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
