DRILLS AND DRILLING. Drilling as a mechanical operation is often confused with boring. In machine shop practice, drilling is making a round hole through metal or the like with a rotating pointed tool, that forces its way through by pressure and rotation, taking off spiral chips of the material drilled and bringing them out of the hole through channels cut in the drill for clearance. Boring is enlarging a hole that already exists, as in a cast-iron cylinder, by turning the interior surface with a cutting tool. Cutting a circular hole in wood with a bit and brace, as done by carpenters, is boring, but a similar operation in metal is drill ing. The forming of a hole in stone or rock, by repeated blows of a chisel or free-falling tool, as when making a hole for blasting or sinking an oil-well, is drilling. A hollow cylindrical tool for cutting out a core of rock is also called a drill. The forcing of holes in thin metal, as structural steel, by means of opposed dies and sudden pressure, is termed punching.
The machinists' drills are of two sorts: (1) Hand drills, which may resemble the car penter's bit and brace except that a metal-cut ting twist-drill serves for a bit; or which may be made on the ratchet-and-pawl principle, for working in confined spaces, as drilling a hole in the web of a section of laid railway track.
(2) Power drills or drilling machines or drill presses —.the latter being the more common catalogue name — which are of a great variety of forms, sizes and styles. They range from the light jeweler's drill, that makes a tiny per foration in a watch-case, to the heavy gang drill that cuts a dozen large holes at one opera tion in a heavy I-beant. The typical machinist's drill-press has an upright post with curved top like a letter f_ At the overhanging top is a driving mechanism with a rotating chuck, made to hold securely any one of a series of steel• drills of varying sizes and to position it perpen dicularly for work. In the centre of the machine is a platform, usually swung lower than the cross bar on die f, that is adjustable for height and arranged to swing in the arc of a circle, and to this platform the work to be drilled is clamped. When in position the operator lowers the drill by a lever, or wheel and worm motion, and throws on a belt to start the drill. When the hole is cut through he throws up the drill, and is then ready to repeat the operation.
The miner's drill is known as the rock-drill, and consists of a tripod frame, having an up right steam-cylinder, on the piston of which is mounted the drill, which is simply a hardened chisel arranged to rotate as it strikes the rock, so that the cutting edge is turned for every blow. Several hundred blows a minute may be delivered by drill, which is the standard machine of the miner as well as the contractor, for forming holes for blasting rock. (See
MINING AND MILLING MACHINERY). The oil well drill or drilling rig, for sinkingpetroleum wells, operates on a similar principle, but a vastly larger and heavier outfit is required, as the hole has to go down perhaps hundreds or even thousands of feet. See PETROLEUtt.
The diamond drill is used by prospectors to take out a core of rock that they may analyze its content and judge of its value. This tool has enabled mining companies to test their ore in the various levels, and thus know far in advance what they can depend• upon for future reserves. The original diamond drill was a hollow cylinder with black diamonds on the lower edge for cut ters. These being expensive a serrated edge of hardened steel was next tried. This was fol lowed by the shot-bit or shotted drill. This bit is a soft steel cylinder in connection with which small chilled steel shot are used. The bit grinds the shot into the rock, thus gradually wearing it away by repeated minute crushings or small particles. The hollow drill rods are rotated and lowered by a machine solidly at the • opening. The lower drill rod is sur rounded by a or cup-like tube and the two are joined at their lower ends by a plug. The centre portion of the plug serves as a bear ing for a protecting ring and on its lower end a ring is threaded, while to this the core barrel is attached on which either the shot bit or the cutter can be threaded. The shot bit is pro vided with a series of triangular notches in its lower end, one of the walls of the notch being vertical and the other forming an angle of 30* therewith. The steel shot, which are fed through the hollow rods from the top, are car ried by a current of water under this notch, and the inclined wall allows them to pass under the edge of the shot bit. The sizes of shot used vary with the nature of the rock to be drilled, some being as large as duck shot, though mostly finer. The working edge of the shot bit is rounded, so that the shot grinds not only directly beneath the drill, but also to a certain extent at the inner and outer sides, thus cutting out proper clearance for the operation of the drill. Water into the hollow drill rods through a pipe and passes out under the bit and up the annular space outside the core barrel, carrying with it the sludge or fine par tides ground up by the shot. The current of water flows with great strength up as far as the top of the calyx, where the annular space widens considerably, so that the current is re duced and the sludge it carries drops by gravity. See BORING.