revolution, a finely-cut screw furnishes an instrument well adapted to
impart a slow motion, or to measure minute spaces. Suppose a screw is
cut so as to have fifty threads in an inch, then each turn will advance
it 1/50 in.; half a turn 1/100 in.; a quarter of a turn, 1/200, and so
on. It is quite easy to attach a graduated circle to the head of the
screw, so that, by a fixed pointer at the circumference, any required
fraction of a revolution may be read off. Thus if the circle had two
hundred equal parts, we could, by turning the screw so that one division
passed the index, cause the screw to advance through 1/200 of 1/50 inch,
or 1/10000 part of an inch. This is the method adopted for moving the
cross-wires of the instruments for measuring very small spaces under the
microscope. Sir Joseph Whitworth, who has done so many great things in
mechanical art, was the first mechanician to perceive the importance of
extreme accuracy of workmanship, and he invented many beautiful
instruments and processes by which this accuracy might be attained. Fig.
30 represents one of his measuring machines, intended for practical use
in the workshop, to test the dimensions of pieces of metal where great
precision is required. The base of the machine is constructed of a rigid
cast iron bed bearing a fixed headstock, A, and a movable one, B, the
latter sliding along the bed, C, with a slow movement, when the handle,
D, is turned. This slow motion is produced by a screw on the axis, _a_,
working in the lower part of the headstock, just as the slide-rest is
moved along the bed of the lathe. The movable headstock, when it has
been moved into the position required, is firmly clamped by a
thumbscrew. The face of the bed is graduated into inches and their
subdivisions. Here it should be explained that the machine is not
intended to be used for ascertaining the absolute dimensions of objects,
but for showing by what fraction of an inch the size of the work
measured differs from a certain standard piece. Each headstock carries a
screw of 1/20 inch pitch, made with the greatest possible care and
accuracy. To the head of the screw in the movable headstock is attached
the wheel, _b_, having its circumference divided into 250 equal parts,
and a fixed index, _c_, from which its graduations may be counted. An
exactly similar arrangement is presented in connection with the screw
turning in the fixed headstock, but the wheel is much larger, and its
circumference is divided in 500 equal parts. It follows, therefore, that
if the large wheel be turned so that one division passes the index, the
bar moves in a straight line 1/500 of the 1/20 of an inch, that is,
1/10000 an inch. The ends of the bars, _d_ and _e_, are formed with
perfectly plane and parallel surfaces, and an ingenious method is
adopted of securing equality of pressure when comparisons are made. A
plate of steel, with perfectly parallel faces, called a _gravity-piece_,
or _feeler_, is placed between the flat end of the bar and the
standard-piece, and the pressure when the screw-reading is taken must be
just sufficient to prevent this piece of steel from slipping down, and
that is the case when the steel remains suspended and can nevertheless
be easily made to slide about by a touch of the finger. Thus any piece
which, with the same screw-readings, sustains the gravity-piece in the
same manner as the standard, will be of exactly the same length; or the
number of divisions through which the large wheel must be turned to
enable it to do so tells the difference of the dimensions in
ten-thousandth parts of an inch. By this instrument, therefore, gauges,
patterns, &c., can be verified with the greatest precision, and pieces
can be reproduced perfectly agreeing in their dimensions with a standard
piece. Thus, for example, the diameters of shafting can be brought with
the greatest precision to the exact size required to best fit their
bearings.
In another measuring machine on the same principle the delicacy of the
measurement has been carried still farther, by substituting for the
large divided wheel one having 200 teeth, which engage an endless screw
or worm. This will easily be understood by reference to Fig. 31, where a
similar arrangement is applied to another purpose. Imagine that a wheel
like P, Fig. 31, but with 200 teeth, has taken the place of E in Fig.
