Drill bit


A drill bit is a cutting tool used with a drill to remove material and create holes, typically with a circular cross-section. Drill bits are available in various sizes and shapes, designed to produce different types of holes in a wide range of materials. To function, drill bits are usually mounted in a drill, which provides the rotational force needed to cut into the workpiece. The drill will grasp the upper end of a bit called the shank in the chuck.
Drills come in standardized drill bit sizes. A comprehensive drill bit and tap size chart lists metric and imperial sized drills alongside the required screw tap sizes. There are also certain specialized drill bits that can create holes with a non-circular cross-section.

Characteristics

Drill geometry has several characteristics:
  • The spiral in the drill bit controls the rate of chip removal. A fast spiral drill bit is used in high feed rate applications under low spindle speeds, where removal of a large volume of chips is required. Low spiral drill bits are used in cutting applications where high cutting speeds are traditionally used, and where the material has a tendency to gall on the bit or otherwise clog the hole, such as aluminum or copper.
  • The point angle, or the angle formed at the tip of the bit, is determined by the material the bit will be operating in. Harder materials require a larger point angle, and softer materials require a sharper angle. The correct point angle for the hardness of the material influences wandering, chatter, hole shape, and wear rate.
  • The lip angle is the angle between the face of the cut material and the flank of the lip, and determines the amount of support provided to the cutting edge. A greater lip angle will cause the bit to cut more aggressively under the same amount of point pressure as a bit with a smaller lip angle. Both conditions can cause binding, wear, and eventual catastrophic failure of the tool. The proper amount of lip clearance is determined by the point angle. A very acute point angle has more web surface area presented to the work at any one time, requiring an aggressive lip angle, where a flat bit is extremely sensitive to small changes in lip angle due to the small surface area supporting the cutting edges.
  • The functional length of a bit determines how deep a hole can be drilled, and also determines the stiffness of the bit and accuracy of the resultant hole. While longer bits can drill deeper holes, they are more flexible meaning that the holes they drill may have an inaccurate location or wander from the intended axis. Twist drill bits are available in standard lengths, referred to as Stub-length or Screw-Machine-length, the extremely common Jobber-length, and Taper-length or Long-Series.
The majority of drill bits intended for consumer use are designed with straight shanks. For heavy duty drilling in industry, bits with tapered shanks are sometimes used. Other types of shank used include hex-shaped, and various proprietary quick release systems.
The diameter-to-length ratio of the drill bit is usually between 1:1 and 1:10. Much higher ratios are possible, but the higher the ratio, the greater the technical challenge of producing good work.
The best geometry to use depends upon the properties of the material being drilled. The following table lists geometries recommended for some commonly drilled materials.
Workpiece materialPoint angleHelix angleLip relief angle
90–13532–4812–26
90–1180–2012–26
90–11824–327–20
118–13524–327–24
118–13524–327–24
60–900–2012–26

Materials

Many different materials are used for or on drill bits, depending on the required application. Many hard materials, such as carbides, are much more brittle than steel, and are far more subject to breaking, particularly if the drill is not held at a very constant angle to the workpiece; e.g., when hand-held.

Steels

  • Soft low-carbon steel bits are inexpensive, but do not hold an edge well and require frequent sharpening. They are used only for drilling wood; even working with hardwoods rather than softwoods can noticeably shorten their lifespan.
  • Bits made from high-carbon steel are more durable than low-carbon steel bits due to the properties conferred by hardening and tempering the material. If they are overheated they lose their temper, resulting in a soft cutting edge. These bits can be used on wood or metal.
  • High-speed steel is a form of tool steel; HSS bits are hard and much more resistant to heat than high-carbon steel. They can be used to drill metal, hardwood, and most other materials at greater cutting speeds than carbon-steel bits, and have largely replaced carbon steels.
  • Cobalt steel alloys are variations on high-speed steel that contain more cobalt. They hold their hardness at much higher temperatures and are used to drill stainless steel and other hard materials. The main disadvantage of cobalt steels is that they are more brittle than standard HSS.

