Page:EB1911 - Volume 04.djvu/268

 By using the sliding link the cross-section and weight of the rods may be greatly reduced, the only strain being that of tension. To deliver a sharp, effective blow, however, the rods must drop with a quick stroke, which brings a heavy strain upon the operating machinery. For overcoming this difficulty, various “free-falling tools” have been devised. By these the bit is allowed to fall by gravity; the rod follows on its measured down stroke, and picks up the bit. Free-falling tools are of two classes: (1) those by which the bit is released automatically; (2) those operated by a sudden twist imparted to the rod by the drillman. One of the best known of the first class is the Kind free-fall (fig. 6). The shank of the bit is gripped and released by the jaws J,J, worked through a toggle joint by movements of the disk D. When the rod begins its downward stroke, the resistance of the water in the hole slightly raises D, thus opening the jaws and releasing the bit, which falls by gravity. On reaching the end of the stroke the jaws again catch the shank of the bit and raise it for delivering another blow. The Fabian free-fall may be noted as an example of the second class (see Köhler, Lehrbuch der Bergbaukunde, p. 57). Tools are sometimes used for cutting an annular groove in the bottom of the hole, and raising to the surface the core so formed, for observing the character of the rock.

4. Rope and Drop Tools.—This method was long ago used in China. Because of its extensive application in the oil-fields it is generally designated in the United States as the “oil-well system.” In its various modifications it is often employed also in general prospecting of mineral deposits and in sinking artesian, natural gas and salt wells. One of its forms is known in England as the Mather &amp; Platt system.

The chief point of difference from rod-boring is the substitution of rope for the jointed rods. For deep boring it possesses the advantage of saving the large amount of time consumed in raising and lowering the rods, as required whenever the hole is to be cleaned out, or a dull bit replaced, since the tools are rapidly run up or down by means of the rope with which they are operated while drilling. The speed of rope-boring is therefore but little affected by increase of depth, while with rod-boring it falls off rapidly. In its simplest form the so-called “string of tools,” suspended from the rope, is composed of the bit or drill, jars and rope-socket. The jars are a pair of sliding links, similar to those used for rod-boring, but serving a different purpose, viz. to produce a sharp shock on the upward stroke, as the jars come together, for loosening the bit should it tend to stick fast in the hole. A heavy bar (auger stem) is generally inserted between the jars and bit, for increasing the force of the blow. The weight of another bar above the jars (sinker-bar) keeps the rope taut. The length of stroke and feed are regulated by the “temper-screw” (fig. 7), a feed device resembling that used for rod-boring. Clamped to it is the drill rope, which is let out at intervals, as the hole is deepened. The bits usually range from 3 to 8 in. diameter, the speed of boring being generally between 20 and 40 ft. per 24 hours, according to the kind of rock. A great variety of special “fishing tools” are made, for use in case of breakage of parts in the hole or other accident. 5. Diamond Drill.—The methods described above are capable of boring holes vertically downward only. By the diamond drill, holes can be bored in any direction, from vertically downward to vertically upward. It has the further advantage of making an annular hole from which is obtained a core, furnishing a practically complete cross-section of the strata penetrated; the thickness and character of each stratum are shown, together with its depth below the surface. Thus, the diamond drill is peculiarly well adapted for prospecting mineral deposits from which samples are desired. The first practical application of diamonds for drilling in rock was made in 1863 by Professor Rudolph Leschot, a civil engineer of Paris.

The apparatus consists essentially of a line of hollow rods, coupled by screw joints, an annular steel bit or crown, set with diamonds, being attached to the lower end. By means of a small engine on the surface the rods are rapidly rotated and fed down automatically as the hole deepened. The speed of rotation is from 300 to 800 revolutions per minute, depending on the character of the rock and diameter of the bit. While boring a stream of water is forced down the hollow rods by a pump, passing back to the surface through the annular space between the rods and the walls of the drill hole. The cuttings are thus carried to the surface, leaving the bottom of the hole clean and unobstructed. For recovering the core and inspecting the bit and diamonds, the rods are raised at every 3 to 8 ft. of depth. This is done by a small drum and rope, operated by the driving engine.

Diamond drills of standard designs (fig. 8) bore holes from 1 to 2 in. diameter, yielding cores of 1 to 1 in. diameter, and are capable of reaching depths of a few hundred to 4000 ft. or more. They require from 8 to 30 boiler horse-power. Large machines will bore shallower holes up to 6, 9 or even 12 in. diameter. For operating in underground workings of mines, small and compact machines are sometimes mounted on columns (fig. 9). They bore 1 to 1 in. holes to depths of 300 to 400 ft., cores being to 1 in. diameter. Hand-power drills are also built. In the South African goldfields several diamond drill holes from 4500 to 5200 ft. deep have been successfully bored. Rates of advance for core-drilling to moderate depths range usually from 2 to 3 ft. per hour, including ordinary delays, though in favourable rock much higher speeds are often attained. In deep holes the speeds diminish, because of time consumed in raising and lowering the rods. If no core is desired a “solid bit” is used. The drilling then proceeds faster, as it is only necessary to raise the rods occasionally, for examining the condition of the bit.

The driving engine has two inclined cylinders, coupled to a crank-shaft, by which, through gearing, the drill-rod is rotated. The rods are wrought iron or steel tubes, in 5 to 10 ft. lengths. For producing the feed two devices are employed, the differential screw and hydraulic cylinder. For the differential feed (fig. 9) the engine has a hollow left-hand threaded screw-shaft, to which the rods are coupled. This shaft is driven by a spline and bevel gearing and is supported by a threaded feed-nut, carried in the lower bearing. Geared to the screw-shaft is a light counter-shaft. By properly proportioning the number of teeth in the system of gear-wheels, the feed-nut is caused to revolve a little faster than the screw-shaft, so that the drill-rod is fed downward a small fraction of an inch for each revolution. To vary the rate of feed, as suitable for different rocks, three pairs of gears with different ratios of teeth are provided. The screw-shaft and gearing are carried by a swivel-head, which can be rotated in a vertical plane, for boring holes at an angle.

The hydraulic feed is an improvement on the above, in that the rate of feed is independent of the rotative speed of the rods and can be adjusted with the utmost nicety. There are either one or two feed cylinders, supplied with water from the pump. The rod, while rotating freely, is supported by the feed cylinder piston and caused to move slowly downward by allowing the water to pass from the lower to the upper part of the cylinder. A valve regulates the passage of the water and hence the rate of feed.

The bit (fig. 10 and fig. 11, B) is of soft steel, set with six to eight or more diamonds according to its diameter. The diamonds, usually from 1 to 2 carats in size, are carefully set in the bit, projecting but slightly from its surface. Two kinds of diamonds are used, “carbons” and “borts.” The carbons are opaque, dark