Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/465

Rh MACHINE DRILLS.] MINING 447 brought down about 400 fathoms from surface, at a pressure of 50 to 55 ft to the square inch, in wrought-iron pipes 2 inches in diameter in the shaft, and 1| Jnches in the level, and admitted through a flexible tube into the inlet I on the left-hand side of the cylinder. The cost of the pipes is rather under 7d. a foot, or about 3s. 3d. per fathom. The air is compressed at the surface by a 14-inch compressor, worked by a 12-inch horizontal engine, capable, however, of working two machine drills. The gross weight of the machine, including the bed-plate and gudgeon, is about 115 lt&amp;gt;.&quot; The method of fixing the machine for work is as follows : &quot;The bed-plate A of the machine is formed with a gudgeon A which fits into, and can be adjusted to any position in, a socket formed in or on a clamp B, which can be fixed on any part of the wrought-irou bar or column B, thus forming a universal joint. This bar or column Scale, figs. 29, 30. Inches 12 2 Feet. FIG. 29. Side Elevation of Darlington s Rock-Drill. Scale T VV can be placed in position either horizontally or vertically, as may be most convenient, but is generally placed across the level, against the sides of which it is secured by means of the clamp L, and adjusting screw M, and claws N and N. If necessary, wooden wedges 0, are driven in between the claws and the wall to make it still firmer. The weight of the bar is about 120 Ib.&quot; 1 FIG. 30. Horizontal Section of Darlington s Rock-Drill. Scale Air-compressing plant of greater size has now been erected at Dolcoath mine, to which the above description refers. At Snail- beach mine in Shropshire they have two air-compressors of 18 inches Fig. 31. diameter and 5 feet stroke ; the air-main is at first 9 inches in dia meter, then 6 inches, whilst 2-inch gas-pipe is used in the levels. A rock-drill which has done, and is doing, excellent work is that of Mr John Darlington. Its construction will be understood by Darling - referring to figs. 29, 30, and 31 ; a is the cylinder, b the piston rod, ton drill. c the borer ; d, d are two openings for bringing in compressed air, either of which may be used according to the position of the drill ; e is the inlet hose with a stopcock, /drill -holder, g stretcher bar, h piston, j rifled bar for turning piston and drill, k ratchet wheel attached to rifled bar, I rifled nut fixed in the piston head, m wood for lessening weight of piston rod and blocking space, n portway for allowing the compressed air to pass to the top of the piston and give the blow, o exhaust portway. The action of the drill is as follows. The compressed air is always acting on the underside of the piston, and when the upper side of the piston communicates with the outer atmosphere the piston moves rapidly backwards and uncovers the portway n. The compressed air rushes through and presses against the upper side of the piston, which has a greater area than the lower side, the difference being equal to the area of the piston rod. The piston is driven rapidly downwards and the drill strikes its blow. At the same time it uncovers the exhaust port o and then the con stant pressure on the annular area on the underside of the piston produces the return stroke. The number of blows per minute is from six hundred to eight hundred. The rotation of the drill is effected by the rifled bar. On the down-stroke of the piston the bar with its ratchet wheel is free to turn under a couple of pawls, and consequently the piston moves straight whilst the bar and ratchet wheel turn. When the up-stroke is being made the ratchet wheel is held by the pawls and the piston is forced to make part of a revolution. As the hole is deepened the cylinder is advanced forwards by turning the handle p ; this works an endless screw q Proc. Mining Institute of Cornwall, vol. i., 1877, p. 12.