Drill bits are tips placed on the end of drills that cut holes in material. This is nearly always a circular hole, though some specialised bits can cut holes in other shapes for specialised purposes. The drill bit is attached to a rotating piece on the end of the drill, which is in turn attached to a motor, usually powered by electricity. The electricity is supplied via a cable or battery. The drill has a piece called a chuck, which can be tightened onto the upper end of the bit, called the shank, to hold it firmly in place during drilling.
There are standard sizes for drill bits, in both metric and imperial units. Charts are available for these, as are tables indicating required screw tap sizes for each. Lists of non-circular cross-section drill bits are also available.
Sometimes the term ‘drill’ is used for the machine portion of the tool, and sometimes for the bit and machine together. For clarity, this article will use the term ‘drill’ for the machine itself, without the bit, and ‘bit’ for the bit itself, without the machine.
Characteristics of Drill Bits
There are several standard aspects of drill bit geometry: the spiral, the point angle, the lip angle, and the functional length.
The spiral bit
the spiral (sometimes called the rate of twist) is the steepness of the groove that spirals along the edge of the bit. If this is steep, the bit will tend to cut more quickly (per rotation). This is called an ‘elongated flute.’ It usually uses lower spindle speeds. A ‘compact flute’ is a flatter spiral, resulting in more loops of the groove along the length of the bit. This tends to cut more slowly (per rotation) and uses a higher spindle speed. This is usually used in situations where the material being drilled out tends to gall on the bit or clog the hole. Aluminium and copper are common examples.
The point angle bit
This is the angle of the tip edge. This has an effect on the way the bit will cut through certain materials. Softer materials require a sharper point angle (a duller point angle will tend to skitter along the surface, failing to bite into the material). Harder materials require a larger point angle, as a larger angle will bite into the material, and will also hold up better to the higher impact and resistance of a harder material. Sharper point angles will tend to dull when used with harder materials, or may bite in and bind the bit to a halt – possibly breaking the tip or shaft.
The lip angle bit
This is the angle of the wedge of the tip after the cutting edge. A greater lip angle will bite deeper (and therefore more quickly) into the material through a rotation, causing a more aggressive cut for a given amount of pressure. A lesser lip angle will create a less aggressive cut for the given amount of pressure. The point angle will determine the proper amount of lip angle a bit requires. An acute point angle requires more aggressive lip angle to ensure adequate web surface area. A flat bit has a smaller surface area and will therefore be more sensitive to changes in lip angle.
The functional length bit
This is the length of the bit beyond the chuck, or the length of the bit that can be extended into the hole being cut. The longer the functional length, the deeper the hole that can be drilled. Longer functional length is achieved by the drill bit having a longer shaft. This advantage has its drawbacks or limitations, however, as a longer shaft will be more flexible and may cause warping of the hole trajectory, and has a higher risk of breakage. A shorter functional length results in a more rigid shaft (in relation to the girth) and the axis of cut will therefore be truer and there will be less chance of breakage under a certain amount of torque resistance. Bits are available in a variety of functional lengths, including standard lengths (called ‘stub-length’ or ‘screw-machine-length), which are relatively short; ‘jobber lengths’, which are considered medium-length; and ‘taper-length’ or ‘long-series’ which are, as the name indicates, the longest of the three categories.
Most drill bit shanks are straight – they are the same diameter at the top of the shaft as they are along the length and at the bottom. Some shanks, used mainly in heavy-duty industrial situations, have tapered shanks to cut down on overall weight and cost. There are also specialty shanks, such as those with hex-shaped shafts, and various proprietary quick-release systems for situations in which several bits are used in rapid succession. These systems reduce work time by bypassing the need to loosen and tighten the chuck during bit transitions.
Most drill bits have a diameter-to-length ration of 1:1 to 1:10. There are situations that use different ratios, such as ‘aircraft-length’ twist bits and pressured-oil gun drill bits, but higher ratios mean an increase in the technical skill required to produce high-quality results. The main factors that determine a proper ratio include the material being drilled and the nature of the drill bit geometry and drill speed desired.
What are drill bits made from?
