HOW TO MAKE AN ENGINE.
The very first mechanical work of difficulty, but of pre-eminent
importance, in making an engine, is boring the cylinder, that is, if
the same is a casting, and not a piece of tube ready made and smooth on
the inside. This is, properly speaking, lathe work, yet may be done in
a different way. Suppose you have bought your entire set of castings,
which is the best way, and that the cylinder is half an inch diameter
inside, which is a manageable size to work upon. Get a half-inch rosebit,
which is very like the countersinks sold with the carpenter’s brace and
bits. Mount it in the lathe in a chuck, A, Fig. 65. Unscrew the point of
the back poppit, and slip over the spindle a boring-flange, B, which is
merely a flat plate like a surface chuck, only the socket is not screwed
but bored out, generally large enough to slip over the spindle. Sometimes
there is, however, a screw at the back, to screw _into_ the spindle, the
same as the points or centres. On the face of this lay a piece of board
of equal thickness, but it is as well if not planed, as its object is
partly to prevent the cylinder from slipping about during the operation,
as it is sometimes inclined to do upon the smooth metal flange, and
partly to prevent the borer or rosebit from coming in contact with the
flange when it has passed through the cylinder. Grasp the latter in the
left hand, and you can easily prevent it from revolving with the drill,
which will go through rapidly, and leave the hole beautifully finished
and quite true from end to end,—indeed, I have bored iron also, rapidly
and with great ease, with this tool.
[Illustration: Fig. 65.]
It is absolutely necessary, remember, that this hole bored in the
cylinder should be at right angles to the _ends_ of the same, and to
secure this you must now make use of it to mount the cylinder in the
lathe to turn these ends or flanges. I will show you a simple and easy
way to do this. C is a bar of iron or steel, preferably of the latter,
about 6 inches long, and three-eighths diameter, filed into six sides.
It is a good plan to have three or four sizes of such bars, with centre
holes drilled carefully into each end, so that you can mount them with a
carrier-chuck, as you would if you were going to turn them. Taking one of
about the size named, mount upon it a piece of wood, and turn this down
until your cylinder will just go tightly upon it. Being a six-sided bar,
it is easy to mount the wood upon it by boring the latter with a gimlet
and then driving the bar into it. It will hold tightly, and not turn
round upon the metal. The cylinder being fixed in this way, you must turn
the two flanges with a graver if the cylinder is of iron, but with a flat
tool or the four-sided brass tool if of the latter metal; and also turn
the edges of the flanges. The rest of the cylinder will be left in the
rough, and may be painted green or black. I should advise you always to
bore the cylinder first when possible, and then to mount it as described
and turn it on the ends, which are thus sure to be correctly at right
angles to the bore. Some cylinders, however, especially short ones, may
be squared up first, and then mounted on a face-plate and bored. Unless,
however, you have either a grip-chuck, which is self-centring, or some
clamps properly constructed for this particular work, you will find the
first method the easiest, especially for small light work.
You should now make the ports for steam and exhaust. Mark them upon the
flat part of the casting, after you have filed this as level as you can,
and do not mark them so long as not to leave you room beyond the _ends_
of the ports for the steam-box or case which has to be placed here. The
upper and lower ports are to be the same size, but the middle one may be
a trifle larger with advantage. In larger engines these are cast in the
metal, and have only to be trimmed and faced; but in the small models
you have to drill them out in the boss cast on the cylinder. Drill down
from the top, as shown at D by the dotted lines, but take great care
not to go farther than the _outer_ ports, which are to be therefore
first made, so that you can tell when the drill has gone far enough. If
you pierce the middle port from either end, the cylinder is spoiled.
To cut the middle one, you merely drill a hole straight in towards the
cylinder, and meet it by another drilled from the side, into which the
pipe for the exhaust is to be screwed. You also drill straight through
into the cylinder at _a b_, and you then plug the end of _f_, and that at
the other end of the cylinder. Your port faces, however, are generally
oblong, and not round. Make a row of holes with the drill, and then, with
a little narrow steel chisel and light hammer, chip out the superfluous
metal, and finish with a small file. You can always make narrow channels
with squared sides by thus drilling two or more holes, and throwing them
into one with a file; but in reality, for these small engines, it is very
little matter whether the ports are round in section or square.
The bottom and top of the cylinder demand our next attention. E and F
show these. They are easily and instantly mounted in a self-centring
chuck, but can be held very well in one of wood carefully bored with a
recess of the right size and depth. You must here, nevertheless, be very
particular, else you will get your work untrue at this point, and then
your piston-rod will stand awry, and all your subsequent fitting will
be badly done. I therefore give you at G a section of the chuck bored
to take the cover truly. Recess the part down to the line _a b_, to fit
the cover exactly, taking care to level very carefully the bottom of
the recess. Below this cut a deeper hole, to allow the flange in which
the stuffing-box will be to go into it. It need not, however, _fit_ the
flange. The rough casting will hold very well in a chuck like this, even
if it is of iron. You now carefully face the bottom of the cover, and
turn the slight flange exactly to fit into the cylinder; then reverse it
in the chuck, so as to get the stuffing-box outside; and in doing so,
take the greatest care that it beds flat upon the bottom of the chuck.
Turn off level the top of the flange first at _x_ of fig. E, and then
place a drill with its point against the middle of this, and its other
end (with a little hole punched in it to keep it steady) against the back
poppit centre, and carefully drill a hole down to the level of _c_, large
enough to admit the gland of the stuffing-box or nearly so; but remember
that you must not go too far, because the rest of the hole must only just
allow the piston-rod to go through it. Therefore, after you have drilled
about three-fourths of the distance, replace this drill by a smaller one,
and with it bore quite through. The advantage of beginning in this way
is, that you can now bring up the back centre of your lathe to steady
the cylinder cover while you finish turning it; and as you will have to
make a chuck only to take hold of the flange _b_, while you turn the
edge, you will need probably some extra support of this kind. I have,
nevertheless, turned an iron cylinder cover 2½ inches diameter without
any such support; the actual strain not being very severe, provided you
understand how a tool should be made and held.
