The lip and spur drill bit is a variation of the twist drill bit which is optimized for drilling in wood. 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 bit. In wood, the lip and spur drill bit is another solution: The centre of the drill bit is given not the straight chisel of the twist drill bit, 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.
Metals are typically isotropic, and an ordinary twist drill bit shears the edges of the hole cleanly. Wood drilled across the grain has long strands of wood fiber. 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 maximizes the chance that the fibers can be cut cleanly, rather than having them pull messily out of the timber.
Lip and spur drill bits are also effective in soft plastic. Conventional twist drill bits in a hand drill, where the hole axis is not maintained throughout the operation, have a tendency to smear the edges of the hole through side friction as the drill bit vibrates.
in stock / 25 €