_Machine Rock-Drills._--The most remarkable advance, which in recent, or perhaps in any, times has been made in the practice of mining consists in the substitution of machine for hand labour in rock boring. The importance of this change is obvious, and very great. Not only is the miner relieved thereby of the labour of boring, but the speed with which the shot-holes may be bored is increased a hundredfold. This gain of speed offers many practical advantages. The ability to sink a shaft or to drive a heading rapidly may ensure the success of an undertaking, and save indirectly the expenditure of large sums of money; and, in all cases, it allows the time spent in preparatory work to be materially shortened. Indeed, it would be difficult to over-estimate the magnitude of the advantage accruing from the increased rate of progress due to the substitution of machine power for hand labour, and in the future we may expect to see its application greatly extended. In making this substitution, numerous difficulties have had to be overcome, and in encountering these many failures have had to be recorded. But it must now be conceded by the most prejudiced that rock-boring machines have successfully passed through what may be described as the tentative stage of their existence, and have taken a foremost place among the mechanical appliances which experience has shown to be capable of effectually performing the work required of them. In the author’s work on ‘Mining Engineering,’ the requirements of a rock drill will be found fully discussed, and the principles and the construction of the most important machines now in use carefully explained and described. In the present work, only one example can be given.
Machine drills penetrate rock in the same way as the ordinary hand drills already described, namely, by means of a percussive action. The cutting tool is in most cases attached directly to the piston rod, with which it consequently reciprocates. Thus the piston with its rod is made to constitute a portion of the cutting tool, and the blow is then given by the direct action of the steam, or the compressed air, upon the tool. As no work is done upon the rock by the back stroke of the piston, the area of the forward side is reduced to the dimensions necessary only to lift the piston, and to overcome the resistance due to the friction of the tool in the bore-hole. The piston is made to admit steam or air into the cylinder, and to cut off the supply, and to open the exhaust, as required, by means of tappet valves, or other suitable devices; and provision is made to allow, within certain limits, a variation in the length of the stroke. During a portion of the stroke, means are brought into action to cause the piston to rotate to some extent, for the purposes that have been already explained. To keep the cutting edge of the tool up to its work, the whole machine is moved forward as the rock is cut away. This forward or “feed” motion is usually given by hand, but in some cases it is communicated automatically. The machine is supported upon a stand or framing which varies in form according to the situation in which it is to be used. This support is in all cases constructed to allow of the feed motion taking place, and also of the cutting tool being directed at any angle. The support for a rock drill constitutes an indispensable and a very important adjunct to the machine, for upon the suitability of its form, material, and construction, the efficiency of the machine will largely depend.