I now propose to assist the young mechanic in special work, instead of
continuing general directions. This will enable me to explain to him
various lathe appliances, and other details of mechanical work hitherto
passed by.
Of all models which boys (and very big boys too) are desirous to
construct, the steam-engine holds the chief place, and deservedly so; for
every boy calling himself mechanical, ought to know how this is made,
and the general _principles_ of its construction as well. However, I am
aware, from experience, that many a youngster, who is even in possession
of a model engine, is utterly ignorant of the cause of its motion;
although it is a great delight to them to see the steam puffing out, and
the wheel revolving “nineteen to the dozen,” as schoolboys say. Now, an
engine is a very simple affair, and can be easily explained; and, as I
wish my readers to work rationally, I shall show them what they have to
do before I tell them how to do it.
[Illustration: Fig. 56.]
A, Fig. 56, represents a cubical vessel of tin or any other substance.
By cubical, I mean that all its sides are squares, and all exactly equal;
each side in the present case is to be 1 inch wide and long, or a square
inch. B is a similar vessel, 1 foot cube. It contains, therefore, 1728
cubic inches, or is 1728 times as large in capacity as the first. Now, if
I were to fill the little vessel with water and tip it into the second,
and put a lamp under it, the water would all soon boil away, as it is
called. It would be converted into steam; and the quantity of steam it
would produce would exactly fill the larger vessel, without exciting any
particular pressure upon its sides.
Steam, thus allowed plenty of elbow room, is like a lazy boy; it will
play and curl about very prettily, but will do no work. We must put some
sort of pressure, therefore, upon it—confine it, and we shall soon see
that, by struggling to escape, it will serve our purpose, and become
a most obedient workman. We have, therefore, only to put double the
quantity of water into our larger vessel, that is, _two_ cubic inches.
We will put on a cover tightly, adding a pipe through which to pour in
the water. Soon we shall have the steam formed as before; but it has no
longer room enough, and out it comes fizzing and roaring, very savage at
having been shut up in so small a cage. And we can make it work too, for
if we set up a little fan-wheel of tin right in its way, we shall see
it spin round merrily enough; or if we cork the tube lightly, we shall
find this cork soon come out with a bang. We have, in fact, already
constructed a steam-engine and a steam-gun on a small scale. The pressure
in this case is, indeed, not great, but what it is I must now try to
explain.
The air or atmosphere, which surrounds us on all sides, exercises a
pressure upon everything of 15 lbs. on every square inch of surface. If
our little cubical inch box of tin had no air inside it, and no steam,
but was absolutely empty, each side, and top, and bottom would have 15
lbs. pressure upon it; which would be evident if it were not very strong,
for it would sink in on all sides directly, just as much as if you
were to _add_ a weight of 15 lbs. when it was full of air, as it would
ordinarily be.
When I spoke of the larger box being exactly filled with steam from the
evaporation of the cubic inch of water poured from the smaller box,
I supposed it empty of air. The steam from that quantity of water,
occupying the place of the air, would also be of the same pressure, 15
lbs. per square inch of surface; and as this only balances the pressure
of the atmosphere, which would be, in such a case, pressing in on all
sides, the steam would not show any pressure; just as, if you put equal
weights into each scale of a balance, the beam of it would remain
horizontal, neither scale showing to the outward senses that it had any
pressure upon it. But in the second case, we have doubled the quantity of
steam, but compelled it to occupy the same space; therefore we have now
real, visible pressure of 15 lbs. upon each square inch; and if we again
halve the space which the steam has to occupy, or double the quantity of
water, we shall obtain a pressure of 30 lbs. beyond the pressure of the
atmosphere.
Let us now disregard atmospheric pressure, and fit up such an apparatus
as Fig. 56, D. Here we have first our small box, closed on all sides,
and from it a small tube rising and entering into the bottom of a larger
one, which is very smooth in the inside; in this is a round plate or
disc, called a piston, which fits the tube nicely, but not so tight as to
prevent it from moving up and down easily; and let a weight of 15 lbs. be
laid upon it. Let us suppose this large tube or cylinder to be 1700 times
larger than the cubic inch box, into which water is to be poured till
full. Now we heat it as before, and when 212° of heat are attained by the
water (which is its boiling-point) when it begins to be converted into
steam, the piston will be seen to rise, and will gradually ascend, until
quite at the top of the tube, because the steam required exactly that
amount of room.
Now we have arrived at the same conclusion which we came to before;
for you see that not only has the cubic inch of water become a cubic
foot of steam (_about_ 1700 to 1728 of its former volume), but it is
supporting 15 lbs. weight, which represents that of the atmosphere, and
if we could get rid of the latter, a solid weight of 15 lbs. would be
thus supported. Now, still neglecting the atmospheric pressure, suppose
instead of 15 lbs. we add another 15 lbs., making the weight 30 lbs.,
down goes our piston again, and stands at about half the height it did
before. We have thus, as we had previously, a cubic foot of steam made to
occupy half a cubic foot of space, giving a pressure (which is the same
as supporting a weight) of 30 lbs.
