WOOD DRILL BITS

Lip and Spur Drill Bit

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.

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.

Metals are typically isotropic, 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.

Lip and spur drill bits are also effective in soft plastic. Conventional twist drills 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 vibrates.

In metal, the lip and spur drill is confined to drilling only the thinnest and softest sheet metals in a drill press. The drills have an extremely fast cutting tool geometry: no point angle and a large (considering the flat cutting edge) lip angle causes the edges to take a very aggressive cut with relatively little point pressure. This means these drills tend to bind in metal; given a workpiece of sufficient thinness, they have a tendency to punch through and leave the drill's cross-sectional geometry behind.

Lip and spur drill bits are ordinarily available in diameters from 3 mm (1/8") to 16 mm (5/8").


Spade bits are used for rough boring in wood. They tend to cause splintering when they emerge from the workpiece. They are flat, with a centering point and two cutters. The cutters often are equipped with spurs in an attempt to ensure a cleaner hole.

Having small shank diameters relative to their boring diameters, spade bits shanks often have flats forged or ground into them to prevent slipping in drill chucks. Some bits are equipped with long shanks and have a small hole drilled through the flat part, allowing them to be used much like a bell hanger bit. Intended for high speed use, they are used with electric hand drills. They are also known as paddle bits.

Forstner bit

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 glued 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. Unlike most other types of drills, they are not practical to use as hand tools.

The bit has a centre point which guides it during the cut (and incidentally spoils the otherwise 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 (5/16") to 50 mm (2") diameter. Sawtooth bits are available up to 100 mm (4") diameter.

Step bit

A step bit, step drill, speed bit, or Unibit 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. They are now available in fractional inch and metric sizes.

Step bits are most commonly used in general construction and plumbing. One drill bit can drill the entire range of holes necessary on a counter top, speeding up installation of fixtures. They are most commonly used on softer materials - plywood, particle board, drywall, acrylic, laminate, etc. They can be used on very thin sheet metal, but metals tend to cause premature drill wear and dulling. A metal hole saw is more appropriate for large-hole applications in thicker metals.

An additional use of step bits 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.

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