Charles George Warnford Lock.

Economic mining: a practical handbook for the miner, the metallurgist and ... online

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numing.

When it is necessary to clean the interior of the commutator and
brushes, the locking ring is unscrewed, and the disc (with brushes
attached) is slid towards the bearing as far as possible, thus leaving
both commutator and brushes clear for cleaning and inspection ; means
are provided for attaching a small slide rest, so that by the use of a
crank handle bolted to the pulley, the commutator can be turned up
in position, should this be required.

Carbon brushes of a special type, admitting of very easy renewal,
are used, thus obviating any trouole that might occur from accumula*
tion of copper dust within the commutator with the ordinary copper
brushes.

Compre99ed Air, — Compressed air is almost exclusively used in pre-
ference to steam as the direct motive power of drills. While the
direct application of steam as the power to drive drills is far more



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30 ECONOMIC MINING.

economical, there are many serious objections to its use nndergronnd,
the chief of which is the exoessive heat which its use causes when
employed in confined or close places. In addition to this objection,
there is a great loss of pressure in transferring the steam to the point
of application, because of condensation in the pipes ea route to this
point. Hardly more than 40 per cent, of the power consumed in
compressing the air to the required degree (usually 60 to 70 lb. per
sq. in.) is utilised by the drill. This loss of power arises chiefly from
the fact that none of the power expended in the compression of the
air can be utilised, inasmuch as tne air when applied to the drills
does not act expansively. Power is thus wasted in the oompression
of the air by the transformation of this power into heat, which is
subsequently lost by conduction and by radiation. Heat generated in
the compression of air, not only results in loss of power directly, but
is a further disadvantage, for the reason that the air during compres-
sion in the cylinder is cooled in the various compressors u^d, by the
introduction of a spray of water into the cylinder, or by means of a
flowing stream of cool water enveloping the cylinder. The loss
throng heat arises from the cooling contraction of the air, and the
consequent decrease of the tension of the air as it passes from the
compressor to the air reservoir. The heated air likewise, because of
its increased tension due to the heat, reacts upon the piston causing
a resistance, and consequently far less of the power is applied to the
piston. The friction of the compressed air passing through the
valves also causes a loss of power. These, with the other causes be-
fore adduced, will account for the small amount of power utilised by
the application of compressed air to the drills. In short the power
expended by the piston during the first part of its stroke is wa^^ted in
the compression of air, since, as above stated, the air cannot be applied
expansively to the drills. This loss is unavoidable. The latter part
of the stroke however, is utilised in driving the compressed air into a
reservoir, under the pressure from which reservoir or receiver it is
distributed to the drills. The lowest pressure in the tran8missi<m of
the air from the receiver to the drills should not exceed 1 to 3 lb. per
sq. in. for a distance of 1000 to 2000 ft., where pipes of sufficient dia-
meter are used. Where pipes are too small, the loss due to friction
may be very considerable. The proper diameter of the pipes will
depend upon the number of drills used and the distance of the drills
from the receiver.

In addition to its use as a power to drive the drills, the air per-
forms a valuable service after it accomplishes the above work, when
it is discliarged in the drifts, or stopes, or wherever it may be used,
as exhaust air. In confined places, as at the face of a drift for ex-
ample, this feature of the use of compressed air is often of great
importance, though the use of compressed air for purposes of general
ventilation of mines is inadmissible from an economical point of
view.

The most perfect modem forms of air-compressor are made by the
Ingersoll-Sergeant Co., and the following particulars of the pattern
known as '* class A '* cannot fail to be interesting. The machine is



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POWER,



31



complete in itself, both stoam and air cylinders being sitnated on one
solid bed. All strains are direct. The chieif features are indicated in
the annexed table : —



1

StauB

CyUadar.


DUm.

Air

Cylinder.


Stroke.


Iter, per
Minate.


Free Air
ll£me.


No.of3-ln.
Drlilsruu.


Weight
complete.


Actual
BoUer
li.p.


Cost.


in.


In.


in.




cab. ft.




lb.