The above directions apply equally to the cylinder bottom, the great
secret in this and all similar work being to take care to bed the work
well and truly against the bottom of the recess, turned in the chuck;
this being neglected, will result in the two faces not being parallel,
which will terribly throw out of truth the rest of your work. Indeed, in
all fitting of this kind, it is absolutely necessary to be exact in the
squaring and truing of each several piece that has to be turned or filed;
otherwise no planning or clumsy arrangement will make your mechanism work
as it ought to do. Take a week, if necessary, over any part, and don’t be
content until it is _well_ done.
Your cylinder ought now to have a finished appearance when the cover and
bottom are placed in position, but the latter have to be permanently
attached by small screws, and these I strongly advise you to buy. They
cost about 50 cents a dozen, including a tap with which to make a thread
in the holes made to receive them; or, if you prefer it, you can buy
miniature bolts and nuts at almost as cheap a rate, which would cost you
much time and trouble to make for yourself, if, indeed, you succeeded
at all. You will want four of these for the top, and the same for the
bottom, the holes for which you will make with a small archimedean or
other drill.
The mention I have made of this reminds me that I am gradually adding
considerably to your list of tools, and it is necessary to do so if you
take up model-making. Set down, at any rate, the following:—
ARCHIMEDEAN DRILL-STOCK and 6 DRILLS.
TABLE-VICE.
HAND-VICE or PIN-VICE.
SMALL BRASS-BACK SAWS for METAL.
PAIR of SMALL PLIERS.
And for use in the lathe, either two or three sizes of rose-bits, or
engineer’s half-round boring bits, of which I shall have to speak
presently; and, unless you buy _all_ screws and nuts, you will want
screw-plate and taps, or small stock and dies. Files of square, round,
and oblong section are matters of course. Remember, too, that after a
file has been used on iron and steel, it is useless for brass; so use
new ones on the latter metal first, and after such use they will answer
for cast iron and then for wrought iron. You will find the cost of files
rather heavy unless you attend to this. Have neat handles to all your
smaller files, with ferules to prevent splitting.
When you purchase the castings of the engine, you will find a valve-box
to enclose the slide and become a steam-chest, as explained. It is like
a box with neither top nor bottom, but with a flange, or turned-out
edge all round, for the screws by which it is to be attached to the
valve-facings of the cylinder. This box must have its flanges filed up
bright on their flat sides and edges—the rest may be painted. It will
exercise your skill to get the two faces flat and true, to fit upon the
cylinder; and at last you will find it expedient to put a brown paper rim
or washer between the surfaces, or a bit of very thin sheet lead, to make
a steam-tight joint. Do not solder it, if it is possible to use screws,
because this is nearly certain to get melted off; and, if not, it is not
nearly so neat and workmanlike a way of uniting the parts. You should,
indeed, in all models, put them together in such a way as to be able at
any time to separate the different pieces again, either for the purpose
of cleaning or repair; and, if you solder, you cannot easily do this.
The valve-casing and its back are generally put on together; four screws
at the corners passing through the back and _both_ flanges into the flat
side of the cylinder. This depends, however, upon the exact shape of
these different pieces; and I can give you no special directions for a
particular case unless I could see the castings which you have to fit
together. The stuffing-box you will make quite separate, both its outer
and inner part, and screw or solder the former into place. It is seldom
cast upon the valve-casing, because of the difficulty of chucking a
cubical object safely so as to turn any part of it.
You are not to screw or solder the valve-box to the cylinder until you
have carefully filed up the valve itself to slide upon the port face,
without the possibility of any escape of steam taking place. This needs
the greatest possible care; and probably, after doing what you can with
a flat file, you will have to put a little emery and oil between the
surfaces, and grind them to a perfect fit, by rubbing them together. This
grinding with emery is an operation frequently required in mechanical
engineering. Lathe-mandrels are fitted in this way into the collars; the
cylinder is also ground into the back poppit-head. It is not a very long
or difficult operation, but whenever you have had to use it, take care
to wipe off the emery, or it will keep on grinding. It is indeed very
difficult to do this perfectly; and for very fine work, such as fitting
the mandrel of a screw-cutting lathe (_i.e._, a _traversing_ mandrel),
oilstone powder and crocus are used, in place of emery. These, however,
cut very slowly, making the operation of grinding exceedingly tedious;
and in the present instance, emery will answer quite well enough. In
_very_ small engines, a stroke or two of a file is all that is needed to
fit the valve, which is so small as hardly to be worthy of the name; but
in an engine with cylinder of 1 or 2-inch bore, it will be impossible to
do with file alone, as well as you can with grinding.
The piston and piston-rod should be turned at the same time, as already
suggested in treating of the atmospheric engine of Newcomen. By this, you
will avoid getting the piston “out of square” with its rod, as if you had
bored the hole for the latter askew—a not unusual occurrence.
I do not mean to say that it is absolutely necessary for you to turn
the piston-_rod_ at all, for, in models, it is generally of round iron
or steel-wire, which is as cylindrical as you can possibly make it.
Knitting-needles are in general use for this, as being well finished and
equalised from end to end. But these are rather hard, being tempered only
to about the degree of steel-springs; therefore you must never attempt
to cut a screw on them until you have first heated the end to be screwed
red-hot, and allowed it to cool again very slowly. If you do this, a
screw-plate will put a sufficiently good thread to allow you to attach
either the piston, or the small piece of brass necessary to form the
hinge, upon the other end of the rod—that is to say, the piece marked
H in Fig. 64. Leave this for the present, however, not attempting at
present to cut either the piston-rod or valve-rod to its intended length.
