Section II.—Machine Boring.
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.
The foregoing is a general description of the construction and mode of action of percussive rock-drills. The numerous varieties now in use differ from each other rather in the details of their construction than in the principles of their action, and the importance of the difference is, of course, dependent upon that of the details. It is but just to remark here that the first really practical solution of the rock-drilling problem is due to M. Sommeiller, whose machine was employed in excavating the Mont Cenis tunnel.
The Darlington Drill.
—The machine which, in England, has stood the test of experience most satisfactorily, and which, consequently, is surely working itself into general favour in this country, and also in some of the important mining districts of the Continent, is the invention of John Darlington, and is known as the “Darlington drill.” This drill is remarkable as the attainment of the highest degree of simplicity of parts possible in a machine. The valve gear of a machine drill is especially liable to derangement. It must necessarily consist of several parts, and these parts must as necessarily be of a somewhat fragile character. Besides this, when actuated by the piston through the intervention of tappets, the violence of the blow delivered at each stroke is such as to rapidly destroy the parts. In some machines, the force of these blows and their destructive tendency have been reduced to a minimum; but when every means of remedying the evil has been employed, there remains a large amount of inevitable wear and tear, and a liability to failure from fracture or displacement exists in a greater or less degree. Moreover, as these effects are greatly intensified by increasing the velocity of the piston, it becomes at least undesirable to use a high piston speed. To remedy these defects, which are inherent in the system, Darlington proposed to remove altogether the necessity for a valve gear by radically changing the mode of admitting the motor fluid to the cylinder. This proposal he has realized in the machine which is illustrated on [Plate IV.]
The Darlington rock-drill consists essentially of only two parts: the cylinder A, [Figs. 20 and 21], with its cover; and the piston B, with its rod. The cover, when bolted on, forms a part of the cylinder; the piston rod is cast solid with the piston, and is made sufficiently large at its outer end to receive the tool. These two parts constitute an engine, and with less than one fixed and one moving part it is obviously impossible to develop power in a machine by the action of an elastic fluid. The piston itself is made to do the work of a valve in the following manner: The annular space affording the area for pressure on the fore part of the piston gives a much smaller extent of surface than that afforded by the diameter of the cylinder, as shown in the drawing; and it is obvious that by increasing or diminishing the diameter of the piston rod, the area for pressure on the one side of the piston may be made to bear any desired proportion to that on the other side. The inlet aperture, or port C, being in constant communication with the interior of the cylinder, the pressure of the fluid is always acting upon the front of the piston, consequently when there is no pressure upon the other side, the piston will be forced backward in the cylinder. During this backward motion, the piston first covers the exhaust port D, and then uncovers the equilibrium port E, by means of which communication is established between the front and back ends of the cylinder, and, consequently, the fluid is made to act upon both sides of the piston. The area of the back face of the piston being greater than that of the front face by the extent occupied by the piston rod, the pressure upon the former first acts to arrest the backward motion of the piston, which, by its considerable weight and high velocity, has acquired a large momentum, and then to produce a forward motion, the propelling force being dependent for its amount upon the difference of area on the two sides of the piston. As the piston passes down, it cuts off the steam from the back part of the cylinder and opens the exhaust. The length or thickness of the piston is such that the exhaust port D is never open to its front side, but, in the forward stroke, it is opened almost immediately after the equilibrium port is closed, and nearly at the time of striking the blow. It will be observed that the quantity of fluid expended is only that which passes over to the back face of the piston, since that which is used to effect the return stroke is not discharged.
The means employed to give a rotary motion to the tool are deserving of special attention, as being simple in design, effective in action, and well situate within the cylinder. These means consist of a spiral or rifled bar H, having three grooves, and being fitted at its head with a ratchet wheel G, recessed into the cover of the cylinder. Two detents J, J, [Fig. 22], also recessed into the cover, are made to fall into the teeth of the ratchet wheel by spiral springs. These springs may, in case of breakage, be immediately renewed without removing the cover. It will be observed that this arrangement of the wheel and the detents allow the spiral bar H to turn freely in one direction, while it prevents it from turning in the contrary direction. The spiral bar drops into a long recess in the piston, which is fitted with a steel nut made to accurately fit the grooves of the spiral. Hence the piston, during its instroke, is forced to turn upon the bar; but, during its outstroke, it turns the bar, the latter being free to move in the direction in which the straight outstroke of the piston tends to rotate it. Thus the piston, and with it the tool, assumes a new position after each stroke.
