Drill Bit

By :

B. Ramesh Associate Professor of Mechanical Engineering,

St. Joseph’s College of Engineering, Chennai-119

Ph.D. Research Scholar

College of Engineering, Guindy campus,

Anna University,

Chennai-25.

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Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a chuck, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes.

The drilling process is primarily used to produce cylindrical holes. Through various tools (twist drills, combination drills, spade drills, etc.) different hole shape can be produced as cylindrical holes, drilled and counterbored, drilled and countersunk, multiple diameter holes, etc.

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From top to bottom: Spade, lip and spur (brad point), masonry bit and twist drill bits

• Drilling is most common single machining operation

• Drilling makes up 25% of machining

• Drilling occurs at the end of a tool within the material, four actions take place a the drill tip

– A small hole is formed by the web—chips are not cut here in the normal sense.

– Chips are formed by the rotating lips.

– Chips are removed from the hole by the screw action of the helical flutes.

– The drill is guided by lands or margins that rub against the walls of the hole

Types of drills :

– Twist drill: most common drill

– Step drill: produces holes of two or more different diameters

– Core drill: used to make an existing hole bigger

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23/64” Drill 0.375” Reamer

7/32” DrillCenter Drill

Countersink toolCounterbore tool

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Types of holes can be made:

– through holes, in which the drill exits the opposite side of the work

– blind holes , in which the drill does not exit

(a) (b)

Figure depicting (a) through holes and (b) blind holes

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The figure below illustrates the various operations related to drilling.

(a) Reaming(b) Tapping(c) Counterboring(d) Countersinking(e) Center drilling(f) Spot facing

Operations Related to Drilling• Reaming: Reaming is used to slightly enlarge a hole, to provide a

better tolerance on its diameter and to improve its surface finish. The tool is called a reamer and it usually has straight flutes.

Tapping: This operation is performed by an tap and is used to provide internal screw threads on an existing hole.

Counterboring: Counterboring provides a stepped hole, in which a larger diameter follows a smaller diameter partially into the hole. A counterboring hole is used to seat bolt heads into a hole so the heads do not protrude above the surface.

Operations Related to Drilling• Countersinking: This is similar to Counterboring, except that the

step in the hole is cone-shaped for flat head screws and bolts.

Spotfacing: Spotfacing is similar to milling. It is used to provide a flat machined surface on the workpart in a localized area.

Centering: Also called center drilling, this operation drills a starting hole to accurately establish its location for subsequent drilling. This tool is called a centerdrill.

Drills and Drilling Operations

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Universal bits

• These drill bits can be used in wood, metal, plastic, and most other materials.

Twist drill bits

• The twist drill bit is the type produced in largest quantity today. It comprises a cutting point at the tip of a cylindrical shaft with helical flutes; the flutes act as an Archimedean screw and lift swarf out of the hole.

• The twist drill bit was invented by Steven A. Morse of East Bridgewater, Massachusetts in 1861. The original method of manufacture was to cut two grooves in opposite sides of a round bar, then to twist the bar (giving the tool its name) to produce the helical flutes. Nowadays, the drill bit is usually made by rotating the bar while moving it past a grinding wheel to cut the flutes in the same manner as cutting helical gears.

• Twist drill bits range in diameter from 0.051 to 89 mm and can be as long as 650 mm.

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• The most common twist drill (sold in general hardware stores) has a point angle of 118 degrees, acceptable for use in wood, metal, plastic, and most other materials, although it does not perform as well as using the optimum angle for each material. In most materials it will not tend to wander or dig in.

• A more aggressive (acute) angle, such as 90 degrees, is suited for very soft plastics and other materials; it would wear rapidly in hard materials. The bit will generally be self-starting and cut very quickly.

• A shallower angle, such as 150 degrees, is suited for drilling steels and other tougher materials. This style of bit requires a starter hole, but will not bind or suffer premature wear so long as a suitable feed rate is used.

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The body of a twist drill has two spiral flutes which usually have a 30° helical angle. These flutes act as a passageway for chip extraction from the hole and for coolant to enter the hole (however, cooling is not effective since chips and coolant move in opposite directions).

The thickness of the drill between the flutes, also called the web, provides support over the length of the drill body.

