CHAPTER I. THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN BLASTING ROCKS. Section I.--Hand Boring. Drills.

—The operations of blasting consist in boring suitable holes in the rock to be dislodged, in inserting a charge of some explosive compound into the lower portion of these holes, in filling up, sometimes, the remaining portion of the holes with suitable material, and in exploding the charge. The subjects which naturally first present themselves for consideration are: the nature, form, and construction of the tools, machines, and other appliances used. Of these tools, the “drill” or “borer” constitutes the chief. To understand clearly the action of the rock drill, we must consider the nature of the substance which has to be perforated. He who has examined the mineral constitution of rocks will have recognised the impossibility of cutting them, using that term in its ordinary acceptation, inasmuch as the rock constituents are frequently harder than the material of the tools employed to penetrate them. As a rock cannot be cut, the only way of removing portions of it is to fracture or to disintegrate it by a blow delivered through the medium of a suitable instrument. Each blow so delivered may be made to chip off a small fragment, and by this means the rock may be gradually worn away. To effect this chipping, however, the instrument used must present only a small surface to the rock, in order to concentrate the force, and that surface must be bounded by inclined planes or wedge surfaces, to cause a lateral pressure upon the particles of rock in contact with them. In other words, the instrument must be provided with an edge similar to that possessed by an ordinary cutting tool.

The conditions under which the instrument is worked are obviously such that this edge will be rapidly worn down by attrition from the hard rock material, and by fracture. To withstand these destructive actions, two qualities are requisite in the material of which the instrument is composed, namely, hardness and toughness. Thus there are three important conditions concurring to determine the nature and the form of a cutting tool to be used in rock boring—1, a necessity for a cutting edge; 2, a necessity for a frequent renewal of that edge; and 3, a necessity for the qualities of hardness and toughness in the material of the tool.

In very hard rock, a few minutes of work suffice to destroy the cutting edge, and then the tool has to be returned to the smithy to be re-sharpened. Hence it is manifest that the form of the edge should not be one that is difficult to produce, since, were it so, much time would be consumed in the labour of re-sharpening. Experience has shown that the foregoing conditions are most fully satisfied in the steel rod terminating in a simple chisel edge, now universally adopted.

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.

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.

The stock is octagonal in section; it is made in lengths varying from 20 inches to 42 inches. The shorter the stock the more effectively does it transmit the force of the blow, and therefore it is made as short as possible. For this reason, several lengths are employed in boring a shot-hole, the shortest being used at the commencement of the hole, a longer one to continue the depth, and a still longer one, sometimes, to complete it. To ensure the longer drills working freely in the hole, the width of the bit should be very slightly reduced in each length. It has already been remarked that the diameter of the stock is less than the width of the bit; this difference may be greater in coal drills than in rock or “stone” drills; a common difference in the latter is ³/₈ of an inch for the longer. The following proportions may be taken as the average adopted:—

	Width of the Bit.			Diameter of the Stock.
	1		inch		5/8	inch
	1	1/8	„		3/4	„
	1	1/4	„		7/8	„
	1	1/2	„	1		„
	1	3/4	„	1	1/8	„
	2		inches	1	3/8	„
	2	1/4	„	1	1/2	„
	2	1/2	„	1	5/8	„

The striking face of the drill should be flat. The diameter of the face is less than that of the stock in all but the smallest sizes, the difference being made by drawing in the striking end. The amount of reduction is greater for the largest diameters; that of the striking face being rarely more than one-eighth of an inch.

The making and re-sharpening of rock drills constitute an extremely important part of the labour of the mine smith. The frequent use of the drill, and its rapid wear, necessitate a daily amount of work of no trifling proportions, and the judgment and skill required in proper tempering render some degree of intelligence in the workman indispensable; indeed, so much depends upon the smith whose duty it is to repair the miners’ tools, that no pains should be spared to obtain a man capable of fulfilling that duty in the most efficient manner possible.