Drill bits tend to be made from steel, cobalt and carbide. Each material has its unique set of advantages and disadvantages. Carbides, for example, are very hard materials suitable for making drill bits, but the hardness of the material also means and increase in brittleness, which may mean catastrophic failure (breakage) under certain conditions in which a softer bit may hold up. Steel is often hard enough to do the job, and flexible enough to hold up against breakage where carbides would fail. This is especially important in situations in which the drill is hand-held, and therefore subject to small changes in angle during drilling. A frame-stabilised drill will not vary so much and will therefore have less variance on the angle (and subsequent negative impact) on the bit. This makes steel the best material for a drill bit for use in handheld drills.
Softer, low-carbon steel bits are common, as they are inexpensive, but they do not hold up well to wear and need to be sharpened or replaced more frequently, decreasing the value for the money. These are used mostly for drilling wood, though even the harder woods will wear them out significantly more quickly than softer ones.
High-carbon steel bits are more durable at the onset, but if they are overheated during use (which can be caused by friction when drilling harder materials) the temper of the edge can be lost, resulting in a softer tip and subsequent loss of performance. These are normally used to drill wood and some metals.
High-speed steel (HSS) bits are hard, like high-carbon, but are also more resistant to a loss of tempering due to heat. They are commonly used to drill hardwoods, some metals and other materials, and can be run at higher speeds without fear of frictional heating and loss of edge tempering. These have mostly taken over the market share once held by carbon steel bits.
Cobalt steel alloy bits are a type of high-speed bit that contains more cobalt than normal. These can be used to drill materials such as stainless steel and other hard materials, without losing temper due to frictional heat. They are, however, more brittle than standard HSS drill bits, and so have an increased chance of breakage, especially if used in a hand-held situation.
Tungsten Carbide (and other carbide) bits are able to drill almost any material, as they are extremely hard and capable of holding an edge for longer than other metals, against even very hard materials. Tungsten carbide is, however, quite expensive, so it has often been added to the cutting edge of bits made from softer metals. In recent times, however, solid tungsten carbide bits have become more common, especially in the case of very thin bits, like those used in drilling printed circuit boards which requires the drilling of many small holes with diameters of less than 1mm each.
Even harder than Tungsten carbide is polycrystalline diamond (PCD). This is a very hardy materials and resists wear well. It is made up of a tungsten carbide support, with a sintered mass of diamond particles bonded to it. This layer is typically 0.5mm (0.020in) thick. The PCD nib can be ground to whatever geometry is best suited to the purpose. These bits are used in many industries, including the demanding automotive and aerospace industries, to drill abrasive aluminium alloys, carbon-fibre-reinforced plastics, and other abrasive materials. The hardness of this drill bit material is also useful in reducing machine downtime, and thus avoiding costly delays or suspension of factory running times. PCD is not, however, suitable for use with ferrous metals, as a reaction occurs between the carbon in the PCD and the iron in the metal.
Coatings of Drill Bits
- Titanium nitride (TiN) is a hard ceramic material that extends the cutting life of high-speed drill bits by 300-400%, even with sharpening.
- Titanium aluminum nitride (TiAlN) is a can also extend useable tool life by 500% or more.
- Black oxide is a black coating, as the name indicated, and is an inexpensive option which provides both heat resistance and lubricity, and can resist corrosion. The usable life of high-speed steel bits is increased if coated in black oxide.
- Titanium carbon nitride (TiCN) is also a coating for drill bits, and is considered better than TiN.
- Diamond powder is often used for cutting stone, tiles, and other hard, brittle materials. It fuctions as an abrasive and so it can get very hot if not coupled with a watering system to keep the bit and surrounding material cool.
- Zirconium nitride is a drill-bit coating from the Craftsman brand.
- Al-Chrome Silicon Nitride (AlCrSi/Ti)N is made of an alternating nano-layer of super-hard ceramic. It performs very well against other coated and uncoated bits, and is used mainly for drilling carbon fibre reinforced polymer (CFRP) and CFRP-Ti stack.
- BAM coating stands for Boron-Aluminum-Magnesium (BAlMgB14) coating, and is a ceramic coating used commonly in composite drilling.
Universal Drill bits
Universal bits are used for a variety of materials, such as wood, metal, plastic, and other materials, usually in situations where high-performance results are not necessary.
Twist drill bits
The most common type of drill bit produced today is the twist drill bit. It has a cutting point at the tip, is cylindrical in shaft shape, and has helical flutes. The flutes function to drive the bit into the substance (Archimedean screw) and to expel the swarf from the new hole that creates.
