Fig. 54.
Fig. 54.
The actual construction of such rest varies somewhat, but Fig. 54, H, shows it in its most ordinary form. The lower part is, of course, to be clamped down securely to the lathe-bed, there being a projection below which is made to fit accurately between the bearers similar to that beneath the poppits. This projection secures the correct position of the rest, of which one frame or plate will travel lengthwiseof the bed, while the other will move exactly at right angles to it. But in thecompoundslide-rest, which is very general, there is also a third circular motion, by which the upper part can be set at any angle with the lower, instead of being permanently fixed at right angles to it. By this the tool can be made to approach the work more and more as it passes along it; and it will therefore cut deeper at one end of its traverse than at the other. The result will be that what is thus turned will not be a true cylinder, but a cone,i.e., it will be larger at one end than the other, although otherwise smooth and even.
We are thus provided with the most valuable addition to the lathe ever devised by mechanics, and it is no longer a question of the strength and skill of the workman whether we can produce a perfect piece of work, but simply of the accuracy with which the lathe and rest are constructed, and of the form and condition of the tools to be used. The latter are not exactly like those ordinarily used, although the principle of the cutting angles already laid down needs to be adhered to even with more unfailing attention than that required for the correct formation of hand-tools. Moreover, it is plain that—here we shall no longer feel whether the tool is working as it ought to do—we shall be unconscious of the precise amount ofstrainthat is being brought to bear against its edge, and if it is by chance working in a bad position, at a wrong angle, we cannot re-adjust it in amoment as we could a hand-tool by a slight movement of the fingers or wrist.
Hence you will see at once how very important it is that tools for the slide-rest should be shaped with themost rigid adherenceto correct principles; and, further, that they should be so fixed in the slide-rest as to meet the work at the precise angle, and at the height exactly suited to the material of which it is composed. As regards the latter point, it may be taken as an almost invariable rule that the work should be attacked on its axial line (that is, a line that would run from end to end of it dividing it lengthwise into equal parts, or, as it would commonly be named, itsmiddleline). If the tool meets it above this, it is most likely that it will rub against it, and the point will be out of cut. If it meets it below, there will be a tendency for the point to catch in, and the work to roll up upon the face of the tool, which, in fact, it very often does with careless workmen, and then there comes a smash of some kind—lathe centres snapped off, the tool broken, the bar bent beyond remedy, and possibly the operator’s toes made unpleasantly tender.
The most common slide-rest tool for outside work is that given at H². It is made straight, as shown, or bent sideways to right or left to cut shoulders on the work, or enter hollows, or creep sneakingly round corners, or any other of those crooked ways in which man delights; butwhether straight or not, these tools have all most commonly the cranked form shown here. This gives the tools aslightdegree of elasticity—not very much, because that would only injure the perfection of the work; therefore they are not very considerably cranked. The angles are ground as directed in the table of tool-angles, and if the point is too low, slips of iron are placed below the shank upon the tool-plate of the slide-rest; if too high, the grindstone must be resorted to; and the advantage of these cranked tools is, that they can be ground down several times without being brought too low to be packed up with iron slips to the right level. Thus a cranked tool is by far more advantageous for the slide-rest than one made straight like those used for hand-turning. For inside work, however, or “holing,†the crank form is not possible, unless the hole is of large size, and so, for this purpose, straight side-tools are used, like K.
If the tool is well placed, as well as correctly made, nothing can be more easy and delightful than slide-rest work. You merely advance the tool to take the required cut (beginning generally at the right-hand end of the bar), and then gently turning the other handle, you will see it traverse along, as if work was a pleasure to it, as it ought to be to all young mechanics. Not infrequently, however, instead of this even, steady work, the tool jumps and catches, or rubs and shrieks: it is out of temper, I suppose;at any rate, in some one or more particulars it needs correction.
Although with the slide-rest you can generally venture upon taking a deeper cut than you could with hand-tools, it is by no means well to hurry the work. At first, especially before it has become cylindrical, the tool will only cut partly round its surface, and the work is done in an uncomfortable, jerking, dissatisfied sort of way, and the deeper you drive the tool the worse it is; but as soon as the outer skin is off, and the work has become cylindrical, a long, clear, bright shaving curls off pleasantly from end to end, and the surface ought, if the tool is wetted, to become at once of a finished appearance.
You should always, with a slide-rest, take the whole run of the piece from end to end to a certain depth, and then, commencing again at the end, repeat the same process, and so on until the required size is almost attained. When it is, take out the tool with the pointed end which has been in use, and insert one freshly sharpened with a broad point, getting it so placed as to cut the shaving both from the surface below, and from the shoulder to which it is attached at the side, as I explained to you in the chapter on grinding and setting tools, and which must be well understood before you can hope to make good work with tools rigidly fixed in a slide-rest. With this tool, kept wet with soap and water (or soda water, which is better for this than foryour stomach), take averylight shaving from end to end, taking especial care to turn the handle which gives the traverse slowly andevenly. If you stop, or almost stop, the tool will be sure to draw a little deeper into cut, which will make a scratch upon the work, or, it may be, plough a groove, and so far spoil the appearance of it.
