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Encyclopedia Britannica - Main :: FLA-FRA |
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FOUNDING (from Lat. fundere, to pour) , the process of casting in metal, of making a reproduction of a given object by running molten metal into a mould taken in sand, loam or plaster from that object. To enable the founder to prepare a mould for the casting, he must receive a pattern similar to the casting required. Some few exceptions occur, to be noted presently, but the above statement is true of perhaps 98% of all castings produced. The construction of such patterns gives employment to a large number of highly skilled men, who can only acquire the necessary knowledge through an apprenticeship lasting from five to seven years. A knowledge of two trades at least is involved in the work of pattern constructionthat of the craft itself and that of the moulder and founder. Patterns have to be constructed strongly. They are generally of wood
A single example must illustrate the main principles which have just been stated. The object selected is a bracket which involves questions of joints, of cores, of pattern construction and of moulding. The casting, the pattern, and its mould are illustrated. Fig. i illustrates in plan the casting of a double bracket, the end elevation
difficulties in the way of withdrawal from a mould, supposing it were made just like its casting. But if it be made as in fig. 3, with the open spaces A, B, in fig. 2, occupied with core prints, and the pieces A, A in fig. 3 left loosely skewered on, everything will " de- liver " freely. Moreover the pattern might be made solidly as shown in fig. 3, or else jointed and dowelled in the plane a-a, as in fig. 4, or along the upper faces of the prints bb, fig. 3. The744 timber shadings in figs. 3 and 4 illustrate points in the tnost suitable arrangement of material. The prints are " boxed up." Fig. 4 shows a certain stage of the moulding, in which one half of the pattern has been " rammed " in sand, and turned over in the " bottom box," and the upper half is ready to be rammed in the top box," with " runner pin " or " git stick " A, set in place. The lower loose piece has had its skewer removed during the ramming. Fig. 5 illustrates the mould completed and ready for pouring. The boxes have been parted, the pattern has been withdrawn, cores inserted in the impressions left by the prints, vents taken from the central body
Every single detail now briefly noted in connexion with this bracket is applied and modified in an almost infinite number of ways to suit the ever varying character of this process does not touch some of the great subdivisions of moulding and casting. There is a large volume of large and heavy work for which complete patterns and core boxes are never made, because of the great expense that would be involved in the pattern construction. There are also some cases in which the methods adopted would not permit of the use of patterns, as in that group of work in which the sand or loam is " swept " to the form required for the moulds and cores by means of striking boards, loam boards, core boards or strickles. In these classes of moulding the loose green sands and core sands are not much used; instead, loama wet and plastic sand mixtureis employed, supported against bricks (loam moulds) or against core bars and plates, and hay ropes (loam cores). All heavy marine engine
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of the separate parts in course of construction, and for closing them up, or putting them together in their due relations. These joints also are swept by the boards, generally cut to produce suitable " checks," or " registers " to ensure that they accurately fit together. Fig. 6, showing a portion of a swept-up mould, illustrates the general arrangement. A plate, A, carries a quantity of bricks, B, which are embedded in loam, and break joint. To a striking bar, C, sup-ported in a step, a striking board or sweeping board, D, is bolted, and is swept round against plastic loam, which is afterwards dried. The check on the board at A corresponds with a similar check on the board which strikes the interior of the pan, and by which top and bottom portions of the mould are registered together. This is indicated in dotted outline. Its mould also is swept on bricks, and turned over into place, and the metal is poured into the space b, b, between the two moulds. There is also a large group of swept-up work which is not symmetrical about a centre of rotation. Then the movements of the sweeping boards are controlled by the edges of " core plates," or of " core irons " (fig. 7). Bend pipes, and the Core-making is a special
Machine-moulding.T here is a development of modern methods of founding which is effecting radical changes in some departments of foundry practice, namely, mouldingby machines. The advantages of this method are manifold, and its limitations volute casings of centrifugal pumps and pipes, afford examples of this kind. In fig. 7, A is the core iron, held down by weights, and B the strickle," sweeping up the half bend C, two such halves pasted together completing the core. 9_.l r ^Tr~I^ M.1 Iri.ET^-W -OW Ililll- join OE MIA r Ys , -b are being lessened continually. There are. two broad departments between which machine-moulding is divided. One, of less importance, is that of toothed wheels; the other is that of general work, except of a very massive character. Gear-wheel moulding machines are essentially a special
