Drill bit

Drill bits are the cutters of drill tools. Bits are interchangeable, meaning that they can be removed from the end of the drill, either to replace a worn part or to change the size of the part.

Lip and spur drill

The lip and spur drill bit is a variation of the twist drill which is optimised 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 centre 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.

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Lip and spur drill bits are also effective in soft plastic and sheet metal. Conventional twist drills can, in some kinds of plastic, smear the edges of the hole, perhaps through local heating. When used on thin sheet metal the lips cut a disc from the sheet rather than tearing and deforming the edge of the hole

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

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