Drill bits are cutting tools used to create cylindrical holes, almost always of circular cross-section. Drill bits come in many sizes and have many uses. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes.
The shank is the part of the drill bit grasped by the chuck of a drill. The cutting edges of the drill bit are at one end, and the shank is at the other.
Drill bits come in standard sizes, described in the drill bit sizes article. A comprehensive drill bit and tap size chart lists metric and imperial sized drill bits alongside the required screw tap sizes.
The term drill may refer to either a drilling machine or a drill bit for use in a drilling machine. In this article, for clarity, drill bit or bit is used throughout to refer to a bit for use in a drilling machine, and drill refers always to a drilling machine.
Exceptionally, specially-shaped bits can cut holes of non-circular cross-section; a square cross-section is possible.
Drill bit geometry has several aspects:
- The spiral (or rate of twist) in the drill bit controls the rate of chip removal. A fast spiral (high twist rate or “compact flute”) drill bit is used in high feed rate applications under low spindle speeds, where removal of a large volume of swarf is required. Low spiral (low twist rate or “elongated flute”) 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 controls wandering, chatter, hole shape, wear rate, and other characteristics.
- The lip angle 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 length of a bit determines how long a hole can be drilled, and also determines the stiffness of the bit and accuracy of the resultant hole. Twist drill bits are available in standard lengths, referred to as Stub-length or Screw-Machine-length (short), the extremely common Jobber-length (medium), and Taper-length or Long-Series (long).
Most drill bits for consumer use have straight shanks. For heavy duty drilling in industry, bits with tapered shanks are sometimes used.
General-purpose drill bits can be used in wood, metal, plastic, and most other materials.
- 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. Twist drill bits range in diameter from 0.002 to 3.5 in (0.051 to 88.900 mm) and can be as long as 25.5 in (650 mm). 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 inexpensive. 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.
- A step drill bit is a drill bit that has the tip ground down to a different diameter. The transition between this ground diameter and the original diameter is either straight, to form a counterbore, or angled, to form a countersink. The advantage to this style is that both diameters have the same flute characteristics, which keeps the bit from clogging when drilling in softer materials, such as aluminum; in contrast, a drill bit with a slip-on collar does not have the same benefit. Most of these bits are custom-made for each application, which makes them more expensive.
- A unibit (often called a step drill bit) is a roughly conical bit with a stairstep profile. Due to its 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.
- Hole saws take the form of a short open cylinder with saw-teeth on the open edge, used for making relatively large holes in thin material. They remove material only from the edge of the hole, cutting out an intact disc of material, unlike many drills which remove all material in the interior of the hole. They can be used to make large holes in wood, sheet metal and other materials.
The shank is the end of a drill bit grasped by the chuck of a drill. The cutting edges of the drill bit contact the workpiece, and are connected via the shaft with the shank, which fits into the chuck. In many cases a general-purpose arrangement is used, such as a bit with cylindrical shaft and shank in a three-jaw chuck which grips a cylindrical shank tightly. Different shank and chuck combination can deliver improved performance, such as allowing higher torque, greater centering accuracy, or moving the bit, but not the chuck, with a hammer action.