I ought, perhaps, to add in this place, however, that under 30 lbs.
pressure, or atmospheric weight and 15 lbs. additional, the water would
not become steam at a temperature of 212°, but it would have to be made
much hotter, until a thermometer placed in it would show 252°.
So far we have seen what a cubic inch of water will do when heated to a
certain degree, and at first sight it may not seem a great deal. Far from
being light work, however, this is actually equal to the work of raising
a weight of 1 ton a foot high. Let us prove the fact. Suppose the tube or
cylinder to be square instead of round, and that its surface is exactly 1
square inch, how can we give 1700 times the room which is occupied by the
water? It is plain that the piston must rise 1700 inches in the 1-inch
cylinder or tube, carrying with it, as before, its weight of 15 lbs.—that
is, it has raised 15 lbs. 1700 inches, or about 142 _feet_. But this is
the same as 15 times 142 feet raised 1 foot, which is 2130 lbs. raised 1
foot, very nearly a ton, the latter being 2240 lbs. So, after all, you
see that our little cubic inch of water is a very good labourer, doing a
great deal of work if we supply him with sufficient warmth.
Now this is exactly the principle of the ordinary steam-engine: we have
a cylinder in which a piston is very nicely fitted, and we put this
cylinder in connection with a boiler, the steam from which drives the
piston from one end of the cylinder to the other. In the first engine
that was made, the cylinder actually occupied the very position it does
in our sketch; it was made to stand upon the top of the boiler, a tap
being added in the short pipe below the cylinder, so that the steam could
be admitted or shut off at pleasure. But it is plain that although our
little engine has done some work, it has stopped at a certain point;
there is the piston at the top, and it cannot go any farther; we must get
it down again before it can repeat its labour.
How would you do this, boys? Push it down, eh? If you did, you would find
it spring up again when you removed your hand, just as if there were
underneath it a coiled steel spring; by which, however, you would learn
practically what is meant by the _elasticity_ of steam. Besides this,
if you push it down, you become the workman, and the engine is only the
passive recipient of your own labour. Try another plan; remove the lamp,
and see the result—gradually, _very_ gradually, the piston begins to
descend.
Take a squirt or syringe, and squirt cold water against the apparatus.
Presto! down it goes, now very quickly indeed, and is soon at the bottom
of the cylinder. But we may as well try to get useful work done by the
descent of the piston as well as by its ascent.
Set it up like Fig. 56, E. Here is a rod or beam, _b a c_, the middle of
which is supported like that of a pair of scales. From one end we hang
a scale, and place in it 15 lbs.; and as the piston sinks the weight is
raised, and exactly the same work is done as before. Thus was the first
engine constructed; but instead of the scale-pan and weight, a pump-rod
was attached, and as the piston descended in the cylinder this rod was
raised, and the water drawn from the well. This, however, was not called
a steam-engine, because the work is not really the effect of the steam,
which is only used to produce what is called a vacuum (_i.e._, an empty
space, devoid of air) under the piston. In fact, the up-stroke of the
piston was only partly caused by steam, and the rod of the pump was
weighted, which helped to draw it up.
I must get you to understand this clearly, so that the principle may
become plain—“clear as mud,” as Paddy would say. I told you that the
air pressed on every square inch of surface with a force of about 15
lbs. We do not feel it, because we are equally pressed on all sides—from
within as well as from without—so that atmospheric pressure is balanced.
Sometimes this is a very good thing. We should, I think, hardly like to
carry about the huge weight pressing upon our shoulders, if something
did not counteract it for us, so that we cannot feel it. Indeed, if it
were otherwise, we should become flat as pancakes in no time—“totally
chawed up.”
But sometimes we should prefer to get rid of the air altogether—and I
can tell you it is not easy to do so, unless we put something into its
place; and we want perhaps simply to get rid of it, and make use of the
room it occupied. We require to do this in the present instance, and in
fact we have just done it. If the whole space below the piston, when
we begin to work, is filled with water, it is plain there can be no
air below it; and when the steam has raised it, there is still no air
below it, but only steam. We then apply cold to the cylinder by removing
the lamp and squirting cold water against it, which has the effect of
reducing the steam to water again, which will occupy 1 inch of space
only. We, therefore, now have a space of 1600 cubic inches with neither
air nor water in it; and so, if the piston is 1 inch in size, there will
be the 15 lb. pressure of the atmosphere upon it, and nothing below to
balance it, for we have formed a vacuum below it, and of course this
15 lb. weight will press it rapidly down. It did so; and we therefore
were enabled to raise 15 lb. in the scale-pan. You will know, therefore,
henceforth, exactly what I mean by a vacuum and atmospheric pressure. It
is, you see, the latter which does the work when a vacuum is formed as
above; but you can easily understand that it might be possible to use
both the atmospheric pressure _and_ the pressure of steam as well, which
is done in the condensing steam-engine.