When the borer-steel bars are supplied to the smith, he cuts them up, as required, into the desired lengths. To form the bit, the end of the bar is heated and flattened out by hammering to a width a little greater than the diameter of the hole to be bored. The cutting edge is then hammered up with a light hammer to the requisite angle, and the corners beaten in to give the exact diameter of the bore-hole intended. As the drills are made in sets, the longer stocks will have a bit slightly narrower than the shorter ones, for reasons already given. The edge is subsequently touched up with a file. In performing these operations, heavy hammering should be avoided, as well as high heats, and care should be taken in making the heat that the steel should be well covered with coal, and far enough removed from the tuyere to be protected from the “raw” air. Overheated or “burned” steel is liable to fly, and drills so injured are useless until the burned portion has been cut away.

Both in making and in re-sharpening drills, great care is required to form the cutting edge evenly, and of the full form and dimensions. If the corners get hammered in, as shown in Fig. 7, they are said to be “nipped,” and the tool will not free itself in cutting. When a depression of the straight, or the curved, line forming the edge occurs, as shown in Fig. 8, the bit is said to be “backward,” and when one of the corners is too far back, as in Fig. 9, it is spoken of as “odd-cornered.” When either of these defects exist—and they are unfortunately common—not only does the bit work less effectively on the rock, but the force of the blow is thrown upon a portion only of the edge, which, being thereby overstrained, is liable to fracture.

The hardening and tempering of steel is a matter requiring careful study and observation. It is a well-known fact that a sudden and great reduction of temperature causes a notable increase of hardness in the metal. The reason of this phenomenon is not understood, but it is certain that it is in some way dependent upon the presence of carbon. The degree of hardness imparted to steel by this means depends upon the amount of the reduction of the temperature, and the proportion of carbon present in the metal, highly carburetted steel being capable of hardening to a higher degree, under the same conditions, than steel containing less carbon. Thus, for steel of the same quality, the wider the range of temperature the higher is the degree of hardness. But here we encounter another condition, which limits the degree of hardness practically attainable.

The change which takes place among the molecules of the metal in consequence of the change of temperature causes internal strains, and thereby puts portions in a state of unequal tension. This state renders the strained parts liable to yield when an additional strain is thrown upon them while the tool is in use; in other words, the brittleness of the steel increases with its hardness. Here again the proportion of carbon present comes into play, and it must be borne in mind that for equal degrees of hardness the steel which contains the least carbon will be the most brittle. In hardening borer-steel, which has to combine as far as possible the qualities of hardness and toughness, this matter is one deserving careful attention. It is a remarkable fact, and one of considerable practical value, that when oil is employed as the cooling medium instead of water, the toughness of steel is enormously increased.

The tempering of steel, which is a phenomenon of a similar character to that of hardening, also claims careful consideration. When a bright surface of steel is subjected to heat, a series of colours is produced, which follow each other in a regular order as the temperature increases. This order is as follows: pale yellow, straw yellow, golden yellow, brown, brown and purple mingled, purple, light blue, full clear blue, and dark blue. Experience has shown that some one of these colours is more suitable than the rest for certain kinds of tools and certain conditions of working.

The selection of the proper colour constitutes a subject for the exercise of judgment and skill on the part of the smith. For rock drills, straw colour is generally the most suitable when the work is in very hard rock, and light blue when the rock is only of moderate hardness.

The processes of hardening and tempering drills are as follows: When the edge of the bit has been formed in the manner already described, from 3 to 4 inches of the end is heated to cherry redness, and dipped in cold water to a depth of about an inch to harden it. While in the water, the bit should be moved slightly up and down, for, were this neglected, the hardness would terminate abruptly, and the bit would be very liable to fracture along the line corresponding with the surface of the water. In cold weather, the water should be slightly warmed, by immersing a piece of hot iron in it, before dipping the steel. When a sufficient degree of hardness has been attained, the remainder of the hot portion is immersed until the heat is reduced sufficiently for tempering. At this stage it is withdrawn, and the colours carefully watched for. The heat which is left in the stock will pass down to the edge of the bit, and as the temperature increases in that part the colours will appear in regular succession upon the filed surface of the edge. When the proper hue appears, the whole drill is plunged into the water and left there till cold, when the tempering is complete. When the edge is curved or “bowed,” the colours will reach the corners sooner than the middle of the bit. This tendency must be checked by dipping the corners in the water, for otherwise the edge will not be of equal hardness throughout. As the colour can be best observed in the dark, it is a good plan to darken that portion of the smithy in which tempering is being carried on.