The mode of fixing the cutting tool to the piston rod is a matter deserving some attention. As the tool has to be changed more than once during the progress of a bore-hole, it is important that the change should be accomplished in as short a time as possible; and as the vibration of the machine and the strain upon the tool are necessarily great, it is equally important that the tool be firmly held. It is also desirable that the mode of fixing the tool shall not require a shoulder upon the latter, a slot in it, or any peculiarity of form difficult to be made in the smithy. The Darlington machine fulfils the requirements of expedition in fixing, firmness of retention, and simplicity of form most satisfactorily. The means and the method are the following: The outer end of the rod or holder is first flattened to afford a seat for the nut, as shown in [Figs. 21 and 25]. The slot is then cut and fitted tightly with a piece of steel K forged of the required shape for the clamp, and the holder is afterwards bored to receive the tool while the clamp is in place. This clamp K is then taken out, its fittings eased a little, and its end screwed and fitted with a nut. When returned to its place in the holder, the clamp, in consequence of the easing, can be easily drawn tight against the tool, by which means it is firmly held in position. The shank of the tool is turned to fit the hole easily, and the end of it is made hemispherical to fit the bottom of the hole, upon which the force of the reaction of the blow is received.
It would seem impossible to attain a higher degree of simplicity of form, or to construct a machine with fewer parts. The absence of a valve or striking gear of any kind ensures the utmost attainable degree of durability, and allows a high piston speed to be adopted without risk or injury. As the piston controls its own motion, there is no liability to strike against the cylinder cover. The stroke may be varied in length from half an inch to four inches, and as the machine will work effectively with a pressure of 10 lb. to the inch, holes may be started with the greatest ease. With a pressure of 40 lb., the machine makes 1000 blows a minute, a speed that may be attained without causing undue strains or vibration. This alone constitutes a very great advantage. It must indeed be conceded that an unprejudiced consideration of the merits of this drill shows it to be admirably adapted to the work required of it.
Borer-Bits.
—The form and the dimensions of the cutting tools, variously described as “drills,” “borers,” and “bits,” used with machine rock-perforators are matters of great practical importance. The dimensions are determined mainly by two conditions, namely, the necessity for sufficient strength in the shank of the tool, and the necessity for sufficient space between the shank and the sides of the hole to allow the débris to escape. Experience has shown that the latter condition is best fulfilled when the distance between the sides of the hole and the shank of the tool is from 3⁄16 inch to 1⁄4 inch, regard being had to the former condition.
The form of the cutting edge is determined by several conditions, some of which have been already discussed in relation to hand drills. The form first adopted was naturally that possessed by the hand drill, namely, the chisel edge. To increase the useful effect of the blow, the cutting edge was subsequently doubled, the bit being formed of two chisel edges crossing each other at right angles. This bit, which from its form was called the “cross” bit, was found to penetrate the rock more rapidly than the straight or chisel bit. The gain in speed was very marked at the commencement of the hole; but it diminished gradually as the hole progressed in depth, owing to the difficulty with which the débris escaped. To remedy this defect, the cutting edges were next made to cross each other obliquely, so as to form the letter X. In this way, the two chisel edges were retained, while the breadth of the bit was considerably reduced. This form, described as the X bit, cleared the hole much more effectively than the cross, but not in a manner that was altogether satisfactory. Another modification of the form was, therefore, made, and this time that of the Z was adopted, the upper and the lower portions of which were arcs of circles struck from the centre of the bit in the direction contrary to that of the rotation.
This form of tool, which is known as the Z bit, readily cleared itself of the débris. But besides this advantage, it was found to possess others of an important character. With the chisel-edge forms, the corners of the bit were rapidly worn off by friction against the sides of the hole. With the Z form, this wearing no longer occurred, by reason of the large surface exposed to friction. Another advantage of the Z form of bit lies in its tendency to bore the hole truly circular. Generally then, it may be stated that this form satisfies most fully the determining conditions. The form of bit, however, that is most suitable in a given case will, in some degree, be determined by particular circumstances. Of these, the nature and the character of the rock will operate most strongly to influence the choice. Thus the cross bit will generally be found the most suitable in fissured rock, while the single chisel edge may be used with advantage in rock of a very solid and hard character. Indeed, on the judicious selection of the most suitable form of cutting edge, the success of machine boring largely depends. The chisel bit, the cross bit, the X bit, and the Z bit, are shown in [Figs. 24] to [27].
Fig. 24.
Fig. 25.
Fig. 26.
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.
The Stretcher Bar.
—The simplest kind of support is the “stretcher bar.” This consists essentially of a bar so constructed that it may be lengthened or shortened at pleasure, by means of a screw. It is fixed in position by screwing the ends into firm contact with the sides, or with the roof and the floor, of a heading. The machine is fixed to this bar by means of a clamp, which, when loosened, slides along the bar, and allows the drill to be placed in the required position, and to be directed at the required angle. The bar illustrated in [Fig. 26, Plate V.], is that which is used with the Darlington drill; in it, lightness and rigidity are combined in the highest possible degree by the adoption of the hollow section. The mode of setting the bar in a heading is shown in the drawing; the end claws are set against pieces of wood on the floor and the roof, and are tightened by turning the screw with a common bar.
