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 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.

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.