Drill bit
From Freepedia
Drill bits are the cutters of drill tools. Bits are interchangeable, meaning that they can be removed from the end of the drill, either to replace a worn part or to change the size of the part.
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.
Twist drill
The twist drill bit is the type produced in largest quantity today. It can be used to bore in metal, plastic, wood and stone.
The twist drill bit was invented in the US by Steven A. Morse in the 1860s. 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 with new, because the bits are inexpensive and sharpening well is difficult. For larger bits, special grinding jigs are available.
Manufacturers can produce specialist 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, it is still true that even for industrial users, most holes are still driled with a conventional bit of high speed steel.
 Twist drill cutting edges |
 Twist drill |
Long series drill
Image:LongSeriesDrill-11 32.jpg
long series drills are extended length drill bits. They are not optimized for drilling deep holes as they require frequent withdrawal to clear the flutes of swarf and prevent drill breakages, however used carefully they are more than satisfactory. Gun drills are the preferred drills for deep hole drilling.
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 centre 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 drill
- This section currently under construction
Spade bits are used for rough boring in wood. They tend not to leave clean holes. They are flat, with two cutters and a point and often have spurs in an attempt to ensure a cleaner hole.
 Spade bits |
 Tiny spade bit |
Forstner drill
Forstner bits 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 bored 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 bored 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 bore, 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.
Center 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.
The small starting tip has a tendency to break, and it is economical and practical to make the drill bit double ended.
Core drill
Image:DrillCore.jpg 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.
Masonry 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.
 25 mm * 400 mm SDS-plus masonry bit |
 Masonry bit tip |
Diamond core drill
Image:Drill diamond core.jpg Diamond core drill bits are used to bore large holes in brick, concrete and stone. They are not generally used in other materials. The bit consists of a metal cylinder, usually relatively soft steel mounted on an arbor. Industrial diamonds are embedded at the open end of the cylinder. In the image, the diamonds are on the metal segments attached to the end. The segments are thicker than the cylinder wall, so most of the bit does not rub in the hole being bored. The sloping slots in the cylinder wall help carry the dust out. Diamond core drills can be used with or without water lubrication. The drill shown can cut a 115 mm diameter hole through a single-thickness brick wall in less than a minute, running at about 300 RPM. The resultant hole is very cleanly cut.
This form of core drill wanders hopelessly when presented to a flat surface, and needs a centring mechanism. The arbor can carry a drill bit to bore a centring hole, but the version shown has a plain 10 mm rod. A 10 mm masonry twist drill must first be used to drill the centring hole for the rod. A wooden or stone template, a close fit for the cylinder, can also be used to guide the bit at the start of the cut. After the first few millimeters of cut, the centring mechanism may no longer be needed, although it will help the bit to bore without wandering in a deep hole.
Diamond core drill bits for use with portable drills are commonly available in diameters from 20 mm to 130 mm. The only limit on length of the cylinder, and thus depth of the hole, is the need to remove the bit from the hole to clear dust. 300 mm cylinder length is not uncommon, although shorter bits are usual.
Holesaw
Holesaws have the same general mechanical construction as the diamond core drill bit, but, instead of the abrasive effect of diamonds, the holesaw uses the cutting effect of saw teeth. The open end of the saw's cylinder is milled with saw teeth. Instead of masonry, the holesaw is suitable for cutting wood, plastic, soft plaster or soft metal.
The set of the saw teeth makes the cut annulus slightly wider than the cylinder wall thickness, so the cylinder doesn't rub in the cut. Just as in the diamond core drill bit, the cylinder is mounted on an arbor with a centre pilot drill, and has sloping slots to clear sawdust.
Adjustable holesaw
An adjustable holesaw consists of a number of thin metal saw blade-like strips, and a flat disc with a large number of grooves in one side and a shank on the other. By snapping the blades into different grooves on the disc, a hole saw of a wide variety of sizes can be constructed.
Circle cutter
Another type of adjustable hole saw, also called a circle cutter, is formed by having one, two, or three adjustable teeth on a platform with a pilot bit. To cut out a hole of any size, the teeth need only be adjusted to the proper position. This type is available in sizes up to a foot and larger, and can be used to accurately cut large circles.
 52 mm holesaw made by Sandvik of Sweden circa 1988 |
 Holesaw dismounted from arbor |
 Adjustable holesaw |
 Disston circle cutter |
Brace drill
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. 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 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.
 3/4 inch brace drill bit |
 3/4 inch brace drill bit tip detail |
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.
 20 mm auger bit for wood |
 auger bit tip detail |
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.
 gimlet bit for wood |
 gimlet bit tip detail |
Spoon bit
- This section currently under construction.