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Three Main Partsof a Drill

Shank

Body

Point

• Drill can be defined as a rotary end cutting tool having one or more cutting lips, and having one or more helical or straight flutes for the passage of chips and the admission of a cutting fluid.

Nomenclature and geometry of conventional twist drill

The figure below depicts a twist drill – the most commonly used drill bit.

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Twist drill bit

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The figures below depicts a twist drill – the most commonly used drill bit.

Twist drill with internal coolant hole

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Figure : Geometry and shape of general purpose drill

Tool Geometry

– Rake angle – controls direction of chip flow

– Cutting edges – affects surface finish and tool-tip strength

– Nose radius – affects surface finish

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Excessive clearance results in lack of support behind cutting edge with quick dulling and poor tool life.

Clearance angle behind cutting lip for general purposes is 8º to 12º.

Nomenclature and Geometry of a Twist Drill

• Three main components are the:– Shank– Body– Point

Shank - This is the portion which is “clamped” to provide the drive. Straight shank for drill up to ½”. Shank is equal to body diameter. Above ½”, shank can be tapered or reduced.

Body Flutes - Helical grooves cut around the

body which form the cutting edges

Allow coolant to flow to the cutting edge

Allow chips to be withdrawn Margin- Narrow raised section of the body.

Provides full body to hole support to help keep it aligned as it drills.

Body clearance - Reduced section of the drill between the flutes and margin. Used to reduce friction between drill and workpiece

– Web- Thin section in the center of the drill which forms a “core” for the drill. This feature increases as it extends to the shank. Forms the chisel edge of the drill.

• Point - Chisel edge of the drill.

– The spiral, or rate of twist in the drill, controls the rate of chip removal in a drill. A fast spiral drill is used in high feed rate applications under low spindle speeds, where removal of a large volume of swarf is required. Low spiral drills are used in cutting applications where high cutting speeds are traditionally used, and where the material has a tendency to gall on the drill or otherwise clog the hole, such as aluminum or copper.

– The point angle is determined by the material the drill will be operating in. Harder materials require a larger point angle, and softer materials require a sharper angle. The correct point angle for the hardness of the material controls wandering, chatter, hole shape, wear rate, and other characteristics.

– The lip angle determines the amount of support provided to the cutting edge. A greater lip angle will cause the drill to cut more aggressively under the same amount of point pressure as a drill with a smaller lip angle. Both conditions can cause binding, wear, and eventual catastrophic failure of the tool. The proper amount of lip clearance is determined by the point angle.

Classifications• Classification Based on Construction

– Solid Drills: Those made of one piece of material such as high speed steel

– Tipped Solid Drills: Those having a body of one material with cutting lips made of another material brazed or otherwise bonded in place

– Composite Drills: Those having cutting portions mechanically held in place

• Classification Based on Methods of Holding or Driving

– Straight Shank Drills: Those having cylindrical shanks which may be the same or different diameter than the body of the drill; the shanks may be made with or without driving flats, tang, grooves or threads

– Taper Shank Drills: Those having conical shanks suitable for direct fitting into tapered holes in machine spindles, driving sleeves or sockets; tapered shanks generally have a driving tang

– Taper Shank Square Drills: Those having tapered shanks with four flat sides for fitting a rachet or brace

– Shell Core Drills: Core drills mountable on arbors specifically designed for the purpose; commonly used with shell reamer arbors

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– Threaded Shank Drills: Those made with threaded shanks generally used in close center multiple spindle applications or portable angle drilling tools

– Beaded Shank Bits: Drills with flat shanks having raised beads parallel to the axis

• Classification Based on Number of Flutes

– Two-Flute Drills: The conventional type of twist drill used for originating holes

– Single-Flute Drills: Those having only one flute commonly used for originating holes

– Three-Flute Drills (Core Drills): Drills commonly used for enlarging and finishing, drilled, cast, or punched holes; they will not produce original holes

– Four-Flute Drills (Core Drills): Used interchangeably with three-flute drills; they are of similar construction except for the number of flutes

• Classification Based on Hand of Cut

– Right-Hand Cut: When viewed from the cutting point the counterclockwise rotation of a drill in order to cut; the great majority of drills are made right hand

– Left-Hand Cut: When viewed from the cutting point the clockwise rotation of a drill in order to cut

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Cutting Tools for Drilling Operations

• Twist Drill: The twist drill is the most common type of drill. It has two cutting edges and two helical flutes that continue over the length of the drill body. The drill also consists of a neck and a shank that can be either straight or tapered. A tapered shank id fitted by the wedge action into the tapered socket of the spindle and has a tang that goes into a slot in the spindle socket, thus acting as a solid means for transmitting rotation. Straight shank drills are held in a drill chuck that is, in turn, fitted into the spindle socket in the same way as tapered shank drills.

