RIVETS are small fastenings made of the best wrought iron or mild steel either by hand or by machinery, and before they are fixed consist each (see Fig. 206) of a small spindle or shank surmounted by a head, which may be pan-shaped as in Fig. 208, or formed like a cup or button as in Fig. 207.
About half of the shank of the rivet farthest from the head is slightly tapered.
When the rivet has been made red-hot, and put through the hole it is to occupy, the tail end of the shank is formed into a button or point of the shape required, which differs in the various kinds of rivets, as described at page 86.
Rivets are chiefly used to connect plates of iron. They are preferable to small bolts, because, being hammered close to the face of the plate, they hold more tightly, and the shanks of rivets are not so likely to become oxidised as those of bolts; moreover, as rivets are nearly always fixed when hot, they contract in cooling, and draw the plates together with great force.
The actual handiwork connected with different processes in building has not, as a rule, been described in these Notes, because they can generally be seen by the student, and thus better understood than by any written description, - but with riveting the case is different. A student connected with ordinary building work is in many instances not likely to see riveting actually going on, and some knowledge of the process is necessary in order to understand the precautions which should be observed in good work.
The process of hand-riveting will therefore be briefly described.
A riveting gang consists of three men and two boys. The latter heat the rivets; the men insert and clench them.
The heating is generally effected in a portable smith's forge by placing a few rivets in a plate bored with holes, so that their tails stick through the holes into the heart of the fire, while their heads upon the upper side of the plate are comparatively cool.
1 Neither riveting, rolled beams, nor plate girders are mentioned in the Syllabus for this Course, but they are included in this volume because it is desirable to understand them in connection with floors and iron roofs.
When the men require a rivet, the hottest is selected and handed to them by means of pincers.
Of the three men, two, the "riveters," stand on one side of the plate, and the third, the "holder-up," on the other side.
The riveters are armed with riveting hammers, two or three tapering steel punches, an iron "snap" i.e. a short bar having its end hollowed out to form a cup that will fit the point (when finished) of the rivet, and a heavy sledge-hammer with which to strike this snap, called a "cupping hammer."
The holder-up has a heavy bar, or holding-up iron, the end of which is also hollowed out to fit the rivet head, or a heavy hammer fitted upon a long stave may be used for the purpose.
The riveters drive a punch through the holes for the rivet about to be inserted, so as to make those in the several plates coincide with one another.
The holder-up knocks the punch back out of the hole, picks up a red-hot rivet and places it in the hole with its red-hot tail sticking out toward the riveters, and places his iron upon its head.
The riveters then hammer the iron immediately round the rivet, so as to bring the plates close together. If this is not done, the rivet, when hammered, will bulge out between the plates, and keep them apart.
They then hammer down the tail of the rivet neatly, so as to form a point of the shape required.
This last operation should be performed with heavy hammers having flat ends; and by it not only should the end of the rivet be formed, but the whole shank should be "upset," that is, squeezed up and made thicker, so as to fill the hole completely.
When the rivet is to be formed with a convex point, it is generally finished with the snap.
After the point has been neatly formed by the hammers, and is just losing its red heat, the snap is held upon it by one of the riveters, and struck by the other with the cupping hammer (the holder-up pressing his hammer against the head of the rivet on the other side), so that the point formed is made smooth and even. and all superfluous metal round the edges is cut off.
Machine-riveting is done with rivets of the same form as those clenched by hand, - generally with snap heads and points. The rivets are inserted red-hot, and the points clenched by means of a die which moves forward and presses it into shape.
In some machines there are two dies, so that both ends of the rivet are formed at the same time.
Some of the largest machines are worked by hydraulic power; these not only hold the plates tight together, but bring a pressure of 50 or 60 tons upon the head of the rivet.
The ordinary machines can only be used to rivet such work as can be brought to them, but there are also machines which are adapted for riveting up girders in situ.
Machine-riveting is cheaper and better than that done by hand. The steady pressure brought by the machine upon the rivet not only forms the head; but compresses and enlarges the shank, so that it is squeezed into, and thoroughly fills up all the irregularities of the holes.
The superiority of machine-riveting is strikingly shown when rivets have to be taken out.
After the head is cut off, a hand-clenched rivet may be easily driven out, but a machine-clenched rivet must, as a rule, be drilled out.
Rivets clenched by machines can generally be easily distinguished from those done by hand; the latter are covered with marks caused by the shifting of the snap during riveting; while on a machine-riveted head there is generally a burr like the peak of a jockey's cap, caused by the die having caught the rivet a little out of the centre.