Fig. 60.
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 ino. 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 thesame 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 thesteam-pipe, which you have alreadyfixed 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 inhotwater; 8 parts bismuth, 5 lead, and 3 tin, will melt inboilingwater. You can buy these at anyoperativechemist’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, andthus 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 theinsideof 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 pipeinsideit.
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 meltoff 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 foreachside, 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 anadvantage 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 tofitover-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 wiresoldered 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.
I have already told you that Watt suggested the use of steam alternately on each side of the piston; and carried it out by closing the top of the cylinder, and allowing the rod of the piston to pass through a stuffing-box or gland. I now have to explain to you how this alternate admission of the steam may be effected.
You evidently require first an opening at the top and bottom of the cylinder, communicating with the boiler, one only being open at a time; but in this case, where is the steam to escape that was on one side of the piston when the opposite side was being acted upon? It must go somewhere, but evidently must not return to the boiler. Hence, some method has to be contrived by which, when one end of the cylinder is open to the boiler, the other may be open to the air or to the condenser (in which the steam is cooled under Watt’s plan). Fig. 61 will, I think, render clear one or two of these arrangements.
Fig. 61.
Fig. 61.
The first is the four-way cock, a very simple contrivance, easily and frequently used in models. You must first understand how a common water or beer tap is made. Fig. 61, A, represents one in section, turned so as to open the passage along the pipe to which it is attached; C is the pipe in which is the tap, a conical tube of brass set upright, and with a hole right and left made through it, fixed into a short horizontal tube (generally cast with it in one piece). Into this fits very exactly the conical plug B, also with a hole through it sideways. When this is put into place, no water or other liquid can pass, unless the hole in the plug is in the same direction with the hollow tube forming an open passage. If a key is put on the square part of the plug, and it is turned half round, the passage through the pipe will be closed. A steam tap would be made in a similar manner, if its only office were to open and close a passage in a tube. But we now want two passages closed and two opened, and then the alternate pair closed and opened. This is cleverly effected by a four-way cock.
At D is shown a section of the steam cylinder and piston, with the stuffing-box and all complete. A pipe enters this at the top and bottom, and another crosses it in the middle, making four passages. Shaded black is the four-way cock,the white places showing the open channels through the plug. When this plug stands as at D, steam can pass from the boiler to the top of the cylinder only, above the piston, which it drives downward; the steam below the piston escapes through the other open-curved channel into the air, or to the condenser. Just as the piston reaches the bottom of the cylinder, the tap is turned, and the passage stands as seen at E. Steam now passes to the bottom below the piston, driving it upward, and the steam above it, which has done its work, passes outward through the other open channel of the tap.
You must understand that when Newcomen first set up his engine, a man had to turn the taps at the proper moment; and it is said that one Humphrey Potter, a boy, being left in charge, and getting tired of this work, first devised means to make the engine itself do this, by connecting strings tied to the handles of the taps to the beam that moved up and down above his head. Beighton and others improved on this, and very soon it became unnecessary for the attendant to do anything but keep up a good fire, and attend to the quantity of water in the boiler, and the pressure of the steam.
In the model I gave you of Newcomen’s engine, I purposely left the taps to be moved by hand; but F of the present figure shows how, by bringing them near together, and adding cogged wheels or pulleys, you would make onehandle answer for both; and I shall leave you to devise an easy method of making the engine work this one handle for itself. When Watt made his first engine, therefore, this work had been already done, and he only had to improve upon it, and to make it work more accurately to suit the engine designed by himself.
If you should chance to pay a visit to the Museum at South Kensington, you may see, I believe, Watt’s original engine, if not Newcomen’s. The cylinders are so large and cumbrous, that the wonder is they were ever bored by the inefficient means then in use; and the beam is a most unwieldy mass of timber and iron, that looks as if no power of steam could ever have made it oscillate. Yet it was in its day a successful engine, the wonder of the age; and did good work for its inventor and purchaser. I strongly advise my readers to try and visit Kensington, for there are many interesting models there, besides engines and appliances of older days. They will thus learn what rapid progress has been made since the days of Savery, Newcomen, and Watt; not only in the improvement of the arrangement of the parts, but in the workmanship, which last is mainly due to the invention of the slide-rest and planing-machine.
