Bore-holes 14 to 16 deep can be drilled by this size of machine easily. It is the kind used in many stone quarries, and by the County Councils in Scotland. A machine for drilling deeper holes is also used in sale quarries, the cylinder of which is 3 inches in diameter. This latter machine weighs 260 lbs. without the tripod, aud can easily bore holes 20 to 25 feet deep, which in some quarries is a decided advantage.

176. 'Hirnant' Drill.—The other rock drill to be described is known as Larmuth's or the 'Hirnant' drill, a longitudinal section of which is shown in fig. 55.

The leading feature of this drill is, that the valve is actuated by a tappet rocking on a pivot, which works in contact with the piston. The valve box, which, in the earlier type of this machine, was made very long, has one of three arms of the tappet fitting into a recess in the valve spindle which carries the D-shaped valves. This arrangement, however, proved defective, owing to the breaking of the valve spindle by the sudden shock which it was subjected to when at actual work.

A shorter valve box is now substituted, the spindle dispensed with, and a solid piston valve a used in its place, obviating the defect existing in the former drill. The tappet c is, however, retained to ensure certainty of action and to lock the valve in position.

The piston d is turned to fit the cylinder b exactly, neither piston rings nor packing being made use of. The wearing of the piston and cylinder must in time admit of steam or compressed air leaking between them, but this does not materially affect the working parts, although its efficiency generally must necessarily be impaired. India-rubber buffers, n, are employed at either end of the cylinder to destroy the momentum of the piston in case of overshooting. When compressed air is the motive power employed to work this class of drill, these buffers wear quickly by being acted upon by the oil used in lubricating the cylinder, and when steam is used to operate it, the combined action of these destructive agents wears out the buffers quickly, and necessitates frequent renewals.

The action of a tappet-actuated valve, as already pointed out, must to a certain extent strike a cushioned blow, thereby expending a considerable amount of the force on the machine itself, instead of striking the rock directly. For the same reason, in a tappet drill a variable stroke can only be obtained by working on a cushion of air or of steam.

The rotary motion in the older type of this drill was obtained by means of a twisted square bar; this arrangement, however, not proving quite satisfactory, a grooved or spiral bar f has been substituted. This works through a nut in the back head of the drill, turning the cutting tool round at each stroke, and is secured by means of a ratchet and pawl 9.

The feed or forward motion is provided for by a long screw, i, having a square thread passing through nuts i1, and supplied with an arrange

of fixing the cutting tool or steel to the

lower end of piston was attained in the
earlier machines by means of a bolt pass-
ing through the chuck in which the
shank of the bit was inserted, and likewise
through the steel itself, being tightened by
a nut as shown at h in fig. 55. A new
method of securing the drilling-bit is made
by forming the shank with a taper and
fitting this into a conical recess in the
chuck end of piston, wedge fashion, as
represented in fig. 56.
This arrangement,
however, is not satisfactory when drilling
in seamy rock, as the steel works loose on
being hammered, a process which is neces-
sary under these circumstances.

The Hirnant' drill is constructed in a very solid manner, the object of the makers being steady and permanent work. It is not a fast boring machine in large or deep holes, and, generally speaking, requires few repairs. Very good work can be accomplished with this machine, providing the drill-runner exercises skill and intelligence SO as to utilize the full power of the machine; if he works with a reduced stroke on a cushion of steam or of compressed air, he is simply spending power in pounding the drill itself instead of applying it against the rock to be bored.

The force of the blow in a tappet drill is materially checked by the too early admission of the steam or compressed air, which forms a cushion in front of the piston; the work accomplished will therefore be small in proportion to the power consumed, although the piston may be travelling at a great speed.

a

Greater skill is required for operating 'Hirnant' drill than is necessary for working one on the 'Ingersoll-Sergeant' principle. There can be no question that

the results claimed for the 'Hirnant' machine have been attained, but the fact undoubtedly remains that rock drills which have a tappet action strike a blow on a cushion of air or steam, and, therefore, cannot deliver a dead blow. Further, a cylinder of larger size in proportion to the width of bit is required for drilling the bore-hole, or if it be of the same size, the cutting capacity must necessarily be reduced. The Hirnant' drill, notwithstanding the defects common to all drills of the tappet type, has been largely used by contractors in this country for tunnelling and other public works.

177. Compound Rock Drills.-The principle of expansion has been introduced for the purpose of further economizing the motive power in rock drills, but hitherto without much success. The difficulty has been the greatly increased size and consequently heavier machine necessary, which counterbalances the advantages which lightness gives for quick and easy handling. What may be gained in economy of power by compounding is lost in the time occupied in moving the heavier machine from one bore-hole to another. In addition to this the expanded steam frequently has insufficient power to withdraw the drill steel from a bore-hole, in seamy rock, without admitting live steam, thereby counteracting the benefit derived from compounding.