This type of drill bit was invented by Steven A. Morse, in Massachusetts USA, in 1861. Initially, two grooves were cut into a bar that was then twisted, causing the grooves to run around the shaft in a helical-type fashion. Ore modern production techniques cut the grooves into the bar while turning it, bypassing the need to twist the bar once cut.
This type of bit ranges in diameter from 0.051mm to 88.900mm (0.002in to 3.5in). They range in length from very short, stubby shafts to some as long as 650mm (25.5in).
The performance of the bit is determined in large part by the geometry of the cutting edges. Sharpening them correctly is very important, and so smaller bits are often discarded when dull, as properly shaping their edges is extremely difficult and not cost effective. Larger bits, however, are more expensive to replace and are better suited to special grinding jigs and specialised tool grinders that enable proper sharpening and shaping.
Special version of drill bits are available for specialised purposes, machinery, jobs and materials, but most jobs can be accomplished using standard high-speed bits.
The most common point angle is 118 degrees, which works well for wood, metal, plastic and many other materials. Each material will have an optimal angle, however.
When drilling soft plastics, a more aggressive angle may be suitable, such as 90 degrees. Steels and similar materials require a shallower angle (150 degrees, for example) for best results and will likely need a starter hole for accuracy and control.
If a blind, flat-bottomed hole is needed, you can use a drill bit with no point angle at all.
Long drill bits can be necessary for very deep holes, but should be avoided for drilling unless needed, as they require frequent withdrawal to clear out the flutes, and they tend to wander and even break. For deep hole drilling, gun drill bits are more effective.
Step drill bits
A step drill bit has the tip ground down toa smaller diameter. This transition can be abrupt and straight, to form a counterbore, or it can be angled to form a countersink – either way, you will benefit from the fact that both diameters will have the same flute characteristics. This reduces clogging and performs better than options that employ slip-on collars or multiple bits.
A unibit, or step drill bit, has a stairstep profile almost like a cone. This type of bit can be used to drill a range of hole sizes. Smaller ones can be self-starting, but larger ones will require a starter hole and are best used for enlarging existing ones.
These bits are often used on sheet metal and general jobs around a construction site. The versatility of it speeds up progress by eliminating the constant changing of bits during work. They are ideal for drilling thin steel, aluminium and plastic, and so are common in the electrical trades.
Other bits sometimes leave burrs, and unibits can be used to remove these, especially at the entrance side of the hole.
The unibit was invented by Harry C. Oakes in 1973 (and is a trademark of Irwin Industrial Tools) but the stepped drill was first produced by Bradley Engineering in Wandsworth, London in the 1960s, when it was called the Bradrad. The patent was sold to Halls Ltd. (UK) and is still produced by that company.
Hole saws are hollow, open-cylinder bits that make larger holes in thin materials. They drill out a centre hole and a round saw-cut, leaving a waste disc with a hole in its centre when finished.
Metal drill bits
Centre drill bits, sometimes called Slocombe drill bits, are used to create starting holes for larger bits when working with metal, or to create indentations for lathe centring. The conical shape of the resulting hole pulls the second bit (or lathe centre) into the centre of the hole, allowing for very precise hole placement.
Technically, a centre drill bit and a spotting drill bit are different, but in common usage, the names are often interchanged, even by industry experts and suppliers. Suppliers often refer to centre drill bits as ‘combined-drill-and-countersinks’ to make it clear which tool they mean. They come in a range of sizes of course, which are numbered from 00 (the smallest) to 10 (the largest).
A spotting drill bit (discussed in more detail below) is really the proper tool to use for centring holes.
Holes for lathe centres
When a manufacturing process requires holes to be drilled at certain intervals, with measurements given between centres, centre drill bits are a suitable tool for the job. This usually includes lathe or cylindrical-grinder processes. The use of these holes allows a component to be used on one machine, such as a lathe, and then removed from it to be placed on a different machine, such as a grinder, without losing the precise positioning requires for accurate work. The loss of co-axiality (loss of precision) when moving between machines is usually less than 0.05mm (or 0.002 in), and a TIR of less than 0.003mm (or 0.0001 in) is maintained – if other conditions are not interfering with accuracy of course.