You cannot do this until you have laid down the exact plan of the engine,
and marked on the bed-plate the position of all the parts.
I shall now suppose that you have finished the cylinder, with its
slide-valve, casing, stuffing-boxes, and piston, so that you have these
in exactly the state in which you might buy them at Bateman’s and
elsewhere, if you preferred, to spare yourself the trouble of boring the
cylinder and fitting it. You can buy them just in this condition, with
the rest of the castings in the rough; but I rather hope you may prefer
to try and do for yourself the not _very_ heavy or difficult work which I
have described.
I suppose you, indeed, to have bought the forked connecting-rod, either
arranged for brasses, or with holes drilled (or to be drilled) in the
ends, which would probably be the case for a model of the size named, and
also the various bearings, guides, and so forth required—some of which
would have to be turned, and some filed, but which ought now to present
little difficulty to our young mechanic.
Try to keep sharp edges to all your filed work, unless _evidently_
intending to round them; for surfaces pretending to be flat, but
partaking of a curved sectional form, characterise the workman as
undeniably a bad hand with the file, and not worth his wages. Still I
may tell you at once that nothing is so difficult as to use a file well.
It has a knack of rounding off edges, which always get more than their
proper share of its work. But this being the case, you will know what to
try and avoid. Therefore, always endeavour in filing a flat surface to
make it slightly hollow in the middle, which it is scarcely possible,
however, for you to do; but the endeavour to effect this by filing the
middle more than the edges will help you wonderfully in keeping the
latter sharp. Those, for instance, on the fork of the connecting-rod,
especially the inside ones, should be as straight and sharp as possible;
and if you round the outside edge, take care to do it so that it shall be
evident you intended it; and so with all edges, whether turned or filed.
The disc of the eccentric can only be turned by letting it into a chuck
to something less than half its thickness, and levelling one side and
half the edge, and then reversing it; unless you prefer to drill and
mount it on a spindle upon its centre. If you do this, you will of course
eventually have two holes in it; because this first one is not that by
which it will be mounted when in place. This second hole is not, however,
of the least importance, and may be left without plugging, and, if
preferred, may become in part ornamented by drilling additional holes,
and filing them into some pattern; or if it is desired to conceal the
one it was turned upon, this can be plugged and faced off, and will then
not be the least apparent. If the outer ring, or strap, as it is called,
is to be made in two pieces, with projecting lugs, it is evident the
outside edge cannot well be turned; and, unless you have that most useful
addition to the lathe, a grip or jaw-chuck, you will have some little
difficulty in letting the ring into a wooden chuck, so as to turn the
inside. The solid ring is, therefore, preferable (if you use the first,
however, you turn it up as a single ring, and then saw it across through
the lugs), which can be let into a common chuck, with a place chiselled
out to allow the boss to project, into which the eccentric rod has to
be screwed. This boss also has to be drilled and turned on the outside.
There are several modes of chucking it which can be applied, but the
simplest is to use the carrier-chuck, and to let the ring become its own
carrier by coming against the pin, as shown in Fig. 66, A.
When the ring is _very_ small, I should first drill the hole for the wire
rod, and then screw and mount it upon a little wire spindle, as in fig.
B, aiding this, if necessary, by the back centre. But the smallest models
require to be put into a watchmaker’s lathe or throw, and, except as
curiosities, are scarcely worth making.
I have already told you never to undertake engine-making without first
laying down a full-sized plan on paper, with centre lines through the
principal parts, from which to take all measurements, and to mark these
upon the base-plate, as a guide to the perfect adjustment of the various
parts. Some of these are capable of a little extra adjustment after
being put in place: the eccentric rod, for instance, can be lengthened
or shortened by screwing into or out of the eccentric ring; and the
piston-rod, too, may be similarly lengthened or shortened slightly; but
try to work as near as you can to precise measure without such adjustment.
To turn the fly-wheel, which is the last operation (including the
crank-axle), it is better carefully to drill the boss, if not already
done, marking the centre on each side, and working half through from
each, so as to insure the squareness of the hole with the side of the
wheel, which is very important. Then mount it at once upon its axle,
previously turned slightly conical, where the wheel is to be placed, and
run both together in the lathe. This will insure the wheel running true
when the engine is put together.
In the horizontal engine which I have sketched, the crank is quite
separate from the axle; and this is the easiest way to make it. The crank
itself is filed up, like C of fig. 66, and drilled for the axle and the
pin upon which the brasses on the connecting-rod work. Turn down the end
of the crank-shaft _very_ slightly conical, until the crank will _almost_
go over it. Then heat the crank, which will expand it and enable you to
slip it on the shaft. Dip it in cold water, and it will be as firm as if
made in one piece with the axle. This is called shrinking it on, and the
operation will often stand you in good stead, and save the trouble of
filing key-ways and making the small wedges called keys. The pin D can
in this case be turned up separately, and screwed in, which will complete
the work.
The eccentric must evidently be placed in position before the crank is
added, and this, too, might be shrunk on, were it not that it cannot
easily be fixed in a model until the engine is set up. The best way,
therefore, is, in this case, to turn the eccentric with a little
projecting boss to take a set screw, E, Fig. 66.
Where the axle has to pass through bearings, it must be turned down at
these parts, so that the whole will be like F. First on the right is
the journal, _e_, then the place for the fly-wheel, _d_, very slightly
conical—the smallest part being towards _e_—then the second journal, and
then another slightly conical part, the smallest end towards _a_, to take
the eccentric and crank. The fly-wheel you will key on shaft, thus:—G
represents the boss or centre of the wheel bored for the axle, and a
key-way or slot filed on one side at _a_. There is a flat place filed on
the axle, and the wheel is turned round to bring this opposite to the
key-way. A wedge or key, _b_, is then driven in, which keeps the wheel
secure, and prevents it from turning round or working loose on the axle.