In the earliest engine, called the _Atmospheric_ for the reason above
stated, the top of the cylinder was left entirely open, as in our sketch;
but the condensing water was not applied outside the cylinder, but
descended from a cistern above, and formed a little jet or fountain in
the bottom of the cylinder at the very moment that the piston reached its
highest point. Down it, therefore, came, drawing up the pump-rod. When
at the bottom the jet of water ceased. Steam was again formed below the
piston, which raised it as before; and the process being repeated, the
required work was done. A boy, to turn a couple of taps, to let on or off
the water or steam, was all the attendance required.
For some time the atmospheric engine, the invention of Newcomen, was the
only one in general use; and even this was, in those days (1705-1720),
so difficult to construct that its great power was comparatively seldom
resorted to, even for pumping, for which it was nevertheless admirably
suited. The huge cylinder required to be accurately bored, while there
were no adequate means of doing such work; and although the piston was
“packed,” by being wound round with hemp, it was difficult to keep it
sufficiently tight, yet at the same time to give it adequate “play.”
Then, another drawback appeared, which, though of less importance in some
districts, absolutely prevented the introduction of this engine into many
parts of the country. The consumption of coal was enormous in proportion
to the power gained. We can easily understand the reason of this, when
we consider the means used for producing a vacuum in the cylinder below
the piston. The water introduced for the purpose, chilled, not only the
steam, but cylinder and piston also; and therefore, before a second
stroke could be made, these had to be again heated to the temperature of
boiling water. The coal required for the latter purpose was therefore
wasted, causing a dead loss to the proprietor.
So matters continued for some time, until a mathematical instrument-maker
of Glasgow, named Watt, about the year 1760, began to turn his attention
to the subject; and having to repair a model of Newcomen’s engine
belonging to the University of Glasgow, the idea seems to have first
struck him of condensing the steam in a separate vessel, so as to avoid
cooling the cylinder after each upward stroke of the piston. This was the
grand secret which gave the first impetus to the use of steam-engines;
and from that day to this these mighty workmen, whose muscles and sinews
never become weary, have been gradually attaining perfection. Yet it
may be fairly stated that the most modern form of condensing engine in
use is but an improvement upon Watt’s in details of construction and
accuracy of workmanship. For Watt did not stand still in his work; but
after having devised a separate condenser, he further suggested the idea
of closing the top of the cylinder, which had hitherto been left open to
the influence of the atmosphere; and rejecting the latter as the means
of giving motion to the piston, he made use of the expansive power of
steam on each side of the piston alternately, while a vacuum was also
alternately produced on either side of it by the condensation of the
steam.
The atmospheric engine was thus wholly displaced. The saving of fuel in
the working of the machine was so great, that the stipulation of the
inventor, that one-third of the money so saved should be his, raised
him from comparative poverty to affluence in a very short time. Watt,
however, had still to contend with great difficulties in the actual
construction of his engines. He was in the same “fix” as some of my young
readers, who are very desirous to make some small model, but have little
else than a pocket-knife and gimblet to do it with. For there were no
large steam-lathes, slide-rests, planing and boring machines, procurable
in those days, and even the heaviest work had to be done by hand, if
indeed those can be called hand-tools which had frequently to be _sat
upon_ to keep them up to cut. It was therefore impossible for Watt to
carry out his designs with anything like accuracy of workmanship, else
it is probable that he would have advanced the steam-engine even further
towards perfection than he did. In spite of these drawbacks, however,
this great inventor lived to see his merits universally acknowledged, and
to witness the actual working of very many of these wonderful and useful
machines.
The first necessity which occurred from closing the cylinder at both
ends was the devising some means to allow the piston-rod to pass and
repass through one end without permitting the steam to escape. This was
effected by a stuffing-box, which is represented in Fig. 57, A, B,—the
first being a sectional drawing, which you must learn to understand, as
it is the only way to show the working details of any piece of machinery.
We have here a cylinder cover, _a_, which bolts firmly to the top of the
cylinder, there being a similar one (generally without any stuffing-box)
at the other end or bottom of the same. On the top of this you will
observe another piece, which is marked _b_, and which is indeed part of
the first and cast in one piece with it. Through the cylinder cover, _a_,
is bored a hole of the exact size of the rod attached to the piston,
which has to pass through it, but which hole, however well made, would
allow the steam to leak considerably during the working of the piston-rod.
[Illustration: Fig. 57.]
To obviate this, the part _b_ is bored out larger, and has a cup-shaped
cavity formed in it, as you will see by inspecting the drawings. Into
this cavity fits the gland, _c_, which also has a hole in it, to allow of
the passage of the piston-rod. This gland is made to fit into the cavity
in _b_ as accurately as possible; and it can be held by bolts as in the
fig. A, or be screwed on the surface as shown at B, in which latter case
the greater part of the interior of _b_ is screwed with a similar thread.