Use in spotting hole centres
Twist drill bits have a tendency to wander when they are started on a flat or unprepared surface. Correcting a loss of trueness is difficult, if not impossible, once the proper location has been lost. The purpose of a centre drill bit is to give the twist bit a seat in which to drill, preventing wandering and accidental slippage from the desired position. Once started in the proper place, the twist bit is less likely to wander, and will wander less when it does happen.
In the case of serious production, however, centre drill bits aren’t the tool of choice for preparing accurate starting points for drilling. The correct tool for traditionally drilled holes is the spotting drill bit (best used in preparation for drilling with a high speed steel (HSS) twist drill bit). In the US, these are usually called spot drill bits.
The included angle of the spotting drill bit needs to match or be greater than the twist bit’s included angle. If it isn’t, then the stress on the twist bit’s corners is unduly high and will cause a loss of accuracy and hole quality, as well as a good deal of premature wear on the twist bit itself.
Most modern solid carbide drill bits do not need a centre drill bit – in fact should not be used with one – as they are specifically designed to function without one; they start their own holes. Using a spotting bit with solid carbide drill bits will cause undue wear and premature failure of the carbide bit, and may cause a decrease in hole quality as well. If a process calls for chamfering a hole with a spot or centre drill bit, it is best to do so after the hole has been drilled, if using a solid carbide bit.
Since hand-held tools increase the risk of inaccuracy in drilling, a centre punch can be used to spot the centre position and then a pilot hole can be drilled.
Core drill bit - what is it?
Two very different tools are commonly called by this term, so let’s start with some clarification.
A core drill bit can refer to a bit used to enlarge an existing hole. Such a hole may be previously drilled, or it may a punched hole or a core created during casting. It is called a core drill bit because its original purpose was drilling out the hole left by a foundry core. Foundry core holes tend to be irregular, and therefore need to be bored out. A core drill bit, in this case, is solid and is similar in look to reamers, with no cutting point and no means of starting their own holes. Core drill bits also have three to four flutes to improve the finish of the hole and maintain an evenness of cut, but they are meant to remove a different amount of material than reamers. Where a reamer is meant to increase an existing large hole by a small amount (0.1mm to 1mm or so), a core drill bit may increase the size of a hole up to double the original size.
An ordinary two-flute twist drill may be able to enlarge a hole made by a casting core, but the result will not be as clean or accurate, may be out of round, off centre, and have a poor finish. Any flash or other protuberance may also result in the two-flute drill bit catching and creating a very noticeable and functionally negative flaw.
An annular cutter is also often called a core drill bit. These are hollow bits and so they leave a hollow centre core intact after drilling is complete. The purpose of these drill bits is often more about the core than the hole it leaves behind. Common uses include core sampling for geological and other scientific work, coring ice or sediment layers for strata analysis, etc.
Gun drill bit
Straight-fluted drills which allow compressed air or other suitable cutting fluids to be injected through the hollow body of the bit, are called gun drills, or gun drill bits.
Indexable drill bit
These are used mainly in high precision production, such as CNC, as they are expensive in comparison with less accurate options. They use replaceable carbide or ceramic inserts as their cutting edges, so they don’t need to be ground or sharpened. The outside radius of the cut is achieved by one insert and another takes care of the inner radius. The bit is far more hard-wearing than the average bit, with a non-consumable shank and multiple coolant channels to lengthen usable life. Varied configurations are also common, including straight flute, fast spiral, multi-flute and other cutting face geometries. These bits cut very quickly, bear high axial loads, and so are typically used for holes no deeper than about 5 times the bit diameter.
A countersink bit creates a conical hole, most often used in a manufactured object to hide the head of a screw or bolt, making it flush with the flatter surface of the item being fastened. A variation on this tool is the counterbore, which created a flat-bottomed hole for making hex screwheads, like capscrews, flush with the surrounding surface. Countersink screws can also remove burring left behind during previous drilling or tapping.
Ejector drill bit
Ejector drill bits are used for deep hole drilling of holes from 19mm to 102mm (3/4 to 4 in) in diameter. These bits are constructed as a tube within a tube, allowing water to be flushed down between them, washing chips through the centre of the bit. They also have a special carbide cutter point.
