all the plates by a single operation. The top plate is again used as a template or guide for drilling the holes in the next batch of plates.
The plates have also to be marked off at the edges all round so that they can be finished to the exact sizes. They may be done from the drawings in case only one or two boilers are being made, but when a large number are in hand the marking-off is usually done by template. The template is a sheet of metal cut out like a dressmaker’s pattern to the exact size of the
Fig. 3—Radial Drilling Machine for Boiler Plates.
by Messrs. William Asquith, Ltd. (Halifax.)
plate, and in most cases it has also the rivet and
other holes drilled through it to show their positions. Finally the edges of the plates are machined to size in a special plate-edge planing machine, in which a cutting tool moves along the edge for the full length or width of the plate.
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The next operation is to bend the barrel plates into circular rings. This is done in another form of “mangle,” or
plate-bending rolls.
In this machine there are two bottom rolls (Fig. 4) which rotate in fixed bearings, and one top roll, the bearings of which can be moved up and down vertically. By gradually bringing the top roll down each time the plate is passed backwards and forwards through the rolls, a pressure is brought to bear on the plate, and by passing it through a sufficient number of times it is gradually bent to a true circle, except at the ends where the curvature is completed by pressing or hammering it into a block. This last operation is of great importance, and has to be done because it is impossible to finish the edges of the plates to a true circle in the rolls, and unless a true circular form be obtained, the boiler will strain at the joint, and give endless trouble in service.
Fig. 5.—Boiler Shell Drilling Machine.
By Messrs. Campbells and Hunter, Ltd. (Leeds).
In many works the bulk of the rivet-holes are not drilled until the plates have been bent and put together, only a few holes being made through which bolts are passed to hold the plates together in place. The boiler is afterwards raised on end and secured on the revolving table of a boiler shell drilling machine (Fig. 5), and all the rivet-holes drilled through the plates in position. This produces holes which come accurately
opposite each other.
Boiler Tube and Back Plates, Firebox Casing.
The
smokebox tubeplate
shown at
C
on the left-hand side of Fig. 2, is a flat plate circular in shape except at the bottom, where there is a projection
D
to fasten it to the cylinders. Its edge is flanged out at right angles all round except along the base of the projection mentioned. The operation of flanging requires considerable skill and great care. It consists in heating the plate in a large furnace to a good red heat, and then sqeezing it between shaped cast iron blocks in a hydraulic press of the type shown in Fig. 6 capable of developing a total pressure of 300 to 400 tons. The table of the press slides up and down on four columns, being actuated by the plungers of the hydraulic rams seen underneath. Heavy cast iron blocks or dies in halves are machined to the shape of the finished plate and jointed together, the corners inside the flanges being rounded to the proper radii. The lower blocks are bolted to the bottom table of the press, and the upper
Fig. 6.—Hydraulic Flanging Press.
By Messrs Fielding & Platt, Ltd. (Gloucester).
blocks to the cross-head at the top. The curves and shape of the blocks are so made that when they are pressed together, there is just sufficient space between them for the thickness of the plate. There are two sets of rams, the larger or main rams, and the smaller or vice rams. When the blocks or dies are fixed and the plate to be flanged has been placed in the proper position (in which it is held by steady pins which enter holes in the plate), the vice rams move the table up, and grip the plate firmly between the bottom and top blocks. Then the pressure is put on to the main rams, which squeeze the plate into the required shape between the dies. The whole operation is done at one heat. The firebox back plate is flanged in a similar manner, the dies in this instance being shaped so that the firehole is also pressed to the necessary form.
The “throat plate” E E (Fig. 2), which connects the firebox shell to the barrel of the boiler, is of a complicated shape, and has to be flanged in two opposite directions, since the upper portion has to have its semicircular flange turned forward to join the boiler barrel, and the bottom part is flanged backwards to connect to the firebox shell wrapper plate. This plate requires extreme care when flanging to prevent damage to the material.
The sides and top F of the firebox shell are frequently made in one piece called the wrapper plate, which requires no flanging. The top portion is usually circular, and forms a continuation of the boiler barrel. This circular portion is produced by means of the plate bending rolls.
Assembling the Boiler. The boiler barrel is attached to the smokebox tube-plate by means of a continuous weldless angle iron ring G (Fig. 2). This is faced and bored in a large lathe or on a boring mill to fit over the end of the circular barrel plate, on to which it is shrunk by being heated slightly so that it expands. When put into place it contracts and grips the barrel plate, to which it is riveted all round the circumference.
The foundation ring H at the bottom of the firebox (Fig. 2) is a steel casting or forging of rectangular shape, the sides of which must be perfectly “square,” as it has to fit accurately between the inner copper firebox and the firebox outer shell or casing. It is machined and ground to accurate dimensions, the rivet-holes marked off and drilled, and then it is fitted to the firebox shell.
The whole of the plates and parts are then put together, and temporarily held in place by bolts through some of the rivet-holes. The structure has to be levelled carefully, so that the different plates are “square” and in line with one another. Careful measurements are made to see that the parts come together in accordance with the drawings or templates,