30, and that the wheel, T, on the axis of the endless screw is shaped
like E, Fig. 30. One turn of the axis carrying the endless screw,
therefore, turns the wheel through 1/200 of a revolution, and as this
axis bears a graduated head, having 250 divisions, the screw having 20
threads to the inch, is, when one division passes the index, advanced
through a space equal to 1/250 × 1/200 × 1/20, or 1/1000000 an inch;
that is, the one-millionth part of an inch. This is an interval so small
that ten times its length would hardly be appreciated with the highest
powers of the microscope, and the machine is far too delicate for any
practical requirements of the present day. It will indicate the
expansion caused by heat in an iron bar which has merely been touched
with the finger for an instant, and even the difference of length
produced by the heat radiated from the person using it. A movement of
1/1000000 of an inch is shown by the gravity-piece remaining suspended
instead of falling, and the piece falls again when the tangent-screw is
turned back through 1/250 of a revolution, a difference of reading
representing a possible movement of the measuring surface through only
2/1000000 an inch. This proves the marvellous perfection of the
workmanship, for it shows that the amount of play in the bearings of the
screws does not exceed one-millionth of an inch.
A good example of a machine-tool is the _Drilling Machine_, which is
used for drilling holes in metal. Such a machine is represented in Fig.
31, where A is the strong framing, which is cast in a single piece, in
order to render it as rigid as possible. The power is applied by means
of a strap round the speed pulley, B, by which a regulated speed is
communicated to the bevel wheel, C, which drives D, and thus causes the
rotation of the hollow shaft, E. In the lower part of the latter is the
spindle which carries the drilling tool, F, and upon this spindle is a
longitudinal groove, into which fits a projection on the inside of E.
The spindle is thus forced to rotate, and is at the same time capable of
moving up and down. The top of the spindle is attached to the lower end
of the rack, G, by a joint which allows the spindle to rotate freely
without being followed in its rotation by the rack, although the latter
communicates all its vertical movements to the spindle, as if the two
formed one piece. The teeth of the rack are engaged by a pinion, which
carries on its axis the wheel H, turned by an endless screw on the
shaft, I, which derives its motion by means of another wheel and endless
screw from the shaft, K. The latter is driven by a strap passing over
the _speed pulleys_, L and M, and thus the speed of the shaft K can be
modified as required by passing the strap from one pair of pulleys to
another. The result is that the rack is depressed by a slow movement,
which advances the drill in the work, or, as it is technically termed,
gives the _feed_ to the drill. By a simple piece of mechanism at N the
connection of the shafts K and I can be broken, and the handle O made to
communicate a more rapid movement to I, so as to raise up the drill in a
position to begin its work again, or to bring it quickly down to the
work, and then the arrangement for the self-acting feed is again brought
into play. By turning the wheel, P, the table, Q, on which the work is
fastened, is capable of being raised or lowered, by means of a rack
within the piece R, acted on by a pinion carried on the axle, P. The
table also admits of a horizontal motion by the slide S, and may besides
be swung round when required.
[Illustration:
FIG. 31.—_Whitworth’s Drilling Machine._
]
The visitor to an engineer’s workshop cannot fail to be struck with the
operation of the powerful _Lathes_ and _Planing Machines_, by which long
thick flakes or shavings of iron are removed from pieces of metal with
the same apparent ease as if the machine were paring cheese. The figure
on the opposite page represents one of the larger forms of the planing
machine, as constructed by Sir J. Whitworth. The piece of work to be
planed is firmly bolted down to the table, A, which moves upon the
Ꮩ-shaped surfaces, running its whole length, and accurately fitting into
corresponding grooves in a massive cast iron bed. The bevel wheel, of
which a portion is seen at B, is keyed on a screw, which extends
longitudinally from end to end of the bed. This screw works in nuts
forming part of the table, and as it turns in sockets at the ends of the
bed, it does not itself move forward, but imparts a progressive movement
to the table, and therefore to the piece of metal to be planed. As this
table must move backwards and forwards, there must be some contrivance
for reversing the direction of the screw’s rotation, and this is
accomplished in a beautifully simple manner by an arrangement which a
little consideration will enable any one to understand. It will be
observed that there are three drum-pulleys at C. Let the reader confine,
for the present, his attention to the nearest one, and picture to
himself that the shaft to which it is attached is placed in the same
horizontal plane as the axis of the screw and at right angles to it,
passing in front of bevel wheel B. A small bevel wheel turning with this
shaft, and engaging the teeth of the wheel B, may, it is plain,
communicate motion to the screw. Now let the reader consider what will
be the effect on the _direction_ of the rotation of B of applying the
bevelled pinion to the nearer or to the farther part of its