We must now return to the double-acting or real steam engine, and consider a second means whereby the steam can be alternately admitted and exhausted.
The four-way cock, already explained, was found to wear very considerably in practice, and hence work loose, and a new contrivance, called the slide-valve, soon took its place. Of this there are two patterns, the long D-valve and the short one, which latter is used for locomotives. There is also a form called a tappet-valve, often used for large stationary engines, but which is noisy and subject to rapid wear. I shall describe the long D first, in the form in which it would be most easily made for a model engine.
The two ports by which steam passes to the cylinder are shown atd,e, of H, Fig. 61. C is the passage to the boiler, K is that to the condenser. These are openings in a tube smoothly bored within, and having at the top a stuffing-box like that on the cylinder. Within this tube works an inner one,b, having rings or projections at the ends fitting perfectly, and which are packed with india-rubber, hemp (or, in modern days, with metal), to make a close fit. In a model, two bosses of brass, K, soldered on the tube and then turned, make the best packing. These packed portions of the inner tube form the stoppers to the steam ports,e e, alternately, at the top and bottom. The upper part of the inner tube has a cross arm, 3, affixed, from the centre of which rises the valve-rod by which it is moved up and down. In the position 1, the steam can pass fromcround the tube tod, and thence to the top of the cylinder to whichdis attached. The exhauststeam passes fromebelow the piston bykto the condenser. In the second position, 2, the steam is evidently shut off fromd, but can pass out ate ebelow the cylinder, while the communication is still open to the condenser fromd, through the middle of the tube to K. This is a very good form of valve, because the exhaust is always open, and the motion is smooth and equal.
Fig. 62.
Fig. 62.
There are many modifications of the long D-valve, but the principle of all is the same; I shall therefore describe the short slide-valve which is nearly always used in the models which are purchased at the shops. This, too, is the usual form of valve in locomotives, traction-engines, and the majority of those in use for agricultural and similar purposes. A, Fig. 62, is the cylinder as before in section with piston. A thick piece is cast with the cylinder, on one side of it, having steam ports also cast in it, which are here left white. The two as before go to the top and bottom of the cylinder, and have no communication with the central one, which is bored straight into the boss, and generally is turned at right angles and connected with the condenser, or with a pipe opening into the chimney of the engine to increase the draught by means of the jets of steam, as is the case always in locomotives, or into the air, which is less usual. Seen from behind, these ports are like B, being cast and cut rectangular; and the face, B, is planed quite level, which is absolutely necessary to theproper action of the slide-valve which has to work upon it. This valve is a box of iron, C, with a wide flange or rim, this flange being of sufficient width to close either port. If this valve is placed as it stands when the engine is at rest,bcovers the upper steam port, andathe lower; while the exhaust or middle port is open to the hollow part of the box. Now, if we slide the valve downwards until the upper port is open, the other two will be in communication, being united by being both together in the inside of this box or valve. Suppose the valve then cased in, and that steam is admitted from the boiler into the case, it is evident that such steam could freely pass to the top of the cylinder above the piston to force it downwards, while that which was below would escape by the lower port into the box, and thence pass to the condenser. If, instead of pushing down the valve, we had drawn it upwards, the lower port would have been opened, and the upper and middle would have been brought into communication inside the valve, and the contrary effect would have been produced upon the piston.
