Rock-Excavating Machines.
—At one period during the work of constructing the Hoosac tunnel considerable attention was devoted to the development of a rock excavating, boring, or tunneling machine. This device was designed to cut a groove around the circumference of the tunnel thirteen inches wide and twenty-four feet in diameter by means of revolving cutters. It proved a failure, as did one of smaller size, eight feet in diameter, tried subsequently. During and before the Hoosac tunnel work a number of boring-machines of similar character were experimented with at the Mont Cenis tunnel and elsewhere in Europe; but, like the American devices, they were finally abandoned as impracticable.
Hand Drills.
—Briefly described, a drill is a bar of steel having a chisel-shaped end or cutting-edge. The simplest form of hand drill is worked by one man, who holds the drill in one hand, and drives it with a hammer wielded by his other hand. A more efficient method of hand-drill work is, however, where one man holds the drill, and another swings the hammer or sledge. Another form of hand drill, called a churn drill, consists of a long, heavy bar of steel, which is alternately raised and dropped by the workman, thus cutting a hole by repeated impacts.
In drilling by hand the workman holding the drill gives it a partial turn on its axis at every stroke in order to prevent wedging and to offer a fresh surface to the cutting-edge. For the same reason the chips and dust which accumulate in the drill-hole are frequently removed. The instruments used for this purpose are called scrapers or dippers, and are usually very simple in construction. A common form is a strong wire having its end bent at right angles, and flattened so as to make a sort of scoop by which the drillings may be scraped or hoisted out of the hole. It is generally advantageous to pour water into the drill-hole while drilling to keep the drill from heating.
Power Drills.
—When the conditions are such that use can be made of them, it is nearly always preferable to use power drills, on account of their greater speed of penetration and greater economy of work. Power drills are worked by direct steam pressure, or by compressed air generated by steam or water power, and stored in receivers from which it is led to the drills through iron pipes. A great variety of forms of power drills are available for tunnel work in rock, but they can nearly all be grouped in one of two classes: (1) Percussion drills, and (2) Rotary drills.
Percussion Drills.
—The first American percussion drill was patented by Mr. J. J. Couch of Philadelphia, Penn., in March, 1849. In May of the same year, Mr. Joseph W. Fowle, who had assisted Mr. Couch in developing his drill, patented a percussion drill of his own invention. The Fowle drill was taken up and improved by Mr. Charles Burleigh, and was first used on the Hoosac tunnel. In Europe Mr. Cavé patented a percussion drill in France in October, 1851. This invention was soon followed by several others; but it was not until Sommeiller’s drill, patented in 1857 and perfected in 1861, was used on the Mont Cenis tunnel, that the problem of the percussion drill was practically solved abroad. Since this time numerous percussion drill patents have been taken out in both America and Europe.
A percussion drill consists of a cylinder, in which works a piston carrying a long piston rod, and which is supported in such a manner that the drill clamped to the end of the piston rod alternately strikes and is withdrawn from the rock as the piston reciprocates back and forth in the cylinder. Means are devised by which the piston rod and drill turn slightly on their axis after each stroke, and also by which the drill is fed forward or advanced as the depth of the drill-hole increases. The drills of this type which are in most common use in America are the Ingersoll-Sergeant and the Rand. There are various other makes in common use, however, which differ from the two named and from each other chiefly in the methods by which the valve is operated. All of these drills work either with direct steam pressure or with compressed air. Workable percussion drills operated by electricity are built, but so far they do not seem to have been able to compete commercially with the older forms. No attempt will be made here to make a selection between the various forms of percussion drills for tunnel work, and for the differences in construction and the merits claimed for each the reader is referred to the makers of these machines. All of the leading makes will give efficient service. It goes almost without saying that a good percussion drill should operate with little waste of pressure, and should be composed of but few parts, which can be easily removed and changed.