Drill (tool)
A drill is a cutting tool for making holes in solid material, although especially handheld power drills can be used as a general-purpose power source for rotating tools such as screwdriver bits and sanding disks. Its most common implementation is a means to rotate and press a drill bit against, and into, the material to be drilled. Some drills do not rotate the bit, but hammer it against the piece being worked; these variants are called hammer drills or rotary hammers.
The term is often used loosely, not distinguishing the actual cutting element (i.e., the drill bit) from the power source.
Power source
Primitive drills, which might be no more than a sharpened rock hammered against the work, are among the earliest human tools. Drills for some applications are still muscle-powered, but the majority of industrial drills are driven by an electrical motor or compressed air. Very large drills, such as those used to make oil wells, may be powered by a gasoline or diesel motor. Some hammer drills use an explosive charge to drive the bit.
Drill bits
Drill bits are usually replaceable, since they wear out faster than the drill motor. For many hand and industrial processes, the drill accepts different sizes, point types, and materials of drill bits, for different applications.
Materials
For most applications, drill bits are made of tough, sharpened steel. They need to have more strength against transverse movement than a tap (tool). Useful hybrids can come from tipping a steel shaft with a hard cutter of tungsten carbide or industrial diamond.
Shapes
Perhaps the most common general-purpose shape for a drill bit is a rod with a sharpened point, and spiral flutes machined into the rod behind the point. The flutes provide cooling and a means for waste to move away from the work. The back end of the drill rod is round, or sometimes a formed shape to match a receptacle, to anchor it in the power source.
Besides material, spiral drills vary with the angle of the cutting edge. Different angles work more efficiently for drilling hard metal versus soft plastic. In many drilling applications, the drill tip and the work need to be cooled, usually by fluid, to avoid melting the work or damaging the heat treatment of the cutter.
There are a wide range of wood drills more specialized than spirals, but they tend to share the property of having a central guiding point and somewhat shallower, more widely separated edges to enlarge the hole.
Waste or core removal
Many drills grind the material removed into small waste. Some drills, however, keep the removed material as intact as possible. One type is called a core drill, which is used in geology to take soil samples. Hole saws also produce intact cores,