Whenever you finish turning any bar that has been centred at each end, be careful to leave the centre marks just as they were when the work was in the lathe. The ends will have been otherwise trimmed off at the very commencement, and it may happen that at some future day it may be desired to re-mount the piece for repair, when, if these marks are gone, and new centres have to be drilled, the whole will run so much out of truth that it will have to be entirely re-turned from the commencement. Do not, therefore, fancy that these centre marks are unsightly, and forthwith file them out, but be content to leave them.
Slide-rest tools, made in the ordinary way, are so far troublesome in use that if they get broken you must have them re-forged, and few country smiths know anything about such matters. I have a tool now lying by me made by a smith (true, it was a Welsh smith), and although I stood by and explained how it should be done, and cut one out of a piece of wood, it never arrived at a proper shape, and was never even placed upon the rest. I keep it as old Izaak Walton kept the Londoner’s artificial fly, viz., “tolaugh at,†and as a caution to all concerned, never to go to a country blacksmith for slide-rest tools. The following plan answers very well for many kinds of outside work, and is on the whole a plan that may be satisfactorily followed by the young mechanic.
Instead of having the tools constructed from a large bar of steel half an inch or so in the square, they are made of short pieces about an inch long, fitted into a peculiar holder.
The advantage of this arrangement consists in the ease with which you can make your own tools out of broken round, triangular, or square pillar files, small chisels and such like. These can be shaped by the grindstone alone, and the blacksmith will not have to be called into requisition. I shall give you two forms of tool-holders, more or less simple, because I may suppose my young mechanic to be fast growing into an old hand, and able to appreciate differences in these arrangements.
Fig. 55, A, B, represents two of such holders, one for round, the other for flat steel cutters. You can see at once that when these are upon the bed of the rest, they form a tool with cranked end, as previously described, and can therefore be used in precisely the same manner. I shall give no directions formakingthese tool-holders, which are, nevertheless, very simple affairs, and can be readily understood from the drawings here given.
Fig. 55.
Fig. 55.
Another form is shown at C. The partd eis a clamp, which is separately drawn atf. This, like the last, enables one to use all sorts of odds and ends for tools. There are several other patterns of tool-holders, arranged either to use the little pieces of square, round, or triangular steel bars, so that one side, at least, of these may remain without grinding, and others in which two entirely new faces must be given to the tool by the grindstone. The latter are, perhaps, generally the best, because you can then, with theaid of the table of tool-angles, shape your cutters very accurately to the work required of them.
Although such tool-holders and cutters are generally used for metals, there are others intended for wood; and constructed to hold miniature gouges and chisels, which perform their work admirably. A capital tool for outside work, Fig. 55, E, which was used extensively at Portsmouth dockyard for brass turning, is made simply by filing off at an angle of about 45° a round short bar of steel. This angle, however, is unusually small for brass and gun-metal, 80° being better. For iron it will answer better, because though filed, or rather ground at 45°, the cutting edge, a little way from what may be called the point of the tool, is nearer 60°.
Similar to these last are the tube gouges, short bits of steel tube ground off and sharpened. These fixed in a holder answer beautifully for soft wood, and do not “catch in.†If the holder is bent so as to bring the tool into proper position, inside work can be rapidly effected by these, such as hollowing out large bowls and similar heavy work. All this can, of course, be done rapidly with the slide-rest, so far as regards the removal of the greater part of the wood. But in the case of a bowl, in which a curve predominates over a straight line, hand-tools must be used to finish it (generally the inside hook-tool). This last is, in fact, almost identical with the tube gouge; for theslide-rest, and that which makes it so difficult a tool to use, is that, being a hand-tool, and subject to slight unintentional changes of position upon the part of the workman, it catches in, and is either wrenched out of the hand, or a piece is chopped off the wood. Rigidly held in the slide-rest, the exact angle, once found, is of course maintained.
I now propose to assist the young mechanic in special work, instead of continuing general directions. This will enable me to explain to him various lathe appliances, and other details of mechanical work hitherto passed by.
Of all models which boys (and very big boys too) are desirous to construct, the steam-engine holds the chief place, and deservedly so; for every boy calling himself mechanical, ought to know how this is made, and the generalprinciplesof its construction as well. However, I am aware, from experience, that many a youngster, who is even in possession of a model engine, is utterly ignorant of the cause of its motion; although it is a great delight to them to see the steam puffing out, and the wheel revolving “nineteen to the dozen,†as schoolboys say. Now, an engine is a very simple affair, and can be easily explained; and, as I wish my readers to work rationally,I shall show them what they have to do before I tell them how to do it.
Fig. 56.
Fig. 56.