circumference, supposing the direction of the rotation of this pinion to
be always the same. He will perceive that the direction in one case will
be the reverse of that in the other. The shaft to which the nearest
pulley is attached carries a pinion engaging the wheel at its farther
edge, and therefore the rotation of this pulley in the same direction as
the hands of a watch causes the wheel B to rotate so that its upper part
moves towards the spectator. The farthest pulley, _a_, turns with a
hollow shaft, through which the shaft of the nearest pulley simply
passes, without any connection between them, and this hollow shaft
carries a pinion, which engages the teeth of B at the nearer edge, and,
in consequence, the rotation of the farther pulley, _a_, in the
direction of the hands of a watch, would cause the upper part of B to be
moving from the spectator. The middle pulley, _b_, runs loosely on the
shaft, and the driving-strap passes through the guide, _c_, and it is
only necessary to move this, so as to shift the strap from one drum to
another, in order to reverse the direction of the screw and the motion.
This shifting of the strap is done by a movement derived from the table
itself, on which are two adjustable stops, D and E, acting on an
arrangement at the base of the upright frame when they are brought up to
it by the movement of the table, so as not only to shift the strap, but
also to impart a certain amount of rotation to upright shaft, F, in each
direction alternately. The piece which carries the tools, G and H, is
placed horizontally, and can be moved vertically by turning the axis, I,
thus causing an equal rotation of two upright screws of equal pitch,
which are contained within the uprights and work in nuts, forming part
of the tool-box. The pieces carrying the tools are moved horizontally by
the screws which are seen to pass along the tool-box, and these screws
receive a certain regulated amount of motion at each reversal of the
movement of the table from the mechanism shown at K. The band-pulley, L,
receives a certain amount of rotation from the same shaft, and the
catgut band passing round the tops of the cylinders which carry the
cutters is drawn in alternate directions at the end of each stroke, the
effect being to turn the cutters half round, so as always to present
their cutting edges to the work. There are also contrivances for
maintaining the requisite steadiness in the tools and for adjusting the
depth of the cut. The cutting edge of the tools is usually of a Ꮩ-shape,
with the angle slightly rounded, and the result of the process is not
the production of a plane, but a grooved surface. But by diminishing the
amount of horizontal _feed_ given to the cutters, the grooves may be
made finer and finer, until at length they disappear, and the surface is
practically a plane. Planing machines are sometimes of a very large
size. Sir J. Whitworth has one the table of which is 50 ft. in length,
and the machine is capable of making a straight cut 40 ft. long in any
article not exceeding 10 ft. 6 in. high or 10 ft. wide.
[Illustration:
FIG. 32.—_Whitworth’s Planing Machine._
]
[Illustration:
FIG. 33.—_Pair of Whitworth’s Planes, or Surface Plates._
]
The copying principle is evident in this machine; for the plane surface
results from the combination of the straightness of the bed with the
straightness of the transverse slide along which the tools are moved. It
should, moreover, be observed that it is precisely this machine which
would be employed for preparing the straight sliding surfaces required
in the construction of planing and other machines, and thus one of these
engines becomes the parent, as it were, of many others having the same
family likeness, and so on _ad infinitum_. Thus, having once obtained
perfectly true surfaces, we can easily reproduce similar surfaces. But
the reader may wish to know how such forms have been obtained in the
first instance; how, for example, could a perfectly plane surface be
fashioned without any standard for comparison? This was first perfectly
done by Sir J. Whitworth, forty-five years ago. Three pieces of iron
have each a face wrought into comparatively plane surfaces; they are
compared together, and the parts which are prominent are reduced first
by filing, but afterwards, as the process approaches completion, by
scraping, until the three perfectly coincide. The parts where the plates
come in contact with each other are ascertained by smearing one of them
with a little oil coloured with red ochre: when another is pressed
against it, the surfaces of contact are shown by the transference of the
red colour. Three plates are required, for it is possible for the
prominences of No. 1 exactly to fit into the hollows of No. 2, but in
that case _both_ could not possibly exactly coincide with the surface of
No. 3; for if one of them did (say No. 1), then No. 3 must be exactly
similar to No. 2, and consequently when No. 2 was applied to No. 3,
hollow would be opposed to hollow and prominence to prominence. A little
reflection will show that only when the three surfaces are truly plane
will they exactly and entirely coincide with each other. The planes,
when thus carefully prepared, approach to the perfection of the ideal
mathematical form, and they are used in the workshop for testing the
correctness of surfaces, by observing the uniformity or otherwise of the
impression they give to the surface when brought into contact with it,
after being covered by a very thin layer of oil coloured by
finely-ground red ochre.
