Electric Rock Drill

Electric Rock Drill

Electric power in mining-operations is now successfully applied to haulage, hoisting, lighting and pumping; and until lately, drilling was the one department of mining in which an electric source of energy had not been introduced. Drilling by electric power has been the field for much investigation and several forms of electric-drills have been tried; but in the severe service of rock-drilling many difficulties have been encountered, and the heavy cost of repairs has caused a deep prejudice against this system of drilling.

In the majority of electric rock-drills previously placed on the market, two principal factors have been considered; first, a portable electric motor usually of heavy pattern, coupled to the driving-mechanism of the drill proper by means of a flexible shaft; and second, the coupling of some form of reciprocating cross-head to a piston by means of a spiral spring, the drill-bit being fastened to the piston-rod by some suitable form of chuck with fastening bolts and nuts. In this latter form of drill, the cross-head has a fixed stroke, while the piston with drill-bit attached is allowed a variable stroke through the medium of the spiral spring. The principal difficulties in this type of drill are as follows:

electric rock drill

  • expensive repairs in the flexible shaft;
  • breakage in the spiral springs;
  • large space required in setting-up;
  • and loss of time in adjustment.

In the effort to overcome these objections, the Box electric rock-drill is designed with the electric motor mounted on the guide-shell, directly behind the drill proper; the troublesome spiral springs are replaced by an air-cushion; and the drill-bit, instead of being reciprocated back and forth in the hole of the rock, is simply turned in the hole, while a hammer strikes a blow on the end of the drill-bit,—similar to the manner in which a miner would turn the drill with his left hand and strike a blow with the hammer in his right hand.

Electric Rock Drill Mining

Construction & Operation 

Development.—The elements of the earlier models are given in Fig. 1, which shows the cross-sections of the Model 4 drill. The electric motor, designed to be waterproof, is shown at the left, mounted on the guides of the shell and coupled to the drill proper by means of machine-cut forged-steel gears. A taper-pin, with nut, fastened the motor to the drill and the removal of this pin allowed the motor to be removed. The crank was connected to the cross-head by means of a connecting-rod; the drill-hammer was coupled to the cross-head by means of a spiral spring; one end of the hammer formed a piston which was fitted to a small cylinder solid with the cross-head; the drill-bit was held with small end-play, in a revolving chuck driven by a train of gearing from the crank-shaft.

“ Model 6 ” drill is the latest improved form, the important change being the replacement of the spiral spring by an air-cushion. The relation of the electric motor, transmission-gears, crank, and connecting-rod remains practically the same as in the Model 4 drill, but Fig. 2 shows the details by which a perfect air-cushion is placed between the cross-head having a constant stroke, and the variable-stroke hammer.

It will be noted in Fig. 2 that the element corresponding to the cross-head of an engine becomes a cylinder in which is fitted the hammer of the drill. This cylinder of cast-iron is machine-finished inside and out, and the forged-steel hammer is machine-finished all over and fitted to the bore of the cylinder. The form of the hammer resembles the piston and rod of a steam-engine, leaving an air-space between the piston of the hammer and the heads of the cylindrical cross-head. When the drill is in motion, the air on each side of the hammer-piston is alternately compressed and rarefied, giving exactly the effect of a spring between the cylindrical cross-head and hammer, thus furnishing an element which is indestructible.

To compensate for possible leakage and to insure a full supply of air, two ports are cut in the side of the moving cylinder which are so located that as soon as the piston moves from its central position, one port is opened to the atmosphere and a supply of air rushes in; then the other port opens and furnishes an air-supply to the other side of the piston. Thus, these two ports serve to equalize the air-pressure on both sides of the piston as it passes its central position. Oil-grooves are turned in both piston and rod to furnish an oil-packing between the piston and the cylinder-walls; also between the rod of the hammer and the cylinder-head. These parts of the drill have been described in detail, for the reason that springs were found to be objectionable and certain to break; their replacement by the air-cushion seems to have solved the question perfectly. Fig. 3 shows a drill with the cover removed in order that the internal mechanical construction can be seen.

It was thought that in time, the piston might wear and destroy the air-cushion by allowing the air to leak from one side of the piston to the other side. In order to ascertain the effect of wear, one of the pistons was turned down 1/32 in. smaller in diameter and the drill then used on hard rock. No. appreciable difference in speed of drilling was noted. An amount of wear equal to 1/32 in. in diameter would require several months of the hardest service.

