MACHINE DRILLING: POWER TRANSMISSION; COMPRESSED AIR _VS_. ELECTRICITY;
AIR DRILLS; MACHINE _VS_. HAND DRILLING. WORK-SHOPS. IMPROVEMENT
For over two hundred years from the introduction of drill-holes
for blasting by Caspar Weindel in Hungary, to the invention of
the first practicable steam percussion drill by J. J. Crouch of
Philadelphia, in 1849, all drilling was done by hand. Since Crouch's
time a host of mechanical drills to be actuated by all sorts of
power have come forward, and even yet the machine-drill has not
reached a stage of development where it can displace hand-work
under all conditions. Steam-power was never adapted to underground
work, and a serviceable drill for this purpose was not found until
compressed air for transmission was demonstrated by Dommeiller
on the Mt. Cenis tunnel in 1861.
The ideal requirements for a drill combine:--
a. Power transmission adapted to underground conditions.
c. Simplicity of construction.
e. Rapidity and strength of blow.
f. Ease of erection.
h. Mechanical efficiency.
i. Low capital cost.
No drill invented yet fills all these requirements, and all are
a compromise on some point.
POWER TRANSMISSION; COMPRESSED AIR _vs_. ELECTRICITY.--The only
transmissions adapted to underground drill-work are compressed
air and electricity, and as yet an electric-driven drill has not
been produced which meets as many of the requirements of the metal
miner as do compressed-air drills. The latter, up to date, have
superiority in simplicity, lightness, ease of erection, reliability,
and strength over electric machines. Air has another advantage in
that it affords some assistance to ventilation, but it has the
disadvantage of remarkably low mechanical efficiency. The actual
work performed by the standard 3-3/4-inch air-drill probably does
not amount to over two or three horse-power against from fifteen to
eighteen horse-power delivered into the compressor, or mechanical
efficiency of less than 25%. As electrical power can be delivered to
the drill with much less loss than compressed air, the field for a
more economical drill on this line is wide enough to create eventually
the proper tool to apply it. The most satisfactory electric drill
produced has been the Temple drill, which is really an air-drill
driven by a small electrically-driven compressor placed near the
drill itself. But even this has considerable deficiencies in mining
work; the difficulties of setting up, especially for stoping work,
and the more cumbersome apparatus to remove before blasting are
serious drawbacks. It has deficiencies in reliability and greater
complication of machinery than direct air.
AIR-COMPRESSION.--The method of air-compression so long accomplished
only by power-driven pistons has now an alternative in some situations
by the use of falling water. This latter system is a development
of the last twelve years, and, due to the low initial outlay and
extremely low operating costs, bids fair in those regions where
water head is available not only to displace the machine compressor,
but also to extend the application of compressed air to mine motors
generally, and to stay in some environments the encroachment of
electricity into the compressed-air field. Installations of this
sort in the West Kootenay, B.C., and at the Victoria copper mine,
Michigan, are giving results worthy of careful attention.
Mechanical air-compressors are steam-, water-, electrical-, and
gas-driven, the alternative obviously depending on the source and
cost of power. Electrical- and gas- and water-driven compressors
work under the disadvantage of constant speed motors and respond
little to the variation in load, a partial remedy for which lies
in enlarged air-storage capacity. Inasmuch as compressed air, so
far as our knowledge goes at present, must be provided for drills,
it forms a convenient transmission of power to various motors
underground, such as small pumps, winches, or locomotives. As stated
in discussing those machines, it is not primarily a transmission
of even moderate mechanical efficiency for such purposes; but as
against the installation and operation of independent transmission,
such as steam or electricity, the economic advantage often compensates
the technical losses. Where such motors are fixed, as in pumps
and winches, a considerable gain in efficiency can be obtained by
It is not proposed to enter a discussion of mechanical details of
air-compression, more than to call attention to the most common
delinquency in the installation of such plants. This deficiency
lies in insufficient compression capacity for the needs of the
mine and consequent effective operation of drills, for with under
75 pounds pressure the drills decrease remarkably in rapidity of
stroke and force of the blow. The consequent decrease in actual
accomplishment is far beyond the ratio that might be expected on
the basis of mere difference of pressure. Another form of the same
chronic ill lies in insufficient air-storage capacity to provide
for maintenance of pressure against moments when all drills or
motors in the mine synchronize in heavy demand for air, and thus
lower the pressure at certain periods.
