In drilled-work there is no loss, and the holes are not only accurately located but are accurately cut. But drilled-work is very expensive, as it has to be done by hand or by machine-drills, the process being slow at best and consequently meaning a very large addition to the charge for labor. In riveted girders it would probably double the expense of the girder.

Punching of Holes.

Loss by Punching.

The advantages of drilling are, that the holes can be cut after the plates have been partially secured together, thus assuring a perfect fit of the holes over each other; and that the holes being perfectly smooth and even bear more evenly on the rivet, and the work is less apt to fail by compression, than where the bearing of plate against rivet is ragged ami uneven, as in punched work. On the other hand, the edges of drilled holes are so sharp that they promote shearing, and for this reason the edges of drilled holes in plates should be filed or reamed off. As a rule, however, the architect will find the bearing and cross-breaking strengths of rivets less than their shearing, excepting where rivets are small in comparison to thickness of plates being riveted, which is not often the case.

To settle, then, whether work should be drilled or punched, is mainly a matter of expense. Drilled-work, of course, is far preferable as regards strength and it costs aceordingly.

The rule of the mills is to punch all holes, excepting for countersunk rivets, which, after punching, of course, have to be drilled, to obtain the slanting sides of the hole.

A medium course between drilling and punching would be to punch the holes smaller than desired and then drill or ream them to accurate size when partially secured together.1 Steel should always be drilled unless annealed after punching.

In most work, however, the architect will have to be satisfied with punching, and must therefore allow sufficient material to make good the damage done and to allow for inaccuracies.

In riveting proper, that is, filling the holes, there are also the two methods of doing it, by hand (hand-riveting), or by machinery (machine-riveting), but unlike the making of the hole, in this case, the machine-work is both better and cheaper.

A machine-driven rivet is driven and completed by one powerful squeeze of the steam (or compressed-air) riveting-machine; this squeeze not only forces the plates more closely together, but more completely fills the hole with the rivet metal, besides the great advantage of doing the entire work while the rivet is hottest, and while it is, of course, at one temperature.

Advantages of Drilling.

Machine-Riveting.

1 If this is not done the " drift pin " will be used to force all the holes into line, and this means crushing and possibly buckling of the plates.

In hand-riveting these advantages are lost, the power being only-equal to that of the mechanic's blow, and as in hand-work the process consumes some time, the rivet changes its temperature and cools considerably.

In riveting, the entire rivet, including the head, should be heated to at least a red heat. It should not be heated beyond this for fear of "burning," particularly with steel rivets. Rivets that have been heated once and allowed to cool without working should be discarded. If rivets are driven at a lower heat than a red one, they will be greatly damaged, unless riveted cold.

In hand-riveting at least two men are required, one to hold the head, the "holder-up," the other to do the riveting ; but generally there is a boy to heat and bring the rivets, one holder-up, and two riveters, whose strokes alternate and thus accelerate the process.

The riveter puts a punch or drift-pin through the holes to clear them and force them into line; the holder-up seizes the hot rivet with his pincers and puts its shank through the hole, he then covers the head with a holding-up-iron shaped to fit it, and the riveters at the other end begin hammering down the projecting end of shank. When this is roughly shaped the use an iron (called "button set," for round-headed rivets), which is properly shaped to make the head. Before beginning to hammer down the end of shank, the riveters should always thoroughly' hammer the plates around the hole, to bring the plates closely together.

Hand-riveted work can sometimes be distinguished from machine-riveted work by the many marks at the made head. In machine-work there is but one mark, and this may be a little out of the centre with the shank and so show the squeezed material around its edge. But usually the work cannot be distinguished in this way. If hand-riveting has been conscien-tiously done and by careful mechanics, it is difficult to distinguish it from machine-riveting. But the shanks of hand-riveted work, as a rule, do not fill the hole as well as those in machine-riveted work, and they can more easily be "backed out" after the head has been cut off with a chisel and hammer. The only reliable test in both methods is to hold the hand one side of the head and strike the other side with a light hammer - the hammer test-when the sound will quickly disc-lose loose shanks. If in machine-riveting the plate has been sprung, and the pressure is quickly removed while the rivet is still hot, the plate may settle back, lift the hot head, and so form a loose shank.

Hand-Riveting.

How to Distinguish Riveting.

In designing riveted work, whether to be hand or machine riveted, the architect should bear in mind the necessity of placing the rivets so that they can be inserted in the holes from one side and hammered from the other, and for machine-work, that the machine can reach them.1

Steel rivets are very seriously damaged during the process of riveting. Box gives as the average of a number of tests of bar steel a tensional-strength of 47,84 tons (gross), which, after riveting, was equal in shearing-strength to only 23,77 tons (gross), a loss of 50 per cent as between the tensional-strength of the steel and shearing-strength of the riveted work, whereas in ordinary steel work this loss never exceeds 33 1/3 per cent, as between tension and shearing.