Figure 1 Standard jobbers length twist drill
STRAIGHT SHANK DRILLS, JOBBERS LENGTH
| There is a wide variety of drilling conditions in industry today due to the use of so many different materials and types of equipment. With this in mind, the general purpose drill has been designed to perform satisfactorily under as many different conditions as possible (Figure 1). | Figure 1 Standard jobbers length twist drill |
This drill may be considered a high production tool for all jobs except those where some element of material or set-up presents a particularly difficult problem. Even under such conditions, this type of twist drill can be used with some degree of efficiency.
HIGH-HELIX DRILLS
| Helix is a word that means the twist of a drill. The twist or helix of the drill dictates how the chip evacuates the hole you are drilling. High-helix drills have been developed especially for drilling deep holes in materials of low tensile strength, such as aluminum, magnesium, copper, die casting materials, wood, some plastic materials, and stacked aluminum sheets. A high helix drill is also known as a fast helix drill. Its wide flutes and high-helix help to quickly clear the chips (Figure 2). High-helix drills have been used successfully in drilling softer steels, free-machining brasses and bronzes, and free-machining stainless steels. |  Figure 2 High-helix drill |
LOW-HELIX DRILLS FOR BRASS
| Low-helix drills were manufactured primarily for use in brass, although they can be used when drilling shallow holes in some of the aluminum and magnesium alloys. The design of these drills allows for heavy feed rates and will readily remove the large volume of chips associated with production drilling operations (Figure 3). |  Figure 3 Low-helix drill |
Drill Finishes
| There are two common finishes found on twist drills in industry. (Figure 4). Black oxide is the most common. It is for use on hard materials, such as metals. These drills are dull black in appearance. |  Figure 4 Black oxide drill finish |
 Figure 5 Polished flute drill finish |
Another common finish is a polished finish. These drills are shiny silver in appearance (Figure 5). The polished finish allows for quick chip evacuation and is best for soft materials such as plastics and woods. |
| Coated twist drills are high speed steel or cobalt twist drills that have been coated with a very thin coating of aluminum oxide or titanium nitride (Figure 6). The thin coating of these very hard, abrasion-resistant materials allows the drills to operate at much faster cutting speeds than non-coated drills. Coated drills will hold up much better in abrasive materials, such as plastics, hard rubber, asbestos and some varieties of fiberboard, than an un-coated drill will. |  Figure 6 Titanium nitride coated drill |
DRILLS FOR PLASTICS top
| Drills designed for use in most rigid plastics, hard rubber, asbestos and some varieties of fiberboard have wide flutes and a slow helix (Figure 7). This provides maximum space for chip ejection while their sharper drill points (angle of 90° or less) provide the corner wear resistance required in these often abrasive materials. |  Figure 7 Drills designed for plastic |
LEFT HAND DRILLS, JOBBERS LENGTH
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Left hand drills are produced with a general purpose design. They have the same flute and overall lengths as general purpose jobbers length drills (Figure 8). Left hand drills are used on multiple spindle gear driven drilling heads, where adjacent spindles operate alternately left and right hand. They are also commonly used on screw machines, where the sequence of operations forces you to drill while the spindle is revolving in the left hand direction. |
 Figure 8 Left-handed twist drill |
STRAIGHT FLUTED DRILLS
| Straight fluted drills were originally produced for drilling in brass or other soft materials, as they will not run ahead or "grab." They are also suitable for use in thin sheet material for the same reason (Figure 9). |
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COBALT H. S. S. HEAVY DUTY DRILLS
These heavy duty drills were developed to drill the difficult-to-machine high strength and thermal resistant materials. Cobalt H. S. S. has a harder, stronger makeup, which increases abrasion resistance and has a higher red hardness needed due to the high localized temperatures developed when machining these tough materials. These drills are usually equipped with split points to reduce the high-end thrust required in drilling high strength materials.
SCREW MACHINE LENGTH DRILLS
| Screw machine drills are primarily used in screw machines and small turning centers, where the size of the machine requires the use of short drills (Figure 10). Screw machine drills eliminate the need of reworking regular length drills-cutting-off, thinning the webs, and re-sharpening. |
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The drills are made with short flutes and short overall length to maintain the maximum rigidity without sacrificing any of the cutting ability of these tools.