£


10


10^


12


160


183


2


5,500


30


250


12


12:


14


155


295


3


7,400


40


313


14


14:


18


120


398


5


10,800


55


417


16


1^:


18


120


518


7


12,000


70


521


18


18


24


94


683


10


20,050


100


636


20


20}


24


94


840


12


21,400


130


709


22


22}


30


75


1011


16


29,000


155


917


24


24}


80


75


1202


20


34,500


200


1042



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33



DRILLING.

A MOD deal of oonfosion arises from the applioation of a single word
to ODQTej two distinct meanings. The term " drill " is used to indi-
cate an instrument which removes a solid core of the strata through
wkieh it passes — this is the diamond drill already alluded to under
Sroi^iectiiig (see p. 3) ; and to the common tool, simply a steel chisel,
Igf vhich holes are made for the insertion of an explosive— which
^ ~* under notice in this section.

]>QBpite modem progress in all kinds of machinery, a very large
'on of the drilling done at this day is still performed by Hand
Sometimes the drill is worked single-handed, the workman
^^W»»*g and rotating the drill with one hand and wielding the hammer
iatta other, and sometimes the work is shared by two men or even
tlttee. In small undertakings hand drilling is always the cheapest
nAbeet.

In soft ground, or indeed, in tolerably stiff ground, drilling can be
done cheaper or fully as cheap by hand as by power drills. This is
tme even where the power for the drills costs but little. But where
the ground is very hard, drilling with power drills is more economical
than by hand. The comparative cost of this system of drilling depends
upon the hardness of the ground, and also upon the cost of tiie power
for the air drills. The cost of power is of course exceedingly variable.
Where steam is used for the power to compress air, the expense of the
engineer must be added to the cost of runniDg the air drill. Where
the time of the engineer is to be charged entirely to this account, the
cost of drilling will evidently be greater than when this item is
apportioned among several charges, as hoist, pumping, &c., as well as
where many power drills are utsed at the mine.

The same relation exists where water is used for power, some
adaM having free water, while others pay for all they use.

Tlie drills generally used in California are 3 to 3^ in. diam. of
cgriinder, and require about 10 to 12 h.p. per drill. Where speed is
Sft •Iject, air dmls are used to advantage, doing the same work as
kwi drills ill about two-thirds the time in soft ground, to one-fifth
Ife^ftiBie in hard ground.

•Qbt drifting main tunnels, from which a large output of ore is to

e, two drills are employed in the face of the tunnel. This

expedites the progress of the tunnel, with not a greatly

d cost per ft. of tunnel run. The consumption of powder

to drifts run is greater when air drills are used than by hand

From two to four times the quantity of powder is used

ine drills as when hand drilling is employed, for the same

of drift run.

D




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34 ECONOMIC MINING.

Air drills are often used to advantage in stoping. Where the
vein is wide and the ground is hard, that method is preferable to hand
drilling, but it cannot be used to advantage in small or flat veins.

Some mines employ as many as 6 to 10 drills in the stope. In
the Idaho mine, Grass Valley, California, nearly the entire stoping is
done by machine drills. Two men are required to run a drill. Self-
feeding drills have, as yet, been introduced into but few localities.

The progress of drifting with drills varies with the hardness of the
ground, &c. With a single drill, 100 to 150 ft. per month in bard
ground is good speed. The cost per ft in California varies from 14f.
to 609. in £rifts about 4 ft wide by 7 ft high, exclusive of timbering
and track. This is reckoned on the basis that two Si^in. drills con-
sume daily 3 cords of firewood, which costs 7«. to 29^. a cord, but
generally about 149. to 18<.

It is almost unnecessary to remark, especially when one remembers
the very confined spaces where it is so often desirable to run rock
drills, that the length should be reduced to a minimum, for in narrow
workings it is impossible with a long drill to place the holes as a
skilled miner would do in hand labour. When this vital point (as
regards economy in explosives) is kept in view, holes can be as advan-
tageously placed as in hand boring, whereby the same economy in
explosives is obtained, with the highest advance of machine drilling.

The advantage of a short drill is by no means confined to stoping,
for it is quite as marked in the main headings, as no hard-and-fast
system of holes need any longer be followed, for, as in hand boring,
all holes can be so placed as to take full advantage of all joints and
cleavages, whereby greater progress is made and a large saving in
explosives is effected.

