Fig. 27.
The sharpening of bits of a form other than that of the chisel is done by means of “swages.” The tempering is effected in the way already [described] in reference to hand drills. As in the latter case, the degree of temper must be suited to the hardness of the rocks to be penetrated. Generally the straw colour will be found to be the best degree. It is a remarkable fact that the wear of the cutting edge of a machine drill is, for a given length of boring, five or six times less than that of a hand drill. Steel of the best quality should always be used.
As in the case of hand boring, each successive length of drill must diminish slightly in the width of its cutting edge; a diminution of about 1⁄32 inch may be considered sufficient. Care should, however, be taken to ensure the proper dimensions being given to the edge, and it will be found advantageous to have at hand an accurate gauge through which the tool may be passed previously to its being fixed to the machine. It is important that the tool be truly “centred,” that is, the centres of the edge of the bit, of the shank, and of the piston rod, should be perfectly coincident.
Rock-Drill Supports.
—A machine rock-drill may satisfy every requirement, and yet, by reason of the defective character of the support to which it is attached, it may be unsuitable to the work required of it. Hence it becomes desirable to carefully study the design and construction of a drill support, and to consider the requirements which it is needful to fulfil. Assuming the necessity for a high degree of strength and rigidity in the support, a primary condition is that it shall allow the machine to be readily adjusted to any angle, so that the holes may be bored in the direction and with the inclination required. When this requirement is not fulfilled, the machine is placed, in this respect, at a great disadvantage with hand labour. If a machine drill were not capable of boring in any position and in any direction, hand labour would have to be employed in conjunction with it, and such incompleteness in the work of a machine would constitute a serious objection to its adoption.
Besides allowing of the desired adjustment of the machine, the support must be itself adjustable to uneven ground. The bottom of a shaft which is being sunk, or the sides, roof, and floor of a heading which is being driven, present great irregularities of surface, and, as the support must of necessity in most cases be fixed to these, it is obvious that its design and construction must be such as will allow of its ready adjustment to these irregularities. The means by which the adjustment is effected should be few and simple, for simplicity of parts is important in the support as well as in the machine, and for the same reasons. A large proportion of the time during which a machine drill is in use is occupied in shifting it from one position or one situation to another; this time reduces, in a proportionate degree, the superiority of machine over hand labour, in respect of rapidity of execution, and it is evidently desirable that it should be shortened as far as possible. Hence the necessity for the employment of means of adjustment which shall be few in number, rapid in action, and of easy management.
For reasons similar to the foregoing, the drill support must be of small dimensions, and sufficiently light to allow of its being easily portable. The limited space in which rock drills are used renders this condition, as in the case of the machine itself, a very important one. It must be borne in mind that, after every blast, the dislodged rock has to be removed, and rapidity of execution requires that the operations of removal should be carried on without hindrance. A drill support that occupies a large proportion of the free space in a shaft or a heading is thus a cause of inconvenience and a source of serious delay. Moreover, as it has to be continually removed from one situation to another, it should be of sufficiently light weight to allow of its being lifted or run along without difficulty. In underground workings, manual power is generally the only power available, and therefore it is desirable that both the machine and its support should be of such weight that each may be lifted by one man. Of course, when any endeavour is made to reduce the weight of the support, the necessity for great strength and rigidity must be kept in view.
In spacious headings, such as are driven in railway tunnel work, supports of a special kind may be used. In these situations, the conditions of work are different from those which exist in mines. The space is less limited, the heading is commenced at surface, and the floor is laid with a tramway and sidings. In such a case, the support may consist of a more massive structure mounted upon wheels to run upon the rails. This support will carry several machines, and to remove it out of the way when occasion requires, it will be run back on to a siding; but for ordinary mining purposes, such a support is suitable.