CENTER DRILLS top
| Center drills are very short stubby drills, designed for centering work, where their extreme rigidity reduces runout and eccentricity. For this reason, on the larger sizes they have been quite successful as starting drills on both milling machines and lathes. |
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SPOTTING, CENTERING DRILLS
| These drills are used to produce accurate and true position for starting follow-up drills, and obtaining perfect alignment. Their short flute and overall lengths and no body clearance permit chucking close to the point so that they will produce a true start or center (Figure 12). |
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They are also made with a constant web in order that subsequent re-sharpening of the drills requires no web thinning. In addition, they are useful in spotting holes for tap drilling and will reduce tap breakage on tapped holes.
COMBINED DRILLS AND COUNTERSINKS
When metal parts are to be machined on centers, as in turning or cylindrical grinding, it is always necessary to drill and countersink the center holes to serve as seats or bearings for the machine centers. In making this type of center hole, care must be taken to drill only deep enough to give the necessary protection. If the drill penetrates too far, the machine-center may ride on the corner of the straight, large diameter rather than on the 60° surface.
There are two types of combined drills and countersinks: the plain type and the bell type. The plain type will produce center holes as indicated in Figure 13. The plain type of combined drill and countersink is, by far, the most common.
 Plain type |
 Bell type |
Figure 13 Combined drill and countersinks
If the parts are to be handled excessively, where the edges of the center holes will get beat up, it is advisable to use the bell type. The bell type center-drill should also be used to produce center holes on parts that are going to be subjected to high heat. The sharp edges of the drilled holes have a tendency to break down under high heat. The flat produced on the bell type drilled hole will not break down like the plain type center drilled hole.
THREE AND FOUR FLUTE CORE DRILLS top
| These drills are designed for enlarging and/or finishing holes previously produced by drilling, casting, or punching. They can not produce original holes. Overall and flute lengths and shank diameters are made to the same proportions as taper length drills. |
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The purposes of using core drills instead of ordinary two-fluted drills for enlarging are two-fold. First, because of greater productivity. If we use drills with three or four cutting edges instead of two, we can use a greater feed per revolution, and more holes can be drilled in a given time. The second reason for using core drills is that better finishes can be obtained. The cutting action of core drills is much the same as that of a reamer.
EXTRA-LENGTH DRILLS
| Quite often, because of the inaccessibility or the extreme depth of a hole, it is found that even long series drills are not long enough. The drilling of such holes generally requires the use of an extra-length drill. Such drills are usually made with a general purpose flute design that will perform satisfactorily under the widest range of operating conditions and materials. |
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DEEP-HOLE OR CRANKSHAFT DRILLS
These drills are designed for drilling the oil holes in forged crankshafts and connecting rods and the like. They have, however, found a great many more applications in the drilling of deep holes in other tough materials. The mechanical construction of this drill is special, as it has a heavy web, a helix angle somewhat higher than normal, and usually a split-point type of web thinning.
PARABOLIC FLUTE DRILLS
| These drills are designed with open fluting, heavy parallel web, and high helix (Figure 16). They usually have split points to reduce the thrust required by the thick web. |
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Because of their heavy web construction, parabolic flute drills can be used under heavy feed conditions in most materials, in shallow holes, as well as deep holes. They are very popular for deep hole drilling because they work best under heavy feed conditions, with minimal reductions in feed required even in holes 12 or more diameters deep, and usually require one-third or less of the number of retractions required by conventionally fluted drills.
HALF-ROUND DRILLS
| These drills are used mostly in screw machines for horizontal drilling of deep holes in free-machining brass. The hole is usually started with a short, rigid center drill, after which the half-round drill penetrates the balance of the metal. |
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OIL/COOLANT HOLE DRILLS top
| Originally used largely in screw machine and turret lathe production work, recent cutting fluid supply developments have made these coolant hole drills adaptable to a wider variety of drilling operations. In addition to conventional pump cutting fluid systems, these drills are now used in conjunction with air operated systems that supply the fluid in pulsed bursts or as a mist. The cutting fluid is fed through the drill to the cutting region where it improves cooling and lubrication, and assists in ejecting chips from the drilled hole. Properly applied coolant hole drills of optimum design can be useful in drilling difficult-to-machine materials and can improve production rates in ordinary materials. |
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SPECIAL PURPOSE TWIST DRILLS
These drills are somewhat specialized in their application but are used sufficiently to deserve mention and potential consideration for certain hole-producing operations.
