THE former illustrations of forging have been principally descriptive of such works as could be made from a single bar iron, on purpose that the examples to be advanced in welding or joining together two pieces of iron by heat, technically called "shutting together" or " shutting up," might be collected at one place.
There are several ways of accomplishing this operation, and which bear some little analogy to the joints employed in carpentry; more particularly that called scarfing, used in the construction of long beams and girders by joining two shorter pieces together endways, with sloping joints, which in carpentry are interlaced or mortised together in various ways, and then secured by iron straps or bolts. In smith's work likewise, the joinings are called scarfs, but from the adhesive nature of the iron when at a suitable temperature, the accessories called for in carpentry, such as glue, bolts, straps and pins, are no longer wanted. It would be desirable that the remarks on the fire and degrees of heat, page 206, should be again perused before the reader attempts the process of welding.
The example, fig. 106, page 216, was left unfinished, but we will proceed to show the mode of joining the two cylindrical ends of the work. The scarfs required for the "shut," are made by first upsetting or thickening the iron by blows upon its extremity, to prepare it for the loss it will sustain from scaling off, both in the fire and upon the anvil, and also in the subsequent working upon the joint. It is next rudely tapered off to the form of a flight of steps, as shown in figs. 116 and 117, and the sides are slightly bevelled or pointed, as in fig. 117, the proportions being somewhat exceeded to render the forms more apparent.
The two extremities are next heated to the point of ignition; and when this is approached, a little sand is strewed upon each part, which fuses and spreads something like a varnish, and partially defends them from the air; the heat is proper when, notwithstanding the sand, the iron begins to burn away with vivid sparks. The two men then take each one piece, strike them forcibily across the anvil to remove any loose cinders, place them in their true positions, exactly as in fig. 116, and two or three blows of the small hammer of the principal or fireman stick them together; the assistant then quickly joins in with the sledge hammer, and the smoothing off and completion of the work are soon accomplished.
It is of course necessary to perform the work with rapidity, and literally "to strike whilst the iron is hot; " the smith afterwards jumps the end of the rod upon the anvil, or strikes it endways with the hammer; this proves the soundness of the joint, but it is mostly done to enlarge the part, should it during the process have become accidentally reduced below the general size. The sand appears to be quite essential to the process of welding, as although the heat might be arrived at without its agency, the surfaces of the metal would become foul and covered with oxide when unprotected from the air; at all events common exprience shows that it is always required. The scarf joint, shown in figs. 116 and 117, is commonly used for all straight bars, whether flat, square or round, when of medium size.
In very heavy works the welding is principally accomplished within the fire: the two parts are previously prepared either to the form of the tongue or split joint, fig. 118, or to that of the butt joint, fig. 119, and placed in their relative positions in a large hollow fire. When the two parts are at the proper heat, they are jumped together endways, which is greatly facilitated by their suspension from the crane, and they are afterwards struck on the ends with sledge hammers, a heavy mass being in some cases held against the opposite extremity to sustain the blows; the heat is kept up, and the work is ultimately withdrawn from the fire, and finished upon the anvil.
The butt joint, fig. 119, is materially strengthened, when, as it is usually the case for the paddle shafts of steam-vessels and similar works, the joint whilst still large is notched in on three or four sides, and pieces called stick-in pieces, dowels, or charlins, one of which is represented by the dotted lines, are prepared at another fire, and laid in the notches; the whole, when raited to the weldig; heat, is well worked together and reduced to the intended size; this mingles all the parts in a very substantial manner. For the majority of works however, the scarf joint, fig. 116, is used, but the stick-in pieces are also occasionally employed, especially when any accidental deficiency of iron is to be feared.
When two bars are required to form a T joint, the transverse piece is thinned down as at a, in fig. 120; for an angle or corner the form of b may be adopted; but c, in which each part is cut off obliquely, is to be preferred. The pieces a, b, c, are represented upside down, in order that the ridges set down on their lower surfaces may be seen. In most cases when two separate bars are to be joined, whatever the nature of the joint, the metal should be first upset, and then set down in ridges on the edge of the anvil, or with a set hammer, as the plain chamfered or sloping surfaces are apt to slide asunder when struck with the hammer, and prevent the union. When a T joint is made of square or thick iron, the one piece is upset, and moulded with the fuller much in the form of the letter; it is then welded against the flat side of the bar: such works are sometimes welded with dowel or tenon joints, but all the varieties of method cannot be noticed.
There are many works in which the opposite edges, or the ends of the same piece, require to be welded; in these the risk of the two parts sliding asunder scarcely exists, and the scarfs are made with a plain chamfer, or simply to overlap or fold together without any particular preparation.