A, Fig. 56, represents a cubical vessel of tin or anyother substance. By cubical, I mean that all its sides are squares, and all exactly equal; each side in the present case is to be 1 inch wide and long, or a square inch. B is a similar vessel, 1 foot cube. It contains, therefore, 1728 cubic inches, or is 1728 times as large in capacity as the first. Now, if I were to fill the little vessel with water and tip it into the second, and put a lamp under it, the water would all soon boil away, as it is called. It would be converted into steam; and the quantity of steam it would produce would exactly fill the larger vessel, without exciting any particular pressure upon its sides.
Steam, thus allowed plenty of elbow room, is like a lazy boy; it will play and curl about very prettily, but will do no work. We must put some sort of pressure, therefore, upon it—confine it, and we shall soon see that, by struggling to escape, it will serve our purpose, and become a most obedient workman. We have, therefore, only to put double the quantity of water into our larger vessel, that is,twocubic inches. We will put on a cover tightly, adding a pipe through which to pour in the water. Soon we shall have the steam formed as before; but it has no longer room enough, and out it comes fizzing and roaring, very savage at having been shut up in so small a cage. And we can make it work too, for if we set up a little fan-wheel of tin right in its way, we shall see it spin round merrily enough; or if we cork the tube lightly, we shall find this cork sooncome out with a bang. We have, in fact, already constructed a steam-engine and a steam-gun on a small scale. The pressure in this case is, indeed, not great, but what it is I must now try to explain.
The air or atmosphere, which surrounds us on all sides, exercises a pressure upon everything of 15 lbs. on every square inch of surface. If our little cubical inch box of tin had no air inside it, and no steam, but was absolutely empty, each side, and top, and bottom would have 15 lbs. pressure upon it; which would be evident if it were not very strong, for it would sink in on all sides directly, just as much as if you were toadda weight of 15 lbs. when it was full of air, as it would ordinarily be.
When I spoke of the larger box being exactly filled with steam from the evaporation of the cubic inch of water poured from the smaller box, I supposed it empty of air. The steam from that quantity of water, occupying the place of the air, would also be of the same pressure, 15 lbs. per square inch of surface; and as this only balances the pressure of the atmosphere, which would be, in such a case, pressing in on all sides, the steam would not show any pressure; just as, if you put equal weights into each scale of a balance, the beam of it would remain horizontal, neither scale showing to the outward senses that it had any pressure upon it. But in the second case, we have doubled the quantity of steam, but compelled it to occupy the samespace; therefore we have now real, visible pressure of 15 lbs. upon each square inch; and if we again halve the space which the steam has to occupy, or double the quantity of water, we shall obtain a pressure of 30 lbs. beyond the pressure of the atmosphere.
Let us now disregard atmospheric pressure, and fit up such an apparatus as Fig. 56, D. Here we have first our small box, closed on all sides, and from it a small tube rising and entering into the bottom of a larger one, which is very smooth in the inside; in this is a round plate or disc, called a piston, which fits the tube nicely, but not so tight as to prevent it from moving up and down easily; and let a weight of 15 lbs. be laid upon it. Let us suppose this large tube or cylinder to be 1700 times larger than the cubic inch box, into which water is to be poured till full. Now we heat it as before, and when 212° of heat are attained by the water (which is its boiling-point) when it begins to be converted into steam, the piston will be seen to rise, and will gradually ascend, until quite at the top of the tube, because the steam required exactly that amount of room.
Now we have arrived at the same conclusion which we came to before; for you see that not only has the cubic inch of water become a cubic foot of steam (about1700 to 1728 of its former volume), but it is supporting 15 lbs. weight, which represents that of the atmosphere, and if we could get ridof the latter, a solid weight of 15 lbs. would be thus supported. Now, still neglecting the atmospheric pressure, suppose instead of 15 lbs. we add another 15 lbs., making the weight 30 lbs., down goes our piston again, and stands at about half the height it did before. We have thus, as we had previously, a cubic foot of steam made to occupy half a cubic foot of space, giving a pressure (which is the same as supporting a weight) of 30 lbs.
I ought, perhaps, to add in this place, however, that under 30 lbs. pressure, or atmospheric weight and 15 lbs. additional, the water would not become steam at a temperature of 212°, but it would have to be made much hotter, until a thermometer placed in it would show 252°.
So far we have seen what a cubic inch of water will do when heated to a certain degree, and at first sight it may not seem a great deal. Far from being light work, however, this is actually equal to the work of raising a weight of 1 ton a foot high. Let us prove the fact. Suppose the tube or cylinder to be square instead of round, and that its surface is exactly 1 square inch, how can we give 1700 times the room which is occupied by the water? It is plain that the piston must rise 1700 inches in the 1-inch cylinder or tube, carrying with it, as before, its weight of 15 lbs.—that is, it has raised 15 lbs. 1700 inches, or about 142feet. But this is the same as 15 times 142 feet raised 1 foot, which is 2130 lbs. raised 1 foot, very nearly a ton, the latterbeing 2240 lbs. So, after all, you see that our little cubic inch of water is a very good labourer, doing a great deal of work if we supply him with sufficient warmth.
