Forging

FORGING is the art of forming or shaping metal by heat ing and hammering. The art is as old as history. Forging differs from founding or casting in the fact that the metal is never melted, but is worked while hot enough to be in a plastic or pasty condition. The tools used are in the main counterparts of the shapes desired and they mould by impact. Forging is divided into two broad classifications, hand forging and machine forging (see DROP FORGING), according to the way in which the work is done rather than to the work itself ; for the operations performed on the metal are of the same general character and the resulting pieces are mainly the same, the difference being that the hand forger, smith or blacksmith works with a hand hammer and variously formed hand tools, whereas machine forging employs to the full est extent the use of automatic power-driven machines and gener ally one or more special tools or dies designed and made for the production of the particular piece which it is desired to produce.

Hand forging does employ to some extent power-driven ma chines to deliver blows instead of relying completely upon muscu lar effort ; the tools used in these machines are of a general nature and can be used on many different forgings rather than being designed for a particular piece. Machine forgings are more uni form and generally of more exact dimensions than those made by hand with hammer and anvil, but the cost of the special tools required for machine forging precludes the use of this method for a small number of forgings, the quantity of equal cost by both methods depending upon the cost of the necessary tools, the cost of production and the possible saving in subsequent operations of machining due to uniformity and exactness. If the requirement be a single round bar bent at right angles on one end, the advantage of hand forging is obvious. If the requirement be several thou sand intricate pieces such as an ornamental fence picket, the ad vantage of machine forging is as readily apparent. One claw ham mer head could be produced by the hand forger in very much less time than that necessary to produce the tools required for the machine forging process, but were a large quantity wanted the cost of the machine tools would be very minor when spread over the quantity of pieces produced, and, since after once making the tools, the machine forger's time for producing one piece would be only a small fraction of the hand forger's time, the cost per piece (which is generally though not always the determining fac tor) would be very much less than if the large quantity were pro duced by the hand process. There is no definite quantity at which the cost per piece is equal. This quantity is different for each different piece together with the specifications imposed but will in general be from 5 to r oo forgings. The perfecting of machines for making the tools for the machine forging process lowers the point of equal cost. The possibility of future requirements of the same piece should be considered as affecting the cost of the entire quantity eventually required.

Hand forging operations are : (I) reducing or drawing down from a larger to a smaller section (fullering and swaging) ; (2) enlargement of a smaller to a larger portion (upsetting) ; (3) bending, or turning to any angle or curvature; (4) uniting one piece of metal to another (welding) ; (5) the formation of holes by punching; (6) severance, or cutting off. In none of these proc esses, the last excepted, is the use of a sharp cutting tool involved, and therefore there is no violence done to the fibre of the metal. Nor have the tools of the smith any sharp edges, except the cutting-off tools or "sets." The flow of the metal, which is viscous when at full red heat, must never be lost sight of, and in forging judgment must be exercised in arranging the direction of the fibre in a way best calculated to secure maxi mum strength.

Fullering is the preliminary roughing-down of the material between tools having convex edges ; swaging, the completion or finishing process between swages, on dies of definite shape, nearly hemispherical in form. When a bar has to be reduced from larger to smaller dimensions, it is laid upon a fuller or round-faced stake, set in the anvil, or, in some cases, on a flat face (fig. I), and blows are dealt upon that portion of the face which lies exactly opposite with a fullering tool and struck on its head by a sledge hammer. The position of the piece of work is quickly changed at brief intervals in or der to bring successive portions under the action of the swages until the reduction is completed; the upper face, and if a bottom fuller is used the under face also, is thus left corrugated slightly. These corrugations are then re moved either by a flatter (fig. I), if the surfaces are plane or by hollow swages (fig. I), if the cross section is circular. Spring swages (fig. I) are frequently used instead of separate top and bottom tools. Frequently swaging is practised at once, without the preliminary detail of fullering. It is adopted when the amount of reduction is slight, and also when a power hammer is available. This process of drawing down or fullering is, when practicable, adopted in preference to either upsetting or welding, because it is open to no objection, and involves no risk or damage to the material, while it improves the metal by consolidating its fibres, but its limitations in anvil work lie in the tediousness of the operation when the part to be reduced is very much less in diameter and very much longer than the original piece of bar, in which case there are other alternatives.

