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FORGING
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FORGING , the See also:craft of the See also: These corrugations are then removed either by a flatter, if the surfaces are See also:plane (fig. 2), or by hollow swages, if the See also:cross section is circular (fig. 3). See also:Spring swages (fig. 4) are frequently used instead of See also:separate " See also:top and bottom tools." Frequently swaging is prac- tised at once, without the preliminary detail of fullering. It is adopted when the amount of reduction is slight, and also when a See also:steam See also:hammer or other type of See also: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 See also:risk of damage to the material, while it improves the metal by consolidating its See also:fibres. But its limitations in anvil tediousness of the operation, when the See also:part to be reduced is very much less in See also:diameter, and very much longer, than the See also:original piece of bar. Then there are. other alternatives. If a See also:long bar is required to have an enlargement at any portion of its length, not very much larger in diameter than the bar, nor of the metho . The Upsetting. to See also:great beienla See also:ged istheated, the parts adjacent remaipart remaining See also:cold, and an end is hammered, or else lifted and dropped heavily on the anvil or on an iron See also:plate, with the result that the heated portion becomes both shortened and enlarged (See also:figs. 5 and 6). This process is only suitable for relatively See also:short lengths, and has the disadvantage that the fibres of wrought iron are liable to open, and so cause weakening of the upset portion. But steel, which has no direction of fibre, can be upset without injury; this method is therefore commonly adopted in steel work, in power presses to an equal extent with drawing down. The alternative to upsetting is generally to weld a larger to a smaller bar or section, or to encircle the bar with a See also:ring and weld the two (fig. 7), 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 fulrrum. It is always, when practicable, preferable to cutting out a curved orangular shape with a hot sett or to welding. The continuity of the fibre in iron is preserved by bending, and the risk of an imperfect weld is avoided. Hence it is a simple and safe gepding, process which is constantly being performed at the anvil. An objection to sharp bends, or those having a small See also:radius, is that the fibres become extended on the See also:outer radius, the cross section being at the same See also:time reduced below that of the bar itself. This is met by imparting a preliminary amount of upsetting to the part to be See also:bent, sufficient to counteract the amount of reduction due to See also:extension of the fibres. A See also:familiar example is seen in the corners of See also:dip cranks. The See also:property possessed by pieces of iron or steel of uniting autogeneously while in a condition of semi-See also:fusion is very valuable. When portions which differ greatly in dimensions have to welding. be See also:united, welding is the only method practicable at the anvil. It is also generally the best to adopt when See also:union has to be made between pieces at right angles, or when a piece on which much work has to be done is required at the end of a long See also:plain bar, as in the tension rods of See also:cranes and other structures with eyes. The See also:art of welding depends chiefly on having perfectly clean See also:joint faces, See also:free from See also:scale, so that metal can unite to metal; union would be prevented by the presence of See also:oxide or of 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 he made so hot. A very few 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-See also:joints include the See also:scarf (figs. 8 and 9), the See also:butt (fig. Io), the V (fig. II) and the glut, one form of which is shown in fig. 12; the illustrations are of bars prepared for welding. These forms give the smith a suitable choice for different conditions. A convexity is imparted to the joint faces in order to favour the See also:expulsion of slag and dirt during the closing of the joint; these undesirable matters become entangled between See also:concave faces. The ends are upset or enlarged in order to leave enough metal to be dressed down flush, by swaging or by flattering. The proportional lengths of the joint faces shown are those which conform to See also:good practice. The fluxes used for welding are numerous. See also:Sand alone is generally dusted on wrought iron, but steel requires See also:borax applied on the joint while in the See also:fire, and also dusted on the joint at the anvil and on the face of the utter itself. Electric welding is largely taking the See also:place of the See also:hand process, but See also:machines are required to maintain the parts in contact during the passage of the current. Butt joints are employed, and a large quantity of power is absorbed, but the output is immensely greater than that of hand-made welds. When holes are not very large they are formed by punching, but. large holes are preferably produced by bending a See also:rod round and welding it, so forming an See also:eye (fig. 13). Small holes punching. are often punched simply as a preliminary See also:stage in the formation of a larger hole by a process of drifting. A piece of work to be punched is supported either on the anvil or on a ring of metal termed a See also:holster, laid on the anvil, through which the See also:burr, when severed, falls. But in making small holes through a thick See also:mass, no burr is produced, the metal yielding sideways and forming an enlargement or See also:boss. Examples occur in the wrought iron stanchions &Via `\\+....