Now this is exactly the principle of the ordinary steam-engine: we have a cylinder in which a piston is very nicely fitted, and we put this cylinder in connection with a boiler, the steam from which drives the piston from one end of the cylinder to the other. In the first engine that was made, the cylinder actually occupied the very position it does in our sketch; it was made to stand upon the top of the boiler, a tap being added in the short pipe below the cylinder, so that the steam could be admitted or shut off at pleasure. But it is plain that although our little engine has done some work, it has stopped at a certain point; there is the piston at the top, and it cannot go any farther; we must get it down again before it can repeat its labour.
How would you do this, boys? Push it down, eh? If you did, you would find it spring up again when you removed your hand, just as if there were underneath it a coiled steel spring; by which, however, you would learn practically what is meant by theelasticityof steam. Besides this, if you push it down, you become the workman, and the engine is only the passive recipient of your own labour. Try another plan; remove the lamp, and see the result—gradually,verygradually, the piston begins to descend.
Take a squirt or syringe, and squirt cold water againstthe apparatus. Presto! down it goes, now very quickly indeed, and is soon at the bottom of the cylinder. But we may as well try to get useful work done by the descent of the piston as well as by its ascent.
Set it up like Fig. 56, E. Here is a rod or beam,b a c, the middle of which is supported like that of a pair of scales. From one end we hang a scale, and place in it 15 lbs.; and as the piston sinks the weight is raised, and exactly the same work is done as before. Thus was the first engine constructed; but instead of the scale-pan and weight, a pump-rod was attached, and as the piston descended in the cylinder this rod was raised, and the water drawn from the well. This, however, was not called a steam-engine, because the work is not really the effect of the steam, which is only used to produce what is called a vacuum (i.e., an empty space, devoid of air) under the piston. In fact, the up-stroke of the piston was only partly caused by steam, and the rod of the pump was weighted, which helped to draw it up.
I must get you to understand this clearly, so that the principle may become plain—“clear as mud,†as Paddy would say. I told you that the air pressed on every square inch of surface with a force of about 15 lbs. We do not feel it, because we are equally pressed on all sides—from within as well as from without—so that atmospheric pressure is balanced. Sometimes this is a very good thing. We should, I think, hardly like to carry about the hugeweight pressing upon our shoulders, if something did not counteract it for us, so that we cannot feel it. Indeed, if it were otherwise, we should become flat as pancakes in no time—“totally chawed up.â€
But sometimes we should prefer to get rid of the air altogether—and I can tell you it is not easy to do so, unless we put something into its place; and we want perhaps simply to get rid of it, and make use of the room it occupied. We require to do this in the present instance, and in fact we have just done it. If the whole space below the piston, when we begin to work, is filled with water, it is plain there can be no air below it; and when the steam has raised it, there is still no air below it, but only steam. We then apply cold to the cylinder by removing the lamp and squirting cold water against it, which has the effect of reducing the steam to water again, which will occupy 1 inch of space only. We, therefore, now have a space of 1600 cubic inches with neither air nor water in it; and so, if the piston is 1 inch in size, there will be the 15 lb. pressure of the atmosphere upon it, and nothing below to balance it, for we have formed a vacuum below it, and of course this 15 lb. weight will press it rapidly down. It did so; and we therefore were enabled to raise 15 lb. in the scale-pan. You will know, therefore, henceforth, exactly what I mean by a vacuum and atmospheric pressure. It is, you see, the latter which does the work when a vacuum isformed as above; but you can easily understand that it might be possible to use both the atmospheric pressureandthe pressure of steam as well, which is done in the condensing steam-engine.
In the earliest engine, called theAtmosphericfor the reason above stated, the top of the cylinder was left entirely open, as in our sketch; but the condensing water was not applied outside the cylinder, but descended from a cistern above, and formed a little jet or fountain in the bottom of the cylinder at the very moment that the piston reached its highest point. Down it, therefore, came, drawing up the pump-rod. When at the bottom the jet of water ceased. Steam was again formed below the piston, which raised it as before; and the process being repeated, the required work was done. A boy, to turn a couple of taps, to let on or off the water or steam, was all the attendance required.
For some time the atmospheric engine, the invention of Newcomen, was the only one in general use; and even this was, in those days (1705-1720), so difficult to construct that its great power was comparatively seldom resorted to, even for pumping, for which it was nevertheless admirably suited. The huge cylinder required to be accurately bored, while there were no adequate means of doing such work; and although the piston was “packed,†by being wound round with hemp, it was difficult to keep it sufficientlytight, yet at the same time to give it adequate “play.†Then, another drawback appeared, which, though of less importance in some districts, absolutely prevented the introduction of this engine into many parts of the country. The consumption of coal was enormous in proportion to the power gained. We can easily understand the reason of this, when we consider the means used for producing a vacuum in the cylinder below the piston. The water introduced for the purpose, chilled, not only the steam, but cylinder and piston also; and therefore, before a second stroke could be made, these had to be again heated to the temperature of boiling water. The coal required for the latter purpose was therefore wasted, causing a dead loss to the proprietor.