The piston-rod being in place, hemp is wound round it (or india-rubber
packing-rings are fitted over it), and the gland is then fitted in upon
it, and screwed down, thus squeezing the hemp or rubber tightly, and
compelling it to embrace the piston-rod so closely, that leakage of steam
is wholly prevented. Whenever you have, therefore, to prevent steam or
water escaping round a similar moving-rod in modelling pumps or engines,
you will have to effect it in this way. The piston was also packed with
hemp or tow, either loosely-plaited or simply wound round the metal in a
groove formed for the purpose. In Fig. 57, C and D, I have added drawings
of a piston, so made, partly for the purpose of again explaining the
nature of sectional drawings. In this one, C, you are shown the end of
the piston-rod passing through the piston, and fastened by a screwed nut
below, a shoulder preventing the rod from being drawn through by the
action of this nut. The hemp packing is also shown in section, but in the
drawing D the groove is left for the sake of clearness.
In all your smaller models you will have to pack your piston in this
way, except in those where you entirely give up all idea of _power_. The
little engines, for example, sold at $1 and upwards, with oscillating
cylinders, have neither packed pistons nor stuffing-boxes; the friction
of those would stop them, and escape of steam is of no great consequence.
It will, however, be found advantageous to turn a few shallow grooves
round these unpacked pistons after they have been made to fit their
cylinders as accurately as possible, like fig. C. These fill with water
from the condensation of steam, which always occurs at first until the
engine gets hot; and thus a kind of packing is made which is fairly
effectual.
In Fig. 58 I have given a drawing of Newcomen’s engine, in case you
would like to make a model of one; but I do not think it will repay you
as well for your labour as some others. There is the difficulty of the
cistern of cold water and the waste-well; and the condensation of the
steam is a troublesome affair in a small model, so that, on the whole,
I should not recommend you to begin your attempts at model-making with
the construction of one of these. I shall, however, add a few directions
for this work, because what I have to say about boring, screwing, and so
forth, will apply to all other models you may desire to construct.
The cylinder, in this case, will be more easily made by obtaining a
piece of brass tubing, which can be had of any size, from 3 or 4 inches
diameter to the size of a small quill. The first you will often use for
boilers, the latter for steam or water pipes. You can also obtain at
the model makers—Bateman, for instance, of High Holborn—small taps and
screws, and cocks for the admission of water and steam, and all kinds of
little requisites which you would find great difficulty in making, and
which would cost you more in spoiling and muddling than you would spend
in buying them ready made.
[Illustration: Fig. 58.]
The drawing is given on purpose to show the best and easiest arrangement
for a model. It has all parts, therefore, arranged with a view to
simplicity. A is the boiler made of a piece of 3-inch brass tubing, as
far as _a_, _b_, _c_, _d_, the bottom being either of brass or copper
at the level of _a_, _b_; the upper domed part may be made by hammering
a piece of sheet brass, copper, or even tin, with a round-ended boxwood
mallet upon a hollowed boxwood block, of which T, T is a section. You
should make one of these if it is your intention to make models your
hobby, as it will enable you to do several jobs of the same kind as the
present. Probably you will not be able to make the dome semicircular, or
rather hemispherical; but at all events, make it as deeply cupped as you
can—after which, turn down the extreme edge one-sixteenth of an inch all
round to fit the cupped part exactly. This requires a good deal of care
and some skill. If you find that you cannot manage it, make your boiler
with a flat top instead. Whichever way you make it, a very good joint to
connect the parts is that shown in section at V.[2] The edge of the lower
part is turned outwards all round; that of the upper part is also turned
outwards, first of all to double the width of the other, and is then bent
over again, first with a pair of pliers and afterwards with a hammer,
a block or support being placed underneath it. All this is done by the
manufacturer with a stamping machine on purpose, and would be completed
by the Birmingham brass-workers before I could write the description. It
can, however, be done without any more tools than shown.
You will often need a tinman’s boxwood mallet with one rounded end and
one flat one, which, of course, you can now turn for yourself, as it is
an easy bit of work. With the rounded end you can cup any round piece of
tin; but it requires gentle work; do it gradually by hammering the centre
more than the edges. I will show you presently how to do similar work by
spinning in the lathe, which is a curious but tolerably easy method of
making hollow articles of many kinds from round discs of metal without
any seam.