are used, and the machine takes charge of the correct pitching or division of the teeth moulded therefrom, leaving to the moulder the work only of turning the handle of the division plate, and ramming the sand around the pattern teeth. The result is accurate pitching, and the use of two or three teeth instead of a full pattern, together with any core boxes and striking boards that are necessary for the arms. The other department of machine moulding includes nearly every conceivable class of work of small and medium dimensions. There are some dozens of distinct types of machines in use, for no one type is suitable for all classes of moulds, while some are designed specially for one or two kinds only. The fundamental principles of operation are briefly these: The pattern parts constitute, by their method of attachment to a plate or table A (fig. Io), an integral portion of the machine, so that they must partake of certain movements which are imparted to it. Often pat-terns mounted, as in fig. to, are moulded by hand, without any aid from a machine, by methods of plate - moulding." The de-livery of the pattern from the sand is in- variably accomplished by a perpendicular movement
machine (fig. II), withdrawing either the pattern from the mould or the mould from the pattern. The important point is that the perpendicular movement
or the hydraulic ram in fig. tt, is free from the unsteadiness which is incidental to withdrawal by the hands of the moulder; and if the machine performed nothing. more than this it would justify its existence. Little or no taper is required in the pattern, and the moulds are more nearly uniform in dimensions than hand-made moulds. But there are other advantages. In machine-moulding the joint faces for parting moulds are produced by the faces of the plates on which the pattern is mounted (figs. to and 1I), instead of by the hands and trowel of the moulder. When the joint face is of irregular outline, as it often is, this item alone saves a good deal of time, which again is multiplied by the number of moulds repeated, often amounting to thousands. Further, provision is generally made on machine plates for the ingates and runners (fig. io) through which the metal enters the mould, the preparation of which in hand work occupies a considerable amount of time. Another great advantage applies especially to the case of deep moulds. These give much trouble in hand-moulding in consequence of the liability of the sand to become torn up during the withdrawal of the pattern. But in machine-moulding such patterns are encircled by a plate, termed a " stripping plate," which is pierced to allow the patterns to pass through, and which, being maintained firmly on the sand during the lifting of the pattern, prevents it from becoming torn up. This is not merely a matter of convenience, but is a necessity in numerous instances. The most familiar example is that of the teeth of gear wheels, in which even a very slight amount of taper interferes with accurate engagement, and this is representative of many other portions of mechanism. These stripping plates are of metal, but in order to save the cost of filing them in iron or steel, many are cheaply made by casting a white metal alloy round the actual pattern itself in the first place, the white metal being enclosed and retained in a plain iron frame
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The highest economies of machine-moulding are obtained when (I) several small patterns are mounted and moulded at once on a single plate (fig. to); (2) when top and bottom parts of a mould are produced on different machines, carrying each its moiety of the pattern; (3) when the machine and pattern details are simplified so much that the labour of trained moulders is displaced by that of unskilled attendants who are taught in a month or two the few simple operations required. That is the direction in which repetitive casting is now rapidly tending. In fig. 11 A is the plate, which in its essentials corresponds with the plate A in fig. to, but which in the machine is made to swivel so as to bring each half of the pattern B, B in turn uppermost for ramming in the box parts C, C. The ramming is done by hand, the final squeeze being imparted against the presser D by the action of the hydraulic ram E pushing the plate, mould and box up against D. The plate being then lowered, and turned over, the further descent of the rain withdraws the bottom box from the pattern, which is the stage seen in the illustration
Though casting in iron, steel, the bronzes, aluminium, &c., is End of Article: FOUNDING (from Lat. fundere, to pour) If you wish, you can link directly to this article.
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