Twist drill

Core Drill: A core drill consists of the chamfer, body, neck and shank. This type of drill may have three or four flutes and an equal number of margins, which ensures superior guidance, thus resulting in high machining accuracy.

Core drilling: to increase diameter of

existing holes

Spade Drill: A spade drill is used for drilling large holes of 90 mm or more. The design of this type of drill results in a marked saving in tool cost as well as in tangible reduction in tool weight that facilitates its ease of handling. Moreover, this drill is easy to grind.

Spade drill: for large, deep holes

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• Step Drill: A multiple diameter drill with one set of drill lands which are ground to different diameters

Gun Drill: A gun drill is used for drilling deep hole. All gun drills are straight-fluted, and each has a single cutting edge. A hole in the body acts as a conduit to transmit coolant under considerable pressure to the tip of the drill.

Gun drill with holes for coolant

Step drill: for stepped holes

Center Drill: Center drill bits are used in metalworking to provide a starting hole for a larger-sized drill bit or to make a conical indentation in the end of a workpiece in which to mount a lathe center. In either use, as the drill is either establishing the center of a hole or making a conical hole for a lathe center. However, the true purpose of a center drill is the latter task, while the former task is best done with a spotting drill.

Characteristics

• The spiral, or rate of twist in the drill, controls the rate of chip removal in a drill.

• A fast spiral drill is used in high feed rate applications under low spindle speeds, where removal of a large volume of swarf is required.

• Low spiral drills are used in cutting applications where high cutting speeds are traditionally used, and where the material has a tendency to gall on the drill or otherwise clog the hole, such as aluminum or copper.

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• The point angle, or the angle formed at the tip of the drill, is determined by the material the drill will be operating in. Harder materials require a larger point angle, and softer materials require a sharper angle. The correct point angle for the hardness of the material controls wandering, chatter, hole shape, wear rate, and other characteristics.

• The lip angle determines the amount of support provided to the cutting edge. A greater lip angle will cause the drill to cut more aggressively under the same amount of point pressure as a drill with a smaller lip angle. Both conditions can cause binding, wear, and eventual catastrophic failure of the tool. The proper amount of lip clearance is determined by the point angle.

• A very acute point angle has more web surface area presented to the work at any one time, requiring an aggressive lip angle, where a flat drill is extremely sensitive to small changes in lip angle due to the small surface area supporting the cutting edges.

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Tool geometry

Workpiece material

Point angle Helix angle Lip relief angle

Brass 90 to 118 0 to 20 12 to 26

Cast iron 90 to 118 24 to 32 7 to 20

Mild steel 118 to 135 24 to 32 7 to 24

Stainless steel 118 to 135 24 to 32 7 to 24

Plastics 60 to 90 0 to 20 12 to 26

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The most important properties in tool materials are:

• Toughness

• Hot Hardness

• Wear resistance

There is always a trade-off between these properties.

For example, increasing the hot hardness and wear resistance of the cutting tool generally results in a reduction in toughness.

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Drill bit shank

• The shank is the part of a drill bit grasped by the chuck of a drill. Different styles of shank/chuck combination deliver different performance, such as allowing higher torque or greater centering accuracy.

  Most drills for consumer use have straight shanks. For heavy duty drilling in industry, drills with tapered shanks are sometimes used.

• The diameter-to-length ratio of the drill bit is usually between 1:1 and 1:10. The higher the ratio, the greater the technical challenge of producing good work.

Types of shank:

• Brace shank , Straight shank, Hex shank, SDS shank

• Triangle shank, Morse taper shank, Square shank

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Brace drill bit shank

• At first, the tapered shank was just rammed into a square hole in the end of the drill. Over time, various chuck designs have been invented, and modern chucks can grasp and drive this shank effectively.