Rotation.—The Box electric rock-drill is the first drill to discard the troublesome pawls, springs and ratchets found in the rotating mechanism of other types. The chuck of the Box drill is of forged steel, mounted in removable composition-bearings. A train of gears, driven from the main crank-shaft, gives a continuous rotation to the chuck by means of a gear cut on the outside surface of the chuck.

In the chuck is mounted an automatic key engaging two flattened surfaces forged in the drill-bit, which allows an end-play in the drill-bit of about 1 in., but compels the bit to turn with the chuck. There are no bolts and nuts to loosen, and the rotating-mechanism connecting the crank-shaft with the chuck has ample strength to turn the drill-bit in any kind of rock.

Hinged Guide-Shell.—Another important improvement consists of a hinge between the guide-shell and the cone of the clamp, which enables the whole drill to be tipped to one side or the other, thereby enabling long drill-steels to be quickly inserted or removed from the drill, or hole, without disturbing, or changing, the set-up of the drill. (Shown in Fig. 4.) When the drill is tilted back into place, it is again in proper alignment with the hole. The guide-shell, drill and motor are mounted on a tunnel-column shaft-bar with clamp and arm (Fig. 4), or on a tripod (Fig. 5).

Controller and Motor.—The electric current is conducted from the main transmission-wires to the controller through heavy rubber-covered mains; the controller is of special design, and includes the necessary resistance for five speeds with proper fuse-blocks, all contained in a compact waterproof aluminum case. Connection is made with the drill-motor by a short heavy rubber-insulated cable. The electric motor was designed by






the General Electric Co. expressly for this drill and service, after most thorough investigation and practical tests. While every means are employed to reduce the weight and size, all precaution is taken for careful insulation and substantial construction. The motor has a waterproof case, and aluminum caps are used on pinion and commutator-ends. Suitable hand-hole plates are provided for inspection and care of the carbon- brushes. The motors are wound for either 110 or 220 volts direct current.

Removal of Crushed Rock by Water.—For the purpose of removing the crushed rock from the drill-hole, a long steel tube is placed over the drill-bit in order to deliver a stream of water to the cutting-edges of the bit. This tube, of an internal diameter slightly larger than the outside diameter of the drill-bit, has a groove cut on the outside surface nearly its entire length, similar to a key-way in a shaft. Another steel tube, whose, internal diameter fits the external diameter of the grooved tube, is pressed over the inner tube, thus leaving a hole through the shell of the finished water-tube. One end of this tube is fastened to a bracket which is bolted to the chuck- end of the drill. In this bracket is a water-duct, which is connected by rubber hose and suitable fittings to a portable tank, holding a supply of compressed-air and water. When the water-valve is open, a stream of water is delivered with a concentrated washing-action to the point of the drill-bit, and the small rock-particles cut out by the drill are washed back from the bottom of the hole on the outside of the tube to the face of the workings.

Dust Prevention.—One of the most serious conditions in rock-drilling is the danger to the operators in breathing the dust produced, but its removal by water as soon as it is formed over-comes this danger. This advantage, alone, makes the additional apparatus for the water-device worthy of use, as well as providing humane methods for the comfort and protection of the drill-men. Another advantage of the water-attachment is that the stream of water, playing on the cutting edges of the drill-bit, preserves its temper and increases its life.

Tube Serves as a Guide.—A reciprocating type of rock-drill, meeting a seam or pocket, is almost sure to be deflected from its true course and finally stick, or “ fitcher,” as the miners say.

This is particularly the case in drilling through seams running in a line diagonal to the axis of the drill-hole. When the Box electric drill encounters such a condition, the drill-bit action is such that the cutting-edges rebound but a small distance from the bottom of the hole, and being supported by the water-tube to a point within about 3 in. of the cutting-edges, any sticking, or “fitchering,” is prevented; even under the worst conditions but a slight deflection is produced. It is possible to drill one hole across another at a very slight angle, and have but a slight deflection in the second hole.