The degree of temper required depends upon the quality of the steel and the nature of the work to be performed. The larger the proportion of carbon present in the metal, the lower must be the temper. Also the state of the blunted edges, whether battered or fractured, will show what degree of hardness it is desirable to produce. From inattention to these matters, good steel is not unfrequently condemned as unsuitable.

To form the striking face, the end of the stock is heated to a dull red, and drawn out by a hammer to form a conical head. The extremity is then flattened to form a face from ¹/₂ inch to 1 inch in diameter. This head is then annealed to a degree that will combine considerable toughness with hardness. The constant blows to which the head is subjected tend to wear it down very rapidly. There is great difference in the lasting qualities of steel in this respect; some drills will wear away more quickly at the striking than at the bit end.

A smith will, with the assistance of a striker, sharpen and temper about thirty single-hand drills of medium size in an hour, or twenty double-hand drills of medium size in the same time. Of course, much will depend on the degree of bluntness in the cutting edge; but assuming the drills to be sent up only moderately blunted, this may be taken as a fair average of the work of two men.

It will be evident from the foregoing remarks, that to enable a drill to stand properly it must be made of good material, be skilfully tempered in the smithy, and provided with a cutting edge having an angle and a shape suited to the character of the rock in which it is used. To these conditions, may be added another, namely, proper handling; for if the drill be carelessly turned in the hole so as to bring all the work upon a portion only of the cutting edge, or unskilfully struck by the sledge, fracture or blunting will speedily result. Improper handling often destroys the edge in the first five minutes of using.

Drills, as before remarked, are used in sets of different lengths. The sets may be intended for use by one man or by two. In the former case, the sets are described as “single-hand” sets, and they contain a hammer for striking the drills; in the latter case, the sets are spoken of as “double-handed,” and they contain a sledge instead of a hammer for striking. It may appear at first sight that there is a waste of power in employing two men, or, as it is termed, the double set, for that two men cannot bore twice as fast as one. This rate of speed can, however, be obtained, and is due less to the greater effectiveness of the stroke than to the fact that two men can, by repeatedly changing places with each other, keep up almost without intermission a succession of blows for an indefinite length of time; whereas, with the single set, the man is continually obliged to cease for rest.

Hammers.

—To deliver the blow upon a rock drill, hammers and sledges are used. The distinction between a hammer and a sledge is founded on dimensions only: the hammer being intended for use in one hand, is made comparatively light and is furnished with a short handle, while the sledge, being intended for use in both hands, is furnished with a much longer handle and is made heavier. The striking face of the blasting sledge should be flat, to enable the striker to deliver a direct blow with certainty upon the head of the drill; and to facilitate the directing of the blow, as well as to increase its effect, the mass of metal composing the head should be concentrated within a short length. To cause the sledge to fly off from the head of the drill in the case of a false blow being struck, and thereby to prevent it from striking the hand of the man who holds the drill, the edges of the striking face should be chamfered or bevelled down till the diameter is reduced by nearly one-half. This requirement is, however, but seldom provided for.

The head of a sledge is of iron; it consists of a pierced central portion called the “eye,” and two shanks or “stumps,” the steeled ends of which form the striking faces or “panes.” The form of the head varies in different localities, but whatever the variations may be, the form may be classed under one of four types or “patterns.” A very common form is that shown in Fig. 10 and known as the “bully” pattern. By varying the width, as shown in Fig. 11, we obtain the “broad bully,” the former being called for the sake of distinction the “narrow” bully. Another common form is the “pointing” pattern, represented in Fig. 12. The form shown in Fig. 13 is designated as the “bloat” pattern; and that given in Fig. 14 the “plug” pattern. Each of these forms possesses peculiar merits which renders it more suitable for certain uses than the others. The same forms are used for hammers. The eye is generally made oval in shape, but sometimes, especially with the bloat pattern, it is made circular, as shown in Fig. 13. The weight of a sledge head may vary from 5 lb. to 10 lb., but a common and convenient weight is 7 lb. The length of the helve varies from 20 inches to 30 inches; a common length for blasting sledges is 24 inches. The average weight of hammer heads is about 3 lb., and the average length of the helve 10 inches.

