Drill (tool)

Drill (tool), cutting tool for making round holes in wood, metal, rock, or other hard material. Tools for drilling holes in wood are commonly known as bits, as are certain specialized types of tools used in rock drilling.

The most common tool for drilling wood is the auger bit, which is provided with single or double helical cutting surfaces and is revolved by a hand-operated cranklike brace or an electric motor. A number of special forms of wood bits are also employed, including the expanding bit, which has a central guide screw and a radial cutting arm that can be adjusted to drill holes of different sizes.

For drilling metal, twist drills are usually employed. A twist drill is a cylindrical rod with two spiral flutes cut around the rod. The flutes meet at the point of the drill in an angle that is usually between 118° and 120°. Twist drills are made in sizes from a few thousandths of an inch in diameter to an inch, and the diameter of the drill governs the size of the hole produced. When a larger hole is required the drilled hole is enlarged by means of a boring tool. Twist drills are usually rotated by motor-driven drilling devices. The simplest form of drilling machine is a small, hand-held electric motor with a chuck that grasps the drill. For precise work and for larger drills a drill press is employed. This machine consists of one or more motor-driven spindles, usually vertical, with chucks at the lower ends for holding drills. The spindle can be raised or lowered with a hand wheel or lever. An adjustable metal table below the spindle holds the work to be drilled. Hand- or power-rotated twist drills can also be used for drilling in glass, plastics, and ceramics.

Small, shallow holes in stone, concrete, brick, and similar materials are usually drilled by hand with a star drill, a steel rod with an X-shaped cutting point. The point is held against the object to be drilled, and the other end of the rod is struck with a hammer or sledge. The drill is revolved slightly after each stroke. Similar rock drills powered by pneumatic hammers are used to drill holes larger in diameter, such as those used for the placement of explosive charges in mining and quarrying. A rotary drill consists of an augerlike bit fastened to a series of connected steel pipes. It is used in drilling to great depths, as in drilling oil wells, both on land and at the bottom of the ocean.