• Easy to make in a forge

• Very wide tolerances allowable (not very precise)

• Moderate torque transmission but without the slippage common to round shanks

• Hard to grasp with any precision without the proper chuck

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Straight drill bit shank

• The straight shank is the most usual style on modern drill bits, by number manufactured. It is almost always made the same diameter as the drill bit, for economy. It's then held in a 3-jaw drill chuck. Very small bits can have straight shanks larger than the drill diameter, often for holding in a standard size collet. Large drill bits can have straight shanks smaller than their drill diameter, so that medium-size chucks can be used to drill large holes. Such a drill bit is called reduced-shank or a blacksmith's drill.

• very accurate centering

• low torque transmission

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Hexagonal shank

• The flats of a hex shank can either be machined on a round shank, as in the photograph, or can be the natural flats of hex bar stock. A hex shank can be grasped by a 3-jaw drill chuck, or can be held in a custom chuck specifically for hex shanks.

• High torque transmission

• moderately accurate centring

• Cannot be held in a collet

Hex drill bit shank

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SDS shank

The SDS bit was developed by Bosch in 1975 and the name comes from the German "Steck – Dreh – Sitz" (Insert – Twist – Stay).

The SDS shank has the advantage of a simple spring-loaded chuck, so that bits can be chucked with a simple and quick hand action. Further, the shank and chuck are uniquely suited to hammer drilling in stone and concrete. The drill bit is not held solidly in the chuck, but can slide back and forth like a piston.

The hammer of the drill acts to accelerate only the drill bit itself, and not the large mass of the chuck, which makes hammer drilling with an SDS shank drill bit much more productive than with other types of shank. So, SDS shanks are most often seen on masonry drills, for which hammer drilling action is most helpful.

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• Can only be held in an SDS chuck

• Not very accurate centering

• High torque transmission

Fig : SDS-plus drill bit shank

SDS Plus (TE-C) SDS Top (TE-T) SDS Max (TE-Y) Hilti TE-S

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Triangle drill bit shank

• The triangle shank is almost always made by machining three flats on round bar stock. It is intended as a minor modification of a straight shank, still allowing it to be held in a 3-jaw drill chuck, but allowing higher torque transmission and limited slipping.

• Moderately accurate centering

• Cannot be held in a collet

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Morse taper shank

• The Morse taper twist drill bits are used in metalworking. The full range of tapers is from 0 to 7.

• The Morse taper allows the bit to be mounted directly into the spindle of a drill, lathe tailstock or (with the use of adapters) into the spindle of milling machines. It is a self locking (or self holding) taper of approximately 5/8" per foot that allows the torque to be transferred to the drill bit by the friction between the taper shank and the socket.

• Cannot be held in a chuck or collet

• High torque transmission provided the bit is driven hard into the workpiece

• Very accurate centering39

Materials:

Steels:

• Soft low carbon steel bits are used only in wood, as they do not hold an edge and require frequent sharpening.

• Bits made from high carbon steel are an improvement on low-carbon steel due to the hardening and tempering capabilities of the material. These bits can be used on wood or metal, but lose their temper, resulting in a soft cutting edge, if overheated.

• High speed steel (HSS) is a form of tool steel; HSS bits are much more resistant to heat. They can be used to drill metal, hardwood, and most other materials at greater cutting speeds than carbon steel bits, and have largely replaced carbon steels in commercial applications.

• Cobalt steel alloys are variations on high speed steel which contain more cobalt. 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.

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Others:

• Tungsten carbide and other carbides are extremely hard materials that can drill in virtually all materials while holding an edge longer than other bits. Due to their brittleness and high cost they are mainly used for drill tips, small pieces of hard material fixed or brazed onto the tip of a bit made of less hard metal. However, it is becoming common in job shops to use solid carbide drills, and in certain industries, most notably PCB drills.

• Polycrystalline diamond (PCD) is among the hardest of all tool materials and is therefore extremely wear-resistant. It consists of a layer of diamond particles, typically about 0.5 mm (0.019") thick, bonded as a sintered mass to a tungsten carbide support. Bits are fabricated using this material by either brazing small segments to the tip of the tool to form the cutting edges, or by sintering PCD into a vein in the tungsten carbide "nib". The nib can later be brazed to a carbide shaft and ground to complex geometries that cause braze failure in the smaller "segments". PCD bits are typically used in the automotive, aerospace, and other industries to drill abrasive aluminum alloys, carbon fiber reinforced plastics and other abrasive materials, and in applications where machine downtime to replace or sharpen worn drills is exceptionally costly.

