Brad point bit The
brad point drill bit (also known as
lip and spur drill bit, and
dowel drill bit) is a variation of the
twist drill bit which is optimized for drilling in wood. 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 brad point drill bit is another solution: the center 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. While drilling, the sharp point of the spur pushes into the soft wood to keep the drill bit in line. Metals are typically
isotropic, so even an ordinary twist drill bit will shear the edges of the hole cleanly.
Wood drilled across the grain, however, produces long strands of wood fiber. These long strands tend to pull out of the hole, rather than being cleanly cut at the hole edge. The brad point 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 to be pulled messily from the timber. Brad point drill bits are also effective in soft plastic. When using conventional twist drill bits in a handheld drill, where the drilling direction is not maintained perfectly throughout the operation, there is a tendency for hole edges to be "smeared" due to side friction and heat. In metal, the brad point drill bit is confined to drilling only the thinnest and softest
sheet metals, ideally with a
drill press. The bits have an extremely fast cutting tool geometry: no point angle, combined with 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. Brad point drill bits are ordinarily available in diameters from .
Wood spade bit 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 a
bell-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 to 36 mm, or to inches. Image:Spade bits.JPG|Spade bits Image:drill spade tiny.jpg|Tiny spade bit
Spoon bit Spoon bits consist of a grooved shank with a point shaped somewhat like the bowl of a spoon, with the cutting edge on the end. The more common type is like a gouge bit that ends in a slight point. This is helpful for starting the hole, as it has a center that will not wander or walk. These bits are used by chair-makers for boring or reaming holes in the seats and arms of chairs. Their design is ancient, going back to Roman times. Spoon bits have even been found in Viking excavations. Modern spoon bits are made of hand-forged carbon steel, carefully heat-treated and then hand ground to a fine edge. Spoon bits are the traditional boring tools used with a brace. They should never be used with a power drill of any kind. Their key advantage over regular brace bits and power drill bits is that the angle of the hole can be adjusted. This is very important in chairmaking, because all the angles are usually eyeballed. Another advantage is that they do not have a lead screw, so they can be drilled successfully in a chair leg without having the lead screw peek out the other side. When reaming a pre-bored straight-sided hole, the spoon bit is inserted into the hole and rotated in a clockwise direction with a carpenters' brace until the desired taper is achieved. When boring into solid wood, the bit should be started in the vertical position; after a "dish" has been created and the bit has begun to "bite" into the wood, the angle of boring can be changed by tilting the brace a bit out of the vertical. Holes can be drilled precisely, cleanly and quickly in any wood, at any angle of incidence, with total control of direction and the ability to change that direction at will. Parallel spoon bits are used primarily for boring holes in the seat of a
Windsor chair to take the back spindles, or similar round-tenon work when assembling furniture frames in
green woodworking work. The spoon bit may be honed by using a slipstone on the inside of the cutting edge; the outside edge should never be touched.
Forstner bit Forstner bits were patented by
Benjamin Forstner in 1886. They 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; for such applications they are normally used in drill presses or lathes rather than in hand-held electric drills. Because of the flat bottom of the hole, they are useful for drilling through veneer already glued to add an inlay. The bit includes a center
brad 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. Forstner bits have radial cutting edges to plane off the material at the bottom of the hole. Bits may have two or more radial edges. 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 diameter. Sawtooth bits are available up to diameter.
Center bit 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 brad point 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. Image:drill brace.jpg|A 19 mm (3/4 inch) center bit, made sometime before 1950 Image:drill tip brace.jpg|Center bit tip detail
Auger bit 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 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 in (6.35 mm), #6 is 6/16 or 3/8 in (9.53 mm), #9 is 9/16 in (14,29 mm), and #16 is 16/16 or 1 in (25,4 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 in) to 30 mm (1 3/16 in). Augers up to long are available, where the chip-clearing capability is especially valuable for drilling deep holes. Image:drill auger.jpg| auger bit for wood Image:drill tip auger.jpg|Auger bit tip detail
Gimlet bit The gimlet bit is a very old design. The bit is the same style as that used in the
gimlet, a self-contained tool for boring small holes in wood by hand. Since about 1850, gimlets have had a variety of cutter designs, but some are still produced with the original version. The gimlet bit is intended to be used in a hand brace for drilling into wood. It is the usual style of bit for use in a brace for holes below about diameter. The tip of the gimlet bit acts as a tapered screw, to draw the bit into the wood and to begin forcing aside the wood fibers, without necessarily cutting them. The cutting action occurs at the side of the broadest part of the cutter. Most drill bits cut the base of the hole. The gimlet bit cuts the side of the hole. Image:drill gimlet.jpg|Gimlet bit for wood, made sometime before 1950. Image:drill tip gimlet.jpg|Gimlet bit tip detail
Hinge sinker bit The hinge sinker bit is an example of a custom drill bit design for a specific application. Many European kitchen cabinets are made from
particle board or
medium-density fiberboard (MDF) with a laminated
melamine resin veneer. Those types of
pressed wood boards are not very strong, and the screws of butt
hinges tend to pull out. A specialist hinge has been developed which uses the walls of a hole, bored in the particle board, for support. This is a very common and relatively successful construction method. A Forstner bit could bore the mounting hole for the hinge, but particle board and MDF are very abrasive materials, and steel cutting edges soon wear. A
tungsten carbide cutter is needed, but the complex shape of a forstner bit is difficult to manufacture in carbide, so this special drill bit with a simpler shape is commonly used. It has cutting edges of tungsten carbide brazed to a steel body; a center spur keeps the bit from wandering.
Adjustable wood bits An adjustable wood bit, also known as an expansive wood bit, has a small center pilot bit with an adjustable, sliding cutting edge mounted above it, usually containing a single sharp point at the outside, with a
set screw to lock the cutter in position. When the cutting edge is centered on the bit, the hole drilled will be small, and when the cutting edge is slid outwards, a larger hole is drilled. This allows a single drill bit to drill a wide variety of holes, and can take the place of a large, heavy set of different size bits, as well as providing uncommon bit sizes. A
ruler or
vernier scale is usually provided to allow precise adjustment of the bit size. These bits are available both in a version similar to an auger bit or brace bit, designed for low speed, high torque use with a brace or other hand drill (pictured to the right), or as a high speed, low torque bit meant for a power drill. While the shape of the cutting edges is different, and one uses screw threads and the other a twist bit for the pilot, the method of adjusting them remains the same. ==Other materials==