Drills and Drilling
A drill is a tool used to make holes in hard substances. The two chief kinds of drills are rotary drills and percussive drills. Rotary drills, which work like dentists' drills, are the most widely used. Percussive drills, which have a hammering action, are used mainly for drilling rock. The term "drill" may be applied either to the entire drilling tool or to the cutting part of the tool, sometimes called the drill bit.
Some drills are powered by hand, and others are powered by electric motors. Large rotary drills are often powered by diesel engines. Most percussive drills are driven by compressed air, as in the air hammer.
Types of Drills
Metal Drill. Metal drills are used to drill holes in metal. The most common type of metal drill is the twist drill, which usually has two cutting edges, called lips, inclined at an angle to the axis of the drill. The twist drill also has a pair of spiral flutes that run the length of the drill. The flutes provide a path for chips to leave the hole and for lubricants to flow down to the tip of the drill, where cutting takes place.
Wood Drill. Drills used to make holes in wood are called wood drills. They are similar in shape to metal drills, but they need not be as strong or as heat-resistant as metal drills because wood is easier to cut.
Core Drill. The core drill is a cylindrical tube that has a hard abrasive substance, such as black diamonds, embedded in its lower rim. As the drill rotates, its rim cuts out a core of earth or rock. The core is brought to the surface and provides information about the ground below.
Rock Drill. The rock, or miner's, drill is a percussive drill. It consists of a hard chisel that is repeatedly pounded into rock. Rock drills are used to drill blast-holes, in which the explosives used to excavate mines and tunnels are placed. The blastholes may be a foot (30 cm) or more in diameter and more than 125 feet (38 meters) long.
Oil-well Drill. An oil-well drill is a rotary arbrasive drill that is used in oil prospecting. The drill is very heavy and strong, since it must drill through as many as several thousand feet of rock.