In gneiss and homogeneous rocks, free from faults, the ordinary
chisel-shaped bit is to be preferred, as, in such rocks, it cuts faster
than any other, and smiths can more readily sharpen it. In schistose
rocks, wherein layers of uneven hardness, with numerous fissures, are
encountered, the chisel bit does not answer so satisfactorilv, as the
drill seeks the softer material, gets out of line, sticks fast in the holes,
and much time is lost in extricating it, and unnecessary jar and strain
are thrown on the machine. In such ground, to obtain the beet
results from machine drilling, the chisel bit should be replaced by
the cross, of which there are two varieties, the + and x • For the
softer rocks, the cutting edge can with advantage be made Z shaped.

The quality of steel used in the tool bits requires careful selection,
as will be readily conceded when it is mentioned that at 4 atoios.
effective pressure a 3-in. diam. cylinder drill will deliver 700^00
blows, each of 175 foot-pounds per minute, and the best brands of tool
steel are required to stand this severe strain in hard rock.

The drill bits, according to the size of hole to be bored, should be
made from 1^ in., 1^ in., 1^ in., and 1 in. octagonal or round steel,
and care must be taken in changing, and that the bit to follow fits
easily and revolves freely in the hole. The borers should be sharpened
in successively diminishing diameters, and a difference of ^ in. in each
successive tool will genendly be found sufficient.

It is well to get a supply of the necessary drill steel for use in



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DRILLING. 35

mftdunes, <mt to length, with the ahanks truly turned, and the bits
sharpened ready for immediate use.

If the shanks are not truly turned in a lathe, but only swaged, it
ifl all important to see that when fitted in the machine the tool forms
a inkQ central extension of the piston rod, for if there be any eccentric
motion in the cutting edge of the tool when revolving — ^to which too
little attention is o&n paid — ^the power of the drill will be wasted
in friction, by the grinding of the bit in the hole, involving serious
waste of time and power.

It is frequently quite as difficult to drill a straight hole as a round
one. The shape of the bit has something to do with the alignment
of the hole. It is an invariable rule that the edge of the bit should
never be tapered in rock of uneven or irregular construction. The
marble bit is of no use except in a material like marble which is
Qiiifbrm. It is obvious that with a tapered bit passing through a
flint seam or other irregularity in rock the tendency would be to
glance, and this would result in " running " of the hole.

Where drill holes tend to run out of Ime the bit should invariably
have a straight edge, that is, at right angles to the axis of the drill
steeL It makes no difference whether the bit is a + or an X bit,
fio £ar as the alignment of the hole is ooncemed. In some difficult
places where the hole passes through soft spots or seams running
diagonally across the hole it is advisable to upset the steel for a dis-
tance of about 6 in. above the bit In other words, the steel should
be very nearly the full diameter of the bit for a distance of about 6 in.
at the bottom. The purpose of this is, that the steel may be caught
by the wall of the hole, thus preventing " running " untU the pocket
or seam has been pamed. This is readily understood when it is
known that the steel used with percussive drills is usually about
1 in. diam. octagon with a bit of about 2^ in. diam., thus there is a

re of about \ in. between the steel and the drill hole, and should
condition of the bottom of the hole be such as to tend to thrust
the bit to one side, it vnll gradually work the steel up against the
Bide of the hole, and will result in a crooked hole, which will give
trouble through binding and sticking. If the bar of steel were nearly
equal in diameter to uiat of the bit, it would, as it were, force the
hole to mn straight. It will not do, of course, to carry so much
weif ht of steel, hence where trouble is met it is best to upset the
steel at the bottom.

In the ordinary course of drilling the runner sometimes finds that
his hole is going crooked, and without waiting to get a special piece
of steel he attempts to pass through the obstruction. The first thing
to do is to reduce the speed of cutting. This is done by either
throttling the steam or shortening the stroke of the drill by dulling
the bit, but whatever is done it is necessary to ** go slow " with the
•iriUing. An eflSdctive means by which to prevent *' innning " is to
poll out the steel and throw some iron filings, or small pieces of iron
ia any shape, into the hole; then put in the steel and go ahead.
This not only reduces the speed of cutting, but the pieces of iron
are thrust into the softer places, and thus the bit cuts through the
obstruction, and keeps the hole in line.