AIR-DRILLS.--Air-drills are from a mechanical point of view broadly
of two types,--the first, in which the drill is the piston extension;
and the second, a more recent development for mining work, in which
the piston acts as a hammer striking the head of the drill. From an
economic point of view drills may be divided into three classes.
First, heavy drills, weighing from 150 to 400 pounds, which require
two men for their operation; second, "baby" drills of the piston type,
weighing from 110 to 150 pounds, requiring one man with occasional
assistance in setting up; and third, very light drills almost wholly
of the hammer type. This type is built in two forms: a heavier
type for mounting on columns, weighing about 80 pounds; and a type
after the order of the pneumatic riveter, weighing as low as 20
pounds and worked without mounting.
The weight and consequent mobility of a drill, aside from labor
questions, have a marked effect on costs, for the lighter the drill
the less difficulty and delay in erection, and consequent less
loss of time and less tendency to drill holes from one radius,
regardless of pointing to take best advantage of breaking planes.
Moreover, smaller diameter and shorter holes consume less explosives
per foot advanced or per ton broken. The best results in tonnage
broken and explosive consumed, if measured by the foot of drill-hole
necessary, can be accomplished from hand-drilling and the lighter
the machine drill, assuming equal reliability, the nearer it
approximates these advantages.
The blow, and therefore size and depth of hole and rapidity of
drilling, are somewhat dependent upon the size of cylinders and
length of stroke, and therefore the heavier types are better adapted
to hard ground and to the deep holes of some development points.
Their advantages over the other classes lie chiefly in this ability
to bore exceedingly hard material and in the greater speed of advance
possible in development work; but except for these two special
purposes they are not as economical per foot advanced or per ton
of ore broken as the lighter drills.
The second class, where men can be induced to work them one man per
drill, saves in labor and gains in mobility. Many tests show great
economy of the "baby" type of piston drills in average ground over
the heavier machines for stoping and for most lateral development.
All piston types are somewhat cumbersome and the heavier types
require at least four feet of head room. The "baby" type can be
operated in less space than this, but for narrow stopes they do
not lend themselves with the same facility as the third class.
The third class of drills is still in process of development, but
it bids fair to displace much of the occupation of the piston types
of drill. Aside from being a one-man drill, by its mobility it
will apparently largely reproduce the advantage of hand-drilling
in ability to place short holes from the most advantageous angles
and for use in narrow places. As compared with other drills it
bids fair to require less time for setting up and removal and for
change of bits; to destroy less steel by breakages; to dull the
bits less rapidly per foot of hole; to be more economical of power;
to require much less skill in operation, for judgment is less called
upon in delivering speed; and to evade difficulties of fissured
ground, etc. And finally the cost is only one-half, initially and
for spares. Its disadvantage so far is a lack of reliability due to
lightness of construction, but this is very rapidly being overcome.
This type, however, is limited in depth of hole possible, for,
from lack of positive reverse movement, there is a tendency for
the spoil to pack around the bit, and as a result about four feet
seems the limit.
The performance of a machine-drill under show conditions may be
anything up to ten or twelve feet of hole per hour on rock such
as compact granite; but in underground work a large proportion of
the time is lost in picking down loose ore, setting up machines,
removal for blasting, clearing away spoil, making adjustments,
etc. The amount of lost time is often dependent upon the width of
stope or shaft and the method of stoping. Situations which require
long drill columns or special scaffolds greatly accentuate the loss
of time. Further, the difficulties in setting up reflect indirectly
on efficiency to a greater extent in that a larger proportion of
holes are drilled from one radius and thus less adapted to the
best breaking results than where the drill can easily be reset from
The usual duty of a heavy drill per eight-hour shift using two men
is from 20 to 40 feet of hole, depending upon the rock, facilities
for setting up, etc., etc.[*] The lighter drills have a less average
duty, averaging from 15 to 25 feet per shift.
[Footnote *: Over the year 1907 in twenty-eight mines compiled
from Alaska to Australia, an average of 23.5 feet was drilled per
eight-hour shift by machines larger than three-inch cylinder.]
MACHINE _vs_. HAND-DRILLING.--The advantages of hand-drilling over
machine-drilling lie, first, in the total saving of power, the
absence of capital cost, repairs, depreciation, etc., on power,
compresser and drill plant; second, the time required for setting
up machine-drills does not warrant frequent blasts, so that a number
of holes on one radius are a necessity, and therefore machine-holes
generally cannot be pointed to such advantage as hand-holes. Hand-holes
can be set to any angle, and by thus frequent blasting yield greater
tonnage per foot of hole. Third, a large number of comparative
statistics from American, South African
, and Australian mines show
a saving of about 25% in explosives for the same tonnage or foot
of advance by hand-holes over medium and heavy drill-holes.
