Drill Accessories

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

The shank is the part of the drill bit grasped by the chuck of a drill. The cutting edges of the drill bit are at one end, and the shank is at the other.

Drill bits come in standard sizes, described in the drill bit sizes article. A comprehensive drill and tap size chart lists metric and imperial sized drills alongside the required screw tap sizes.

The term drill can refer to a drilling machine, or can refer to a drill bit for use in a drilling machine. In this article, for clarity, drill bit or bit is used throughout to refer to a bit for use in a drilling machine, and drill refers always to a drilling machine.

Soft low carbon steel bits are used only in wood, as they do not hold an edge well and require frequent sharpening. Working with hardwoods can noticeably shorten their lifespan. They are cheaper than longer-lived bits.
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.

Step drill bits:
A step drill is a drill bit that has 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, as compared to a drill with a slip-on collar. Most of these bits are custom-made for each application, which makes them more expensive.

Indexable drill bits are primarily used in CNC and other high precision or production equipment, and are the most expensive type of drill bit, costing the most per diameter and length. Like indexable lathe tools and milling cutters, they use replaceable carbide, HSS or ceramic inserts as a cutting face to alleviate the need for a tool grinder. One insert is responsible for the outer radius of the cut, and another insert is responsible for the inner radius. The tool itself handles the point deformity, as it is a low-wear task. The bit is hardened and coated against wear far more than the average drill bit, as the shank is non-consumable. Almost all indexable drills have multiple coolant channels for prolonged tool life under heavy usage. They are also readily available in odd configurations, such as straight flute, fast spiral, multiflute, and a variety of cutting face geometries.

Typically indexable drills are used in holes that are no deeper than about 5 times the drill diameter. They are capable of quite high axial loads and cut very fast.

Klein Tools 3 8 x 72 inch Flex Auger Bit Power Tool Accessory Part Drill Hole
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Unibit:
A unibit, also incorrectly known as a step drill or step bit, is a roughly conical bit with a stair-step profile. Due to their design, a single bit can be used for drilling a wide range of hole sizes. Some bits come to a point and are thus self-starting. The larger-size bits have blunt tips and are used for hole enlarging.

Unibits are commonly used on sheet metal and in general construction. One drill bit can drill the entire range of holes necessary on a countertop, speeding up installation of fixtures. They are most commonly used on softer materials, such as plywood, particle board, drywall, acrylic, and laminate. They can be used on very thin sheet metal, but metals tend to cause premature drill wear and dulling.

Unibits are ideal for use in electrical work where thin steel, aluminum or plastic boxes and chassis are encountered. The short length of the unibit and ability to vary the diameter of the finished hole is an advantage in chassis or front panel work. The finished hole can often be made quite smooth and burr free, especially in plastic.

An additional use of unibits is deburring holes left by other bits, as the sharp increase to the next step size allows the cutting edge to scrape burrs off the entry surface of the workpiece. However, the straight flute is poor at chip ejection, and can cause a burr to be formed on the exit side of the hole, more so than a spiral twist drill turning at high speed.

The unibit was invented by Harry C. Oakes in 1971 and patented in 1973. It was sold by the Unibit Corporation in the 1980s, but since the patent expired it is sold by other companies.

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 which to mount a lathe center. 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 (as explained in detail below). Nevertheless, because of the frequent lumping together of both the terminology and the tool use, suppliers may call center drills combined-drill-and-countersinks in order to make it unambiguously clear what product is being ordered. They are numbered from 00 to 10 (smallest to largest).

Core drill bit:
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.

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 a hole with an annular cross-section. A diamond core drill bit is intended to cut an annulus in the workpiece. Large bits of similar shape are used for geological work, where a deep hole is drilled in sediment or ice and the drill, which now contains an intact core of the material drilled, is retrieved and the strata in the core sampled are studied.

Left-hand bits are almost always twist bits and are predominantly used in the repetition engineering industry on screw machines or drilling heads. Left-handed drills allow a machining operation to continue where either the spindle cannot be reversed or the design of the machine makes it more efficient to run left-handed. With the increased use of the more versatile CNC machines their usage is less common than when specialized machines were required for machining tasks.

Screw extractors are essentially left-hand bits of specialized shape, used to remove common right-hand screws whose heads are damaged, making use of a screwdriver impossible. The drill bit is rotated counter-clockwise and will tend to jam in the damaged head and then turn the screw counter-clockwise, unscrewing it.

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. To pull the bit from the hole, either the female thread in the wood workpiece must be stripped, or the rotation of the bit must be reversed.

The edge of the bit has a sharpened spur to cut the 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 do not cut well in the end grain of wood. The central screw tends to pull out, or to split the wood along the grain, and the radial edges have trouble cutting through the long wood fibers.

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

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