Standard Size.—The Box electric drill is made in but one size at present, which is equivalent to an air- or steam-drill with a 3-in. piston. The dimensions of the drill are as follows:—


The weights of various parts of the drill are as follows:—


Cost of Plant.—A one-drill prospecting-plant, weighing approximately 4,300 lb., costs $1,160 delivered on board cars at Colo., including the following fittings for both drill- and power-plants:

Drill-Plant.—1 Model 6, Box electric drill, with electric motor and guide-shell; 1 6-ft. tunnel-column, shaft-bar, or tripod, complete with clamp, pinch-bar and wrench; 1 portable water- tank, with attached hand-pump, hose and fittings; 1 controller, with resistance and fuse-block, encased in waterproof aluminum case, with 50 ft. of heavy 2-wire electric cable; 1 complete set of drill-steels, including 18 pieces, varying in length from 2 ft. 6 in. to 6 ft.; 1 complete set of water-tubes for the drill-steels.

Power-Plant.—1 electric generator, direct-current, belted-type, with sliding base and rails; 1 semi-portable boiler and engine, both mounted on one cast-iron base, with fittings complete ; 1 endless cemented leather belt to connect engine with generator; 1,000 ft. No. 8 copper insulated wire, for transmission of 500 ft. from generator to drill; 50 insulators for transmission line; 1 ammeter, ranging from 0 to 25 amperes; 1 voltmeter, ranging from 0 to 150 volts; 1 double-pole knife-switch; 1 fuse-block, with fuses.

A four-drill plant including machinery suitable for the increased size weighs approximately 12,500 lb., and costs $3,750 delivered on board cars, Colo.

Where a direct-current electric-light system is employed about the mine, the drills may be run from this circuit. If an alternating-current power-system be available, the current may be transformed to 110 volts, or 220 volts direct-current system, by means of a motor-generator set. A motor-generator set, or a gasoline-engine plant, will cost about the same as the steam-boiler and engine-plant of the same capacity.

Impact, or the Force of Percussion.—In order to give some conception of the force of the blow delivered by a machine-drill, it is well to investigate the principles of this action. Among the mining fraternity, it is customary to measure the blow of a machine-drill in pounds, obtained by multiplying the area of the piston of the air-, or steam-drill by the pressure per square inch. This calculation is not correct, because it does not take into account the weight of piston, chuck, and drill-steel; or the distance in which the moving masses are brought to rest. If the energy stored up in a swiftly-moving hammer could be measured and this quantity, expressed in foot-pounds, be divided by the distance in feet in which the blow is arrested, the result would be the measure in pounds of the impact, force of percussion, or blow. When a miner strikes a blow with a hand-hammer on the end of a drill-steel in drilling a “ down ” hole, two forces are acting; the muscular force in the effort to increase the velocity, and the force of gravity. At the moment the hammer strikes the end of the drill-steel, a velocity of from 20 to 40 ft. per second may be realized, and the effect is the same as though this maximum velocity, at the moment of impact, had been maintained throughout the whole stroke of the hammer. If the velocity at the moment of impact he assumed at 30 ft. per second, the force will be the same as if the hammer had fallen from a height expressed by V²/2g, or the square of the velocity, divided by twice the constant for gravity; or 900/64.33 = 14 ft. With a hammer weighing 4 lb. the stored-up energy will be 14 x 4 = 56 foot-pounds. Assuming that the cutting-edge of the drill penetrates 1/16 in. into the rock before the energy of the blow is fully absorbed, then are 56 foot-pounds of energy being exerted through a distance of 0.0625 ft. Dividing 56 by 0.0625 gives 10,769 lb., the direct pressure which would produce the same effect as the miner striking with a 4-lb. hammer.

As the pressure is a maximum when the hammer first comes in contact with the drill-steel, and reaches a minimum when the penetration of the steel into the rock ceases, 10,769 lb., as found above, would more correctly represent the average pressure throughout the time of the blow.

As the Box drill is driven by an electric motor, it becomes a simple matter to measure the energy delivered to the hammer of the drill. First, the drill-mechanism was run without the hammer, and the current measured to determine the work due to friction within the drill; then the current was measured with the hammer drilling into rock under usual conditions. The difference in these two measurements, neglecting the slight friction of the hammer and the drill-steel, gives the measure of energy exerted in the hammer-blow and amounts to 6 amperes at 110 volts, or 29,195 foot-pounds. As the Box drill will penetrate hard granite at the rate of 3 in. per minute, with a drill-steel of 2.25 in. in diameter, the energy of the blow will be 116,780 lb., or over 58 tons, of static pressure. The hammer weighs 10 lb., has a 4-in. stroke, and strikes 600 blows per minute.