If inconvenient to turn a boss and add a set-screw to the eccentric, this
also may be keyed in its place after its position has been found; but,
for the latter purpose, it should fit rather tightly on the axle, so that
it can be just moved round with the finger stiffly until its position
with respect to the crank is ascertained.
[Illustration: Fig. 66.]
This position I shall now endeavour to explain, using a diagram from an
American work, in which this generally supposed difficult point is thus
ably and satisfactorily explained. First, put your engine together as
if for work, and having cut the eccentric rod to about the length you
seem to require, judging from your plan drawn upon the bed-plate, turn
round the eccentric, with your fingers upon the crank-shaft, and, having
removed the cover of the valve-box, so that you can see the action on
the valve, watch the motion of the latter. Doubtless, the result will be
that one of the steam-ports will be opened clear to the exhaust-port,
while the other is nearly or entirely shut. The rod is then too long or
too short. If in a horizontal engine the port nearest to the crank is
wide open and the other shut, the rod is too long, and must be shortened
_half_ the difference only (_you_ will do this by screwing it farther
into the eccentric hoop). When the valve “runs square,” or opens and
shuts the ports correctly, set the eccentric as in the diagram, H, in
respect to the crank, _i.e._, with its widest part at right angles to
it. By running square is meant that when the eccentric is turned round
as described, the valve opens the ports equally, and does not affect one
more than the other. The line _a_ of the diagram shows that the position
of the eccentric may advantageously be a little _beyond_ the right angle
to the crank, to give what is called “lead,” _i.e._, to open the valve a
little before the piston commences its return-stroke.
The boilers of model engines are made of tin, sheet-brass, or copper;
seldom of the latter, which is, nevertheless, by far the best material,
and one that you can braze, rivet, or solder satisfactorily, or bend into
any shape with a hammer or wooden mallet. When polished, too, its rich
red colour is very handsome. Brass is chiefly used from the facility of
obtaining tubes of it ready brazed or soldered, from which any desired
length can be cut. A brazed copper boiler will stand a great deal of
pressure; will tear, and not fly into pieces when it bursts; and may be
heated after the water has boiled away without suffering any injury. It
would certainly not be worth while to make one for a model engine with a
half-inch cylinder, but for one of 1 inch diameter and 2½ stroke; and for
larger sizes, it will amply repay the trouble; and I will show you how to
make one, with a tube or flue inside to add to the heating surface.
I shall endeavour presently to give the proper dimensions of boilers
to work cylinders of given diameters, but the general directions here
subjoined apply to all boilers of models, whether large or small. The
main body of the boiler is generally cylindrical, and is, in fact, a
tube of sheet-metal, with riveted, brazed, or soldered seams, the last
greatly predominating in the toy engines; the result of which is, that
the first time the water gets too low, out drops the bottom, or, at the
least, divers leaky places appear, and the boiler is obliged to go to the
tinman’s for repair, its beauty being ever after a thing of the past. It
is difficult to braze in an ordinary fire; because even if, by blowing it
with a pair of bellows, you get sufficient heat, you cannot always manage
to apply your work in a good position, as you can over the hot coals of
a forge fire, where there are no bars, hobs, or other parts of the grate
standing in the way. Moreover, you often want both hands free just as the
solder commences to “run,” and forge-bellows will keep up the blast for
a few seconds after your hand is taken from the staff or handle of them.
Still, if you have no forge, which is probable, you should make a fire of
cinders or coke (the latter if possible); and if you can contrive a grate
by putting together a few bricks in some out-house, with a bar or two
of hoop-iron below for the coke to rest upon, you will have a far more
convenient fire to work at than can possibly be obtained in any ordinary
household grate or stove. You will require a pair of light tongs, which
_ought_ to be something like A, Fig. 67; but it is quite possible to
do without these if you can hold your work in any other way; as, for
instance, with a loop of iron wire twisted round it and left long enough
to form a handle.
The first thing to do is to cut a strip of copper large enough to make
the required tube. A piece 6 inches wide will roll up into a cylinder of
about 2 inches diameter (the circumference of a circle being nearly equal
in all cases to three times its diameter, or measure through the centre).
If, therefore, you want one 6 inches across, which is the smallest size
that can be advantageously fitted with a flue or internal tube, you must
cut it out 18 inches wide, and if it is 8 in length to the bottom of the
steam dome, it will be a large and serviceable boiler, fit to work an
engine with a cylinder of 1½ bore by 2½ or 3 inch stroke, which would
drive a small lathe. But observe that if you really have pluck and skill
enough to try your hand upon an engine that will give you real _power_,
you must take care to remember that “the strength of anything is the
strength of its _weakest_ part.” So don’t make the very common mistake
of having a good boiler and ample cylinder, and then fit the engine with
piston-rod, valve-rod, and such like, too small to bear the strain which
you propose to put upon the engine. Remember that every screw and nut
and pin upon which strain is liable to fall, must be of sufficient size
and strength to bear it safely: if not, your engine will not only come
to grief in the heavy trial, but it is quite possible that you also may
become subjected to a bad scald or other disagreeable consequence of your
error.