After you have hammered the joint of the upper and middle parts together,
you must solder them all round with tinman’s solder. For this purpose
you require a soldering-iron represented at W. This is a rod of iron,
flattened and split at the end, holding between the forked part a piece
of copper, which is secured to the iron by rivets. I should not recommend
a heavy one, not so heavy nearly as what you may see at any blacksmith’s
or tinman’s shop, because your work will be generally light, and such
irons are all top heavy to use. The end, which may be curved over as
shown, will require to be _tinned_, for without this it will not work
at all well. File the end bright, and heat it in the fire nearly red
hot. Get a common brick, and with an old knife or anything else, make a
hollow place in it—a kind of long-cupped recess like a mussel shell, if
you know what that is, and put a little rosin into it. Take your iron
from the fire, and holding it down close to the brick, touch it with a
strip of solder, which will melt and run into the cavity. Now rub the
iron well in the solder and rosin, rub it pretty hard upon the brick,
and presently you will see it covered with bright solder, from which
wipe what remains in drops with a piece of tow. The iron is now fit for
immediate use; but remember, the first time you heat it red-hot, you
will burn off the tinning, and you must file it bright again, and repeat
the process. So when you want to solder, heat the iron in a clean fire,
until, when you hold it a foot from your nose, you find it pretty warm;
and avoid a _red_ heat. You will now find, that when the soldering-iron
is hot, it will not only melt but pick up the drop of solder; and as you
draw it slowly along a joint (previously sprinkled with powdered rosin,
or wetted with chloride of zinc, or with Baker’s soldering fluid), the
solder will gradually leave the iron, and attach itself to the work in a
thinly-spread, even coat.
The secret of soldering is to have the iron well-heated, and wiped clean
with a bit of tow, and to apply it along the joint so slowly and steadily
that the tin or other metal will become hot enough just to melt solder.
Try to solder, for instance, a thick lump of brass; file it bright if
at all tarnished—for this must invariably be done with all metals. You
will be unable to do it at first, for the moment the solder touches it,
it will be chilled, and rest in lumps, which you can knock off directly
when cold. Now place the brass on the fire for a few seconds until hot,
and try again; the solder will flow readily as the iron passes along
it, for it is kept up to the melting-point until it has fairly adhered.
This is why in heavy work a large iron is required; it retains heat
longer, and imparts more of it to the metal to be soldered. But you
will find it often better to use a light soldering-iron, and to place
the brass-casting upon the bar of the grate for a short time. You may,
indeed, often work without any soldering-iron as follows:—
Heat the pieces to be soldered (suppose them castings and not thin
_sheets_ of metal) until they will melt solder. Take a stick of the
latter, and just dip it in one of the soldering solutions named, and rub
it upon the work previously brightened. The solder will adhere to both
such pieces. Now, while still hot, put them together and screw in a vice,
or keep them pinched in any way for a few minutes, and you will find them
perfectly secured. In making chucks for the lathe, and in forming many
parts of your models, you will find it advantageous to work in this way;
but, notwithstanding, you will often require a light soldering-iron, and
sometimes also a blowpipe, which I shall likewise teach you to use, as
also how to make a neat little fireplace or furnace to stand on your
bench by which to heat the iron.
I must now suppose that you have carefully soldered the dome to the
middle of your boiler; and as the solder will be underneath, the joint
will be concealed even if (as is likely) you should not have made a
very neat piece of work. Before you put on the bottom of the boiler,
you will have to make two holes in the top—one for the steam-pipe
three-eighths of an inch in diameter, the other for the safety-valve also
three-eighths—because this will require a plug of brass to be soldered
in, which plug will have a hole drilled through it of a quarter of an
inch diameter. These may be punched through from the inside, or drilled;
they are easily made, but should be as round and even as possible.
Take a piece of three-eighths-inch tubing, with a stop-cock soldered into
the middle of it. I shall suppose you have bought this. It need not be
over an inch in length altogether; and you must put it through the hole
in the top of the boiler, and solder it round on the inside of the same.
The nearer you can get the stop-cock to the bottom of the cylinder the
better the engine will work, because the steam will have to rise through
whatever water is left in this pipe from the jet used to cool the steam.
You will see that it cannot run off by the pipe C into the pump well,
like that which collects in the cylinder itself. In a real engine the
steam-tap was a flat plate which slid to and fro sideways, level with
the bottom of the cylinder; but this you would not make easily at present.
The plug for the safety-valve you must turn out of a little lump of
brass. It must be about three-eighths of an inch long; and you must drill
a quarter-inch hole through it, and countersink one end of the hole
(that is, make it wider and conical by turning a rosebit or larger drill
round in it a few times), to make a nice seat, as it is called, for the
valve itself, which need not be now attended to. Remember you can buy
at Bateman’s, or any model-maker’s in London, beautiful safety-valves
ready-made, as well as any part of a model engine that you cannot make
yourself; and indeed it is so far a good plan at first that it saves
you from becoming tired and disgusted with your work, owing to repeated
failures. If you buy them, therefore, you must do so before you make the
holes above alluded to, but in some respects it will be more to your
advantage to try and make all the details for yourself. I cannot call it
making an engine, if, like many, you buy all the parts and have little
left to do but screw them, or solder them, together. Don’t do this, or
you will never become a modeller.
Your boiler from _c_ to _a_ is, in height, maybe 2 inches, the dome 1½ or
thereabout. This will slip inside the part that you see in the drawing,
and which I here sketch again separately.[3]
[Illustration: Fig. 59.]