So matters continued for some time, until a mathematical instrument-maker of Glasgow, named Watt, about the year 1760, began to turn his attention to the subject; and having to repair a model of Newcomen’s engine belonging to the University of Glasgow, the idea seems to have first struck him of condensing the steam in a separate vessel, so as to avoid cooling the cylinder after each upward stroke of the piston. This was the grand secret which gave the first impetus to the use of steam-engines; and from that day to this these mighty workmen, whose muscles and sinews never become weary, have been gradually attaining perfection. Yet it may be fairly stated that the most modernform of condensing engine in use is but an improvement upon Watt’s in details of construction and accuracy of workmanship. For Watt did not stand still in his work; but after having devised a separate condenser, he further suggested the idea of closing the top of the cylinder, which had hitherto been left open to the influence of the atmosphere; and rejecting the latter as the means of giving motion to the piston, he made use of the expansive power of steam on each side of the piston alternately, while a vacuum was also alternately produced on either side of it by the condensation of the steam.
The atmospheric engine was thus wholly displaced. The saving of fuel in the working of the machine was so great, that the stipulation of the inventor, that one-third of the money so saved should be his, raised him from comparative poverty to affluence in a very short time. Watt, however, had still to contend with great difficulties in the actual construction of his engines. He was in the same “fix†as some of my young readers, who are very desirous to make some small model, but have little else than a pocket-knife and gimblet to do it with. For there were no large steam-lathes, slide-rests, planing and boring machines, procurable in those days, and even the heaviest work had to be done by hand, if indeed those can be called hand-tools which had frequently to besat uponto keep them up to cut. It was therefore impossible for Watt tocarry out his designs with anything like accuracy of workmanship, else it is probable that he would have advanced the steam-engine even further towards perfection than he did. In spite of these drawbacks, however, this great inventor lived to see his merits universally acknowledged, and to witness the actual working of very many of these wonderful and useful machines.
The first necessity which occurred from closing the cylinder at both ends was the devising some means to allow the piston-rod to pass and repass through one end without permitting the steam to escape. This was effected by a stuffing-box, which is represented in Fig. 57, A, B,—the first being a sectional drawing, which you must learn to understand, as it is the only way to show the working details of any piece of machinery. We have here a cylinder cover,a, which bolts firmly to the top of the cylinder, there being a similar one (generally without any stuffing-box) at the other end or bottom of the same. On the top of this you will observe another piece, which is markedb, and which is indeed part of the first and cast in one piece with it. Through the cylinder cover,a, is bored a hole of the exact size of the rod attached to the piston, which has to pass through it, but which hole, however well made, would allow the steam to leak considerably during the working of the piston-rod.
Fig. 57.
Fig. 57.
To obviate this, the partbis bored out larger, and hasa cup-shaped cavity formed in it, as you will see by inspecting the drawings. Into this cavity fits the gland,c, which also has a hole in it, to allow of the passage of the piston-rod. This gland is made to fit into the cavity inbas accurately as possible; and it can be held by bolts as in the fig. A, or be screwed on the surface as shown at B, in which latter case the greater part of the interior ofbis screwed with a similar thread. The piston-rod being in place, hemp is wound round it (or india-rubber packing-ringsare fitted over it), and the gland is then fitted in upon it, and screwed down, thus squeezing the hemp or rubber tightly, and compelling it to embrace the piston-rod so closely, that leakage of steam is wholly prevented. Whenever you have, therefore, to prevent steam or water escaping round a similar moving-rod in modelling pumps or engines, you will have to effect it in this way. The piston was also packed with hemp or tow, either loosely-plaited or simply wound round the metal in a groove formed for the purpose. In Fig. 57, C and D, I have added drawings of a piston, so made, partly for the purpose of again explaining the nature of sectional drawings. In this one, C, you are shown the end of the piston-rod passing through the piston, and fastened by a screwed nut below, a shoulder preventing the rod from being drawn through by the action of this nut. The hemp packing is also shown in section, but in the drawing D the groove is left for the sake of clearness.
In all your smaller models you will have to pack your piston in this way, except in those where you entirely give up all idea ofpower. The little engines, for example, sold at $1 and upwards, with oscillating cylinders, have neither packed pistons nor stuffing-boxes; the friction of those would stop them, and escape of steam is of no great consequence. It will, however, be found advantageous to turn a few shallow grooves round these unpacked pistonsafter they have been made to fit their cylinders as accurately as possible, like fig. C. These fill with water from the condensation of steam, which always occurs at first until the engine gets hot; and thus a kind of packing is made which is fairly effectual.
In Fig. 58 I have given a drawing of Newcomen’s engine, in case you would like to make a model of one; but I do not think it will repay you as well for your labour as some others. There is the difficulty of the cistern of cold water and the waste-well; and the condensation of the steam is a troublesome affair in a small model, so that, on the whole, I should not recommend you to begin your attempts at model-making with the construction of one of these. I shall, however, add a few directions for this work, because what I have to say about boring, screwing, and so forth, will apply to all other models you may desire to construct.