This is the arrangement adopted, and which will be clearly understood from the following sectional drawing, D.a,a, is the thick casting upon the cylinder, with the upper and lower steam ports, which end towards the middle of the cylinder, with the third port lying between; thenbis a section of the valve, in such a position that the flangeof it no longer covers the lower steam port, while the other two are open together on the inside of the valve. The latter is cased in by the valve-box,e e, in the back of which is the steam pipefcoming from the boiler. The valve-rod, which is moved by the engine, passes atothrough a stuffing-box. It is evidently necessary that this slide-valve should fit, and work very smoothly and correctly against the face of the ports, so as not to allow any escape of the steam. It is not, however, packed in any way at the back (although springs have been sometimes added), because, as the back is subjected to the full pressure of the steam from the boiler, this keeps it quite close to its seat. The rod, however, by which it is worked, might prevent this close contact of the two surfaces if it was screwed into the valve; it is therefore made with a cross, E, at the end, which falls into a notch in a boss cast upon the back of the valve as seen at F. This allows a certain degree of play in one direction, and permits the steam to press it close even after it has become worn by use.
You will, I think, now clearly understand how steam can be admitted alternately to the top and bottom of a cylinder, and how the exhausted steam that has done its work escapes. I must therefore now tell you how the rod of the slide-valve is moved up and down by the engine, but to do this, I must draw such engine complete.
Fig. 63.
Fig. 63.
The cylinder, A, is screwed down on its side upon the bed-plate, R R, out of which are cut two holes, one for the fly-wheel, P, of which part only appears for want of space, the other for the crank, L, on the end of the axle, M M, running through bearings, N N. The slide-valve-box is at B, C being the steam-pipe from the boiler. The piston-rod has necessarily to move only in a straight line in the direction of its length, but the crank which it has to work to turn the fly-wheel must needs move round in a circle. Hence, a poker-and-tongs joint, F O F, is arranged. The connecting-rod, H, which is attached to the crank by brasses at K, divides or is attached to a forked piece, at the lower end of which are a pair of bearings or brasses, F F. The piston-rod carries the piece O, the cross-bar of which is turned, being, in fact, the pin which passes into these bearings at F F. This forms, therefore, a hinge-joint at this place, so that although the piston-rod cannot leave the right line, and can only slide in the guide, E, the rod, H, has an up-and-down motion upon this hinge, allowing the revolution of the crank-pin to take place. D is the valve-rod, in which is a hinge at S, which suffices for the slight movement required in the rod, as it rises and falls by the action of the eccentric, T, the motion and effect of which I now have to explain.
V is a round disc of metal with a recess on its edge, so that it is like an ordinary pulley, but large in proportion toits thickness. A hole for the main crank axle, to which it has to be firmly keyed, is made through it, butnot in its centre(hence its name, eccentric—out of the centre). As the axle revolves, it is evident that this disc revolving with it will carry any point, Y, of its surface round in a circle; the centre of which is on the central line or axis of the crank-shaft. I have drawn such circle as described by the point Y, farthest from the axis; but any and all points describe larger or lesser circles round the same centre. The point Y may, therefore, be considered as the centre of a crank-pin; and the eccentric might, so far as its effects are concerned, be replaced by a crank. Now, if you turn the fly-wheel of your lathe by hand, the crank will revolve, but the treadle will rise and fall only in a straight line; and you will presently see how the eccentric, in its revolution, gives just such a to-and-fro motion to the rod D, and consequently also to the slide-valve, which it has to move.
Round the disc V, closely encircling it, is a flat ring, shown separately at X, with a rod, W, attached to and part of it. This ring is generally made in separate halves, united by bolts passing through projecting lugs or ears. The ring also fits into the groove turned on the edge of the disc V, so that it cannot slip off sideways. This outer ring is turned quite smooth and true on the inside, so that the eccentric disc can revolve within it. In doing so, it isplain that the whole ring will rise and fall, and that the rod W will move up and down, or to and fro, like the treadle of the lathe, thereby giving motion to the valve-rod, which is a continuation of the rod W. As the upper end, however, of this rod has an oscillating, or up-and-down motion, this is imparted, in a certain degree, to its other end, at the farthest distance from the eccentric; and hence the necessity for a hinged joint at S, to prevent the valve-rod from partaking of this movement. It is, however, very slight, so that the rod of the valve is not often made to pass through guides like the piston. The whole movement of the valve-rod is very limited, its traverse only being required to be sufficient to shift the valve the width of one of its ports at each stroke. The length ofstrokeor traverse which can be obtained by the eccentric is always equal to twice the distance between its real centre, and that on which it turns, which will always be a guide to you in making an engine.
