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Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes. This article describes the types of drill bits in terms of the design of the cutter. The other end of the drill bit, the shank, is described in the drill bit shank article. Drill bits come in standard sizes, described in the drill bit sizes article. 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.

Wood drills

The lip and spur drill bit is a variation of the twist drill which is optimized for drilling in wood. It is also called the brad point bit or dowelling bit.
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Conventional twist drill bits do tend to wander when presented to a flat workpiece. For metalwork, this is countered by drilling a pilot hole with a 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.

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Metal has no long-distance structure, and an ordinary twist drill shears the edges of the hole cleanly. Wood drilled across the grain has long strands of wood fibre. These long strands tend to pull out of the wood hole, rather than being cleanly cut at the hole edge. The lip and spur drill bit has the outside corner of the cutting edges leading, so that it cuts the periphery of the hole before the inner parts of the cutting edges plane off the base of the hole. By cutting the periphery first, the lip maximises the chance that the fibres can be cut cleanly, rather than having them pull messily out of the timber.

Forstner bits 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. Because of the flat bottom to the hole, they are useful for drilling through veneer already fixed, to add an inlay. They require great force to push them into the material, so are normally used in drill presses or lathes rather than in portable drills. They are impractical to use other than in power tools. The bit has a centre point which guides it during the cut (and incidentally spoils the flat bottom of the hole). The cylindrical cutter around the perimeter shears the wood fibres at the edge of the bore, and also helps guide the bit into the wood precisely. The tool in the image has a total of two cutting edges in this cylinder. Sawtooth Forstner bits are available, with many more cutting edges in the cylinder. These cut faster but produce a 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 must be pulled out periodically to do this. Bits are commonly available in sizes from 8 mm to 50 mm diameter. Sawtooth bits are available up to 100 mm diameter.

Center bit The center bit is optimised for drilling in wood with a hand brace. Many different designs have been produced. The centre of the bit is a tapered screw thread. This screws into the wood as the drill is turned, and pulls the bit into the wood. There is no need for any force to push the bit into the workpiece, only the torque to turn the bit. This is ideal for a bit for a hand tool. 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 fibres of the wood, as in the lip and spur drill. A radial cutting edge planes the wood from the base of the hole. In this version, there is 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 fibres. Center bits are made of relatively soft steel, and can be sharpened with a file. The drill bit shown was made sometime before 1950, and still works to drill holes in 2005. It drills a hole of diameter 3/4 inch.
 
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