GUN DRILLS
| Gun Drills are usually used for drilling long, straight, accurate holes. Gun drills are special drills and are usually used on special gun drilling machines. Gun drills are a single-flute, straight-fluted drill head, usually cutting to center. |
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| Some gun drills have a cutting edge that does not cut to the center of the drill, thereby leaving a core that is easily removed on through holes. This method of drilling, where metal is left in the center, is known as Trepanning (Figure 20). |
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The cutting edges of the gun drill may be high-speed steel or brazed carbide. Properly applied gun drills are capable of producing holes that are straight and accurate, as well as of long length. While long length is implied by the word "gun," it is not a necessary requirement. Gun drills can often produce holes that are so accurate and straight that secondary hole finishing operations can be reduced or eliminated.
MULTIPLE-DIAMETER DRILLS
| It is often possible to drill a hole of two or more diameters with a single drill of proper construction. Such drills are, in fact, used quite extensively in mass production industries and affect many notable economies. The simplest form of multiple-diameter drill is the combined drill and countersink. Its uses are well known. |
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A Two-diameter Step Drill
A step drill may have two or more diameters, produced by grinding various successive steps on the lands of the drill (Figure 21). These steps are usually separated by square or angular cutting edges, as the individual jobs may require. The step drill is useful for most of the jobs requiring multiple-diameter drilling. It is extensively used because it can be made by grinding down and stepping an ordinary drill. Some thinning of the web is usually required when so made.
Subland drills, like step drills, can be made in more than two diameters and are not necessarily restricted to two-fluted tools.
SPADE DRILLS top
| Spade drills are an economical way to drill large holes. Spade drills are made in sizes ranging from one to five inches. A spade drill consists of a holder made from a heat-treated steel and a high speed steel or cemented carbide-tipped blade (Figure 22). |
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Each holder can hold a range of different size blades. New style spade drills have coolant holes drilled in them, which supply coolant to the cutting edges of the drill. Notches or chip breakers are molded or ground into the cutting edges to reduce the width of the chip. Special purpose blades for core drilling and counterboring are available.
CARBIDE DRILLS
Production drilling may be done either on special machines that are designed for production work, such as gang drills, turret drills, multiple spindle drills, NC or CNC drilling machines, or by using drill jigs on conventional drilling machines. Production drilling machines will use production type tooling.
CARBIDE-TIPPED TWIST DRILLS
| Carbide-tipped twist drills have been in existence for many years. These drills are basically twist drills with carbide brazed to them. They look like a spade drill, but are usually made in smaller diameters (Figure 23). These types of carbide tipped drills are less expensive than solid carbide drills and are more accurate than carbide insert drills. |
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SOLID CARBIDE DRILLS
| (Figure 24). Solid carbide twist drills are usually found in smaller diameters because of the cost of the carbide material. |
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Solid carbide drills offer the advantage of extreme rigidity for greater drilling accuracy. They can only be used where the spindle speed of the machine can turn the drill at the proper cutting speed. Small diameter solid carbide drills need to be turned at very high spindle speeds to work effectively. Solid carbide drills also need a rigid, stable setup. If these two factors cannot be attained, a high speed steel drill is a better choice.
SOLID CARBIDE CIRCUIT BOARD DRILLS top
| Short solid carbide drills with wide flutes of relatively high helix angle are used to drill the abrasive reinforced plastic-copper laminates used in electronic circuit boards. Frequently used in specialized circuit board drilling machines, all sizes of drills in a series have the same overall length. For this reason, another series of these drills has a common 1/8-inch shank diameter for all sizes. |
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INDEXABLE INSERT CARBIDE DRILLS
| In medium size and larger hole diameters an indexable carbide drill may
be the production answer (Figure 26). Carbide insert drills are very cost
effective because as the drill becomes dull, the insert may either be
indexed or changed.
The cutting speeds and feeds are up to ten times faster than those of conventional high-speed steel drills. Carbide insert drills need machines that have high-horsepower capabilities as well as rigid construction. Most or all of the carbide type drills can be equipped with coolant or oil holes manufactured in them. An oil hole drill allows coolant to get down in the hole where the cutting is taking place in order to cool the drill point. The coolant is under some pressure, which also forces it, along with the chips, back up the flutes, both cooling the drill and flushing the chips out of the hole. |
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CARBIDE TIPPED MASONRY DRILLS
| Masonry drills are used in portable electric drills, preferably slow speed. They are used to drill holes in concrete, sandstone, cinderblock, and similar materials (Figure 27). |  Figure 27 . Carbide-tipped masonry drill |