Power Required.—In drilling a hole 2.25 in. in diameter in hard granite from Platte Canon, Colo., at a speed of 3 in. per minute, the Box electric drill required an electric current of 12 amperes at 110 volts, equivalent to 1.32 kw., or about 1.77 h.p. This means that 11.92 cu. in. per minute of granite chips have been removed with an expenditure of 1.77 h.p., or about 6.74 cu. in. of material per horse power per minute. A 3-in. air-drill on the same rock, and drilling at the same speed, will require at least 23 h.p., when drilling at an altitude of 10,000 ft. above sea-level. This gives only 0.518 cu. in. of material cut by the air-drill per horse power per minute; or 1 h.p. expended on the Box electric drill will cut 13 times as much material as the same amount of power expended on a 3-in. air-drill.

Under the average conditions that exist in the Rocky Mountain section, the power required by the Box electric drill in drilling a hole 2.25 in. in diameter at the speed mentioned above, varies from 1 to 1.5 horse power.

Field of Operation.—The Box electric drill was developed to meet the severe service in mining-operations, and after repeated trials under trying conditions it has shown that it will stand the test. There is a certain amount of prejudice to overcome, as is the case in any marked change in methods, but as the same amount of work may be accomplished in the same time, with one-tenth the power used in former methods, the electric system is bound to be recognized by progressive operators, and in good time it will have universal application.

Considering the many advantages in the installation of electric drills for excavations, quarries, sewers, subways and rail-road construction, and the economy to be realized in the electrical transmission of power by the reduction in size of plant and facility in changing the position of drills to accommodate the work, the future for this type of drill is very promising.

The remarkable results attained in the methods of long-distance electrical transmission of power, which renders available the great power-sources of water, or of coal- and oil-fields; the high standards in the manufacture of modern electric generators, motors and appliances; the increasing knowledge among operators concerning the care and control of electric apparatus; the desire for more economical methods in the work of excavation and mining;—all present most favorable conditions for the success of electric rock-drilling.

Comparison With Other Electric Rock Drill

Relative Advantages.—The advantages of an electric drill over one operated by steam, or air, may be appreciated when attention is paid to the great loss by condensation, radiation, or from leaky joints of pipes, as well as the heavy and expensive pipes and fittings necessary in the steam- or air-plants; whereas, the insulated electric cable may be very readily and cheaply installed.

The relative power required to operate the several types of drills is shown graphically as follows:


The objection is sometimes raised to the electric drill in that it provides no means for ventilating the workings in which the drill is operating. In reply it may be said that a perfect system of exhaust- or pressure-ventilation with fans can be designed, requiring very little power as compared with the enormous waste from compressed-air systems in which it is the custom at times to open wide a hose 1 in. or 1.25 in. in diameter. Furthermore, the liberation of compressed air from the drill, or the air-pipe, forces the foul air from the face of the workings back through the other workings of the mine, and furnishes an atmosphere for the miners to breathe, which is vitiated by atomized lubricating oil. Surely such air is not health-giving.

The electric drill does not vitiate the air; and, in addition, its use furnishes a satisfactory system of electric lighting, for the reason that incandescent lamps may be run from the drill- circuit.

The loss in efficiency of air-compressors at high altitudes is another strong argument in favor of the electric drill. The loss for air-compressors delivering air at 100 lb. gauge-pressure, at the different altitudes, compared with the same compressor at sea-level, is as follows:—


There are no such corresponding losses in using the electric drill.

The hammer type of drill possesses a great advantage over the reciprocating forms, from the great saving in power due to the blow being transmitted through the drill-steel in the hammer-drill, instead of moving the drill-steel back and forth in the hole, as is the case in reciprocating-drills. A drill-steel 6 ft. long, weighing about 18 lb., used in connection with a 3-in. air-drill, with a stroke of 6.5 in., at the rate of from 300 to 400 blows per minute, requires at least 20 h.p. at sea-level, 22 h.p. at 5,000 ft. altitude, and 24 h.p. at 10,000 ft. altitude. The forward stroke of the reciprocating-drill ends in a blow in the rock; the return stroke must end on some form of cushion. The use of spiral springs for this purpose is a great source of trouble on account of their breakage.