Half round bit
- This section currently under construction.
Glass bit
- This section currently under construction.
Step bit
- This section currently under construction.
A step bit, or step drill, 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 very wide variety of hole 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 surface of the workpiece.
Left-hand bit
Left-hand bits are almost always twist bits and are predominately 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 efficent 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.
 An 1/8in left-hand drill bit |
 Three sizes of damaged screw removal bits |
 square shank, screw extractors |
Countersink bit
- This section currently under construction.
Countersink bits are used to make a shallow recess with angled sides in the material, to contain a screw head, so the head of the screw will be flush with the surface instead of protruding.
This bit style may be combined with a twist pilot bit, to allow a pilot hole for the screw and the countersink for the head to be drilled in one step, with no bit change required.
Drill 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.
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.
Expansive bit
- This section currently under construction.
Tungsten Carbide Core Bit
- This section currently under construction.
Gun drill
Gun drills are straight fluted drills which allow coolant to be directed through the drills body, directly to the cutting face. They are used for deep drilling of which gun barrels are the obvious example. The coolant provides lubrication and cooling to the cutting edges as well as ejecting the swarf or chips back out the drills length. Modern gun drills take advantage of carbide tips to reduce expense and prolong life. The images show the coolant holes in the drills shank and tip, as well as full length (12mm dia x 635mm long) gun drill and the carbide tip of a second (25mm) gun drill.
 12x640mm gun drill and 25mm gun drill tip |
 Coolant holes in tip and base of gun drill |
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.
 Two PCB drill bits. |
Installer bit
Installer bits are a type of twist bit, but with a sharper angle, intended for high-speed drilling (always with a power drill, usually cordless) in wood, drywall, and similar soft materials, and can be damaged if used to drill metal. Typically the flutes extend about 1/4th to 1/3rd the length of the bit, with a smooth shank the remainder of the length. A typical length for an installer bit is 18in, although longer ones are available.
As their name implies, they are typically used by installers to run wiring through walls, and as such they have a hole drilled between the flutes to aid in fishing a cable. Compared to spade bits (also often used by installers), installer bits drill faster, leave a cleaner hole, are less prone to tangling fiberglass insulation or carpet, and are less likely to crack redwood shingles. In addition, while hitting a nail is usually instantly fatal for a spade bit, an installer bit usually deflects to the side, or if not, usually is not damaged in the time it takes the installer to realize there's a problem and cease drilling.
A different type of installer bit has a very long (usually 4 foot) flexible shank with a small twist bit at the end, 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 of 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 you're drilling still remains.
 An 3/8in, 18in long installer bit |
 Closeup of above bit |
30 mm 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.
Milling bits
Rather than being used in a drill, these bits are used in a milling machine, and are typically capable of horizontal as well as vertical motion while in a workpiece. The most common are endmills, which, once plunged into the workpiece, can move sideways to carve out complex shapes, with different shapes of the tip affecting the shape of the cut. A face mill can produce extremely smooth planes in a work piece, while a form mill can cut specialty shapes, often similar to a router bit. For more information on these bits, see milling machine and milling cutter.
Oil drilling bits
There are 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 tear up the rock.
- a rock bit (also called a roller bit) consists of teeth on wheels which turn as the drill stem is rotated. These teeth apply a shearing pressure to the rock, breaking it up into small pieces.
The original patent for the rotary rock bit was issued to Howard Hughes Sr. in 1909. It consisted of two interlocking wheels. The success of this bit lead 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 is still 40% of the worlds drill bit market.
Materials for Bit Construction
Many different materials are used for or on drill bits, depending on the required application.
Steel
Soft 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.
Carbon steel
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
High speed steel 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
Cobalt steel alloys are very hard, and are used to drill stainless steel and other hard materials. In addition, the cobalt alloys dissipate heat very efficiently, and help keep the cutting edge cool.
Tungsten carbide
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 usually only used in small chips brazed onto the cutting edges of the bit. A few bits, however, are solid carbide, most notably PCB through-hole bits.
Titanium nitride coating
Titanium nitride is a very hard ceramic material, and when used to coat a high-speed steel bit (usually twist bits), can extended 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.
Diamond powder coating
Diamond powder is used as an abrasive, most often for cutting tile, stone, and other very hard materials. Large amounts of heat is generated, and diamond coated bits often have to be water cooled to prevent damage to the bit or the workpiece.
Stainless Steel
Stainless steel is rarely used for drill bits, as it does not hold an edge well. However, they are much less brittle than other materials, and are used where a normal drill bit might be prone to breaking, and a shorter-lived but flexible bit is considered a better compromise.
See Also
References
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