A is the boiler lifted out of B, the outer case or stand, which you can
make out of tin, and paint to imitate bricks. It is almost a pity to
waste sheet-brass upon it, because it is not very important, its object
being only to carry the boiler. It is like D before being folded round
and fastened (not with solder, which would soon melt, but) by a double
fold of the joint, similar to that which you made round the boiler
itself, but turned over once more and hammered down. The holes are
punched with any round or square punch with a flat end, and are intended
to give more air to the lamp C, which should have three wicks, or two at
the least, to keep up a good supply of steam. I have shown the _flat_
piece of tin with three legs only, which is as well as if it were made
with four; but you can please yourself in this matter.
The lamp I need hardly tell you how to make, for it is easier than the
boiler, being merely a round tin box, in the top of which are soldered
three little bits of brass tube for the wicks, and a fourth for the oil
to be poured in—the latter being stopped with a cork.
You should remember that no soldered work, like the inside of the boiler,
must come in contact with the heat of the lamp, unless it has water
about it, because if the water should at any time entirely boil away,
the boiler will leak and be spoiled. A little care in this respect will
insure the preservation of a model engine for a long time; but boys
_generally_ destroy them quickly by careless treatment.
Let us now turn our attention to the cylinder. Cut off a piece of
three-quarter-inch brass tube, 2½ inches in length—you can do this with a
three-square file—mount it in the lathe by making a chuck like Fig. 59,
E, of wood, the flange of which is just able to go tightly into one end
of the tube. The other end will probably centre upon the conical point
of the back poppit, over which it will go for only a certain distance.
If your back centre will not answer on account of its small size, you
must make a similar flange to go into the other end; but take care that
when the back centre is placed against it, it runs truly. If the chuck is
well made, it will do so. You can now with any pointed tool turn off the
ends of the tube quite squarely to the side; but you should only waste
one-quarter of an inch altogether, leaving it 2¼ inches long. When this
is done, take it out of the lathe, and in place of it, mount a disc of
brass rather more than one-eighth of an inch thick, or if you have none
at hand, take an _old_ half-penny or penny piece, which is of copper, and
lay it upon the flat face of a wooden chuck, driving four nails round
its edge to hold it, and with a point-tool cut out neatly the centre, of
a size to fit inside your tube. You will scarcely, however, effect this
perfectly without further turning; so take care to cut it too large; but
before you cut it completely through, make the hole for the tube which
you soldered into the top of the boiler, which is three-eighths diameter.
This you can do beautifully in the lathe with a pointed tool, or with
a drill, centred against the point of the back poppit, as I showed you
before.
Cut the disc quite out (too large, mind) and then turn a spindle like G,
mount the disc upon it as shown, by its central hole, and turn the edge
with a graver or flat tool, such as is used for brass, until it will
exactly fit the brass tube. You can cut out round discs of one-eighth
or one-fourth sheet-brass by mounting any _square_ piece on a wooden
face chuck, keeping it down by four nails or screws, and then with a
point-tool cutting a circle in it until the disc falls out. You will
often save time by so doing. You now have a disc of brass or copper
with a hole three-eighths of an inch wide in it; and as the disc is
three-fourths of an inch in diameter (_i.e._, six-eighths), you will have
three-eighths remaining, or three-sixteenths, each way on the diameter
between the edge of the hole and that of the disc. This will just give
room for the two small holes required, one on each side of the central
one, for the pipes from the cold-water cistern and to the well below the
pump. These must both be of brass; and the first should be turned up and
end in a jet, like a blowpipe, so as to make the water rise in a spray
under the piston; the other should be as long as can be conveniently
arranged.
The bottom of the cold-water cistern is drawn a little above the top of
the cylinder, which is 2¼ inches high. A jet would theoretically rise in
the cylinder to nearly the height of the level of water in the cistern;
but with a small pipe, and other drawbacks inseparable from a model, you
must not reckon on more than about half that height, which should be
sufficient to condense the steam. The piston had better be nicely fitted,
but not packed. You cut a disc of brass as before, drill the hole for
the piston, make a spindle, or put in the piston-rod, and centre this
as a spindle, which is the _best_ plan, and then with a flat brass tool
turn the piston accurately to fit the tube. Or, if you think it easier,
or wish to fasten the piston with a nut, as drawn, you can, if you like,
turn it on a separate spindle; and thirdly, you may tap the hole in the
piston, and screw the end of the piston-rod. The great thing to attend to
is, to turn the edge of the piston square to the sides.
For the piston-rod, a steel knitting needle or piece of straight iron
wire will do very well; but it will have to be flattened at the upper
end, or screwed into a little piece of brass, which must be sawn across
to make a fork by which the chain can be attached which goes over the
beam. Do not solder the cistern pipes in just yet, but go on to other
parts.
The cistern itself can be made out of any tin box. A seidlitz-powder box
will answer well, or you can make one about that size, say 4 inches
long, 2½ wide, and 2 deep. The cistern for the pump will, of course,
require to be the same size or a little larger; it may stand on legs or
be fastened to the bed-plate direct.