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Drill Coatings:

• Black oxide is an inexpensive black coating. A black oxide coating provides heat resistance and lubricity, as well as corrosion resistance. These result in a longer drill life than the typical uncoated high-speed steel drill.

• Titanium nitride (TiN) is a very hard ceramic material, and when used to coat a high-speed steel bit (usually twist bits), can extend the cutting life by three or more times. A titanium nitride bit cannot be properly sharpened, as the new edge will not have the coating, and will not have any of the benefits the coating provided.

• Titanium aluminum nitride (TiAN) is another coating frequently used. It is considered superior to TiN and can extend tool life five or more times.

• Titanium carbon nitride (TiCN) is another coating and is also superior to TiN.

• Diamond powder is used as an abrasive, most often for cutting tile, stone, and other very hard materials. Large amounts of heat are generated, and diamond coated bits often have to be water cooled to prevent damage to the bit or the workpiece.

• Zirconium nitride has also been used as a drill bit coating for some Craftsman tools.

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TiAlN Coating:

Titanium aluminum nitride (TiAlN) has proven more effective than TiN or TiCN in protecting tools from heat, because it has a higher oxidation temperature (1450° F). TiAlN can withstand higher temperatures without breaking down and degrading the tool substrate. Furthermore, when TiAlN's oxidation threshold is exceeded, its outer surface transforms into aluminum oxide, which has excellent hot hardness, low thermal conductivity, and high chemical stability. As a result, most of the heat generated by the cutting operation flows into the chips rather than the cutting tool. Furthermore, TiAlN provides the increased lubricity typical of PVD coatings.

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• Multilayer coating technology has become so common that to find a single layer of one material on a tool is unusual. The basic theory underlying use of multiple coating layers is that each layer has its own function. One layer may have high hardness, another chemical wear resistance, and another oxidation resistance. Because machining processes result in many types of wear environments, multilayer coatings can add to a tool's multipurpose capability.

• Generally, layers in CVD coatings are thicker than those in PVD coatings--on the order of microns thick, versus as thin as a few nanometers for PVD coatings.

• Use of PVD has resulted in economical production of coatings with hundreds or even thousands of very thin layers. In PVD coatings, multiple layers can help distribute built-in internal stresses more evenly in the coating to improve fracture toughness.

• Materials used to make up the various layers of a multilayer coating depend on the application. Layers may consist of several different materials, or may be alternating layers of only a couple of materials. A coating with alternating layers of Al2O3 and TiN, for example, is said to be especially effective in high-speed machining of cast irons and steels.

• Most often, the top layer of a multilayer coating is TiN. This material not only provides the gold color familiar to so many machinists, but also allows fast identification of worn tool edges.

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– Various surface treatments such as cyaniding and nitriding are applied to high-speed-steel drills in order to increase the hardness of the outer layer of material.

– Special polishing and black oxiding are beneficial to minimize friction between the drill and the workpiece or the chips in the flutes.

Black Oxide

Titanium Nitride

• Physical vapor deposition is a viable process for applying hard coatings to cemented carbide tools.

• In PVD, the coating is deposited in a vacuum. The metal species of the coating, obtained via evaporation or sputtering, reacts with a gaseous species (nitrogen or ammonia, for example) in the chamber and is deposited onto the substrate.

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• The chief difference between PVD and CVD is the former's relatively low processing temperature--500ºC (930ºF). This lower processing temperature results in multiple benefits for PVD coatings. For example, the grain structure of the coating is very fine. The result is a very smooth, bright coating with a low coefficient of friction. And, PVD coatings are essentially free of the thermal cracks that are common in CVD coatings.

• Another advantage of the PVD process is the ability to coat tools with sharp edges and complex chipbreaker geometries.

• PVD coatings also have very high built-in compressive stresses that help them resist crack initiation and propagation. Minimizing crack formation and propagation can help prevent premature tool failure, improving tool edge security. This is especially important in, for example, untended machining operations.

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Drill wear

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?Questions

Thank You

Twist drills:

• Drills with no point angle are used in situations where a blind, flat-bottomed hole is required.