\a'vfir that carry See also:light hand railing. In such cases the hole has to be punched from each face, See also:meeting in the centre. Punching under power hammers is done similarly, but occupies less time. The cutting-off or severance of material is done either on hot of cold metal. In the first See also:case the See also:chisels used, " hot setts," have Cutting- keener cutting angles than those employed for the second, termed " cold setts." One sett is held in a hole in the off anvil face, the " anvil See also:chisel," the other is handled and struck with a sledge. The difference between iron and steel at the forge is that iron possesses a very marked fibre whereas steel does not. Many forgings therefore must be made differently according as they are in iron or in steel. In the first the fibre must never be allowed to run transversely to the See also:axis of greatest tensile or bending stress, but must be in See also:line therewith. For this See also:reason many forgings, of which a See also:common eye or See also:loop (fig. 13) is a typical example, that would be stamped from a solid piece if made in steel, must be bent round from bar and welded if in wrought iron. Further, welding which is practically uniformly trustworthy in wrought iron, is dis- trusted in steel. The difference is due to the very fibrous See also:character of iron, the welding of which gives much less anxiety to the smith than that of steel. Welds in iron are frequently made without any See also:flux, those in steel never. FIG. 13. Though mention has only been made of iron and steel, other See also:alloys are forged, as those of See also:aluminium, See also:delta metal, &c. But the essential operations are alike, the See also:differences being in temperature at which the forging is done and nature of the fluxes used for welding. For hardening and tempering, an important section of smith's work, see See also:ANNEALING. See also:Die Forging.—The smith operating by hand uses the above methods only. There is, however, a large and increasing See also:volume of forgings produced in other ways, and comprehended under the See also:general terms, " die forging " or " drop forging." Little See also:proof is needed to show that the various operations done at the anvil might be performed in a more expeditious way by the aid of power-operated appliances; for the elementary processes of reducing, and enlarging, bending, punching, &c., are extremely simple, and the most elaborate forged work involves only a repetition of these. The fact that the material used is entirely plastic when raised to a white heat is most favourable to the method of forging in matrices or dies. A white hot mass of metal can be placed in a See also:matrix, and stamped into shape in a few blows under a hammer with as much ease as a See also:medal can be stamped in steel dies under a coining See also:press. But much detail is involved in the See also:translation of the principle into practice. The parallel between coining dies and forging dies does not go far. The See also:blank for the See also:coin is prepared to such exact dimensions that no surplus material is left over by the striking of the coin, which is struck while cold. But the blank used in die forging is generally a shapeless piece, taken without any preliminary preparation, a See also:mere lump, a piece of bar or rod, which may be square or round irrespective of whether the ultimate forging is to be square, or round, or flat or a See also:combination of forms. At the See also:verge of the welding heat to which it is raised, and under the intensity of the impact of hammer blows rained rapidly on the upper die, the metal yields like See also:lead, and flows and fills the dies. Herein lies a difference between striking a coin and moulding a forging. A large amount of metal is squeezed out beyond the concavity of the forging dies, and this would, if allowed to flow over between the joints, prevent the dies from being closed on the forging. There are two methods adopted for removing this " fin," or ` flash " as it is termed, one being that of suppression, applicable to circular work, the other that of stripping, applied to almost all other cases. The suppression of fin means that the circular bar is rotated in the dies (fig. 14) through a small arc, alternating between every few blows, with