasive, would quickly ruin a normal drill bit, especially given the many hundreds or thousands of holes on most circuit boards. To solve this problem, solid tungsten carbide twist bits are almost always used, which drill quickly through the board while providing a moderately long life. Carbide PCB bits are estimated to outlast high speed steel bits by a factor of ten or more.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 RPM these bits are used at (30,000-100,000 or higher is common), their small size, and the brittleness of the material (even the slight wobble of an operator's hand will shatter one, as will accidental contact with most any object), they must only be used with extensive safety precautions, as a shattered drill bit can easily penetrate skin (and be an expensive mistake!). Due to their delicate nature, these bits should absolutely never be used in a hand drill, and even most moderately expensive drill presses will have too low of an RPM and too high of a chuck wobble to use these bits without breaking them.
Installer bit
Installer bits are a type of twist drill bit for use with a hand-portable power tool. They are also known as bell-hanger bits or fishing bits. 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 this transverse hole, and the bit pulled back through the drilled hole. 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 fishtape 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 removed from the chuck, and all pulled forward through the drilled hole. Sinclair Smith of Brooklyn, New York was issued for this invention on 25 January 1898.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 long in the US, with a small twist bit at the end. The shaft is made of spring steel steel instead of hardened steel, and can be flexed and bent while drilling. 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.
=Large bits=
Oil and Gas well drilling bits
Historically there were two types of drill bits used in oil or natural gas drilling rigs, a drag bit, and a rock bit:
- a drag bit is used for soft rocks, like sand and clay. The drill stem is rotated, and teeth on the bit shear the rock.
- a rock bit (also called a roller cone bit) consists of teeth on wheels which turn as the drill stem is rotated. These teeth apply a crushing pressure to the rock, breaking it up into small pieces.
The original patent for the rotary rock bit was issued to Howard Hughes Sr. for his dual cone roller bit in 1909. It consisted of two interlocking wheels. Walter Benona Sharp worked very closely with Hughes in developing the Rock Bit. The success of this bit led to the founding of the Sharp-Hughes Tool Company.
In 1933 two Hughes engineers invented the tricone bit. This bit has three wheels and is still the dominant bit in the market today. The Hughes patent for the tricone bit lasted until 1951, after which time other companies started making similar bits. However, the Hughes’s market share was still 40% of the worlds drill bit market in 2000.
In today's modern industry the two main types of drill bits are now classed as PDC (polycrystalline Diamond Compact) and Roller Cone; although the tri-cone dominates, bi-cone and mono cone bits do exist. Natural and synthetic diamonds are used in coring bits, as well as for very hard rock drilling with mud motors and turbines.
The technology of both bit types has advanced significantly to provide improved durabilaty and rate of penetration of the rock. This has been driven by the economics of the industry, and by the change from the empirical approach of Hughes in the 1930's, to todays time domain Finite Element codes for both the hydraulic and cutter placement software.
In 2005 market shares were roughly 30% each for Hughes Christensen and Smith Bits, and the remainder of the market with Reed-Hycalog, Security DBS, and smaller companies such as Varel, TSK, Walker-Mcdonald et al.
Evaluation of the dull bit grading is done by a uniform system promoted by the International Association of Drilling Contractors (IADC). See Society of Petroleum Engineers / IADC Papers SPE 23938 & 23940. See also PDC Bits
=Materials for Bit Construction=
Many different materials are used for or on drill bits, depending on the required application.
Steels
soft low carbon steel bits are used only in wood, as they do not hold an edge well, and require frequent sharpening. Working with hardwoods can cause a noticeable reduction in lifespan. They are, however, inexpensive.
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.
Exotics
The material referred to as Tungsten carbide is extremely hard, and can drill in virtually all materials while holding an edge longer than other bits. However, due to its high cost and brittleness, it is more frequently used only in smaller pieces screwed or brazed onto the tip of the bit. It is becoming common in job shops to use solid carbide drills, and in certain industries, most notably PCB drills, it has been common place for a long time.
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 thick, bonded as a sintered mass to a tungsten carbide support. Bits are fabricated using this material by brazing small segments to the tip of the tool to form the cutting edges. These bits are typically used in the automotive, aerospace, and other industries to drill abrasive aluminum alloys, carbon fiber reinforced plastics and other abrasive materials.
Coatings
Titanium nitride 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 can not properly be sharpened, as the new edge will not have the coating, and will not have any of the benefits the coating provided.
TiAlN is another coating frequently used. It is considered 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.
=See also=
=References=
Drilling and threading | Woodworking | Petroleum production
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"Drill bit"