Whatever sized strips of copper you use for a boiler, the edges have to
come together to form what is called a butt-joint; _i.e._, they do not
overlap like the ordinary joints you see made in tin. Before you coil
up the strip into a tubular shape, you have to cut out holes for any
boiler fittings you may wish to add, such as safety-valve, steam-dome,
and gauges to ascertain the level of the water. These, however, do not
all come into the cylindrical part of our present boiler; the gauge-taps
and glass water-gauge alone having to be provided for. The man-hole,
too, which is added to all large boilers, may be dispensed with, its
object being to enable one to get at the inside, which will scarcely be
necessary if our work is well done at first. A boiler of the proposed
size should be heated with charcoal, as it would require a very large
lamp; but where gas can be obtained, it may be preferably used, a
ring gas-burner being placed below within the furnace. The object of
a steam-dome, which, in a horizontal boiler, would have to be placed
somewhere on the tube itself, is to prevent what is called priming,
_i.e._, the carrying into the cylinder water as well as steam, which
arises from the spurting caused by the violent boiling of the water. The
dome merely provides a chamber for dry steam above the general level of
the boiler, the steam-pipe passing from it direct to the cylinders. Our
present boiler will be vertical like the last, but with a flue up the
middle, and a grate fitted below. It is shown complete in Fig. 67, B,
with all the fittings usually attached.
Having coiled up the tube by hammering it over a cylinder of wood turned
for the purpose, a little smaller than the intended size of the boiler
(the edges having been previously filed up bright, and a width of a
quarter of an inch of the upper being similarly cleaned on the inside
all along the seam), a few loops of iron wire are tied round it, at
intervals of 1 inch or 1½ inches; there being a short piece put round,
and twisted together at the ends by a pair of pliers. The object of
these is to prevent the seam from opening on the application of heat,
which it is otherwise certain to do by the expansion of the metal.
Some borax, pounded in a mortar, and heated to drive off the water of
crystallisation, is next mixed with a little water to form a creamy
paste, and smeared along the inside of the tube, upon the brightened
part, the full length of the seam. It is generally better to heat this
salt first sufficiently to dry it (or rather fuse it), because it swells
prodigiously by the first application of heat, and if the spelter is laid
on it, it often carries it off; after once fusing, it only melts quietly.
Before applying the little lumps of spelter, turn over the tube to heat
the part opposite to the seam, so as to equalise the expansion. Then hold
it in a pair of light tongs, lay the spelter all along upon the borax,
and expose it without actually touching the coals to the heat of the
fire, urged by a strong blast. Continue this until a blue flame arises,
which shows that the spelter has melted; this blue flame being, in
fact, that caused by the burning of the zinc in the solder—spelter being
copper and zinc fused together, or, if required softer, brass, tin, and
zinc. The former is generally used, however, on copper. When the blue
flame arises, the solder runs into the joint, and the work is done. With
the hardest of these spelters, a red heat will not seriously affect the
joint, and, therefore, if at any time the water should get below the
line of this seam, so that it becomes exposed to the heat, no harm will
be done. Nevertheless, this ought never to occur, as a gauge should be
attached to every boiler to show the exact position of the water at any
given time.
The inside tube of this boiler will be seen, from the section, to be
conical up to the level of the lower part of the chimney. This is of
copper, brazed like the cylindrical part, and is 2 inches wide below,
and 1 inch above; consequently, the strips to make it must be 6 inches
wide at one end, and taper to 3 inches at the other. If the dome
rises 2 inches from the level of the top of the cylinder, it will be
sufficient; and as this is a difficult piece of work for a boy to manage,
a coppersmith should be asked to hammer the dome into the required form,
as he will know from experience the best size of circular disc to use for
the purpose. This part is so far removed from the action of the fire that
it may safely be soldered, but it is, nevertheless, as well to rivet it,
turning _out_ both the edge of the cylinder and that of the dome. Use
copper rivets, and make the holes half an inch apart. If you find any
leakage, you can run a little solder into the joint on the inside. The
bottom of the boiler may be quite flat and brazed, a few rivets being
first put in to hold the parts accurately together. The same may be said
of the tube which passes through both this and the dome. There is nothing
equal to riveting and brazing for this kind of work.
I may as well state however here, that as such a boiler as I have now
described is worth very good work, it would be a great pity to spoil
it; and it will be better to content yourself with smaller boilers and
engines soldered, where necessary, until you have had some practice in
brazing. This indeed is not difficult in reality, but, at the same time,
requires great care, because sometimes the solder and the work melt
at so nearly the same temperature, that, like a bad tinker, you will
sometimes make two holes instead of mending one. The brass, for instance,
used for beer-taps is very soft, and contains lead, and to a certainty
would itself melt before ordinary spelter, and could not therefore be
brazed; but the best Bristol brass, or yellow metal, will braze easily.
A blacksmith, brazing a key or other iron article, will braze it in a
different way, using brass wire, with which he will envelop the parts
thickly which are to be united, after securing their position with _iron_
binding-wire. He then sprinkles with borax, and heats the work until the
wire runs into the joint; after which he files and cleans off level. This
makes a very good medium.
[Illustration: Fig. 67.]
I have spoken of _riveting_ in this place. There is no difficulty in this
work. You can buy copper rivets of all sizes, and have only to punch
holes, put a rivet in place, and hammer it so as to spread the metal to
form a second head. If the rivets are heated before being applied, they
will draw the parts closer together, because they shrink in cooling.
All large boilers are made in this way, but smaller ones of iron are
often _welded_, where such a mode of junction is possible. When you can
rivet boilers water and steam tight, you will find no difficulty in
constructing them, for you can make riveted joints where brazing would be
difficult or impossible.
[Illustration: Fig. 68.]
Fig. 67, B, is a half-section of such a boiler as I have just described.