    Others

  • Tungsten carbide and other carbides are extremely hard and can drill virtually all materials, while holding an edge longer than other bits. The material is expensive and much more brittle than steels; consequently they are mainly used for drill-bit tips, small pieces of hard material fixed or brazed onto the tip of a bit made of less hard metal. However, it is becoming common in job shops to use solid carbide bits. In very small sizes it is difficult to fit carbide tips; in some industries, most notably printed circuit board manufacturing, requiring many holes with diameters less than 1 mm, solid carbide bits are used.
  • Polycrystalline diamond is among the hardest of all tool materials and is therefore extremely resistant to wear. It consists of a layer of diamond particles, typically about thick, bonded as a sintered mass to a tungsten-carbide support. Bits are fabricated using this material by either brazing small segments to the tip of the tool to form the cutting edges or by sintering PCD into a vein in the tungsten-carbide "nib". The nib can later be brazed to a carbide shaft; it can then be ground to complex geometries that would otherwise cause braze failure in the smaller "segments". PCD bits are typically used in the automotive, aerospace, and other industries to drill abrasive aluminum alloys, carbon-fiber reinforced plastics, and other abrasive materials, and in applications where machine downtime to replace or sharpen worn bits is exceptionally costly. PCD is not used on ferrous metals due to excess wear resulting from a reaction between the carbon in the PCD and the iron in the metal.

    Coatings

  • Black oxide is an inexpensive black coating. A black oxide coating provides heat resistance and lubricity, as well as corrosion resistance. The coating increases the life of high-speed steel bits.
  • Titanium nitride is a very hard metallic material that can be used to coat a high-speed steel bit, extending the cutting life by three or more times. Even after sharpening, the leading edge of coating still provides improved cutting and lifetime.
  • Titanium aluminum nitride is a similar coating that can extend tool life five or more times.
  • Titanium carbon nitride is another coating also superior to TiN.
  • Diamond powder is used as an abrasive, most often for cutting tile, stone, and other very hard materials. Large amounts of heat are generated by friction, and diamond-coated bits often have to be water-cooled to prevent damage to the bit or the workpiece.
  • Zirconium nitride has been used as a drill-bit coating for some tools under the Craftsman brand name.
  • Al-Chrome Silicon Nitride N is a multilayer coating made of alternating nanolayer, developed using chemical vapor deposition technique, is used in drilling carbon fiber reinforced polymer and CFRP-Ti stack. N is a superhard ceramic coating, which performs better than other coated and uncoated drill.
  • BAM coating is Boron-Aluminum-Magnesium BAlMgB14 is a superhard ceramic coating also used in composite drilling.

    Universal bits

General-purpose drill bits can be used in wood, metal, plastic, and most other materials.

Twist drill bit

The twist drill bit is the type produced in largest quantity today. It comprises a cutting point at the tip of a cylindrical shaft with helical flutes; the flutes act as an Archimedean screw and lift swarf out of the hole.
The modern-style twist drill bit was invented by Sir Joseph Whitworth in 1860. They were later improved by Steven A. Morse of East Bridgewater, Massachusetts, who experimented with the pitch of the twist. The original method of manufacture was to cut two grooves in opposite sides of a round bar, then to twist the bar to produce the helical flutes. Nowadays, the drill bit is usually made by rotating the bar while moving it past a grinding wheel to cut the flutes in the same manner as cutting helical gears.
Twist drill bits range in diameter from and can be as long as.
The geometry and sharpening of the cutting edges is crucial to the performance of the bit. Small bits that become blunt are often discarded because sharpening them correctly is difficult and they are cheap to replace. For larger bits, special grinding jigs are available. A special tool grinder is available for sharpening or reshaping cutting surfaces on twist drill bits in order to optimize the bit for a particular material.
Manufacturers can produce special versions of the twist drill bit, varying the geometry and the materials used, to suit particular machinery and particular materials to be cut. Twist drill bits are available in the widest choice of tooling materials. However, even for industrial users, most holes are drilled with standard high-speed steel bits.
Image:5mm carbide bit displaying shallow point angle.jpg|thumb|A 5 mm carbide bit displaying shallow point angle
The most common twist drill bit has a point angle of 118 degrees, acceptable for use in wood, metal, plastic, and most other materials, although it does not perform as well as using the optimum angle for each material. In most materials it does not tend to wander or dig in.
A more aggressive angle, such as 90 degrees, is suited for very soft plastics and other materials; it would wear rapidly in hard materials. Such a bit is generally self-starting and can cut very quickly. A shallower angle, such as 150 degrees, is suited for drilling steels and other tougher materials. This style of bit requires a starter hole, but does not bind or suffer premature wear so long as a suitable feed rate is used.
Drill bits with no point angle are used in situations where a blind, flat-bottomed hole is required. These bits are very sensitive to changes in lip angle, and even a slight change can result in an inappropriately fast cutting drill bit that will suffer premature wear.
Long series drill bits are unusually long twist drill bits. However, they are not the best tool for routinely drilling deep holes, as they require frequent withdrawal to clear the flutes of swarf and to prevent breakage of the bit. Instead, gun drill bits are preferred for deep hole drilling.