Plate from 18th century encyclopedia showing illustrations of a cannon boring machine together with a bomb caliber compass. The compass was used to calculate the size, weights and trajectories of different munitions. In machining, boring is the process of enlarging a hole that has already been drilled (or cast), by means of a single-point cutting tool (or of a boring head containing several such tools), for example as in boring a gun barrel or an engine cylinder. Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered hole.