Metal spade bit
A spade drill bit includes a tool holder and am insert – a tip that can be inserted into the tool holder. The inserts come in a range of sizes from 11mm to 64mm (7/16 in to 2.5 in). There is normally a passage running through the tool holder for injecting coolant into the hole. These bits are able to cut to a depth equal to roughly ten times the diameter of the bit and can be used to make stepped holes.
Before the widespread use of CC machines, the repetition engineering industry used left-handed bits for situations in which the spindle could not be reversed or then the specialised machine design made left-handed drilling more efficient than right-handed drilling. This is less common now, though left-handed bits are still sometimes used in these situations. Left-hand bits are almost always twist bits.
Screw extractors are specialised left-hand bits that are used to remove right-hand screws which have damage to the head, or loss of the head entirely, making it impossible to gain a purchase with a screwdriver head. The extractor is pressed into what remains of the head, and rotated in an anti-clockwise direction. The rotation causes the tip to bite in and jam with the material of the screw. The rotational force required to actually drill through the screw is greater than the force with which the screw holds the material around it, and so rather than drill out the core of the screw, the drill bit instead unscrews the screw, extracting it from the surrounding material.
Straight fluted bit
Most bits have helical twist flutes – twist bits are the most common example of this – but straight fluted drill bits have flutes that run straight, from the top of the bit to the bottom, without twisting around the core of the shaft. These have less tendency to dig into the material and bind or grab onto it while drilling, potentially causing damage or burring within the hole. They are commonly used for drilling metals such as brass and copper, for this reason.
A trepan drill bit cuts an annulus and leaves a centre core behind. Trepan bits are sometimes called BTA drill bits (Boring and Trepanning Association bits). The bits most often use multiple carbide inserts. They require water to cool the tips during drilling and to flush out chips from the cutting edge. Trepans are suitable for cutting large diameter holes and/or holes that are very deep. Though bit diameters range from 150mm to 360mm (6 in to 14 in), which seems quite large, they can drill holes from 300mm to 22m (1 ft to 71 ft), almost seventy times the diameter of the bit itself.
Wood drill bits
Lip and spur drill bits
A common variation on the twist drill bit is the lip and spur drill bit, also called the brad point or a dowelling bit. These are made specifically for drilling wood. Rather than requiring a pilot hole, as is commonly done in wood and metal drilling, lip and spur drill bits have a variation on the common straight-chisel cutting edge. The edge instead had a spur on it with a sharp point and four sharp corners that cut into the wood. These sharp points easily bite into the soft wood and guide the rest of the cutting edge and shaft in a straighter path through the material.
Because drilling across the wood grain can come into opposition to long strands of wood fibre (as opposed to drilling into metals which are typically isotropic, allowing for a more even, clean shearing of the material during drilling) the wood tends to pull out of the hole in one piece rather than stay in place and allow only the cut-way portion to be removed. This makes for an unclean, rougher edge within the hole, especially if it happens in the inner portions of the hole first, pulling the bit out of straight alignment as it drills. By cutting the periphery of the hole before the core, the lip and spur drill bit decreases the chances of fibres pulling out from the sides, causing instead a cleaner cut around the edges while the fibres are still held fast by the core, then cutting away the core materials that has already been separated from the portion of the fibres that should be left in place around the core.
Lip and spur drill bits are also commonly used in drilling plastic, as they tend to leave a cleaner hole than conventional twist drill bits. Vibration and side friction cause conventional twist drill bits, especially when used with a hand drill, to wander from the hole axis, which smears the edges. This is less likely with lip and spur drill bits.
Lip and spur drill bits are used to drill metal, but only in certain specialised situations, such as when drilling through the very softest and thinnest sheet metals, and even then, they are used with a drill press, rather than being hand-held. The aggressive cut caused by the lip angle tends to cause these bits to bind in metal or to punch through, rather than drill cleanly through, which leaves the bit’s cross-sectional geometry behind.
These bits commonly come in the relatively modest diameters of 3mm to 16mm (0.12 in to 0.63 in).
Spoon bits look a little like the bowl of a spoon, hence the name. They have a grooved shank with a bowl-like, spoon-shaped cutting edge on the end. The bit ends in a slight point, like a gouge bit, to prevent the bit from wandering off center during drilling and to make beginning the hole easier and more accurate.