The simple stretcher bar is frequently used in narrow drivings and in shafts of small diameter. But a more satisfactory support in drivings is afforded by a bar suitably mounted upon a carriage designed to run upon rails. The carriage consists simply of a trolly, to the fore part of which the bar is fixed usually by some kind of hinge-joint. It is obvious that the details of the construction of this support may be varied greatly, and numerous designs have been introduced and adopted. In [Figs. 27 and 28, Plate VI.], is shown a support of this character designed by J. Darlington. A single vertical bar is carried on the fore part of the trolly, and fixed, by the usual means, against the centre of the roof. This vertical bar carries an arm, which is capable of turning upon it, as upon a centre, and of sliding up and down it. This arm carries the drill. The central bar having been fixed in position, the arm is slid up to the highest position required, and fixed against the side of the heading. A row of holes are then bored from this arm. When these are completed, the arm is lowered the requisite distance, and another row of holes are bored. This is continued until all the holes are bored over one-half the face. The arm is then swung round, and fixed against the other side of the heading, and the holes are bored over that half the face in like manner. In this way, one-half the heading is kept clear to allow the operations of removing the dislodged rock to be carried on at the same time. If desired, two arms may be used. This arrangement gives undoubtedly great facilities for working the drill, and leaves the heading comparatively unencumbered.
In shaft sinking, the same support, slightly modified, is used without the trolly. The arrangement adopted in this case is shown in [Fig. 29, Plate VII.] The central bar is held firmly in its position by a cross stretcher bar set against the sides of the shaft. The arms are made to revolve upon this bar to allow the holes to be bored in the positions required. When all the holes have been bored, the support, with the machines, is hauled up, by means of a chain attached to the central bar, out of the way of the blast. With this support, the time of fixing, raising, and lowering is reduced to a minimum; while the facility with which the machines may be slid along and fixed to the arm, and the positions of the latter changed, allows the boring to be carried on rapidly.
For open work, as in quarrying, where the stretcher bar cannot be used, the tripod stand is adopted.
The Dubois-François Carriage.
—The support commonly used in France and in Belgium consists of a kind of carriage carrying bars upon which the drills are set. This carriage is used in drivings of all kinds; but it is particularly suitable for tunnelling. It has been adopted, with but slight modification, in the St. Gothard tunnel, and in several other important works of the like character.
A modification of the carriage is shown in [Figs. 30 and 31]. Being designed for ordinary mining operations, it carries but two machines; but it will be readily perceived that, by increasing the number of vertical screws, the same support may be made to carry a larger number. It consists essentially of a vertical frame of flat bar iron a b c d, 8 feet in length, and 4 feet 9 inches in height above the rails, the hinder portion of which rests upon a cast-iron plate e f g h, carried upon two wheels; on this are fixed the two uprights l, l′, which, being bound to the upper part by a transverse bar m m′, form a framing to serve as a support to the two vertical screws p′, q′. The front framing is formed of two longitudinals b c and b′ c′ and the uprights a, a′, and the vertical screws p, q, which are connected to the upper part by the single piece a d. This framing is supported below upon a small trolly with four wheels, connected to the two longitudinals of the framing by a pivot bolt n of T form, the bar of the T being inserted into the elongated openings o cut through the middle of the curved portion of the longitudinals. The cast-iron plate behind, the use of which is only to give stability to the carriage, carries above it, by means of the two curved pieces h, h′, a wrought-iron plate V, upon which the small tools needed for repairs are kept. Two screws, s, s′, carried by lugs cast upon the back of the plate, serve, by turning them down upon the rails, to fix the carriage, the latter being slightly lifted by the screws.
Each machine is supported at two points. Behind, the point of support is given by a cast-iron bracket t, having a projecting eye which enters between the two cheeks formed at the back end of the machine by the continuations of the two longitudinals of the framing. A pin bolt, carried by the machine, allows the latter to be fixed to the bracket, while leaving sufficient freedom of motion to allow of its being directed at the required angle. This bracket, shown in plan in [Fig. 33], is supported by a kind of nut, [Fig. 32], having two handles whereby it may be easily turned. By raising or lowering this, the hinder support of the drill may be brought to the requisite height. To prevent it turning upon the screw, a pin is passed through the hole o, which pin forms a stop for the handles aforementioned. The rotation of the bracket itself is rendered impossible by the form of the vertical screw upon which it is set, as shown in [Fig. 33]. In front, the support is a fork, the shank of which slides along in the piece U, [Figs. 30 and 31]. This support, which is not screwed on the inside, rests upon a nut of the same form as that already described, and the same means are employed to prevent rotation as in the case of the hinder supports.