The cylinder, in this case, will be more easily made by obtaining a piece of brass tubing, which can be had of any size, from 3 or 4 inches diameter to the size of a small quill. The first you will often use for boilers, the latter for steam or water pipes. You can also obtain at the model makers—Bateman, for instance, of High Holborn—small taps and screws, and cocks for the admission of water and steam, and all kinds of little requisites which you would find great difficulty in making, and which would cost you more in spoiling and muddling than you would spend in buying them ready made.
Fig. 58.
Fig. 58.
The drawing is given on purpose to show the best and easiest arrangement for a model. It has all parts, therefore, arranged with a view to simplicity. A is the boiler made of a piece of 3-inch brass tubing, as far asa,b,c,d, the bottom being either of brass or copper at the level ofa,b; the upper domed part may be made by hammering a piece of sheet brass, copper, or even tin, with a round-ended boxwood mallet upon a hollowed boxwood block, of which T, T is a section. You should make one of these if it is your intention to make models your hobby, as it will enable you to do several jobs of the same kind as the present. Probably you will not be able to make the dome semicircular, or rather hemispherical; but at all events, make it as deeply cupped as you can—after which, turn down the extreme edge one-sixteenth of an inch all round to fit the cupped part exactly. This requires a good deal of care and some skill. If you find that you cannot manage it, make your boiler with a flat top instead. Whichever way you make it, a very good joint to connect the parts is that shown in section at V.[2]The edge of the lower part is turned outwards all round; that of the upper part is also turned outwards, first of all to double the width of the other, and is then bent over again, first with a pair of pliersand afterwards with a hammer, a block or support being placed underneath it. All this is done by the manufacturer with a stamping machine on purpose, and would be completed by the Birmingham brass-workers before I could write the description. It can, however, be done without any more tools than shown.
You will often need a tinman’s boxwood mallet with one rounded end and one flat one, which, of course, you can now turn for yourself, as it is an easy bit of work. With the rounded end you can cup any round piece of tin; but it requires gentle work; do it gradually by hammering the centre more than the edges. I will show you presently how to do similar work by spinning in the lathe, which is a curious but tolerably easy method of making hollow articles of many kinds from round discs of metal without any seam.
After you have hammered the joint of the upper and middle parts together, you must solder them all round with tinman’s solder. For this purpose you require a soldering-iron represented at W. This is a rod of iron, flattened and split at the end, holding between the forked part a piece of copper, which is secured to the iron by rivets. I should not recommend a heavy one, not so heavy nearly as what you may see at any blacksmith’s or tinman’s shop, because your work will be generally light, and such irons are all top heavy to use. The end, which may be curved over as shown, will require to betinned, for without this it will not work at allwell. File the end bright, and heat it in the fire nearly red hot. Get a common brick, and with an old knife or anything else, make a hollow place in it—a kind of long-cupped recess like a mussel shell, if you know what that is, and put a little rosin into it. Take your iron from the fire, and holding it down close to the brick, touch it with a strip of solder, which will melt and run into the cavity. Now rub the iron well in the solder and rosin, rub it pretty hard upon the brick, and presently you will see it covered with bright solder, from which wipe what remains in drops with a piece of tow. The iron is now fit for immediate use; but remember, the first time you heat it red-hot, you will burn off the tinning, and you must file it bright again, and repeat the process. So when you want to solder, heat the iron in a clean fire, until, when you hold it a foot from your nose, you find it pretty warm; and avoid aredheat. You will now find, that when the soldering-iron is hot, it will not only melt but pick up the drop of solder; and as you draw it slowly along a joint (previously sprinkled with powdered rosin, or wetted with chloride of zinc, or with Baker’s soldering fluid), the solder will gradually leave the iron, and attach itself to the work in a thinly-spread, even coat.
The secret of soldering is to have the iron well-heated, and wiped clean with a bit of tow, and to apply it along the joint so slowly and steadily that the tin or other metal will become hot enough just to melt solder. Try to solder,for instance, a thick lump of brass; file it bright if at all tarnished—for this must invariably be done with all metals. You will be unable to do it at first, for the moment the solder touches it, it will be chilled, and rest in lumps, which you can knock off directly when cold. Now place the brass on the fire for a few seconds until hot, and try again; the solder will flow readily as the iron passes along it, for it is kept up to the melting-point until it has fairly adhered. This is why in heavy work a large iron is required; it retains heat longer, and imparts more of it to the metal to be soldered. But you will find it often better to use a light soldering-iron, and to place the brass-casting upon the bar of the grate for a short time. You may, indeed, often work without any soldering-iron as follows:—
Heat the pieces to be soldered (suppose them castings and not thinsheetsof metal) until they will melt solder. Take a stick of the latter, and just dip it in one of the soldering solutions named, and rub it upon the work previously brightened. The solder will adhere to both such pieces. Now, while still hot, put them together and screw in a vice, or keep them pinched in any way for a few minutes, and you will find them perfectly secured. In making chucks for the lathe, and in forming many parts of your models, you will find it advantageous to work in this way; but, notwithstanding, you will often require a light soldering-iron, and sometimes also a blowpipe, which I shall likewiseteach you to use, as also how to make a neat little fireplace or furnace to stand on your bench by which to heat the iron.