Fig. 33 represents a small pair of Whitworth’s planes. When one of these
is placed horizontally upon the other, it does not appear to actually
come in contact with it, for the surfaces are so true that the air does
not easily escape, but a thin film supports the upper plate, which
glides upon it with remarkable readiness (A). When, however, one plate
is made to slide over the other, so as to exclude the air, they may both
be lifted by raising the upper one (B). This effect has, by several
philosophers, been attributed to the mere pressure of the atmosphere;
but recent experiments of Professor Tyndall’s show that the plates
adhere even in a vacuum. The adhesion appears therefore to be due to
some force acting between the substances of the plates, and perhaps
identical in kind with that which binds together the particles of the
iron itself.
[Illustration:
FIG. 34.—_Interior of Engineer’s Workshop._
]
[Illustration:
FIG. 35.—_The Blanchard Lathe._
]
_THE BLANCHARD LATHE._
This machine affords a striking example of the application of the
copying principle which is the fundamental feature of modern
manufacturing processes. It would hardly be supposed possible, until the
method had been explained, that articles in shape so unlike geometrical
forms as gun-stocks, shoemakers’ lasts, &c., could be turned in a lathe.
The mode in which this is accomplished is, however, very simple in idea,
though in carrying that idea into practice much ingenious contrivance
was required. The illustration, Fig. 35, represents a Blanchard’s lathe,
very elegantly constructed by Messrs. Greenwood and Batley, of Leeds.
The first obvious difference between an ordinary lathe and Blanchard’s
invention is that in the former the work revolves rapidly and the
cutting-tool is stationary, or only slowly shifts its position in order
to act on fresh portions of the work, while in the latter the work is
slowly rotated and the cutting-tools are made to revolve with very great
velocity. Again, it will be observed that the headstock of the Blanchard
lathe, instead of one, bears _two_ mandrels, having their axes parallel
to each other. One of these carries the pattern, C, which in the figure
has the exact shape of a gun-stock that is to be cut in the piece of
wood mounted on the nearer spindle. One essential condition in the
arrangement of the apparatus is that the pattern and the work having
been fixed in similar and parallel positions, shall always continue so
at every point of their revolutions. This is easily accomplished by
placing exactly similar toothed wheels on the two axles, and causing
these to be turned by one and the same smaller toothed wheel or pinion.
