Drill Bits for Woodworking
Lip and spur drill bits
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 doweling 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 metal, the lip and spur drill bit is confined to drilling only the thinnest and softest sheet metals in a drill press. The bits 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 bits tend to bind in metal; given a workpiece of sufficient thinness, they have a tendency to punch through and leave the bit'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").
Wood spade bits
Spade bits are used for rough boring in wood. They tend to cause splintering when they emerge from the workpiece. Woodworkers avoid splintering by finishing the hole from the opposite side of the work. Spade bits are flat, with a centering point and two cutters. The cutters are often equipped with spurs in an attempt to ensure a cleaner hole. With their small shank diameters relative to their boring diameters, spade bit 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 abell-hanger bit. Intended for high speed use, they are used with electric hand drills. Spade bits are also sometimes referred to as "paddle bits."
Spade drill bits are ordinarily available in diameters from 6 mm (1/4") to 36 mm (1 1/2").
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 drill bits, they are not practical to use as hand tools.
The bit includes a center point which guides it throughout the cut (and incidentally spoils the otherwise flat bottom of the hole). The cylindrical cutter around the perimeter shears the wood fibers at the edge of the bore, and also helps guide the bit into the material more precisely. The tool in the image has a total of two cutting edges in this cylinder. 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 therefore must be pulled out periodically.
Sawtooth bits are also available, which include many more cutting edges to the cylinder. These cut faster, but produce a more ragged hole. They have advantages over Forstner bits when boring into end grain.
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.
Originally the Forstner bit was very successful with gunsmiths because of its ability to drill an exceedingly smooth-sided hole.
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 bit 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 bit. 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 bit 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.
A 19 mm (3/4 inch) center bit, made sometime before 1950
Center bit tip detail
The cutting principles of the auger bit are the same as those of the center bit above. The auger adds a long deep spiral flute for effective chip removal.
Two styles of auger bit are commonly used in hand braces: the Jennings or Jennings-pattern bit has a self-feeding screw tip, two spurs and two radial cutting edges. This bit has a double flute starting from the cutting edges, and extending several inches up the shank of the bit, for waste removal. This pattern of bit was developed by Russell Jennings in the mid-19th century.
The Irwin or solid-center auger bit is similar, the only difference being that one of the cutting edges has only a "vestigal flute" supporting it, which extends only about 1/2" (12 mm) up the shank before ending. The other flute continues full-length up the shank for waste removal. The Irwin bit may afford greater space for waste removal, greater strength (because the design allows for a center shank of increased size within the flutes, as compared to the Jenning bits), or smaller manufacturing costs. This style of bit was invented in 1884, and the rights sold to Charles Irwin who patented and marketed this pattern the following year.
Both styles of auger bits were manufactured by several companies throughout the early- and mid-20th century, and are still available new from select sources today.
The diameter of auger bits for hand braces is commonly expressed by a single number, indicating the size in 16ths of an inch. For example, #4 is 4/16 or 1/4" (6 mm), #6 is 6/16 or 3/8" (9 mm), #9 is 9/16" (14 mm), and #16 is 16/16 or 1" (25 mm). Sets commonly consist of #4-16 or #4-10 bits.
The bit shown in the picture is a modern design for use in portable power tools, made in the UK in about 1995. It has a single spur, a single radial cutting edge and a single flute. Similar auger bits are made with diameters from 6 mm (3/16") to 30 mm (1-3/16"). Augers up to 600 mm (2 feet) long are available, where the chip-clearing capability is especially valuable for drilling deep holes.
20 mm (3/4") auger bit for wood
Auger bit tip detail