Fig. 15 represents a blasting sledge used in South Wales. The stumps are octagonal in section, and spring from a square block in the centre. The panes or striking faces, however, are circular and flat. The length of the head is 8³/₄ inches, and that of the helve 27 inches, and the weight of the tool complete 7 lb.

Fig. 16 represents a blasting sledge used in North Wales. The central block is an irregular octagon in section, formed by slightly chamfering the angles of a square section, and the stumps are chamfered down to form a regular octagon at the panes, which are flat. The length of the head is 7³/₄ inches, and that of the helve 22 inches, and the weight of the tool complete 6 lb. 7 oz.

The sledges used in the north of England have shorter heads, and are lighter than the foregoing. Fig. 17 represents one of these blasting sledges. The head is nearly square in section at the centre, and the panes are flat. The length of the head is 5 inches, and that of the helve 24¹/₂ inches, and the weight of the sledge complete 4 lb. 14 oz.

Auxiliary Tools.

—Besides the drill and the hammer, other tools are needed in preparing the hole for the blasting charge. If the bore-hole is inclined downwards, the dÉbris or “bore-meal” made by the drill remains on the bottom of the hole, where it is converted into mud or “sludge” by the water there present. This sludge has to be removed as the work progresses, to keep the rock exposed to the action of the drill. The removal of the sludge is effected by a simple tool called a “scraper.” It consists of a rod of iron from ¹/₄ inch to ¹/₂ inch in diameter, and of sufficient length to reach the bottom of the bore-hole. One end of the rod is flattened out on the anvil and made circular in form, and then turned up at right angles to the stem. The disc thus formed must be less in diameter than the bore-hole, to allow it to pass readily down. When inserted in the hole, the scraper is turned round while it is being pressed to the bottom; on withdrawing the instrument, the sludge is brought up upon the disc. The operation, two or three times repeated, is sufficient to clear the bore-hole. The other end of the scraper is sometimes made to terminate in a ring for convenience in handling, as shown in Fig. 18. Instead of the ring, however, at one end, a disc may be made at each end, as shown in Fig. 19, the discs in this case being of different diameter, to render the scraper suitable for different size bore-holes. Sometimes the scraper is made to terminate in a spiral hook or “drag-twist,” as represented in Fig. 20. The use of the drag is to thoroughly cleanse the hole before inserting the charge. A wisp of hay is pushed down the hole, and the drag end of the scraper introduced after it, and turned round till it has become firmly entangled. The withdrawal of the hay by the drag wipes the bore-hole clean. Instead of the twist drag, the “loop” drag is frequently employed. This consists of a loop or eye, through which a piece of rag or tow is passed. The rag or tow is used for the same purpose as the hay, namely, to thoroughly cleanse and dry the bore-hole previous to the introduction of the charge. Very frequently the “swab-stick” is used instead of the scraper to clear out the bore-hole. This is simply a deal rod bruised at one end by blows with a hammer until the fibres separate to form a kind of stumpy brush or “swab.” When this is pushed down the hole, the sludge passes up around and between the fibres, which are then spread out by being pressed against the bottom of the hole. On withdrawing the swab, the sludge is brought out with it.

When the charge has been placed in the bore-hole, and the fuse laid to it, the hole needs to be tamped, that is, the portion above the charge has to be filled up with some suitable substance. For this purpose, a “rammer,” “stemmer,” or “tamping iron,” as the instrument is variously called, is required. This instrument is illustrated in Fig. 21. It consists of a metal bar, the tamping end of which is grooved to receive the fuse lying against the side of the bore-hole. The other end is flat, to afford a pressing surface for the hand, or a striking face for the hammer when the latter is needed. To prevent the danger of accidental ignition from sparks caused by the friction of the metal against silicious substances, the employment of iron stemmers has been prohibited by law. They are usually made of copper or phosphor-bronze, the latter substance being more resisting than the former.