D 2

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36 ECONOMIC MINING.

Let us assume that a cobble-stone of the size of an egg or larger
is discovered by the bit in the line of the hole, but a little to one side
of the centre. Obviously as the flange of the bit strikes this obstruc-
tion it will be thrown off at a tangent and will gradually eat away
the side of the hole farthest from the cobble. It is now simply
necessary to drill a few inches more of hole without losing the line,
and a few pieces of iron, or even a nut thrown in the hole, will retard
the " running " until the bit cuts through the obstruction.

Perhaps the most diflBcult place to put in a line of straight holes
is through a mass of old masonry or concrete. It is sometimes
necessary to drill holes in masoDry for the purpose of inserting
foundation bolts. The largest drill at hand should be used, no matter
what the depth of hole is, because a large drill gives less trouble
by sticking, and its force of blow may be regidated by the throttle.
It is also adviiiiable to use steel of large diameter — nearly as large as
the diameter of the bit. The legs of the drill should be firmly set,
and the runner should watch the hole, carefully following the
instructions given each time that there is a tendency to get out of
line.

Should the hole get the best of him in this respect, and the steel
bind so as to stick badly, he had, perhaps, better abandon the hole
and start a new one, for a great deal of time is lost in expensive
efforts to straighten a hole.

A drill hole will sometimes " run " in a most unexpected manner,
and in rock of uniform texture. In a case of this kind the runner
should at once stop his machine and see if his bit is in good shape.
Sometimes one of the flanges breaks off and serves the same purpose
in throwing the steel out of line as though a ** hard head " was en-
countered. If the broken piece is large it will sometimes get in one
oomer of the hole and give considerable trouble, even after the bit
has been repaired.

It is of much importance that the hole be well started, that is, it
should be started straight. In dimension stone quarries, the mouth
of the hole should be preserved at about the diameter of the hole, and
not cratered or broken. This can be done by starting with a light
blow and a short stroke, lengthening the stroke and the force of blow
after the hole has been made a little deeper than the length of the
stroke.

The well-known firm of Siemens Brothers & Co., Westminster,
have introduced two new forms of drill, one being an improvement
on percussive drills and the other a substitute for them. Percussive
drills require when working to perform three different motions:—
(a) The drill must receive a percussive or hammering motion in the
direction of its axis; (6) it must at the same time revolve slowly
round such axis ; (c) it must be propelled forward at a speed relative
to its effective work. Siemens*s drill accomplishes the three motions
simultaneously by means of a single electro-motor, and very simple
mechanism.

The other Siemens drill is intended to prevent the rapid wearing
away of boring tools when boring in hard stone and other similar
material, and consists essentially in substituting for the ooncnssive

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DRILLING, 37

motion of peroussion tools, or the abrading action of rotating tools, the
omshing action of hard metal balls, which are caused, while subject
to preBsnre, to roll over the surface of the material to be operated
upon. This rolling action of the balls is produced by forming on the
lower end of the boring rod, which constitutes the abutment of the
baUs, an annular groove of nearly semicircular section, so that on
the rotation of the boring rod round its axis, while subject to end
pressure, the balls will roll freely both in the groove of the rod and
on the surfaces of the stone, and thus in exerting a crushing action
■uoceasively upon comparatively small portions of the surface, they
will grind the material to powder. From the above described
arrangement it follows that the wear, if any, of the abutment will
take place uniformly all round in the annular groove, rcBulting merely
in the deepening of the latter and thus preventing any defective action
being produced by such wear. The arrangement of the balls and
boring rod may be variously modified.

The Jeffrey power drills have an excellent reputation in America
and are now being introduced into Great Britain by John Davis <fe
Son, Derby.

The drilling machinery made by the Ingersoll-Sergeant Drill Co.
is known all over the world, and has an excellent reputation every-
where.

Before the introduction of the Sergeant drill the Ingersoll was the
only rock drill in the market with a variable piston stroke. The
variable stroke is of the utmost importauoe in a rock drill. In start-
ing a hole in hard rook, the stroke is shortened by simply turning
the crank, thus feeding down the bit close to the rock. A hole can
be started with a short stroke in one-half the time as with a full
stroke. Another advantage of the variable stroke is that it enables
the drill to work loose in seamy or broken holes, and to loosen the
mud in muddy holes.