The two axles must thus always turn round in the same direction and with
exactly the same speed, so that the work which is attached to one, and
the pattern which is fixed on the other, will always be in the same
phase of their revolutions. If, for example, the part of the wood which
is to form the upper part of the gun-stock is at the bottom, the
corresponding part of the pattern will also be at the bottom, as in the
figure, and both will turn round together, so that every part of each
will be at every instant in a precisely similar position. The wood to be
operated upon is, it must be understood, roughly shaped before it is put
into the lathe. The toothed wheels and the pinion which drives them are
in the figure hid from view by the casing, _h_, which covers them. The
pinion receives the power from a strap passing over _f_. The cutters are
shown at _e_; they are placed radially, like the spokes of a wheel, and
have all their cutting edges at precisely one certain distance from the
axis on which they revolve, so that they all travel through the same
circle. These cutting-edges, it may be observed, are very narrow, almost
pointed. The shaft carrying the cutters is driven at a very high speed,
by means of a strap passing over _k_ and _i_. The number of revolutions
made by the cutters in one second is usually more than thirty. The great
peculiarity of the lathe consists in the manner in which the position of
the cutters is made to vary. The axle which carries them rotates in a
kind of frame, which can move backwards and forwards, so that the
cutters may be readily put at any desired distance from the axis of the
work. Their position is, however, always dependent on the pattern, for,
fixed in a similar frame, _b_, which is connected with the former, is a
small disc wheel, _a_, having precisely the same radius as that of the
circle traced out by the cutters, and this disc is made by a strong
spring to press against the pattern. The cutters, being fixed in the
same rocking-frame which carries this guiding-wheel, must partake of all
its backward and forward motions, and as the cutting-wheel and the
guide-wheel are so arranged as to have always the same relative
positions to the axes of the two headstocks, it follows that the edges
of the cutters will trace out identically the same form as the
circumference of the guiding-disc. The latter is, of course, not driven
round, but simply turns slowly with the pattern by friction, for it is
pressed firmly against the pattern by a spring or weight acting on the
frame, in order that the cutters may be steadily maintained in their
true, but ever-varying, position. The rocking-frame receives a slow
longitudinal motion by means of the screw, _n_, so that the cutters are
carried along the work, and the guide along the pattern.
The whole arrangement is self-acting, so that when once the pattern and
the rough block of wood have been fixed in their positions, the machine
completes the work, and produces an exact repetition of the shape of the
pattern. It is plain that any kind of forms can be easily cut by this
lathe, the only condition being that the surface of the pattern must not
present any re-entering portions which the edge of the guide-wheel
cannot follow. The machine is largely used for the purposes named above,
and also for the manufacture of the spokes of carriage-wheels. The
limits of this article will not permit of a description of the beautiful
adjustments given to the mechanism in the example before us,
particularly in the arrangement for driving the cutters in a framework
combining lateral and longitudinal motions; but the intelligent reader
may gather some hints of these by a careful inspection of the figure.
The machine is sometimes made with the frame carrying the guide-wheel
and cutters, not rocking but sliding in a direction transverse to the
axes of the headstocks. It is extremely interesting to see the Blanchard
lathe at work, and observe how perfectly and rapidly the curves and form
of the patterns seem to grow, as it were, out of the rudely-shaped piece
of wood, which, of course, contains a large excess of material, or, in
the picturesque and expressive phrase of the workmen, _always gives the
machine something to eat_.
[Illustration:
FIG. 36.—_Vertical Saw._
]
_SAWING MACHINES._
With the exception of the last, all the machines hitherto described in
the present article are distinguished by this—they are tools which are
used to produce other machines of every kind. Without such implements it
would be impossible to fashion the machines which are made to serve so
many different ends. Another peculiarity of these tools has also been
referred to, namely, that they are especially serviceable, and indeed
essential, for the reproduction of others of the same class. Thus, the
accurate leading-screw of the lathe is the means used to cut other
accurate screws, which shall in their turn become the leading-screws of
other lathes, and a lathe which forms a truly circular figure is a
necessary implement for the construction of another lathe which shall
also produce truly circular figures. In these tools, therefore, we find
the copying principle, to which allusion has been already made, as the
great feature of all machines; but in order to bring this principle
still more clearly before the reader, we have described in the Blanchard
Lathe a machine of a somewhat different class, because it embodied a
very striking illustration of the principle in question. We are far from
having described all the implements of the mechanical engineer, or even
all the more interesting ones; for example, we have given no account of
the powerful lathes in which great masses of iron are turned, or of the
analogous machines, which, with so much accuracy, shape the internal
surfaces of the cylinders of steam engines, of cannons, &c. The history
of the steam engine tells us of the difficulties which Watt had to
contend with in the construction of his cylinders, for no machine at
that time existed capable of boring them with an approach to the
precision which is now obtained.
[Illustration:
FIG. 37.—_Circular Saw._
]