An air drill is a tool that uses compressed air to generate rotational force for drilling holes and completing other tasks. Models can range in size from a dental drill to an industrial jackhammer. An air drill is fundamentally the same as any other type of drill, save for the specific type of power used to generate torque.
Compressed air was first harnessed as a power source around the turn of the 18th century, when methods of making spaces airtight advanced to the point that it was possible to generate sufficient levels of pressure to move pistons housed within small chambers. A common use of early pneumatics was to move documents such as telegrams through tubes - technology that is still used in 21st century drive-through banking. Pneumatic force was applied to tools, including drills, around the turn of the 19th century, and was instrumental in the modern construction techniques that made possible things like skyscrapers and automobiles.
Among the smallest tools powered by air is the dental drill. This device is actually not just a drill but rather a rotary tool simply used most commonly as a drill. Various attachments can lend grinding, buffing, and other kinds of functionality. A dental drill can spin at upwards of 800,000 revolutions-per-minute (RPM), and its utility also makes it popular among jewelers and watchmakers, who value its precision.
On the other end of the spectrum, a jackhammer is a kind of air drill that leverages pneumatic power to break up concrete and other types of masonry. Large air compressors feed into the jackhammer itself, which is made up of a hammer and a chisel bit. The hammer portion hits down on the chisel many times a second by virtue of a spring reloading system. Unlike dental drills, which utilize an electric motor to power the compressor, jackhammers generally rely on large diesel generators. Pneumatic jackhammers are typically used in moderate applications, with heavy duty projects requiring hydraulic models.
Relatively few pistol-grip drills - the kind most used in home improvement situations - are of the air drill type. This is largely due to the adequacy of electric drills in these kinds of situations, and the necessary bulk associated with having to use an air compressor. Some contractors prefer an pistol-grip air drill however, and they can make sense on certain job sites where an air compressor may already be in use. Compared to dental drills, pistol-grip models spin at a relatively slow top speed of around 3,000 RPM.