• Long series drills are extended length twist drills. They are not the best tool for routinely drilling deep holes as they require frequent withdrawal to clear the flutes of swarf and prevent drill breakages. Gun drills are the preferred drills for deep hole drilling.

Twist drill bit cutting edges Twist drill bit with Morse taper shank8 mm drills - long-series morse, plain morse, jobber

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Step drill bits

• A step drill is a drill bit that has had the tip ground down to a different diameter.

• The transition between this ground diameter and the original diameter is either straight, to form a counterbore, or angled, to form a countersink.

• The advantage to this style drill is that both diameters have the same flute characteristics, which keeps the drill from clogging when drilling in softer materials, such as aluminum.

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Hole saw

• Hole saws take the form of a small open cylinder with saw-teeth on the open edge parallel to the axis of the drill. They can be used to make large holes in wood, sheet metal and other materials.

Metal drills

Center and spotting drill bits

• Center drill bits are used in metalworking to provide a starting hole for a larger-sized drill bit or to make a conical indentation in the end of a workpiece. In either use, the name seems appropriate, as the drill is either establishing the center of a hole or making a conical hole for a lathe center. However, the true purpose of a center drill is the latter task, while the former task is best done with a spotting drill .

Fig.: Center drills, numbers 1 to 6

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Use in making holes for lathe centers

• Center drills are meant to create a conical hole for "between centers" manufacturing processes (typically lathe or cylindrical-grinder work). That is, they provide a location for a (live, dead or driven) center to locate the part about an axis.

Use in spotting hole centers

• Traditional twist drill bits may tend to wander when started on an unprepared surface. Once a bit wanders off-course it is difficult to bring it back on center. A center drill bit frequently provides a reasonable starting point as it is short and therefore has a reduced tendency to wander when drilling is started.

• While the above is a common use of center drills, it is a technically incorrect practice and should not be considered for production use. The correct tool to start a traditionally-drilled hole (a hole drilled by a high-speed steel (HSS) twist drill) is a spotting drill, or a spot drill, as they are referred to in the U.S. The included angle of the spotting drill should be the same as, or greater than, the conventional drill bit so that the drill bit will then start without undue stress on the drill's corners, which would otherwise cause premature failure of the drill and a loss of hole quality.

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• Most modern solid-carbide drills should not be used in conjunction with a spot drill or a center drill. They are specifically designed to start their own hole. Usually, spot drilling will cause premature failure of the carbide drill and a certain loss of hole quality. If it is deemed necessary to chamfer a hole with a spot or center drill when a carbide drill is used, it is best practice to do so after the hole is drilled.

• The small starting tip has a tendency to break, so it is economical and practical to make the drill bit double-ended.

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• Three-fluted core drill as used on castings

• A core drill bit can be, as pictured, a bit 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 comes from its first use, for drilling out the hole left by a foundry core, a cylinder placed in a mould for a casting that leaves an irregular hole in the product. This core drill bit is solid.

Core drill bit

• These core drill bits are similar in appearance to reamers as they have no cutting point or means of starting a hole. They have 3 or 4 flutes which enhances the finish of the hole and ensures the bit cuts evenly.

• Core drill bits differ from reamers in the amount of material they are intended to remove. A reamer is only intended to enlarge a hole a slight amount which, depending on the reamers size, may be anything from 0.1 millimeter to perhaps a millimeter. A core drill bit may be used to double the size of a hole.

• Using an ordinary two-flute twist drill 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 flash) which may occur in the product.

• A quite different core drill bit is a hollow cylinder which will cut an annular hole. A diamond core drill bit is intended to cut an annulus in the workpiece.

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Straight fluted bit

• Straight fluted drill bits do not have a helical twist like twist drills do. They are used when drilling copper or brass because they have less of a tendency to "dig in" or grab the material.

Trepan

• A trepan, sometimes called a BTA Drill (after the Boring and Trepanning Association), is a drill that cuts an annulus and leaves a center core. Trepans usually have multiple carbide inserts and rely on water to cool the cutting tips and to flush chips out of the hole. Trepans are often used to cut large diameters and deep holes. Typical drill diameters are 6" to 14" and hole depth from 12" up to 71 feet.

