BOILER-MAKING. This industry is remarkable for the immense amount of detail work to be performed, since nothing is really ready to be put together. The plates, angles, bars and other sections arrive in a fairly straight condition, and must be variously sheared, sawn, straightened, bent, flanged, dished, tapered or thinned, planed, turned, welded and drilled before any kind of construction is possible. Then follows the closing up of hundreds of rivets, the caulking, the fitting of stays, gussets, tubes, and finally the attachment of the numerous mountings.
Each great class of boiler has its peculiar constructional fea tures, necessitating the use of specialized types of machines and riveters. The Cornish and Lancashire boilers require a large quantity of plates, with consequent joints and rivets, and the flues involve rolling, welding, and riveting. The marine Scotch boilers have a good deal of flue work also, as well as tubes and ous stays, and the great size, as much as 18 ft. diam., together with the thick plates, require the employment of the most massive shearing, bending, and riveting machines. In the case of tive boilers, the flat fire-box plates necessitate the fitting of a great number of stays to prevent collapse under the pressure; holes for these must be drilled and tapped, and the stays must be screwed in and riveted over. The fire-tubes likewise are many in number, while superheaters in these and other types often add to the work. Water-tube boilers, though possessing no large amount of plating, demand much attention in the preparation of the tubes and their connections.
Although plate flattening and bending are done cold, the shop must be equipped with furnaces for heating certain plates for flanging, thinning, or special bends, as well as angles, tees and bars for bending or forging. There are also smaller rivet-heating furnaces. Special sorts of cranes have to be installed, some to transport pieces from the furnaces and hold them in position during a process, others to suspend portable riveters in various attitudes about a boiler, and yet others to suspend boilers over fixed riveters.
Shearing machines cut the plates to dimensions, the largest sizes being capable of shearing 12 ft. at a stroke through 2 in. cold plate. The shorn edge cannot be used thus, but must be planed on a long machine having a girder and jacks to hold the plate while a tool carriage travels up and down and planes the rough, slightly cracked edge into a smooth finish. This is a neces sary preliminary to the caulking of the joints after riveting.
Some plates require flattening to take out the buckles and un even tension ; this is effected in a "mangle" with f our, five or seven rollers. Bending can be done with three rolls, two driven ones, and a vertically adjustable bending one (fig. i) ; the setting of this with screws in the end framings determines the degree of curvature. The thick heavy plates which make up the shells of Scotch marine boilers are often handled in vertical rolls, to avoid difficulties of floor space and handling with the crane; and there are powerful hydraulic benders that press the plate bit by bit be tween dies, completing the curve more quickly than a rolling ma chine of the requisite size and driving power.
Flanging is performed in dies, with the exception of that for cylindrical flues, which are frequently put into a rotary machine with rollers that turn over the edge at one heat. Otherwise hy draulic presses are utilized that press the shape at one operation or, in the case of large plates, by a succession of squeezes, em ploying relatively small dies and a narrow machine. Fig. 2 de picts the arrangement of the dies for the first method, comprising a matrix, a fixed top die, and a holding or vice disc worked by a central ram. The holding disc is set level with the top surface of the matrix and the hot plate taken from the adjacent furnace and laid on in a central position. The vice ram rises and the plate becomes gripped against the top die, after which the main ram (or rams) actuating the platen with the matrix on causes the latter to ascend and gather in the edges of the plate, forming it up into a perfect flange. The power required ranges from i oo to 500 tons according to the diameter and thickness of plate. Dif ferently-shaped dies are wanted for locomotive fire-box plates and other contours. Some parts of boilers, such as the ends of certain Cornish types, and of steam drums must be dished as well as flanged ; this is seen in fig. 3. Proper finish to edges has to be imparted in a turning or sawing or milling machine, and some end plates have to go to a special machine which cuts large circular or elliptical holes.
Although some amount of drilling is done with portable electric or pneumatic drills, the bulk of the work goes to special fixed machines, either to those with a single spindle and devices for rapid transference from one spot to another, which do about i oo holes in an hour, or to those with multiple spindles driving a lot of drills simultaneously. The largest design of the latter will take a marine boiler shell up to 20 ft. diameter, upon rollers which revolve it as required to bring fresh portions of the periphery opposite the drills. Some machines drill and tap 2 in. stay holes through both shell and combustion-chamber plates and screw the stays into place, tap stay-tube holes up to 4 in. diameter through both plates and screw the tubes in.
Some details, as flues, are riveted up before insertion in a shell, but there is much awkward riveting to be carried on, this some times necessitating the employment of the hand-controlled pneu matic riveters. But all that is practicable goes preferably to the hydraulic riveters, or to the pneumatic yoke riveters, i.e., those having a U-shaped frame carrying the riveting hammer at the termination of one arm and the holder-up at the other. The large fixed hydraulic machines have a gap as deep as 23 ft., in which a locomotive boiler may be lowered and moved about to the various positions. Marine boiler riveters are built on a more massive scale in order to deal with the thick plates and large rivets, one about i s in. diameter requiring 15o tons pressure. The union of an end plate to the shell may be seen in fig. 4. As the plates are so thick and unyield ing, and absolute tightness of joint is essential for the high steam pressures employed, a plate-closing device is incorpora ted, as represented in fig. 5. The closer touches the plate before the snap reaches the rivet tail, and squeezes with a force of from 3o to 5o tons, in opposition to the fixed snap at the back. Then the riveting snap advances and closes up the rivet.
There are many smaller kinds of portable hydraulic or pneu matic riveters slung from a crane and shaped to reach awkward situations, including corners and interiors. The screwed stays in locomotive fire-boxes are closed over by hand-manipulated pneu matic riveters, or with the help of a large U-shaped machine reaching inside the box and outside the shell.
The insertion of other stays and tubes occupies a good deal of time in the later stages. The tubes have to be expanded to fit tight by means of a power-driven tool carrying rollers that are forced hard against the interior of the tube as the expander revolves. Beading is accomplished with a hand or power-driven tool which forces over the projecting end of the tube to snug contact with the plate.
Caulking of the riveted seams, and of some rivets, is essential as a safeguard against leaks. A flat-nosed tool with matted face is driven against the plate edge with a hand hammer or more gen erally a pneumatic hammer, and burrs up the edge slightly, so that it makes intimate contact with its fellow. If, after testing the boiler with the hydraulic pump, leaks reveal themselves, the seams or rivets are marked at such spots; to be attended to again.
Attachment of the numerous mountings on a boiler means a lot of drilling, tapping and riveting, as well as the making of steam tight joints in cases where detachment has to be provided for, for cleaning or inspection purposes. The final process consists in covering the shell with non-conducting composition, and fitting the bands and lagging sheets.