Fig. 68, A, is the lower part, which is separate, and forms the furnace
in which the boiler stands, fitting it closely. This is drawn to scale,
and is half the real size. _a_ is the steam-pipe, fitted high up in the
dome, the tap, _b_, serving to turn on or off the supply of steam for
the cylinder; _c_ is the safety-valve shown in section, and care must
be always taken to make the conical part short and of a large angle,
or it may stick fast, and cause an explosion; _d_ is the glass gauge,
to show the exact height of the water in the boiler. Its construction
will be understood from the other which is attached, where the boiler
is seen in section. There is no need to have two, and this is added
solely to explain the nature of glass-gauges. The top and bottom are
of brass, being tubes screwing into the boiler, or fastened by a nut
inside; a tube, _g_, of thick glass, connects these two, so as to form
a continuous tube, one end of which opens into that part of the boiler
which is full of steam, the other opening below the water-level. Thus the
tube forms practically part of the boiler, and the level of the water is
clearly seen. The lower tap is used for blowing off water, to insure the
communication being kept open, as it might get stopped up with sediment.
Gauge-cocks, _e_, _f_, are generally added, even where the glass
water-gauge is used. One of these should always give steam, the other
water,—the level of the latter being between the two. If the upper one
gives water, the boiler is too full; if both give steam, the boiler needs
to have water added. With these fittings, even a soldered boiler ought
never to get burnt, and will last a long time with care.
The lower part, Fig. 67, is made like that before described, except that,
being intended for charcoal, a circular grate is used, which simply rests
upon little brackets fixed by rivets for this purpose. The flame and
heat play upon the bottom of the boiler, and also pass up the central
tube—the latter adding greatly to the quantity of steam produced. This
furnace, when lighted, may be fed with bits of coke as well as charcoal,
about the size of filberts, and will give plenty of heat. If the draught,
however, is deficient, turn the waste steam into the tube, so as to form
a jet at each stroke, and it will greatly increase it. It is in this way
that the locomotive engines are always fitted, George Stephenson having
first suggested the arrangement. Previously to this a fan had been fitted
below the grate, which was put in rapid motion by the engine, and thus a
sufficient draught was obtained.
THE SAFETY-VALVE.
To find out what pressure is exerted by the safety-valve, it must be
clearly understood upon what principle it acts. I have in a previous
chapter told you that the atmospheric pressure equals 15 lbs. on each
square inch, so that if the surface of the valve which is exposed to the
air is 1 inch in area or surface, it is pressed down with a force of 15
lbs. The steam, therefore, inside the boiler will not raise it until its
elasticity exceeds this atmospheric pressure. If, therefore, we desire
to have only just 15 lbs. per square inch pressing against the inside of
the boiler (_i.e._, a pressure of “one atmosphere,” as it is called),
we have only to load the valve so that, inclusive of its own weight, it
shall equal 15 lbs. But it is plain that we must not load it at all in
reality; for a flat plate, 1 inch square, of _no weight_, is all that
is needed, the atmosphere itself being the load. Suppose, then, that we
_do_ load it with 15 lbs. in addition to the 15 lbs. with which nature
has loaded it, we shall not find the steam escape until it presses with a
force of 30 lbs. on the square inch, or two atmospheres (which, however,
is not 30 lbs. of _useful_ pressure upon one side of the piston, if the
cylinder is open as in an atmospheric engine, but only 15 lbs.) This is
not the _strain_ which the boiler has to stand, because the atmosphere
is pressing upon it and counteracting it up to the 15 lbs., so that
this strain tending to burst it is but 15 lbs. The number of pounds,
therefore, which is straining the boiler can readily be seen; being
always that with which the safety-valve is loaded, and this is also the
useful pressure for doing any required work. Unfortunately, however, even
in the best constructed engines, a pressure of 15 lbs. upon the boiler by
no means represents that in the cylinder. Now it would be inconvenient
to place weights upon the safety-valve itself, and therefore a lever is
added, as seen in the sketch, with a weight hung at one end of it. This
is shown at B, Fig. 68, where a section of the valve is given with its
stem passing through a guide to insure the correct motion of the valve.
The lever is hinged at one end; and the rule of the pressure or weight
which is brought to bear upon the valve is, that it is multiplied by the
distance at which the weight hangs from the valve, compared with its
distance from the hinge or fulcrum. If a weight of 7 lbs. is hung at 1,
_i.e._, at a distance as far on that side of the valve as the fulcrum
is on the other side of it, 7 lbs. will be the actual power exerted; at
2, where it is twice the distance, it will be doubled, and, as shown
in the drawing, a pressure of 14 lbs. will be brought to bear upon the
valve; while, if the weight is hung at 3, it will exercise a force of 21
lbs. This is very easy to understand and to remember. Sometimes (always
in locomotives) the weight is removed and a spring balance is attached
at the long end. Upon this is marked the actual pressure exerted; there
being a nut to screw down, and thus bring any desired strain upon the
spring. Mind, however, in case you should try this in any of your models,
that the scale marked on the balance when you buy it must be multiplied,
as before, according to the length of your lever. Thus, if I attach such
a balance at 3 of the drawing, a real weight of 5 lbs. shown by the
balance will be 3 × 5, or 15 lbs. upon the valve, and a balance _made
for such engine_ would be marked 15 lbs., to prevent the possibility of
dangerous error.
ENGINES WITHOUT SLIDE-VALVES EASY TO MAKE.