Common uses for these drill bits include boring or reaming holes for the seats and arms of chairs., because the angle of the drill hole can be changed or adjusted by eye, even as the drilling is being done. They also lack a lead screw, so there is less danger of the hole delving too deep and poking out the other side of the surface, marring a surface that is supposed to be clean and intact.
This drill bit design in not a new innovation, as it has been used as far back as Roman times, and variations of them have been found in Viking excavations. What has changed, though, is the material from which they are made – and of course the precision of the shaping. Modern spoon bits are made of hand-forged carbon steel, then hand ground to a fine edge.
Just as in ancient times, these drill bits are not used with power tools of any kind.
When using a spoon bit to ream a pre-bored, straight-sided hole the spoon bit needs to be inserted and then turned in a clockwise direction. Use of a carpenter’s brace ensures that the desired taper is produced. Solid wood requires that the bit be first started in a vertical position to create a ‘dish,’ then angled within the dish, which allows the bit to bite into the wood at the desired angle and in the desired place. Using this method, holes can be made cleanly, quickly and precisely, despite being done by hand, in any kind of wood. Direction and angle can be changed at any time during the drilling process.
A modification on this process is the use of parallel spoon bits. These are able to bore holes into the seat of a piece of furniture (for example, a Windsor chair) to take the back spindles in a uniform manner. Round-tenon, green woodworking is also done using parallel bits.
Honing the spoon bit is accomplished using a slipstone on the inside of the cutting edge. It is important never to touch the outside edge of the bit.
Wood spade bits
Spade bits (sometimes also called paddle bits) are flat, with a centring point in the middle of the blade and two cutting edges. They are used to bore through wood. The holes they tend to leave are rough and they can cause splintering around the edges, especially of the exit hole – this is most often avoided or minimised by finishing the hole from the opposite side of the work, stopping as soon as the centring point emerges from the surface, then using it cut from the other side. The outsides of the cutting edges sometimes have small cutting spurs on them to assist in minimising the tendency to splinter the edges of the hole.
Spade bits have small shank diameters relative to boring diameters. To avoid slipping in drill chunks, the spade bit shanks use flats forged or ground into them, and a small hole drilled through the flat face. This makes them act more like bell-hanger bits. They are used with electric hand drills and are intended to be used at high speeds.
Common diameters available for these bits range from 6mm to 36mm (1/4 in to 1 ½ in).
Invented by Benjamin Forstner, these bits can bore precise, flat-bottomed holes in wood, regardless of the orientation of the wood grain, and are even able to cut into the edge of a block of wood. Overlapping of holes is also possible, usually using drill presses or lathes, rather than being held by hand. Using a Forstner bit to drill through veneer that has already been glued to its inlay produces a clean, flat-bottomed hole through the veneer with little or no damage necessary to the underlying material. Forstner bits are guided along their alignment using their centre point, however, so the flat bottom is not in fact perfect. There will be a centring hole.
Around this centring hole, however, the rest of the cut is clean, with the sharp edge of the bit sheering through fibres at the edges, resulting in a clean and precise end product. Some designs rely on one radial edge, others use two or more, but all are designed to create a smooth result.
As Forstner bits have no mechanism to clear out the spoil from the new hole, the operator must pause to remove chips and strips when necessary.
Sawtooth bits do a similar job as Forstner bits, but as they use many small cutting edges, rather than a few larger, smoother ones, they can result in a rougher surface on the inside of the hole. They do cut better into end grain, however, which can be an advantage over Forstner bits.
Sawtooth bits are available up to 100mm (4 in) in diameter, which is larger than the usual 8-50mm (0.3-2.0 in) diameter common for bit sizes.
Gunsmiths favoured the Forstner drill bits for boring exceptionally-smooth-sided holes, and craftspeople who still use more historical methods still use them.
There are a lot of similarities between auger bots and centre bits, with a main difference being that auger bits have the addition of a long, deep spiral flute to remove the chips while drilling.
The Jennings (or Jennings-pattern) bit was developed by Russel Jennings in the 19th century. It uses a self-feeding screw tip and has two radial cutting edges, each with a spur. There is a double flute, each channel starting from a cutting edge and terminating high enough up the bit to remove the waste chips and strips from the hole.
