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.
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.