In the Box electric drill, only the hammer moves, the drill-steel remaining at the bottom of the hole, except for a slight rebound due to the impact on the rock. When a drill-steel is reciprocated in a hole, considerable friction is produced, and in addition, the gauge of the cutting-edges is rapidly worn down. This is a strong point, for, as the gauge is preserved for a longer period, the holes are more perfect and a marked saving is effected in the cost of sharpening the drill-steels.

Water-power at a considerable distance from the mine may be developed, thus saving the transportation to, and the maintenance of, machinery in high altitudes and remote places. The difference in price of fuel between the mine-site and the rail-road may also warrant the building of the power-station on the railroad.

Practical Results

The following is a summary of reports from various mines having the Box electric drill in regular operation:

1. Record in Tunnel- Work in Boulder County, Colo.—“ One of the machines drilled 12 ft. and 6 in. of hole 2.25 in. in diameter, in hard, closed-grained granite, in just two hours. The machine was set up in the tunnel at a point about 1,700 ft. from the generator. While the machine was in operation the ammeter on the switchboard indicated that it was using about one horse power.”

“ Arrived at tunnel about 1.00 p.m. Unpacked drill and set up; started to drill at 4.05 p.m.; drilled till 6.80 p.m.; drilling 3 holes, each 5 ft. deep, tore down and blasted 7.00 p.m. Driving tunnel about 7 ft. by 8 ft.”

The following statement refers to shaft sinking in country- rock at the Ophir Mine, Anaconda, Colo., Cripple Creek District :

“ You ask me how often we are obliged to clean the drill. I do not think we have averaged more than once a week.

“We have never had a drill stick or bind in a hole, or fail to clear itself of waste, or be deflected from its course by change of strata. We can easily begin at 8 o’clock and complete a round of holes, nine in number, 4 ft. deep (each), by 2.30 or 3 p.m., giving ample time for removal of drill and loading and shooting at the four o’clock shift. For instance, we set up the drill at 8 o’clock this morning, I sat by and watched the man complete five 4-ft. holes by 11.30 a.m.

“ Started setting up at 7.30 a.m.; knocked down at 3.00 p.m.; drilled 30 ft.

“ The drills are doing very satisfactory work, cutting as rapidly as air- or steam-drills, and accomplishing the work at a much less expense and power.”

2. Record at the Tunnel of Imogene Basin Gold Mines Co., Ouray, Colo. Rock very Soft with Mud-Slips Running through the Seams, and very Hard to Mix with Water. Altitude 11,000 Ft. above Sea- Level.—“ Cleaned breast and set up 11 a.m., drilled till 12 noon, started 1 p.m., drilled till 3 p.m., drilling 4 holes, making 20 ft.

“ Engine uses about 6 gal. of oil (kerosene) in two 8-hour shifts, running one drill and four lights in daytime, and one drill and 14 lights at night. Cleaned breast and set up 1.50 p.m., drilling 4 holes, making 16 ft. at 3.20 p.m.”

3. Record of Experimental Work with the Box Electric Drill.— Table I. gives the record of a test of a Box electric drill, installed by Mr. Frank Fletcher of Parral, Mexico, which was made on April 14, 1904. The rock used for the test was hard granite from Platte Canon, Colorado. The diameter of the drill-hole was 2.25 inches. The electric-current used was 11 amperes at 110 volts, which is equivalent to 1.62 horse-power.


The total depth drilled in 14 min. and 45 sec. was 33.5 in., which is at the rate of practically 2.23 in. per minute.

The following list of plants at which the Box electric drill is installed may be considered representative for the various localities named:

  • Ophir Mine, Anaconda, and Imogene Basin Gold Mines Co., Ouray, Colo.
  • Keystone Bromide Mining Co., Tres Piedras, N. M.
  • Bagdad Chase Gold Mining Co., Camp Rochester, Cal.
  • Miami Mining Co., Concord, N. C.
  • Akrokerri Mines Co., Ltd., Ashanti, S. Africa.
  • Frank Fletcher, Baca Station, Durango, Mex.
  • Compania Minera El Banco y Annexas, Oaxaca, Mex.
  • Gualterio C. Palmer, Zacatecas, Mexico.
  • Takata & Co., Tokio, Japan.