The Ingersoll drill has but two quick moving parts, the piston
and the valve. No part other than l3ie piston is subject to violent
shocks.

The Ingersoll drill strikes an unoushioned blow. The valve does
out move until the blow is struck, thus the full force of the blow is
delivered on the rock. It does not usq steam or air expansively, but
at full pressure in the blow and the recovery — an important factor,
distinguishing a rock drill from other steam engines. It has elastic
buffers in the front and back heads of the cylinder, their purpose
being to prevent breakage should the runner neglect to feed his
machine, or should the piston, through any cause whatever, strike
the front head. It is claimed to be the lij^htest rock drill made in
proportion to its force of blow. It is easily handled, and is equally
effective in both wet and dry holes. Owing to its simplicity of con-
struction and the independence of its piston from any connection with
moving parts, it has proved itself to be extremely economical in
repairs. Many Ingersoll drills made 15 and even 18 years ago are
nmsing to-day in good condition.

The Sergeant auxiliary valve drill is, strictly speaking, a drill for
hard rock. The design and purpose of the inventor was to strike a



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38 ECONOMIC MINING.

hard blow, and to so build the maohine that it would Btand hard
usage for years. The experience of the last four years has oonclusively
proved that not only is the Sergeant drill remarkably efficient in
cutting capacity, but that it does its work after years of use equally
as well as when new.

The Sergeant is the only rook drill in the world which combines
the independent valve operated through an auxiliary valve, and which
contains a release rotation. These two features are^he most important
as distinguishing the Sergeant from other rock drills.

The Sergeant, like the IngersoU, strikes an uncushioned blow.
The valve is held in such a position that while the piston carrying
the cutting tool is moved toward the rock the exhaust remains open
on one end, while the full pressure acts on the other end until ^e
blow is struck, at which time the valve immediately reverses. It
must hit the rock, and does it before the steam or air enters the front
end. It does not use st^am or air expansively, but has the benefit of
full pressure to strike the blow and to recover from broken or crooked
holes.

The Sergeant has an auxiliary valve operated by shoulders upon
the piston. The auxiliary valve and its valve seat are entirely inde-
pendent of the main valve and seat. The auxiliary is the trigger to
the main valve. It opens or closes the steam or air passages releasing
the pressure from one end or the other of the main valve. The
pressure bears it upon its seat ; hence its wear is uniform and cannot
produce leakage. The auxiliary valve being light, of steel, and
moving on the arc of a circle through contact with the piston
operating tangentially, it is easily moved, does not wear rapidly, and
never breaks. It is inexpensive and readily duplicated.

Using a round piston made of steel and hardened, fitting plug-like
in the ends, a large opening is efifected by a slight movement of tbe
valve. Being perfectly balanced, there is little or no wear.

A short or long stroke can be obtained at will by turning the
crank and feeding the cylinder toward the rock. This is a most im>
portant feature. A short stroke is of great advantage in starting or
olocking out holes.

A new rotating device, with a release movement, prevents twisting
of the spiral bar or breaking of pawls and ratchets. When a rock
drill strikes a hard blow upon an uneven surface there is a tendency
sometimes to twist the steel in the opposite direction to that in whidli
it rotates. The effect of such a blow on Uie Sergeant drill is simply
to turn the back head around, overcoming the friction of the back
head springs, when with a rigid rotation it might twist the rifle bar
or break the pawls and ratchets.

Two strong steel springs are used in place of buffers. These
springs are placed on the back head and are connected with the front
head through the side bolts ; hence a blow upon either the front or
the back head is cushioned by the springs, thus preventing break-
ages.

The volume and pressure of steam or air used is reduced to a
minimum in proportion to the work performed.

Its construction is such that it can be taken apart and put togetiier



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DRILLING.



39



in m few minutes, the parts being few and simple. It is not liable to
get out of order* It is easily handled and understood by the runner.

The mountings are all new, durable, and especially arranged fer



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 5 of 76)