Sometimes in wet ground it becomes necessary to shut back the water from the bore-hole before introducing the charge of gunpowder. This happens very frequently in shaft sinking. The method employed in such cases is to force clay into the interstices through which the water enters. The instrument used for this purpose is the “claying-iron” or “bull,” represented in Fig. 22. It consists of a round bar of iron, called the stock or shaft, a little smaller in diameter than the bore-hole, and a thicker portion, called the head or poll, terminating in a striking face. The lower end of the shaft is pointed, to enable it to penetrate the clay, and the head is pierced by a hole about an inch in diameter to receive a lever. Clay in a plastic state having been put into the bore-hole, the bull is inserted and driven down by blows with the sledge. As the shaft forces its way down, the clay is driven into the joints and crevices of the rock on all sides. To withdraw the bull, a bar of iron is placed in the eye and used as a lever to turn it round to loosen it; the rod is then taken by both hands and the bull lifted out. To allow the bull to be withdrawn more readily, the shaft should be made with a slight taper and kept perfectly smooth. As the bull is subjected to a good deal of heavy hammering on the head, the latter part should be made stout. This tool, which should be considered as an extra instrument rather than as an essential part of a blasting set, is a very serviceable one, and should always be at hand in wet ground when loose gunpowder is employed.

Another instrument of this auxiliary character is the beche, Fig. 23, used for extracting a broken drill. It consists of an iron rod of nearly the diameter of the bore-hole, and hollow at the lower end. The form of the aperture is slightly conical, so that the lower end may easily pass over the broken stock of the drill, and, on being pressed down with some force, may grasp the stock in the higher portion of the aperture with sufficient firmness to allow of the two being raised together. When only a portion of the bit remains in the hole, it may often be extracted by means of the drag-twist end of the scraper, or the swab-stick may be driven down upon the broken portion, and latter withdrawn with the swab.

Sets of Blasting Gear.

—On Plates I., II., and III., will be found three sets of blasting gear; a set of coal-blasting gear; a set of single-hand stone-blasting gear; and a set of double-hand stone-blasting gear. In the first set, the drill, shown in Fig. 1, is 22 inches in length; the cutting edge is straight and 1¹/₂ inch wide, and the weight is 2¹/₂ lb. The other drill, Fig. 2, is 42 inches in length; it has a straight cutting edge 1⁷/₁₆ inch wide, and weighs 4 lb. 10 oz. The hammer used in this set and shown in Fig. 3 weighs 2 lb. 14 oz.; the length of the head is 4¹/₂ inches, and that of the handle 7³/₄ inches. In the second or single-hand stone set, the shorter drill, Fig. 6, Plate II., is 22 inches in length; the cutting edge is strongly curved, and is 1¹/₂ inch in width, and the weight is 3 lb. 10 oz. The longer drill, Fig. 7, is 36 inches in length; the width of the cutting edge, which is curved as in the shorter drill, is 1⁷/₁₆ inch, and the weight is 6 lb. 5 oz. The hammer used with this set, and represented in Fig. 8, weighs 3 lb. 6 oz.; the length of the head is 5 inches, and that of the handle 10 inches. In the third or double-hand stone set, Plate III., the first or shortest drill, Fig. 12, is 18 inches in length, 1³/₄ inch wide on the cutting edge, and weighs 4¹/₄ lb. The second drill, Fig. 13, is 27 inches in length, 1¹¹/₁₆ wide on the cutting edge, and weighs 6 lb. The third or longest drill, Fig. 14, is 40 inches in length, 1⁵/₈ inch wide on the cutting edge, and weighs 9¹/₄ lb. The cutting edges of all these drills are strongly curved as in the preceding set. The sledge used with this set, and represented in Fig. 15, weighs about 5 lb.

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.

bit