When the materials are of best quality, then there only remains to rivet and stay the boiler. Riveting is of two kinds, single and double. [Fig. 37] shows the method of single riveting, and Figs. [38] and [39] show the plan and cross-section of double riveted sheets.
Fig. 37.
Double Riveting consists in making the joints of boiler work with two rows of rivets instead of one—nearly always, horizontal seams are double riveted as well as domes where they join upon the boiler. Usually all girth seams,—those running round the body of the boiler, are single riveted. The size of the rivets is in proportion to the diameter of the boiler, being 5⁄8, 3⁄4 and 7⁄8 as required in the specification.
Rivet holes are made by punching or drilling, according to the material in which they are made. In soft iron and mild steel they may safely be punched, but in metal at all brittle the holes should be drilled.
Fig. 38.
Rivets are driven by hand, by steam riveting machines or by an improved pneumatic machine which holds the sheet together and strikes a succession of light blows to form the head of the rivet while hot. Rivets are made both of iron and steel, and there are certain well-known brands of such excellent quality that they are almost exclusively used in boiler work.
A place where skill is shown in boiler construction is in laying out the rivet holes, with a templet, so that the sheets come exactly together with the holes so nearly opposite that the dreaded drift pin does not have to be used.
In these figures the letters P and p refer to the “pitch of the rivets,” i.e., the part from centre to centre, and the dimensions given at the sides indicate the amount of lap given in inches and tenths of inches—the diameter of the rivet (1″) is also shown, and the turned over portion of the shank of the rivet is shown by dotted lines.