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latter figure also shows how difficult it would be to caulk the internal edge of such a seam if placed higher up in the curve, and also that it should never be placed near a stay. (See also fig. 329.) The corners of all these plates ought to be drawn out as shown in figs. 232 and 233 (p. 247). The various seams should also be slightly bevelled, and it: is as well to do this before fitting them together.
The combustion chamber bottoms and sides are bent cold to their various curves by passing them through the bending rolls, but the corners, particularly where they have to be drawn out, are heated. It is difficult to obtain quite the correct shape at once, and the final setting has to be done in place. Heaters are frequently used, but whether it is that a blue heat is never reached, or whether the work done to the plates at this temperature is not sufficiently severe, it certainly does not as yet seem to have led to any failures in the shop or subsequently.
Drilling Combustion Chamber Plates.—When the various plates have been properly fitted together, their seams are drilled in placegenerally by hand, but also by machinery. In some works all the outer plates are first removed and drilled by machinery, or even punched, and then refitted, and the inner plates drilled. Sometimes the outer plates are perforated before fitting. If punched, there is always the danger that the plates may crack while bending them, or if punched after bending they are liable to warp, and part of the fitting work has to be done over again. Another plan is to drill all the Aanges before fitting, to mark off the holes from the inside on the outer plates, and to drill these by machinery. In any case, all holes in the flanges will have to be countersunk on the inside, for it is difficult to caulk any other heads when so near to a corner. It is also customary, but not necessary, to countersink all rivet holes on the fire side of the various lap joints; but it does not appear that projecting heads burn off. Wherever there is sufficient space for riveting, or when this is done by machinery, the rivets should be inserted om the water side of the plates.
The angle of the countersink varies from 15° to 45°. The smaMer the angle, the smaller is the power of the rivet to draw the plates together while it is cooling. Ît vanishes altogether when the apex of the cone lies beyond the flat base of the head, or if both ends are countersunk this limit is reached when both cones touch each other at their apexes.
Riveting Combustion Chambers.—The remarks made while discussing the riveting of shell plates, &c., apply to a certain extent to this case. The very greatest care should be taken to ensure perfect contact of the plates at the saddle and its corners, for this is almost the hottest part of the boiler, and any air-spaces are sure, sooner or later, to lead to troubles which rapidly extend. On account of the difficulty of flanging the furnace ends, these parts are not always as Hat as the tube plate to which they are connected, and carelessness in riveting easily leads to the condition of things illustrated in fig. 305,
Careless work at these seams is readily exposed by removing a rivet or two. In corners the rivets may be arranged as in fig. 330 or fig. 331, the latter of the two plans being the more generally adopted. In some works the corner rivets are replaced by screws headed over at either end. For remarks on caulking see p. 297.
Planing Edges of Flanges.-All the edges of the plates ought first to have been machined. With the circumferential plates this is or can be done before bending, and in that case they can be bevelled; the flanges, unless they are trimmed with milling tools, are left with square edges. In some works they are subsequently chipped, while in others they are only chipped, and, if little material has to be removed, this is done after the seams have been riveted. These remarks apply as well to the flanged end plates as to those of the internal parts. If much material has to be removed, this can be done by the shearing or the punching machine, usually before the plates are fitted; or the superfluous parts are removed by a cross-cut chisel. The furnace saddle seams are often shaped by slotting machines or band saws.
Planing machine lathes with very large face plates, or turntables, are also used for this work. The cutting tools or knives are made very wide, sometimes more than 12 ins., and it is then possible to deal with all sorts of irregular-shaped flanges. Of course, as there is always only one thickness of plate, only part of the width of the tool is used at a time, and the feed is a pretty heavy one. The cutting speed is
, about 10 ft. per minute, and the depth of a cut about 3 in. Circular saws, which cut off a solid piece of the flange, or milling cutters, have also been tried for removing the superfluous material, but are not generally used. They travel at the rate of about 2 ft. per hour. The furnace front plate fanges can be machined by the same machine (fig. 212, p. 239) which cuts the holes, but a tool with a very wide cutting edge replaces the parting tool shown there.
Fitting Internal Parts into Shell.—The furnaces and combustion chambers having been riveted together, each one has to be fitted into the furnace front plate. If all the furnaces lead into one combustion chamber, their back ends should not be riveted up without the furnace front plate being in position, otherwise the most serious inconveniences will be encountered when trying to put the two together.