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Wood drill bits

Lip and spur drill bits

• The lip and spur drill bit is a variation of the twist drill which is optimized for drilling in wood and plastic. It is also called the brad point bit or dowelling bit.

• Conventional twist drill bits tend to wander when presented to a flat workpiece. For metalwork, this is countered by drilling a pilot hole with a spotting drill. 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.

10.5 mm lip and spur bit

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• In metal, the lip and spur drill is confined to drilling only the thinnest and softest sheet metals in a drill press. The drills have an extremely fast cutting tool geometry: no point angle and a large (considering the flat cutting edge) lip angle causes the edges to take a very aggressive cut with relatively little point pressure.

• Lip and spur drill bits are ordinarily available in diameters from 3 mm to 16 mm.

Wood spade bits

• 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. With their small shank diameters relative to their boring diameters, spade bit shanks often have flats forged or ground into them to prevent slipping in drill chucks. Spade bits are also sometimes referred to as "paddle bits."

59Spade bits

Tiny spade bit

• Forstner bits

• Forstner bits, also known as Forstner flange bits or webfoot augers, named after their inventor, Benjamin Forstner, bore precise, flat-bottomed holes in wood, in any orientation with respect to the wood grain. They can cut on the edge of a block of wood, and can cut overlapping holes. They require great force to push them into the material, so are normally used in drill presses or lathes rather than in portable drills.

• The bit includes a center point which guides it throughout the cut (and incidentally spoils the otherwise flat bottom of the hole). The cylindrical cutter around the perimeter shears the wood fibers at the edge of the bore, and also helps guide the bit into the material more precisely. The tool in the image has a total of two cutting edges in this cylinder. Sawtooth Forstner bits are available, which include many more cutting edges in the cylinder. These cut faster, but produce a more ragged hole.

• Forstner bits have radial cutting edges to plane off the material at the bottom of the hole. The bit in the image has two radial edges. Other designs may have more. Forstner bits have no mechanism to clear chips from the hole, and therefore must be pulled out periodically.

• Bits are commonly available in sizes from 8 mm to 50 mm diameter. Sawtooth bits are available up to 100 mm (4") diameter.

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Center bits

• The center bit is optimized for drilling in wood with a hand brace. Many different designs have been produced.

• The center of the bit is a tapered screw thread. This screws into the wood as the 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.

• The edge of the bit has a sharpened spur to cut the fibers of the wood, as in the lip and spur drill. A radial cutting edge planes the wood from the base of the hole. In this version, there is minimal or no spiral to remove chips from the hole. The drill must be periodically withdrawn to clear the chips.

• Some versions have two spurs. Some have two radial cutting edges.

• Center bits are made of relatively soft steel, and can be sharpened with a file.

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Auger bits

• The cutting principles of the auger bit are the same as those of the center bit above. The auger adds a long deep spiral flute for effective chip 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 flute. Similar auger bits are made with diameters from 6 mm to 30 mm. Augers up to 600 mm (2 feet) long are available, where the chip-clearing capability is especially valuable for drilling deep holes.

20 mm (3/4") auger bit for wood Auger bit tip detail

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Gimlet bits

• 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 fibers, 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.

Gimlet bit for wood, made sometime before 1950 . Gimlet bit tip detail

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Other materials

Masonry drill bits

• 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, and the rotating 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. Hammer drill bits often use special shank shapes such as the common SDS type.

• Masonry bits of the style shown are commonly available in diameters from 5 mm to 40 mm. Masonry bits up to 1000 mm (39") long can be used with hand-portable power tools, and are very effective for installing wiring and plumbing in existing buildings.

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25×500 mm SDS-plus masonry bit Masonry bit tip

PCB through-hole drill bits

• A great many holes of very small diameter must be drilled in printed circuit boards (PCBs) used by electronic equipment. Most PCBs are made of highly abrasive fiberglass, which quickly wears steel drills, especially given the hundreds or thousands of holes on most circuit boards. To solve this problem, solid tungsten carbide twist bits, which drill quickly through the board while providing a moderately long life, are almost always used. Carbide PCB bits are estimated to outlast high speed steel bits by a factor of ten or more. Other options that are out on the market and used in some situation are a diamond drill bit, or a diamond coated drill bit.

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Two PCB drill bits. A box of #76 (0.02in, 0.508mm) PCB drill bits.

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