Having been led on from the atmospheric engine to that of Watt’s, and
to slide-valve engines generally, I am now going backward a little to
a class easier to make, because they have no slide-valves, nor even
four-way cocks; and then I shall have done with engines. But I dare
say some of my readers will wonder why I have said so little about
condensers and condensing engines. I am sure they will wonder at it if
they understood what I explained of the advantage of a vacuum under
the piston; so that 15 lbs. pressure upon the piston means 15 lbs. of
_useful_ work, instead of 30 lbs. being required for that purpose. But
condensing engines are utterly beyond a boy’s power. They require not
only a vessel into which the steam is injected at each stroke, but there
must be a pump to raise and inject cold water to condense the steam, and
a pump to extract from the vessel again this water, after it has been
used, and a cistern, and cold and hot wells; and all this is difficult to
make _so as to act_; and I am sure no boy cares for a steam engine that
will not work. Moreover, I have given you difficult work as it is—work
that many of my readers will no doubt be afraid to try—yet I did it on
purpose; because if small boys are unequal to some of it, their big
brothers are not, or ought not to be; and mechanical boys must look at
difficulties as a trained hunter looks at a hedge—viz., with a strong
desire to go over it, or through it, or any how and some how to get to
the other side of it. Indeed, you must ride your mechanical hobby very
boldly and with great pluck, or you won’t half enjoy the ride. However,
I am quite aware that I have led you into several difficulties, and
therefore now I propose to set before you some easy work as a kind of
holiday task which will send you with fresh vigour to what is _not_ so
easy.
The engines without slide-valves have also no eccentrics and no
connecting-rods. There is just a boiler, a cylinder, piston, piston-rod,
and crank, and you have the sum total, save and except the fly-wheel.
These are direct-action engines, the cylinders of which oscillate like a
pendulum, and the piston-rod itself is connected to the crank, doing away
with the necessity for guides.
Fig. 69, A, shows one of these engines, and you see that the cylinder
leans to the left when the crank is turned to that side; and if you
turn the wheel to the right, the crank will presently cause it to lean
the other way; and thus, as it turns on a pin, or “trunnion,” as it is
called, it keeps on swinging from side to side as the wheel goes round.
Now, when it is in its first position, the piston is at the bottom of the
cylinder, and it then needs to have the steam admitted below it to drive
up the piston; but when this has passed its highest position, and the
cylinder is turned a little to the _right_, the piston must be allowed to
descend, and, therefore, we must let out the steam below it. We _ought_,
at the same time, to admit steam above the piston to force it down; but,
in the simplest models, which are called single-action engines, this is
not done. The fly-wheel, having been set in motion, keeps on revolving,
and, by its impetus, sends down the piston quite powerfully enough to
overcome the slight resistance which is offered by the friction of the
parts.
Now, you can, I daresay, easily understand that it is possible to
make this to-and-fro motion of the oscillating cylinder open first a
steam-port to allow steam to raise the piston, and then an exhaust-port
to let it blow off into the air. This is exactly what is done in
practice, and it is managed in the following manner:—
[Illustration: Fig. 69.]
B, of Fig. 69, shows the bottom of the cylinder, which is a solid piece
of brass filed quite flat on one side, and turned out to receive the end
of the brass tube, which, generally speaking, is screwed into it to form
the cylinder, this being the easiest way to make it. In the middle of
the upper part of the flat side you see a white steam-port, and below it
a round white spot, which is the position of the pin, or trunnion, on
which it oscillates. Fig. 69, C, is a similar piece of brass, which is
fixed to the top of the boiler. In this, on the _left_ of the upper part,
is also a port, which is connected with the boiler by a hole drilled
below it to admit steam. On the right is also a port, which is merely cut
like a notch, or it may go a little way into the boss, and then be met
by a hole drilled to meet it, so as to form the escape or exhaust port.
Between and below these is the hole for the trunnion.
Now, you can, I think, see that if the cylinder stands upright against
this block, as it does when the crank is vertical (or upright) and on
its dead points, the port at the bottom of the cylinder would fall
between the two on this block of brass, and, as they are both flat and
fit closely, no steam from the boiler can enter the cylinder. Nor do we
want it to do so, because, if the crank is on a dead point, no amount
of steam can make the piston rise so as to move it. But now, if we move
the cylinder to the left, which we can do by turning the wheel, we shall
presently get the crank at right angles to its former position, and,
also, we shall bring the steam-ports in the cylinder and block together,
so that steam will enter below the piston. But, practically to get as
long a stroke as possible, steam is not allowed to enter fully until the
crank is further on than in a horizontal position, that is, _approaching_
its lower dead point; and this is the position in which to put it to
start the engine. By altering the shape or the position of the port a
little, we can so arrange matters as to let steam enter at any required
moment.
Steam having entered, the piston will rise rapidly, forcing up the
piston, and presently, by the consequent revolution of the fly-wheel,
the cylinder will be found leaning to the left, and at this moment the
piston must evidently begin to descend. At this very time the steam-ports
will have ceased to correspond, but the port in the _cylinder_ will come
opposite the exhaust-port in the brass block, and this port is made of
such size and shape that the two shall continue to be together all the
time the piston is descending; but, the moment it has reached the end
of its downward stroke, they cease to correspond in position, and the
steam-port begins again to admit a fresh supply of steam.
The pillar attached to the brass boss has nothing to do with it, but is
one of the supports of the axle of the fly-wheel, as you will understand
by inspection of A of this same drawing.
Such is the single-action model engine, _of no power_, but a very
interesting toy and real _steam_ engine.
The double-action engine is very superior to the foregoing, which, I may
remark, has no stuffing-box, and of which the piston is never packed. I
may also add, that the crank is formed generally by merely bending the
wire that forms the axle of the wheel, and putting the bent end through
the hole of a little boss or knob of brass, screwed to the end of the
piston-rod. Here you have no boring of cylinders to accomplish, but the
cylinder cover, piston, and wheel (often of lead or tin) require the
lathe to make them neatly. Many an engine, however, has been made without
a lathe, and I have seen one with a bit of gun-barrel for a cylinder,
and a four-way cock of very rough construction, that was used to turn a
coffee-mill, and did its work very well too.
But I must go at once to the double-action oscillating cylinder, in
which, although a similar mode of admitting steam is used, it is arranged
to admit it alternately above and below the piston, the exhaust also
acting in a similar manner.