This bed-plate is shown below the picture of the engine. It is merely
an oblong plate of iron one-sixteenth inch thick, or in this particular
engine may be of tin neatly fastened to a half-inch mahogany board, which
will keep all firm. The white places show the position of the boiler and
of the pump cistern, the inner rounds indicating the lamp, and pump, and
cylinder. The square is merely made to show a boiler of that shape, which
some prefer;—it is not so good as a cylindrical one.
Whenever you have to make an engine, you should draw upon the bed-plate
the position of each part, as I have done here, because it will serve
you as a guide for measurement of the several pieces. The four small
circles at S S show the positions of the legs of the support C, which
carries the beam. In the drawing only two are given, but there would be
a similar triangular frame upon this side. This may be made very well of
stout brass wire, but in a bought engine it would be a casting of brass,
painted or filed bright.
The beam itself should be of mahogany, 6 inches long, half an inch wide
(on the _side_), and a quarter of an inch thick. The curved pieces you
will turn as a ring 3 inches diameter with a square groove cut in the
edge for the chain. You can then saw into four, and use two of these,
morticing the strip of mahogany neatly into them. Then finish with four
brass wires, as shown, which will keep the curved ends stiff and give a
finished appearance. The pin in the centre should be also of brass, as a
few bright bars and studs of this metal upon the mahogany give a handsome
look to the engine.
The pump will be of brass tube, made like the cylinder, but the bucket
may be of boxwood, and so may the lower valve, each being merely a disc
with a hole in it, and a leather flap to rise upwards. The bucket,
however, should have a groove turned in its edge, to receive a ring of
india-rubber, or a light packing of tow. The end of the pump-rod must
be split to make a fork like Y, to allow the valve to rise. You can get
just such a fork ready to hand out of an umbrella, if you can find an old
one; if not, and you cannot split the wire, make the rod rather stouter,
and bend it, as shown, so as to form only one side of a fork, which will
probably answer the same purpose in so light a pump.
The valve in both of these may be made of a flap of leather—bookbinder’s
calf, or something not too thick—and it may be fastened at one edge by
any cement that will not be affected by water, or by a small pin,—cut
off the head of a pin with half an inch of its shank, and point it up
to form a small tack. If the valve-box is of boxwood, you must drill a
hole;—you may make it, if preferred, of softer wood.
There is no support shown in the drawing for the cold-water cistern; but
you must stand it on four stout wires, or on a wooden (mahogany) frame,
which can be attached to the bed-plate. As this last is always of some
importance, I shall add it again in this place (Fig. 60), to a scale of
three-quarters of an inch to the foot, showing the position of each part.
[Illustration: Fig. 60.]
Always begin with a centre line and take each measure from it, and draw
another across for the same purpose, at right angles to the first. You
will quickly see the use of this. We draw two lines as described A, B,
C, D, crossing in _o_. The longest is the centre line of beam, cylinder,
and pump. The beam is to be 6 inches long to the outside of the middle
of each arc, whence the chain is to hang. We, therefore, from the centre
point, set off 3 inches each way. At the exact 3 inches will be the
centres of the cylinder and pump;—set these off, therefore, on the plan.
The end of the tank we must have near the cylinder, because we have to
bring a pipe from it into the bottom of the cylinder. Set off, therefore,
the end of the tank 2½ inches—_i.e._, 1¼ on each side of the central
line, and draw it 4 inches in length. N shows the position of the pipe
close to the end and on the line. The centre of the boiler is the same
as that of the cylinder, so we draw a circle round it with a radius of 1½
inches, which gives us the 3-inch circle of the boiler. Then we may set
off equal distances, N, N, for the extremities of the legs of the frame
which is to support the beam, and we complete our plan. M is the waste
pipe, and K is the opening for the water to flow into the tank. We now
find, therefore, that the bed-plate must be 13 inches long and 6 inches
wide to take the engine of the proposed size, and we may, of course,
extend this a little, if thought desirable. Mark off on the bed all the
lines of the plan as here given, and always start any measurement from
one of the two foundation lines, or else, if you make one false measure,
you will carry it on, probably increasing the amount of error at every
fresh measurement. Let this be with you a rule without exception. It is
plain that if you work all parts of your engine to size, you can set it
up on the marked bed-plate with perfect accuracy.
The description I have given will not only enable you to make a Newcomen
engine with very little difficulty, but will give you an insight
generally into this kind of work; and you will learn, too, a practical
lesson in soldering, turning, and fitting. I must, nevertheless, help you
a little in putting your work together.