A similar auger bit is the Irwin, or solid-centre auger bit. The Irwin differs from the Jennings in that it has a vestigial flute, for added support, instead of the two flutes of the Jennings. The vestigial flute extends only 13mm (1/2 in) up the shank. Its second flute runs the full length of the shaft and removes the waste chips and strips. This design adds strength and increases the depth of waste removal.
Both styles are still popular choices today.
The diameter of auger bits is indicated by a single number, which indicates the size in 16th of an inch. #6 is 6/16 of an inch, or 9mm, #16 is 16/16 (1 inch), or 25mm, etc. Sets usually come in size ranged from 4-16 or 4-10.
Some modern auger drill bits have been designed for use in portable power tools. These can have only one spur, a single flute, and a radial cutting edge – and of course they are designed to clear the chips from the cutting edge and hole while drilling. They commonly come in diameters from 6mm to 30mm (3/16 in to 1 3/16 inch). Augers can come in shaft sizes of up to 600mm (2 ft) for drilling deeper holes.
Centre bits are optimised for drilling wood in combination with a hand brace. Centre bits come in a wide range of variations and styles to suit a broad array of purposes and preferences. In general though, these bits are tapered into a screw head. The head bites into the wood as the shaft is turned, and this pulls the bit deeper into the material. The rotational force is all that is needed for driving the shaft into the wood – no downward pressure is necessary once the bit has begun to bite and cut through the surface. Each rotation of the bit cuts away a slice of wood equal to the pitch of the thread. This self-driving effect does mean, however, that the female thread in the workpiece must be stripped, or the drill rotation reversed, in order to remove the bit from the hole.
One drawback of centre bits is that they do no function well in the end grain of wood. The wood tends to split along the grain and the radial edges struggle to cut the long wood fibres. Beware if using them in this situation.
Centre bits can be sharpened with a file.
The gimlet bit has been around for a long time, though they saw several new design developments in the 1850s. The gimlet is a self-contained tool for boring small holes in wood using only hand power. Common sizes range from very small to a maximum of about 7mm (0.28 in) in diameter.
The tip of the bit bites into the wood like a screw, forcing the wood fibres to the side, but not necessarily cutting them. The broadest part of the cutter does the actual cutting of the fibres, and it does so at the side of the hole rather than at the base of the hole, as is the case with most other bits.
Hinge sinker bits
A hinge sinker bit is a custom bit designed specifically for making holes in particle board, which in turn act as mooring bases for the screws of butt hinges. This is most commonly used in kitchen cabinetry and other cabinetry that uses particle board and MDF fibreboard construction with laminated melamine resin veneers. Since typical drilling and fastening would cause damage and disintegration to this building material, the sinker bits, used to install the proper specialist hardware, is the best way to go when working with these products. A centre spur keeps the bits from wandering during drilling.
Note that these holes could be bored by a Forstner bit too, but the cutting edges of the bit would be quickly dullened by the abrasive nature of the MDF boards’ interiors. A tungsten carbide Forstner drill bit might be a solution to this, but as carbide is not ideal for the complex shape of the Forstner bits, they are not a common choice.
Adjustable wood bits
Adjustable wood bits, also sometimes called expansive wood bits, have a small centre pilot bit with a cutting edge mounted above it. The cutter can be adjusted up or down, and usually consists of a cutting edge with a single, sharp point at the outer extremity of the edge. A set screw locks this cutter into position.
The cutting edge can be centred on the bit, resulting in a smaller hole, or it can be slid downwards, which results in a wider hole. This means that this one tool can do the job of many other set-sized bits, including rare or even unique diameter sizes. Precision in sizing is attained using a ruler or vernier scale, which is usually provided with the bit itself at point of purchase.
Adjustable wood bits can be made as auger bits or brace bits, for high or low torque drilling, for hand tools or power tools, and the method of adjusting each of them is functionally the same.
Diamond core bits
Diamond masonry mortar bits are designed to function both as a router and as a drill bit – for this reason they are often called hybrid drill bits. The steel shell of the bit had diamonds embedded in it, set in metal segments at the cutting edge. Drill speeds tend to be slow, but these bits are very tough.
Glass drill bits
Glass bits are not made of glass of course – they’re made for drilling glass, which can be a very tricky business. Glass bits use a carbide point, shaped like a spade, and are usually used at low speeds. Small bits are used first, then successively larger ones until the hole is the desired diameter. Wear is quick for these bits, and their usable life is not long.
