This form of drill is exhibited in [Fig. 1], which represents a common “jumper” borer. It consists of a rod terminating at each end in a chisel edge, and having a swell, technically described as the “bead,” between the extremities to give it weight. The bead divides the jumper into two unequal portions, each of which constitutes a chisel bit, with its shank or “stock.” The shorter stock is used while the hole is shallow, and the longer one to continue it to a greater depth.
Fig. 1.
Fig. 2.
Fig. 3.
With the jumper, the blow is obtained from the direct impact of the falling tool. The mode of using the instrument is to lift it with both hands to a height of about a foot, and then to let it drop. In lifting the jumper, care is taken to turn it partially round, that the edge may not fall twice in the same place. By this means, the edge is made to act most favourably in chipping away the rock, and the hole is kept fairly circular. So long as the holes are required to be bored vertically downwards, the jumper is a convenient and very efficient tool, and hence in open quarrying operations, it is very commonly employed. But in mining, the shot-holes are more often required to be bored in some other direction, or, as it is termed, “at an angle;” that is, at an angle with the vertical. Or it may be that a shot-hole is required to be bored vertically upward. It is obvious that in any one of these directions the jumper is useless. To meet the requirements of such cases, recourse is had to the hammer wherewith to deliver the blow, and the drill is constructed to be used with the hammer. We have a suitable form of tool for application in this wise when we cut out the bead of the jumper and leave the ends flat for a striking face, as shown in [Figs. 2] and [3]. The form of the two chisels thus obtained is that adopted for the ordinary rock drill.
It will be understood from these descriptions that a rock drill consists of the chisel edge or bit, the stock, and the striking face. Formerly drills were made of wrought iron, and steeled at each end to form the bit and the striking face. Now they are commonly made of cast steel, which is supplied for that purpose in octagonal bars of the requisite diameter. The advantages offered by steel stocks are numerous. The superior solidity of texture of that material renders it capable of transmitting the force of a blow more effectively than iron. Being stronger than the latter material, a smaller diameter of stock, and, consequently, a less weight, are sufficient. This circumstance also tends to increase the effect of the blow by diminishing the mass through which it is transmitted. On the other hand, a steel stock is more easily broken than one of iron.
The cutting edge of a drill demands careful consideration. To enable the tool to free itself readily in the bore-hole, and also to avoid introducing unnecessary weight into the stock, the bit is made wider than the latter; the difference in width may be as much as 1 inch. It is evident that in hard rock, the liability of the edge to fracture increases as the difference of width. The edge of the drill may be straight or slightly curved. The straight edge cuts its way somewhat more freely than the curved, but it is weaker at the corners than the latter, a circumstance that renders it less suitable for very hard rock. It is also slightly more difficult to forge. The width of the bit varies, according to the size of the hole required, from 1 inch to 21⁄2 inches. [Figs. 4], [5], and [6] show the straight and the curved bits, and the angles of the cutting edges for use in rock.