Fig. 64.
Fig. 64.
The drawing here described is a plan,i.e., a drawing viewed directly from above; therefore I cannot show you the perspective view of the parts, which are, indeed, in many cases only suggested by the shading. I have, therefore, added a second drawing of the several details. This engine is, in construction, the simplest that can be devised with a slide-valve, there being no additions beyond what are absolutely necessary to make it work; the exhaust-portis below, opposite to the letter B on the valve-box. A, Fig. 64, is the forked connecting-rod, marked H in the previous drawing. This is cast with forked ends,x, andxY (the latter being F F of Fig. 63). These ends receive brasses in the following way, the endxbeing represented on a larger scale at B, with such brasses in place; of these there are two shaped like D. One of these lies in the fork of the connecting-rod end. A second similar one lies in the strap of iron C, which reaches beyond the first. A cotter or key, which is, in fact, a wedge of iron, is then passed through a slot in the strap, and a similar one in the rod; and being driven home, draws the two brasses tightly together, causing them to embrace the crank-pin, L, Fig. 63, or any similar bearing. All shafts that revolve in bearings are made to pass through brasses, and whenever these occur at the end of a rod, they are fitted as here described. E is another bearing of cast-iron, also fitted with brasses; but in a case like this, a plate lies on the upper one, and is screwed down by bolts and nuts as required. This bearing would do very well at E, Fig. 63, as a guide for the piston-rod; but in models such guide is commonly made without brasses, like F or G of the present drawing.
At H, I have shown the part F O F of the drawing 63. The middle is of brass or iron; if of the former,g gmust be separate, as these gudgeons would not be substantial enough, unless of iron or steel. It is essential that K L, the piston-rod, should be in one right line; but, if this is attended to, they need not necessarily be one piece; and frequently the piston-rod, L, is fixed into one end of thecentral casting, and another rod, K, is screwed into the other. In a model, the piston-rod should pass quite through, andg gshould be two separate gudgeons screwed in, and then turned together in the lathe, to insure their being exactly in one line. These go into the brasses in the forked ends of the connecting-rod, to form a hinge at that part, as will be understood by a reference to Fig. 63.
At M, I have shown another simple eccentric and rod, which is less trouble to make in a model than the other. In this the ring is made in one piece, with a round rod screwing into it. The disc has a slight groove turned in its edge, and a small screw, P, passes through the ring and falls into this groove. This suffices to prevent the ring from falling off sideways, and of course is not screwed down so tight as to prevent the disc from revolving. This is a very easy way to fit the eccentric, and is generally followed in small engines. The lattice eccentric rod is nearly always used in large beam engines.
I do not think the reader will now have any difficulty in understanding the precise arrangement of the various parts in the simple horizontal engine of which I have given a sketch. It is a neat and convenient form, easily arranged as a model, and I shall proceed at once to the practical work of constructing this, and engines in general, presupposing a knowledge of the use of the lathe, and of the few tools required.
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 atthe back, to screwintothe 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.
Fig. 65.
Fig. 65.
It is absolutely necessary, remember, that this hole bored in the cylinder should be at right angles to theendsof 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 theendsof 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 onlyto 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 theouterports, 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 ata b, and you then plug the end off, 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 veryparticular, 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 linea 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,fitthe 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 atxof 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 ofc, 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 backcentre 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 flangeb, 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 iswelldone.
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 preferit, 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:—
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 buyallscrews 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 andbothflanges 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 makequite 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., atraversingmandrel), 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. Inverysmall 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 thename; 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-rodat 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 notveryheavy 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, unlessevidentlyintending 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 endeavourin 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 tothe 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 isverysmall, 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 intoor 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-shaftveryslightly conical, until the crank willalmostgo 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 wedgescalled 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 towardse—then the second journal, and then another slightly conical part, the smallest end towardsa, 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 ata. 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.