FIG. 56.

178. Tripods. The support or mounting to which a rock drill is attached for the system of quarrying under review is that of a tripod, which constitutes a very important adjunct to the machine. For many reasons it must be of light weight for convenience in handling, and of strong construction, so that great rigidity may be attained. A point

of the utmost importance is that it should admit of the drill being readily adjusted to any angle so that the holes may be bored in the direction and with the inclination required, and when so fixed permits of the feed or forward movement taking place.

It is also necessary that the tripod should be constructed so as to admit of its ready adjustment to the irregularities of the surface of the rock met with in quarry work. A drill support which does not possess these distinctive requirements is placed at a great disadvantage, and under certain circumstances may necessitate the employment of hand-boring to supplement the work done by the machine drill. It is obvious that a rock drill and mounting which cannot be so placed as to accommodate itself to the extreme irregularities of rock surfaces is placed at a disadvantage, and would under such circumstances be restricted in its operations as a laboursaving machine.

A rock drill may have a perfect valve motion, and be otherwise of good design, but may fail to do efficient work by reason of the defective

construction of the tripod, which incompleteness constitutes in many cases a serious objection to the adoption of a rock-drilling machine.

The form of tripod used in connection with the 'Ingersoll-Sergeant drill is shown in fig. 57.

The saddle bolt o which carries the hip rings p and to which the legs are attached ties the whole appliance together, and by loosening it the legs of the tripod can be thrown into any position, at right angles to the bolt. The front legs q are further provided with a bolt r (attached to the hip ring), the loosening of which enables the position of the legs to be altered from that of a right angle to a parallel position in relation to

the saddle bolt. The front legs q and back leg v, consisting of tubing, are pointed at the lower end; they are made telescopic and are provided with bands and set screws s, so that the legs can be adjusted to the irregularities of the rock surface.

When the machine is working the tripod should be perfectly rigid, for which purpose weights t are attached and kept in position by weight hangers u fixed to the legs at an angle as shown in fig. 57.

The arrangement of the tripod in connection with the 'Hirnant' drill, fig. 58, and indeed with most of the other types of rock drills used, is somewhat similar to that described, but the front legs q have only one movement, namely, at right angles to the saddle bolt o. In addition to this the use of slides and bolts greatly complicates and, at the same time, weakens the tripod, and naturally makes it non-efficient. By having an arrangement similar to that of the tripod for the Ingersoll-Sergeant' drill, and known as the universal joint, the drill can be so placed as to drill holes

in the most advantageous position for blasting, not attainable by other drills. of which the tripods have only one movement.

The weight of the 'Hirnant' drill and tripod described is 3 cwts., and the three weights about 1 cwt. each. The cylinder is 3 inches in diameter, and can bore holes 10 to 12 feet deep, the width of drill-bit or size of borehole at commencement being 2 inches.

179. Drill-bits.-The width of the cutting edge, the form given to a drill-bit, and the diameter of the steel used, vary according to the nature and character of the rock to be bored and the capacity of the drilling machine employed. A judicious selection of the best shape of bit, suitable for the rock, adds considerably to the success of the boring operations with drills actuated by steam or compressed air. The chisel-pointed bit usually adopted in hand-boring has no place in machine drilling when the rock is fissured. A modified form of this kind of bit is shown in fig. 59; it answers very well for sandstone and slate of a solid nature. The flattened or grooved portion at the centre of the bit is made for the purpose of discharging cuttings and sludge.

The part forming the centre of any drilling-bit performs but little work, so that the reducing of the steel, as shown, does not affect its efficiency. The steels for the smaller sizes of rock-drills are generally made from ths of an inch to 1 inch in diameter, while for the larger-sized machines the diameter varies from 1 inch to 13 inch. They are made from octagonal steel bars; the shank, or part which fits into the bush of the chuck at the end of the piston, is turned so as to fit it exactly, the top end being made of a hemispherical shape. The width of the cutting edge is determined by the size of rock drill used, that is, by the diameter of the cylinder, or in other words, by the force of the blow transmitted by the machine to the bit. It varies from of an inch to 3 inches for general quarrying, while in special circumstances it may be as wide as 5 inches. The proper shape to be given to a drillbit, under varying conditions of work, is a subject on which a considerable amount of attention has been bestowed.

8+X

FIGS. 59-61.-Drill-bits.

The cross-bit shown in fig. 60 is a decided improvement on the older form or chisel-pointed bit, and does very good work in solid rock. There is, however, a difficulty with this form of cutting edge, arising from the circumstance that when rotated by means of the spiral bar it has a