"Machine riveting," of course, has to be done in the shops at the ironbuilder's yard or works; so that only certain parts, though often the majority, can be riveted together, by machines, which work on similar lines to the process of hand riveting, but have the advantage of making both ends of the rivet at the same time, while the very great pressure, oftentimes from 50 to 60 tons, compresses the iron and fills up the rivet holes so much better than when done by hand; and consequently, when the rivets cool and contract, they have greater power to pull the plates together. Indeed, cases have been known where the pins of machine-riveted work have been so tightly fitted in the holes, that when the heads have gone, or been removed, the pins have kept the plates together; and, moreover, the pins have even had to be "drilled out," when required, because they could not otherwise be got out. Hence it will be seen that machine riveting is far superior and better than that clenched by hand; and not only that, but the work is done so much cheaper by machinery.
The employment of bolts and nuts, to connect ironwork together, such as plates, is not to be compared, in point of strength and efficiency, with rivets, which pull the plates together in themselves, when they cool, after having been hammered down, whereas bolts can only be screwed up to the plates, so that .trie tightness of the connection and the perfection of the work depend solely on the thread of the bolt and nut.
The different classes of rivets are distinguished by their different-shaped heads, and the different modes of riveting by the order, in which the rivets are placed, and the number of rows of them. Among the different kinds of rivets the chief are as follows: -
Snap rivets, as fig. 421, have hemispherical heads, which are formed by the die, and are sometimes called button or cupended rivets.
Hammered, conical, or staff rivets, as fig. 422, are made by the hammering of the red-hot iron to the required shape, as shown, no die being used for these.
Pan rivets are similar to fig. 423, and counter-sunk rivets to fig.
424; these latter being used where the plates must take a truly level bearing, which would not be possible with heads which project. The holes for these rivets are always drilled with a splayed shoulder within the plates, as the illustration explains.
The holes in the plates may be either punched in or drilled, the latter being the better mode, because, while ensuring much greater accuracy in the setting out of the pitch, they are cleaner, as will be seen by a comparison of figs. 425 and 426; and they also do not injure the fibres of the plates by the sudden shocks and hammering. Drilled rivet holes also make stronger and better connected work, so that the rivets need not be so large as for punched work.