A drill is a cutting tool used to make or to enlarge holes in metal, wood, plastic or other solid materials. Most drills are rotary drills. They produce holes by rotating about their longitudinal axis while at the same time moving axially through the material being drilled. A few drills, also called punches, are percussive and are used with a hammer to make holes. A succession of rapid hammer blows drives the drill into the workpiece. These percussive types of drills are used on hard, brittle, abrasive materials, such as concrete and rock.
Drills are powered by hand or machine. They consist of three parts: a point, a body, and a shank.
At one end of a drill is the point. As the drill moves into the workpiece, the point cuts away small chips. The point on a twist drill is cone-shaped. It has one or more straight cutting edges at the intersection of one edge of each body flute. Thus a two-fluted drill has two cutting edges.
The points of some woodworking drills, or bits, consist of a pointed screw. The points of rock drills, such as the diamond drill, consist of particles of a hard abrasive embedded at the end of a metal tube. Abrasives commonly used for this purpose are particles of tungsten carbide, boron nitride, silicon carbide, and industrial diamonds. The advantage of using a tube-shaped drill rather than a solid one for the drill body is that a smaller amount of the material being drilled has to be abraded away since the rube leaves a solid core that can be mechanically removed in a solid piece.
When the point of a drill gets dull from usage, the cutting edges are sharpened by re-grinding. Each sharpening reduces the length of the drill by a few thousandths of an inch at a time, or more, in the case of an extremely worn drill.
The shank of a drill is at the driving end of the drill. Its main function is to transmit the torque necessary to rotate the drill and the force necessary to feed the drill into the work-piece. It may have any of various shapes- cylindrical, tapered, splined, or rectangular. The two most common types of shanks used on industrial twist drills are the tapered and the straight, or solid, cylinder.
The center, and major, portion of a drill is the body. The body often contains flutes that run from the point to the shank. Flutes are straight or helical channels through which the chips or cuttings from the bottom of the drilled hole move to the surface of the workpiece. The actual cutting of the workpiece is done at the point of the drill and not along the fluted body of the drill.
Drill presses are motor-driven drilling machines used to operate industrial drills. The name is also used to denote the entire machine, including the cutting tool. Small hand or portable drilling machines are called electric drills or simply drills. They are used mostly in home shops. Drill presses are generally classified according to their weight, position, number of spindles, and means of feeding the drill into the workpiece. The speed of the drill usually decreases with the hardness of the material being drilled. The larger the diameter of the drill, generally the faster it is fed per rotation of the drill.
The most common types of drills used in manufacturing plants to drill holes in metal, wood, and plastics are twist drills. The name derives from the fact that the flutes of the drill are twisted in the form of a helix about its longitudinal axis. In the larger-sized drills the helix is formed by actually twisting a straight-fluted bar, but in die smaller drills, the helix is formed by machining the flutes in a solid cylindrical bar with a milling cutter. The most common helix angle of the twist drill is 30°. The cross section of a twist drill closely resembles a dumbbell with a very short connecting member. This connecting member is called the web.
Among the most common types of twist drills used in industry, in addition to the straight-shank and taper-shank drills, are core drills, center drills, and gun drills.
Core drills are three-fluted drills. They are used for enlarging cast, forged, or punched holes. They cannot be used to originate holes.
Center holes in workpieces are often made with center drills. They have both a drill portion and an adjacent countersink portion. The countersink is used to flare the end of the drilled hole in the workpiece so that it may be held between conical ends during subsequent machining and grinding.
Gun drills are straight-fluted drills that are used for drilling deep holes (the depth of the hole is generally greater than about 10 times the diameter of the drill). Gun drills usually have cutting-fluid passages running longitudinally through the body to both cool the cutting edges and flush out the chips as the drill moves into the workpiece.
The home-shop and cheaper twist drills are made of inexpensive carbon tool steel. The most common material used to make the commercial twist drills is a type of high alloy steel called high-speed steel. Its name is derived from the fact that tools made of this material can cut at relatively higher speeds than tools made of ordinary tool steel. A typical composition of high-speed steel (tungsten type) is 18% tungsten, 4% chromium, 1% vanadium, 0.75% carbon, and the balance iron. If the tungsten is replaced with about 8% molybdenum, it is called molybdenum type high-speed steel.
Large-diameter drills are frequently made by butt-welding a structural steel shank to a highspeed steel body since the cost of high alloy steel is 10 to 20 times as great as ordinary structural steels. In some cases, cast nonferrous or carbide inserts are attached to the cutting end of the body to serve as cutting edges.