BORING, a species of circular cutting in which a cylindrical portion of a substance is gradually removed. When tubes of metal are to be formed, a cast is, in some cases, made in solid metal, and the whole of the bore is pro duced by the boring-machine: in others the cast is made hollow at first, and the borer is only used to give uniformity and finish to the inside of the tube. In boring cannon sometimes the tool is made to revolve while the cannon is at rest, and sometimes the cannon is made to revolve while the tool is at rest. By the latter arrangement the bore is said to be formed with more accuracy than by the other method of putting the borer in motion.
In the jewelry and small metal industries, hand drills, which consist of a spindle with steel bits, to which reciprocating rotation is given, are the implements for piercing small holes. The boring of holes in metal plates is effected by means of drills driven by machinery. The drill is inserted in the end of a vertical spindle, which revolves in a fixed frame and is driven by the bevel wheels. The metal to be bored is placed on a table or other support, below the drill; and the up and down motion, or end pres sure and off action, of the drill is effected by the hand gear turning the screw; which, being coupled to the top of the spindle, presses it down or raises it, according to the way it is turned. The spindle slides vertically in the collar forming the axis of the bevel wheel, but is carried round with it by means of a pin which projects into a groove.
As applied to the earth and to rocks, boring embraces two classes of operations — boring of shot-holes for blasting, and the sinking of bores in prospecting for minerals and in forming wells for water, brine and mineral oils. Blast-holes in rocks are made from one to two more — inches in diameter, and may pierce to the depth of nine feet. Such holes are most simply made in hard rock by a steel-pointed drill, struck by a hammer, and turned partly round after each blow to make the hole cylin drical. The addition of a little water serves to preserve the temper of the boring tool, and makes the hole more easy to cut. In soft rock,
whenever the hole is to be vertical, a jumper is used. This is a weighted drill, which acts merely by its own weight when let fall from about a foot in height. The powdered stone is removed at intervals by a scraper. But in all great engineering undertakings rock-boring machinery now supplants band work. The machines are principally devised to imitate the percussive action of the hand drill, the boring chisel being worked and rotated by compressed air, and sometimes directly by steam. The com pressed-air machines possess the great advan tage of aiding in the ventilation of the work ing—often a most important consideration, seeing the operations are chiefly carried on in confined spaces where large volumes of poison ous gases are evolved from explosions. The earliest practical rock-boring machine was that of Sommeiller, one of the engineers of the Mont Cenis tunnel, at which undertaking the apparatus was first used. Now the forms of percussion machines are very numerous, im provements being directed toward lightness and simplicity of parts, and to the method—auto matic or otherwise — of advancing the boring tool as the work proceeds. Among the best known machines are the Barrow, Burleigh, Darlington, Ferroux, Ingersoll and McKean rock-borers. Diamond drills working in the manner described below are also used. Brandt's rotatory borer is an apparatus similar in action to the diamond drill, but with a crown of hardened steel in place of cutting diamonds. The tool is pressed against and rotated by water power. An apparatus similar in principle to the brace and bits of the carpenter is used with advantage in uniform rock, such as slate.
The bores for deep wells of all kinds, and for discovering the mineral contents of a region, come under one category. As a pre liminary operation in mining, boring is of the utmost importance for discovering the position, thickness and dip of mineral strata or lodes, and for ascertaining the nature of the over lying deposits. Bores are made by three classes of implement — (1) boring-rods, (2) rope borers, and (3) diamond drills.