After the explanation I have given you, however, of the single-action
engine, you will, some of you, I think, jump at a conclusion almost
directly, and perhaps be able to plan for yourselves a very easy
arrangement to accomplish the desired end. All boys, however, are not
“wax to receive, and adamant to retain” an impression; for I have known
some who need an idea to be driven into their brains with a good deal of
hard hammering. Stupid?—No. Dull?—No, only slow in _getting hold_, and
none the worse for that generally, if the master will but have a little
patience; for when they _do_ get hold, they are very like bulldogs, they
won’t let go in a hurry, but store up in most retentive minds what they
learned with such deliberation.
THE DOUBLE-ACTION OSCILLATING ENGINE.
The cylinder of the double-action engine is of necessity made with ports
very similar to those of the horizontal engine already described. There
is a solid piece attached to the cylinder as before, which is drilled
down to the upper and lower part respectively of a central boss, turned
very flat upon the face, and which has to work against a similar flat
surface as in the last engine. But the ports in the latter are four
instead of two, and in an engine with upright cylinder would be cut as
follows, and as shown in Fig. 70, C.
[Illustration: Fig. 70.]
Those on the right marked _st_ are steam-ports, which, being drilled into
one behind, are connected with the boiler. The other two marked _ex_,
are similarly exhaust-ports opening into the air. The spaces between _a
b_ and _c d_ of fig. C must be wide enough to close the steam-ports in
the cylinder, when the latter is perpendicular and the engine at rest.
When the cylinder leans to the left, oscillating on the central pin
between the ports in the middle of the circle, the lower port of it will
evidently be in connection with the steam-port in C, while the upper
port of the cylinder will be opposite to the exhaust. As the cylinder
is carried over towards the right, the upper steam-ports will come into
action in a similar way, while the lower exhaust-port is also carrying
off in turn the waste steam. The impetus, therefore, of the fly-wheel
has here only to carry the ports over the spaces _a b_, _c d_, and to
prevent the crank stopping on the two dead points. This, therefore, is
a genuine double-action engine, and will answer, even on a large scale,
very satisfactorily. If you do not quite understand the action of these
ports, cut out two pieces of card, E F. Let E represent the cylinder.
Draw circles, and cut two ports. Cut another piece of card to represent
the brass block, with ports, _c d_; pin them together through the centres
of the circles, and they will easily turn on the pin. Mark the ports, so
that you will see at a glance which are steam and which exhaust. Now cut
out the ports with a penknife, and as you work the two cards together,
swaying that which represents the cylinder to and fro upon the other,
you will see when the ports in each card agree with one another, and
which are opposite to which. This will teach you far better than any
further written explanation. You will also see that, instead of making
the steam and exhaust ports respectively with a division between, the two
steam-ports may be in one curve united, and likewise the two exhausts;
but take care not to unite the exhaust with the steam-ports. There is no
way so easy as this of reversing the action of the steam; it is, in fact,
a circular slide-valve, but wonderfully easy to make, because you have no
steam-case to make, nor any attachments whatever.
The faces of the valve are kept in close contact in one of two
ways—either the centre-pin is fixed into the cylinder face, and after
passing through the brass boss with the ports, is screwed up with a nut
at the back; or else there is fixed a small pillar or upright on the
opposite side of the cylinder, and a little pointed screw passing through
this presses against the cylinder, and makes a point of resistance,
against which it centres, and on which it turns. This is shown at fig.
A. A small indentation is made where the point comes in contact with the
cylinder.
In a locomotive engine there are two such cylinders, working against
opposite faces of the same brass block containing the ports. The cranks
are also two, on the shaft of the driving-wheels, and are at right
angles to each other; so that when one piston is at the middle of its
stroke, the other is nearly or quite at the end of it. Thus, between
the two there is always some force being exerted by the steam; and the
dead points of one crank agree with the greatest leverage of the other.
In locomotives, too, the cylinders generally are made as in the present
drawing, viz., to oscillate on a point at the middle of their length;
but it is just as easy to have the two ports meet at the bottom instead,
so that the point of oscillation may be low down, like the single-acting
cylinders of the last sketch, and this is generally done when the
cylinder is to stand upright.
There is no occasion for me to draw an engine with double-acting
oscillating cylinders, because in appearance it would be like the
single-acting one; but whereas the latter is of absolutely no use,
seeing that the greater part of its motion depends on the impetus of the
fly-wheel, the former can be made to do real work, and is the form to
be used for marine and locomotive engines. For the former, oscillating
cylinders with slide-valves are used in practice; but for real
locomotives fixed cylinders are always used. Of course either will answer
in models, and it will be good practice to try both.
I have now given sufficient explanation of how engines work, and how they
may be made, to enable my young mechanic to try his hand at such work.
The double-action oscillating engines especially are well worthy of his
attention, as he may with these fit up working models of steam-boats and
railway trains, which are far more difficult to construct with fixed
cylinders and slide-valves. I shall therefore close this part of my work
with a description of one or two useful appliances to help him in the
manipulative portion of his labour,—for here, as in most other matters,
head and hand and heart must work together. The heart desires, the head
plans, the hands execute. I think, indeed, I might without irreverence
bring forward a quotation, written a very long time ago by a very clever
and scientific man, in a very Holy Book: “Whatsoever thy hand findeth to
do, do it with all thy might.” Depend upon it, success in life depends
mainly upon carrying into practice this excellent advice. If you take
up one piece of work, and carelessly and listlessly play at doing it,
and then lay it down to begin with equal indifference something else,
you will never become either a good mechanic or a useful man. If you
read of those who have been _great_ men—lights in their generation—you
will find generally that they became such simply by their observance
of that ancient precept of the wise man. They were not so marvellously
clever—they seldom had any unusual worldly advantages; but they worked
“with all their might,” and success crowned their efforts, as it will
crown yours if you do the same.