the result that the fin is obliterated immediately when formed, this being done at the same time that reduction of section is being effected over a portion or the whole of the bar. Stripping means that when a considerable amount of fin hasbeen formed, it is removed by laying the forging on a die pierced right through with an opening of the same shape and See also:area as the. forging, and then dealing the forging a See also:blow with the hammer. The forging is thus knocked through the die, leaving the severed or stripped fin behind. The forging is then returned to the dies and again treated, and the stripping may be repeated twice, or even oftener, before the forging can be completed. Figs. 15 and 16 illustrate the bottom dies of a set for forging in a particular form of eye, the top dies being of exactly the same shape. The first operation takes place in fig. 15, in which a bar of metal is reduced to a globular and cylindrical form, being constantly rotated mean-while. The shank portion is then See also:drawn down in the parallel See also:recess to the left. The shape of the eye is com- pleted in fig. 16, and the FIG. 14. shank in the recess to the left of that. Fig. 17 shows how a See also:lever is stamped between. top and bottom dies. The hole in the larger boss is formed by punching, the punches nearly meeting in the centre, and the centre for the hole to be drilled subsequently in the smaller boss is located by a conical See also:projection in the top die. L It is evident that the methods of die forging, though only explained here in barest outline, constitute a principle of extensive application. An intricate or ornamental forging, which might occupy a smith a See also:quarter of a See also:day in making at the anvil, can often be produced in dies within five minutes (fig. 18). On the other hand, there is the cost of the preparation of the dies, which is often heavy, so that the question of method is resolved into the relative one of the cost of dies, distributed over the number of identical forgings required. From this point of view it is clear that given say a thousand forgings; ordered all alike, the cost of even expensive dies distributed over the whole becomes only an infinitesimal amount per forging. There is, further, the very important fact that forgings which are produced in dies are See also:uniform and generally of more exact dimensions than anvil-made articles. This is seen to be an See also:advantage when forgings have to be turned or otherwise tooled in the engineer's See also:machine See also:shop, since it lessens the amount of work required there. Besides, for many purposes such forgings do not require tooling at all, or only superficial grinding, while anvil-made ones would, in consequence of their slight inaccuracies. Yet again, die forging is a very elastic See also:system, and herein lies much of its value. Though it reaches its highest development when thousands of similar pieces are wanted, it is also adaptable to a See also:hundred, or even to a dozen, similar forgings. In such cases See also:economy is secured by using dies of a very cheap character; or, by employing such dies as supplementary to anvil work for effecting neat finish to more precise dimen- sions than can be ensured at the anvil. In the first case use is made of dies of See also:cast iron moulded from patterns (fig. 19) instead of having their matrices laboriously cut in steel with drills, chisels and milling tools.. In the second, preliminary drawing . down is done under the steam hammer, and bending and welding at the anvil, or under the steam mately to their final shape and dimensions. Then they are reheated and inserted in the dies, when a few blows under the steam or drop hammer suffice to impart a neat and accurate finish. The limitations of die forging are chiefly those due. to large dimensions. The system is most successful for the smallest forgings arid dies which can be handled by one See also:man without the assistance of cranes; and massive forgings are not required in such large See also:numbers as are those of small dimensions. But there are many large articles manufactured which do not strictly come under the See also:term forgings, in which the aid of dies actuated by powerful See also:hydraulic presses is utilized. These include work that is bent, drawn and shaped from steel plate, of which the fittings of railway wagons constitute by far the largest proportion. The dies used for some of these are massive, and a single squeeze from the See also:ram of the hydraulic press employed bends the steel plate between the dies to shape at once. Fairly massive forgings are also produced in these presses. Die forging in its highest developments invades the craft of the skilled smith. In shops where it is adopted entirely, the only craftsmen required are the few who have general See also:charge of the shops. The men who attend to the machines are not smiths, but unskilled helpers. (J. G. Additional information and CommentsThere are no comments yet for this article.
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