I must now suppose that you have carefully soldered the dome to the middle of your boiler; and as the solder will be underneath, the joint will be concealed even if (as is likely) you should not have made a very neat piece of work. Before you put on the bottom of the boiler, you will have to make two holes in the top—one for the steam-pipe three-eighths of an inch in diameter, the other for the safety-valve also three-eighths—because this will require a plug of brass to be soldered in, which plug will have a hole drilled through it of a quarter of an inch diameter. These may be punched through from the inside, or drilled; they are easily made, but should be as round and even as possible.
Take a piece of three-eighths-inch tubing, with a stop-cock soldered into the middle of it. I shall suppose you have bought this. It need not be over an inch in length altogether; and you must put it through the hole in the top of the boiler, and solder it round on the inside of the same. The nearer you can get the stop-cock to the bottom of the cylinder the better the engine will work, because the steam will have to rise through whatever water is left in this pipe from the jet used to cool the steam. You will see that it cannot run off by the pipe C into the pump well, like that which collects in the cylinder itself. In a real engine the steam-tap was a flat plate which slid to and fro sideways,level with the bottom of the cylinder; but this you would not make easily at present.
The plug for the safety-valve you must turn out of a little lump of brass. It must be about three-eighths of an inch long; and you must drill a quarter-inch hole through it, and countersink one end of the hole (that is, make it wider and conical by turning a rosebit or larger drill round in it a few times), to make a nice seat, as it is called, for the valve itself, which need not be now attended to. Remember you can buy at Bateman’s, or any model-maker’s in London, beautiful safety-valves ready-made, as well as any part of a model engine that you cannot make yourself; and indeed it is so far a good plan at first that it saves you from becoming tired and disgusted with your work, owing to repeated failures. If you buy them, therefore, you must do so before you make the holes above alluded to, but in some respects it will be more to your advantage to try and make all the details for yourself. I cannot call it making an engine, if, like many, you buy all the parts and have little left to do but screw them, or solder them, together. Don’t do this, or you will never become a modeller.
Your boiler fromctoais, in height, maybe 2 inches, the dome 1½ or thereabout. This will slip inside the part that you see in the drawing, and which I here sketch again separately.[3]
Fig. 59.
Fig. 59.
A is the boiler lifted out of B, the outer case or stand, which you can make out of tin, and paint to imitate bricks. It is almost a pity to waste sheet-brass upon it, because it is not very important, its object being only to carry the boiler. It is like D before being folded round and fastened (not with solder, which would soon melt, but) by a double fold of the joint, similar to that which you made round the boiler itself, but turned over once more and hammered down. The holes are punched with any round or square punch with a flat end, and are intended to give more air to the lamp C, which should have three wicks, or two at the least, to keep up a good supply of steam. I have shown theflatpiece of tin with three legs only, which is as well as if it were made with four; but you can please yourself in this matter.
The lamp I need hardly tell you how to make, for it is easier than the boiler, being merely a round tin box, in the top of which are soldered three little bits of brass tube for the wicks, and a fourth for the oil to be poured in—the latter being stopped with a cork.
You should remember that no soldered work, like the inside of the boiler, must come in contact with the heat of the lamp, unless it has water about it, because if the water should at any time entirely boil away, the boiler will leak and be spoiled. A little care in this respect will insure the preservation of a model engine for a long time; butboysgenerallydestroy them quickly by careless treatment.
Let us now turn our attention to the cylinder. Cut off a piece of three-quarter-inch brass tube, 2½ inches in length—you can do this with a three-square file—mount it in the lathe by making a chuck like Fig. 59, E, of wood, the flange of which is just able to go tightly into one end of the tube. The other end will probably centre upon the conical point of the back poppit, over which it will go for only a certain distance. If your back centre will not answer on account of its small size, you must make a similar flange to go into the other end; but take care that when the back centre is placed against it, it runs truly. If the chuck is well made, it will do so. You can now with any pointed tool turn off the ends of the tube quite squarely to the side; but you should only waste one-quarter of an inch altogether, leaving it 2¼ inches long. When this is done, take it out of the lathe, and in place of it, mount a disc of brass rather more than one-eighth of an inch thick, or if you have none at hand, take anoldhalf-penny or penny piece, which is of copper, and lay it upon the flat face of a wooden chuck, driving four nails round its edge to hold it, and with a point-tool cut out neatly the centre, of a size to fit inside your tube. You will scarcely, however, effect this perfectly without further turning; so take care to cut it too large; but before you cut it completelythrough, make the hole for the tube which you soldered into the top of the boiler, which is three-eighths diameter. This you can do beautifully in the lathe with a pointed tool, or with a drill, centred against the point of the back poppit, as I showed you before.