You had better begin by soldering into the bottom of the cylinder the end
of the _steam-pipe_, which you have already fixed upright in the middle
of the dome of the boiler, taking care that it stands squarely across
the pipe, or your cylinder will not be upright. Then place the boiler
in position, and you may fix it by turning out slightly the ends of the
legs, and putting a tack through, or screwing, if the bed-plate is of
iron,—or with help of Baker’s fluid you can solder; but this is hardly
safe work, and you had better have a wooden plate, covered with tin, and
tack down the legs. I have drawn you a circular lamp, and given three and
four legs to the boiler-stand; but take care that you so arrange size of
lamp and openings of the stand as to enable you to withdraw the former
for trimming and filling. Now fit in the two small pipes, previously
bent as required. To bend them, if hard soldered or brazed, fill with
melted lead, and then bend; after which melt out the lead again. If soft
soldered, you must fill with a more fusible metal. There is a composition
called “fusible metal,” very convenient for this work, and well worth
making, because you will often need to bend small pipes into various
forms. Melt zinc, 1 oz.; bismuth and lead, of each the same quantity—this
will melt in _hot_ water; 8 parts bismuth, 5 lead, and 3 tin, will melt
in _boiling_ water. You can buy these at any _operative_ chemist’s,
either mixed, ready for use, or separately. Rosin and sand are also
used for bending tin pipes, the sole object being so to fill them that
they will become like a solid strip of metal, and thus bend slowly and
equally, with rounded and not sharp angles.
Pass the two pipes through from beneath the bottom of the cylinder, and
solder them on the upper side of it, so that when the cylinder itself is
added these two joints will not be visible. Then set up the cold-water
cistern; block it up with anything you like so as to keep it in position,
and, inserting the pipe from below, solder this also from above, _i.e._,
on the _inside_ of the cistern. Now, arrange the frame that is to support
it, either stout wire or wood, and set it up so as finally to secure
it in its place. Now, you had better set up the pump cistern, so as to
secure the other small pipe in position, and prevent it from becoming
displaced by any accidental blow. Fix this cistern therefore also, but
leave the cover off for the present, that you may be able to solder the
small pipe _inside_ it.
You will now, at all events, have secured the position of the most
important parts, and you may drop the cylinder into place, and solder
this also round the bottom. This would be facilitated by turning a
slight rebate, Fig. 60, S, round the disc which forms the bottom of the
cylinder, so that the smaller part of it will just fit inside it; but you
will be able to manage it without. Let the cylinder project a very little
beyond the bottom, just to allow a kind of corner for the solder to run
in; it will not show when all is fixed. Do this as quickly as you can, so
as not to melt off the solder round the small pipes. Now, make the pair
of A-shaped supports for the beam. Measure the height of your cylinder
top, above the bed-plate, and allow about another inch, and you will get
the perpendicular height to the axis of the beam. Allow 3 inches more
for each side, that is, in all for _each_ side, 3 inches longer than if
it was to be perpendicular instead of spreading. Take enough brass wire,
about as thick as a small quill, to make two such legs. Bend it in the
middle, like T, Fig. 60, and flatten the bent part by hammering, so as
to allow you to drill a hole to take the pivot on which the beam is to
oscillate. If you like to flatten all of it, and then touch it up with a
file, so as to get quite straight edges, it will look much more handsome.
Make two such pieces exactly alike, and, at distances alike in each, put
cross-bars. File a little way into each, making square, flat notches,
which will just take two flattened bars of the same wire; heat them,
and solder very neatly, so that no solder appears on the outside; file
all flat and true. In this way you can make almost as neat supports as
if they were of cast brass, and you are saved all the trouble of making
patterns. By and by, nevertheless, you must do better.
As I have directed you in this instance to put a wooden bed-plate to your
engine, you may point the ends of the wires, and, making holes sloping at
the same angle in the wooden stand, drive the wires into them. You have
an advantage here, inasmuch as you can raise or lower your stand until
the position of the beam comes exactly right, and you find the ends drop
over the centre of the cylinder and pump-barrel as it ought to do. When
this is the case, you can cut off any wire that projects below the stand
and file it level, for it will not be likely to need more secure fixing.
The pump may now be soldered into the cover of the cistern (before the
cover itself is fastened on), and a hole must be then cut to receive the
water that will flow from the spout, and then the cover can be fitted on.
There is no need to solder it, if it is made to _fit_ over-tightly; and
you may wish, perhaps, to get at the lower valve of the pump now and then.
The only thing left to do is to arrange the safety-valve of the boiler,
which is in many cases the place through which the water is poured to
charge it. In this engine it is, however, plain that you can fill the
boiler by turning both the taps at the same time. A little will run off
by the waste-pipe, but not enough to signify, because the tube below the
cylinder is so much the larger of the two. The safety-valve is a little
bit of brass turned conical to fit the “seat,” made by counter-sinking
the hole. It is shown at K, Fig. 59, N being the seat, O P the dome of
the boiler, and close to O is the gauge-tap for ascertaining the height
of water in the boiler. L M is a lever of flattened wire, pivoted to turn
on a pin at L,—L O being an upright wire soldered to the boiler. A notch
is filed across the top of the valve, on which the lever, L M, rests. The
weight is at M. One, as large as a big pea, hung at the end of a lever 2
inches long, the valve at half an inch from the other end, will probably
suffice for this engine.