The duty of a skilled white man, single-handed, in rock such as
is usually met below the zone of oxidation, is from 5 to 7 feet
per shift, depending on the rock and the man. Two men hand-drilling
will therefore do from 1/4 to 2/3 of the same footage of holes
that can be done by two men with a heavy machine-drill, and two
men hand-drilling will do from 1/5 to 1/2 the footage of two men
with two light drills.
The saving in labor of from 75 to 33% by machine-drilling may or
may not be made up by the other costs involved in machine-work.
The comparative value of machine- and hand-drilling is not subject
to sweeping generalization. A large amount of data from various
parts of the world, with skilled white men, shows machine-work
to cost from half as much per ton or foot advanced as hand-work
to 25% more than handwork, depending on the situation, type of
drill, etc. In a general way hand-work can more nearly compete
with heavy machines than light ones. The situations where hand-work
can compete with even light machines are in very narrow stopes where
drills cannot be pointed to advantage, and where the increased
working space necessary for machine drills results in breaking more
waste. Further, hand-drilling can often compete with machine-work
in wide stopes where long columns or platforms must be used and
therefore there is much delay in taking down, reŽrection, etc.
Many other factors enter into a comparison, however, for
machine-drilling produces a greater number of deeper holes and
permits larger blasts and therefore more rapid progress. In driving
levels under average conditions monthly footage is from two to
three times as great with heavy machines as by hand-drilling, and
by lighter machines a somewhat less proportion of greater speed.
The greater speed obtained in development work, the greater tonnage
obtained per man in stoping, with consequent reduction in the number
of men employed, and in reduction of superintendence and general
charges are indirect advantages for machine-drilling not to be
The results obtained in South Africa by hand-drilling in shafts,
and its very general adoption there, seem to indicate that better
speed and more economical work can be obtained in that way in very
large shafts than by machine-drilling. How far special reasons
there apply to smaller shafts or labor conditions elsewhere have
yet to be demonstrated. In large-dimension shafts demanding a large
number of machines, the handling of long machine bars and machines
generally results in a great loss of time. The large charges in
deep holes break the walls very irregularly; misfires cause more
delay; timbering is more difficult in the face of heavy blasting
charges; and the larger amount of spoil broken at one time delays
renewed drilling, and altogether the advantages seem to lie with
hand-drilling in shafts of large horizontal section.
The rapid development of special drills for particular conditions
has eliminated the advantage of hand-work in many situations during
the past ten years, and the invention of the hammer type of drill
bids fair to render hand-drilling a thing of the past. One
generalization is possible, and that is, if drills are run on 40-50
pounds' pressure they are no economy over hand-drilling.
In addition to the ordinary blacksmithy, which is a necessity,
the modern tendency has been to elaborate the shops on mines to
cover machine-work, pattern-making and foundry-work, in order that
delays may be minimized by quick repairs. To provide, however,
for such contingencies a staff of men must be kept larger than
the demand of average requirements. The result is an effort to
provide jobs or to do work extravagantly or unnecessarily well.
In general, it is an easy spot for fungi to start growing on the
administration, and if custom repair shops are available at all,
mine shops can be easily overdone.
A number of machines are now in use for sharpening drills.
Machine-sharpening is much cheaper than hand-work, although the drills
thus sharpened are rather less efficient owing to the difficulty of
tempering them to the same nicety; however, the net results are
in favor of the machines.
IMPROVEMENT IN EQUIPMENT.
Not only is every mine a progressive industry until the bottom
gives out, but the technology of the industry is always progressing,
so that the manager is almost daily confronted with improvements
which could be made in his equipment that would result in decreasing
expenses or increasing metal recovery. There is one test to the
advisability of such alterations: How long will it take to recover
the capital outlay from the savings effected? and over and above
this recovery of capital there must be some very considerable gain.
The life of mines
is at least secured over the period exposed in
the ore-reserves, and if the proposed alteration will show its
recovery and profit in that period, then it is certainly justified.
If it takes longer than this on the average speculative ore-deposit,
it is a gamble on finding further ore. As a matter of practical
policy it will be found that an improvement in equipment which
requires more than three or four years to redeem itself out of
saving, is usually a mechanical or metallurgical refinement the
indulgence in which is very doubtful.
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