Upsetting.—If a long bar is required with an enlargement at any portion of its length, not very much larger in diameter than the bar, nor of great length, upsetting is the method adopted.

The part to be enlarged is heated, the parts adjacent remaining cold, and one end is hammered, or else the bar is lifted and dropped heavily on end on the anvil, with the result that the heated portion becomes both shortened and enlarged (fig. 1). This process is suitable only for relatively short lengths of upset in hand forging but is commonly adopted in machine forging to an equal extent with drawing down. The alternative to up setting is generally to weld a larger to a smaller bar or section, or to encircle the bar with a ring and weld the two (fig. 2), and then to impart any shape desired to the ring in swages.

Bending is effected either by the hammer or by the simple exercise of leverage, the heated bar being pulled round a fulcrum.

It is always, when practicable, preferable to cutting out a curved or angular shape with a hot set or to welding. The continuity of the fibre is preserved by bending, and the risk of an imperfect weld is avoided. Hence it is a simple and safe process which is constantly being performed at the anvil. An objection to sharp bends, or those having a small radius, is that the fibres become extended on the outer radius, the cross section being at the same time reduced below that of the bar itself. This is met by impart ing a preliminary amount of upsetting to the part to be bent, sufficient to counteract the amount of reduction due to ex tension of the fibres.

Welding

is the very valuable property possessed by pieces of iron or steel of uniting ously while in a condition of semifusion. When long portions which differ greatly in sions have to be united, welding is the only method practicable at the anvil. It is also generally the best to adopt when union has to be made between pieces at right angles, or when a piece on which much work has to be done is quired at the end of a long plain bar, as in the tension rods of cranes and other structures with eyes. The art of welding pends chiefly on having perfectly clean joint faces, free . from scale, so that metal can unite to metal, union being prevented by the presence of oxide or dirt. Also it is essential to have a temperature sufficiently high, yet not such as to overheat the metal. A dazzling white, at which small particles of metal begin to drop off, is suitable for iron, but steel must not be made so hot. A very few light hammer blows suffice to effect the actual union. If the joint be faulty, no amount of subsequent hammering will weld it. The forms of weld-joints include the scarf (fig. 2), the butt (fig. 2), and V (fig. 3) and the glue, one form of which is shown in fig. 3. The illustrations are of bars prepared for weld ing. These forms give the smith a suitable choice for different conditions. A convexity is imparted to the joint faces in order to favour the expulsion of slag and dirt during the closing of the joint ; these undesirable matters would become entangled between concave faces. The ends are upset or enlarged in order to leave enough metal to be dressed down flush, by swaging or by flatting. The proportional lengths of the joint faces shown are those which conform to good practice. The fluxes used for welding are numer ous but the most common is borax. Electric welding is largely taking the place of the hand process, particularly in the United States, but machines are required to maintain the parts in contact during the passage of the current. Butt joints without upsetting are usually employed, and a quantity of power is absorbed, but the output is immensely greater than that of hand welding.

Punching.

When holes are not very large they are formed by punching, but large holes are preferably produced by bending a rod around and welding it, so forming an eye (fig. 3) . Small holes are often punched simply as a preliminary stage in the formation of a larger hole by the process of drifting, which is the gradual enlarging of the hole by tapered punches. A piece of work to be punched is supported either on the anvil or on a ring of metal termed a bolster, laid on the anvil, through which the burr, when severed, falls. But in making small holes through a thick mass, no burr is produced, the metal yielding sideways and forming an enlargement or boss. Examples occur in the wrought iron stanchions that carry light hand railing. In such cases the hole has to be punched from each face, meeting in the centre. Punching under power hammers is done similarly, but occupies less time.

Cutting-off

or severance of material is done either on hot or cold metal. In the first case the chisels used, "hot sets," have keener cutting angles than those employed for the second, termed "cold sets." In one set the anvil chisel is held in a hole in the anvil face, and in the other it is handled and struck with a sledge hammer. (H. K.)