The sides of solid bodies, whatever be the degree of hardness, and
however fine the texture, possess surfaces formed of a succession of
projections and depressions. When two bodies are in contact, these
projections and indentations fit into one another, and the adherence that
results is proportional to the degree of roughness of the surfaces. If,
by a more or less energetic mechanical action, we move one of the bodies
with respect to the other, we shall produce, according as the action
overcomes cohesion, more or less disintegration of the bodies. The
resulting wear in each of them will evidently be inversely proportional
to its hardness and the nature of its surface; and it will vary, besides,
with the pressure exerted between the surfaces and the velocity of the
mechanical action. We may say, then, that the wear resulting from rubbing
two bodies against each other is a function of their degree of hardness,
of the extent and state of their surface, of the pressure, of the
velocity, and of the time.
[Illustration: FIGS. 1, 2 and 3.--APPARATUS FOR SAWING STONE.]
According as these factors are varied in a sense favorable or unfavorable
to their proper action, we obtain variations in the final erosion. Thus,
in rubbing together two bodies of different hardness and nature of
surface, we obtain a wear inversely proportional to the hardness and
state of polish of their surfaces. Through the interposition of a
pulverized hard body we can still further accelerate such wear, as a
consequence of the rapid renewal of the disintegrating element.
The gradual wear effected over the entire surface of a body brings about
a polish, while that effected along a line or at some one point
determines a cleavage or an aperture.
The process usually employed in quarries or stone-yards for sawing
consists in slowly moving a stone-saw backward and forward, either by
hand or machinery, and with scarcely any pressure. Mr. P. Gray has,
however, devised a new process, which is based upon the theoretical
considerations given above. His _helicoidal saw_ is, in reality, an
endless cable formed by twisting together three steel wires in such a way
as to give the spirals quite an elongated pitch.
The apparatus in its form for cutting blocks of stone into large slabs
(Figs. 1, 2, and 3) consists of two frames, A A, five feet apart, each
formed of two iron columns, 7½ feet in height and one foot apart, fixed
to cast iron bases resting upon masonry. At the upper part, a frame, B B,
formed of double T-irons cross-braced here and there, supports a
transmission composed of gearwheels, R R, and a pitch-chain, G G. Along
the columns of the frame, which serve as guides, move two kinds of
pulley-carriers, C C. The pulleys, D D, are channeled, and receive the
cable, a a, which serves as a helicoidal saw. The direction of the saw's
motion is indicated by the arrow. The carriages, C C, are traversed by
screws, V V, which are fixed between the columns. The extremity, v, of
the axle of the pulley to the right is threaded, and actuates a
helicoidal wheel, E, which transmits motion to the wheel, R, through the
intermedium of the vertical shaft, F. This transmission, completed by the
wheels, R R, and the pitch-chains, G G, is designed to move the saw
vertically, through the simultaneous shifting of the carriages, C C. A
tension weight, P, through the intermedium of pulleys, D_{1} D_{1},
permits of keeping the saw taut. A reservoir, H, at the upper part of the
frame, B B, contains the water and sand necessary for sawing. The feeding
is effected by means of a rubber tube, I, terminating in a flattened
rose, J, which is situated over the aperture made by the saw. A small
pump, L. over the reservoir takes water from K, and raises it to H. The
sand is put in by hand.
Above the basin, K, a system of rails and ties supports the carriage, Q,
upon which is placed the block of stone to be sawn. When one operation
has been finished, and it is desired to begin another, it is necessary to
raise the pulley-carriers and the saw. In order to do this quickly, there
is provided a special transmission, M, which is actuated by hand, through
a winch.
The work done by this saw is effected more rapidly than by the ordinary
processes, and certain very hard rocks, usually regarded as almost
intractable, can be sawed at the rate of from one to one and a half
inches per hour.
[Illustration: FIG. 4.--APPARATUS FOR SAWING STONE INTO SLABS.]
For sawing marble into slabs of all thicknesses, the arrangement
described above may be replaced by a system consisting of two drums
having several channels to receive as many saws, or two corresponding
series of channeled pulleys, b b (Fig. 4), independent of each other, but
keyed to the same axles, i i. When the pulleys have been properly spaced
by means of keys, the whole affair is rendered solid by a bolt, g. The
extremity of the axles forms a nut into which pass vertical screws, c c.
These latter are connected above with cone-wheels, l l, which, gearing
with bevel wheels keyed to the shafts, e, secure a complete
interdependence of the whole. The ascending motion, which is controlled
by the endless screws, f, and the helicoidal wheels, m, is in this way
effected with great regularity. Uprights, a a, of double T-iron, fixed to
joists, k k, and connected and braced by pieces, d d, form a strong
frame.
[Illustration: FIG. 5.--APPLICATION OF GAY'S STONE SAW IN A MARBLE
QUARRY.]
The power necessary to run this kind of saw is less than _n_ × ¼ H.P.,
on account of the number of passive parts. The most interesting
application of the helicoidal saw is in the exploitation of quarries.
Fig. 5 represents a Belgian marble quarry which is being worked by Mr.
Gay's method.
_Tubular Perforators_.--Mr. Gay has rendered his saw completer by the
invention of a tubular perforator for drilling the preliminary well. It
is based upon the same principle as the Leschot rotary drill, but differs
from that in its extremity being simply of tempered steel instead of
being set with black diamonds. A special product, called metallic
agglomerate, is used instead of sand for hastening the work.
[Illustration: FIG. 6.--TUBULAR PERFORATOR.]
The apparatus, Fig. 6, consists of an iron plate cylinder, A, 27½
inches in diameter, and of variable length, according to the depth to be
obtained, and terminating beneath in a steel head, B, of greater
thickness. This cylinder is traversed by a shaft, C, to which it is
keyed, and which passes through the center of the aperture drilled. This
shaft is connected with the cylinder, A, through the intermedium of cross
bars, D, and transmits thereto a rapid rotary motion, which is received
at the upper part from a telodynamic wire that passes through the channel
of the horizontal pulley, P. This latter is supported by a frame
consisting of three uprights, Q Q, strengthened by stays, R R, fixed to
the ground.
In order that the cylinder, A, may be given a vertical motion, cords, M
M, fixed to a piece, S, loose on the hub, D, wind round the drum of a
windlass, T, after passing over the pulleys, p p.
The rapid gyratory motion of the cylinder, along with the erosive action
of the metallic agglomerate, rapidly wears away the rock, and causes the
descent of the perforator. During this operation a core of marble forms
in the cylinder. This is detached by lateral pressure, and is capable of
being utilized. The tool descends at the rate of from 20 to 24 inches per
hour, or from 8 to 10 yards per day in ordinary lime rock.--_Le Genie
Civil_.
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