Diamond drill bits can also be used to cut holes in glass, and they last a lot longer.
Masonry drill bits
Masonry bits are made of relatively soft steel, milled into shape rather than ground into shape, and fitted with a tungsten carbide insert brazed into the steel. This insert provides the tougher, longer-lasting cutting edge.
Due to the hardness of the material commonly being drilled, and the difficulty in achieving a cutting action – as opposed to more of a grinding action – masonry bits are usually used with hammer drills. The bit is hammered against the brittle surface, chips are broken free, and the flute carries them up and away from the drilling point. The rotation brings the point of impact for the hammering motion onto a different place each time, resulting in a more uniform pattern of material removal. Some shank shapes allow the bit to slide within the chuck, rather than the whole chuck having to move and execute the hammering, which means a decrease in machine wear and more efficient use of power. These shank shapes are of the SDS type.
Common sizes available for hammer drill bits include a range of from 3mm to 40mm. Core bits are used for diameters larger than this. Masonry bits used with portable hand power tools can be up to about 1,000mm (39 in) long. These are commonly used in electrical and plumbing trades.
A star drill bit looks a lot like a hole punch or chisel, but is used along with a hammer drill to make holes in masonry and/or stone. The bit’s point consists of several blades, joined at the centre – hence the star-shape and the name.
PCB through-hole drill bits
When drilling holes for printed circuit boards (PCBs), many small holes (less than 1mm each) must be drilled in precise locations. PCBs are usually made of fibre glass, which is highly abrasive and can cause rapid wear on drill bits. Since hundreds of thousands of holes must be drilled, steel drill bits are usually not up to the job – at least not for very long. The solution is solid tungsten carbide drill bits. These can last more than ten times as long as steel bits. Sometimes other options are used as well, such as diamond or diamond-coated drill bits.
Drilling in these circumstances is not normally done by hand, but my specialised machines. Since bits do not last a long time under such intensive use (even the tungsten carbide ones wear out relatively quickly), the machines can often replace the bits automatically as they wear, as part of the programmed manufacturing process. Such machines use a collet, rather than a chuck, to mount the bits, and they are set at a pre-determined depth to ensure accuracy without needing adjustment each time new bits are loaded into the machine.
These machines use very high rotational speeds, from 30 thousand to 100 thousand rpm or even more. This means that, even for these tiny diameter bits, the cutter tip speed is relatively high. Add to this the brittle nature of the bits and their tiny shaft diameter, and they are far more vulnerable to breaking if used by hand or in a machine that is not properly calibrated or constructed.
Dull PCB drill bits are, as you might imagine, therefore quite common, bits that have been resharpened are easily available and commonly used I prototyping and home PCB labs. The rake angle and expected feed per revolution are optimised for high-speed automated use on fibreglass PCB substrate, so these bits may not be suitable for use on other materials.
Flexible shaft bit
Another variation on the installer bit (in fact it is commonly called by the same name) is the flexible shaft bit. This has, as the name suggests, a long (1.8m or 72 in), flexible shaft made of spring steel. It can therefore flex while drilling, and not break – like hardened steel would. They can therefore drill through studs from a light box without having to remove material from the wall first. Special tools to aim the bit accurately usually come with the bit. This bit is popular in the USA, and is not usually available in Europe.
Fishing, installer, or bell-hanger bits are twist drill bits commonly used in hand power tools. These bits are most easily identified by the transverse hole drilled through the eb of the bit near the tip. This is there so that the bit can double as a wire feeding tool once a hole is drilled. Simply drill the hole in the material, feed the wire through the hole in the bit, and pull it back through. If something bigger than the hole in the bit needs to be fed through, a wire can be attached to it and once the wire is pulled through, the wire can pull the cable.
This is not only quick and easy, but it can help feed a wire or cable through a space where fish tape is awkward to use.
A variation on this tool is a hole in the shank end of the bit. In this case, the hole is drilled, then the bit is released from the drill, a wire fed through the hole in the bit, and the bit is pulled through the far end of the hole.
These bits can be specialised for drilling through wood, cement, block or brick.
Drill bit shank
Drill bits have different shapes of shanks. Shanks can be shapes to suit certain chucks, or can operate in cooperation with chucks for particular performance needs, such as increased torque, hammering, or accuracy of positioning.
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