Cut the disc quite out (too large, mind) and then turn a spindle like G, mount the disc upon it as shown, by its central hole, and turn the edge with a graver or flat tool, such as is used for brass, until it will exactly fit the brass tube. You can cut out round discs of one-eighth or one-fourth sheet-brass by mounting anysquarepiece on a wooden face chuck, keeping it down by four nails or screws, and then with a point-tool cutting a circle in it until the disc falls out. You will often save time by so doing. You now have a disc of brass or copper with a hole three-eighths of an inch wide in it; and as the disc is three-fourths of an inch in diameter (i.e., six-eighths), you will have three-eighths remaining, or three-sixteenths, each way on the diameter between the edge of the hole and that of the disc. This will just give room for the two small holes required, one on each side of the central one, for the pipes from the cold-water cistern and to the well below the pump. These must both be of brass; and the first should be turned up and end in a jet, like a blowpipe, so as to make the water rise in a spray under the piston; the other should be as long as can be conveniently arranged.
The bottom of the cold-water cistern is drawn a little above the top of the cylinder, which is 2¼ inches high. A jet would theoretically rise in the cylinder to nearly the height of the level of water in the cistern; but with a small pipe, and other drawbacks inseparable from a model, you must not reckon on more than about half that height, which should be sufficient to condense the steam. The piston had better be nicely fitted, but not packed. You cut a disc of brass as before, drill the hole for the piston, make a spindle, or put in the piston-rod, and centre this as a spindle, which is thebestplan, and then with a flat brass tool turn the piston accurately to fit the tube. Or, if you think it easier, or wish to fasten the piston with a nut, as drawn, you can, if you like, turn it on a separate spindle; and thirdly, you may tap the hole in the piston, and screw the end of the piston-rod. The great thing to attend to is, to turn the edge of the piston square to the sides.
For the piston-rod, a steel knitting needle or piece of straight iron wire will do very well; but it will have to be flattened at the upper end, or screwed into a little piece of brass, which must be sawn across to make a fork by which the chain can be attached which goes over the beam. Do not solder the cistern pipes in just yet, but go on to other parts.
The cistern itself can be made out of any tin box. A seidlitz-powder box will answer well, or you can make oneabout that size, say 4 inches long, 2½ wide, and 2 deep. The cistern for the pump will, of course, require to be the same size or a little larger; it may stand on legs or be fastened to the bed-plate direct.
This bed-plate is shown below the picture of the engine. It is merely an oblong plate of iron one-sixteenth inch thick, or in this particular engine may be of tin neatly fastened to a half-inch mahogany board, which will keep all firm. The white places show the position of the boiler and of the pump cistern, the inner rounds indicating the lamp, and pump, and cylinder. The square is merely made to show a boiler of that shape, which some prefer;—it is not so good as a cylindrical one.
Whenever you have to make an engine, you should draw upon the bed-plate the position of each part, as I have done here, because it will serve you as a guide for measurement of the several pieces. The four small circles at S S show the positions of the legs of the support C, which carries the beam. In the drawing only two are given, but there would be a similar triangular frame upon this side. This may be made very well of stout brass wire, but in a bought engine it would be a casting of brass, painted or filed bright.
The beam itself should be of mahogany, 6 inches long, half an inch wide (on theside), and a quarter of an inch thick. The curved pieces you will turn as a ring 3 inchesdiameter with a square groove cut in the edge for the chain. You can then saw into four, and use two of these, morticing the strip of mahogany neatly into them. Then finish with four brass wires, as shown, which will keep the curved ends stiff and give a finished appearance. The pin in the centre should be also of brass, as a few bright bars and studs of this metal upon the mahogany give a handsome look to the engine.
The pump will be of brass tube, made like the cylinder, but the bucket may be of boxwood, and so may the lower valve, each being merely a disc with a hole in it, and a leather flap to rise upwards. The bucket, however, should have a groove turned in its edge, to receive a ring of india-rubber, or a light packing of tow. The end of the pump-rod must be split to make a fork like Y, to allow the valve to rise. You can get just such a fork ready to hand out of an umbrella, if you can find an old one; if not, and you cannot split the wire, make the rod rather stouter, and bend it, as shown, so as to form only one side of a fork, which will probably answer the same purpose in so light a pump.
The valve in both of these may be made of a flap of leather—bookbinder’s calf, or something not too thick—and it may be fastened at one edge by any cement that will not be affected by water, or by a small pin,—cut off the head of a pin with half an inch of its shank, and pointit up to form a small tack. If the valve-box is of boxwood, you must drill a hole;—you may make it, if preferred, of softer wood.
There is no support shown in the drawing for the cold-water cistern; but you must stand it on four stout wires, or on a wooden (mahogany) frame, which can be attached to the bed-plate. As this last is always of some importance, I shall add it again in this place (Fig. 60), to a scale of three-quarters of an inch to the foot, showing the position of each part.