double-riveted butt straps, i.e., there are two rows of rivets on each side of the joint.
Before the barrel rings are made and jointed, the straightened plates are marked off for the rivet holes. In British practice the holes are usually drilled, but in some works, and invariably in American practice, they are punched. Punching boiler plates is injurious to the metal immediately surrounding the holes, and therefore such holes are punched smaller than the finished sizes and afterwards enlarged by passing through them a tool in the form of a round drill with cutting edges, known as a “reamer.” This operation removes the damaged metal.
The exact position of each rivet-hole has to be marked off on each plate when punching is adopted, but when the holes are drilled five or six plates are taken at a time and only the top plate is marked off. The plates are then secured under a radial drilling machine (Fig. 3) and each hole marked on the top plate is drilled through 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