Where each combustion chamber has a separate furnace, these are fitted into the flanged front plate and their holes drilled and riveted, while the stays from one combustion chamber side to the other are screwed into place. Then, when this work has been completed the whole of the combustion chambers, furnaces, and furnace front plate are rigidly connected, and may be lifted into position in the boiler shell, to which the back plate has already been riveted.
If the front tube plate is to be placed inside of the furnace front plate, a little simple manœuvring may be necessary.
In some works the screwing together of the combustion chambers and the riveting of the furnace front seams are carried out inside the boiler shell after the furnace front plate has been riveted up. No advantage is gained, and on account of the confined space the work will be both slow and bad. Besides, if one of the furnaces should crack while its front end is being expanded, all the work just mentioned would have to be done over again.
Fitting Furnaces.—On account of the difficulty of making a furnace front plate with, say, three or four holes, slip easily on to as many
furnaces, these are generally made at least in. smaller in diameter than their respective holes. Sometimes there will be even } in. of difference. Then, if the flange is left square, as shown in fig. 332, the expanded furnace mouth will only bear at the front edge, as shown in fig. 333. It is, therefore, customary to leave the flange slightly conical, as shown in fig. 334. In order to get a good fit, as in fig. 335, the diameter of the front edge of the furnace will occasionally have to be increased even as much as } in. It is dangerous, and in cold weather impossible, to do this expanding without heaters; and when they are used there is the further danger of overheating and of making the plates permanently brittle by hammering them when blue hot, and cracks at these seams are not unknown.
The work is carried out as follows: The furnaces are secured as centrally as possible in their respective holes, and all the holes drilled in place either by hand or by the machine (fig. 210, p. 238). Numerous bolts are then inserted, heaters applied, and the bolts screwed tighter, while the front seam is being hammered, until the plates are in perfect contact. The troubles which these seams sometimes give have led a few engineers to make them treble-riveted, but an apparently safer and more convenient plan would be to make the flanges conical in the
other direction (fig. 336), and either turn the front end of the furnace or give it a slight bevel, as shown. Instead of heaters, portable coke a
, furnaces are sometimes used, which heat the seams almost to redness, and they can be screwed close without hammering.
The Riveting of Furnace Front Seams is not quite so simple as would at first sight appear, particularly if the water-spaces are made narrow, and there is always a very strong inclination to do this in order to gain space. With this object in view the flanges of the furnace holes are sometimes turned the other way to those of the circumferential one (fig. 244, p. 251; fig. 278, p. 268), or the furnace is made taper
(fig. 337) or its front end contracted a few inches, or the furnace is fitted at an angle. Carried to extremes this principle will be recognised in the arrangement shown in fig. 338, which would permit of the furnaces touching each other. When, in addition, the longitudinal seams both of the furnaces and the shell accidentally come together (see fig. 339), the boiler bottom is practically cut off from the upper part, and the consequent want of circulation may make itself seriously felt. Figs. 340, 341, 342 show an arrangement which also permits of the furnaces being placed very near each other and near the shell, except the central one, because here both flanges are turned in the same direction. (See also figs. 84–87, pp. 47, 48.)
In cases where two flanges are so close together that a rivet can
neither be introduced nor subsequently caulked, the holes have to be carefully drilled, tapped, and countersunk, and accurately-fitting screws, with conical heads, inserted. The ends of the screws are
beaded over, and the heads caulked. An arrangement in which all this difficulty is overcome, and in which the front plate holes need not be flanged, is shown in fig. 343. The furnace front flanges will overlap each other. (See also fig. 41, p. 25.)
Furnace Saddles.—The various designs for securing the furnaces to the combustion chambers are shown in the following sketches. Fig. 344 is the most common, except, perhaps, when the back end seam is single riveted all the way round, instead of being double
riveted from below the line of fire bars, as shown. In through combustion chambers of double-ended boilers this seam is sometimes double riveted above and treble riveted below (see fig. 345). The increased width of the lower flange is sometimes met with in the back plate of combustion chambers (see fig. 346). In both the above cases the tube plate is on the water side of the saddle plate, and it is affirmed