Metal casting

“ No doubt many of you have been troubled with the twisting of some special light casting, and will, perhaps, spend hours in the risky operation of bending an iron pattern so as to get a straight casting. A ladleful of lead and tin, melted in a small gas-furnace, will, in a few minutes, give you a pattern which you can bend and adjust to any required shape. It enables you to make trials to any extent, and get castings with the utmost precision. There is also this advantage, that a soft metal pattern can be cut about and experimented with in a way which no other material admits of. ”

“ This is the tendency to “shrink” during the solidification and cooling of the metal of the casting. On account of the freezing of the outer portions of the casting before the metal of the inside, there must result certain hollow places or cavities after the inside metal has cooled unless some channel is kept open through which fluid metal can pass inside to keep cavities from forming. We will not here go into the matter of shrinkage with its great worry to the molder nor the ingenuity and strategy through which he produces castings without shrinkage cavities. ”

“ Some have projections affixed to them; these make prints in the mould to receive cores, which form holes in the casting. Those patterns which enter deeply into the moulding sand are made tapering, to draw out easily. In all cases they must be made sufficiently large to allow for shrinkage in the metal. Ordinary iron castings shrink about one-eighth of an inch to the foot; brass about half as much again. Pattern-makers use a ‘contraction-rule’ to work by ”

“ Metal itself implied metal-working; and metal-working brought about, not only the arts of smelting and casting, but also endless incidental arts of design and decoration. The bronze hatchets, for example, to take our typical implement, begin by being mere copies of the stone originals; but, as time goes on, they acquire rapidly innumerable improvements. First, metal is economized in the upper part which fits into the handle, while the lower or cutting edge is widened out sideways, so as to form an elegant and gracefully curved outline for the whole implement. ”

“ The foregoing refers more particularly to type cast by the regular foundries. The metal used in automatic casting machines, like the Monotype, contains a larger proportion of lead and less antimony; while the metal used for casting line-slugs in the Linotype machine is composed of still larger proportion of lead and very little of the other two metals. Stereotype metal and metals used for leads, slugs, furniture, etc., are largely lead. ”

“ In old times, before the invention and introduction of planing and shaping machines to work metals, the first and most important lesson taught to the machinist’s apprentice was the use of the hand-hammer and cold-chisel. When an outer shell was to be removed from a metal casting and its surface left in condition to be finished by file or scraper, the smoothness and regularity of that surface was essential, not only for economy in working, but accuracy of the file finish. ”

“ This casting, on being removed from the sulphate bath, is ready for attaching to the wire gauze of the dandy-roll. 2. The design is first engraved on a steel die, the parts required to give the greatest effect being cut deepest; the die, after being hardened, is forced by a steam hammer into some yielding material, such as copper, and all of this metal which remains above the plain surface of the steel is subsequently removed by suitable means; the portion representing the design being left untouched would then be attached to the wire-gauze as before. ”

“ The misdirected skill of some modern iron-founders has caused the name of cast iron to be regarded as the very negative of art, and has even thrown suspicion on the process of casting itself as one of questionable honesty. Nevertheless, as a craft capable of giving final shape to metal, it has manifestly an artistic aspect, and, in fact, bronze statuary, a fine art pure and simple, is reproduced from the clay model merely by moulding and casting. We must therefore look for the artistic conditions in the preparation of the model or pattern, the impress of which in sand or loam forms the mould ”

“ From a metallurgical standpoint a die-casting is superior to a sand-casting on account of its density, strength and freedom from blow-holes. Also, when the hot metal comes in contact with the cool dies, it forms a “skin” similar to the scale on an iron sand-casting. As the die-casting requires no machining after leaving the dies, this skin increases the wearing qualities of the casting. ”

“ With small articles in brass, metal patterns are used. For gold articles, such as wedding rings, an iron frame is used, filled with the finest red sand. For small gold work, the cuttle fish bone is used as a mould, the pattern being pressed in the same manner as in sand. [46] A special feature in the process of casting in our district is the production of “chilled” castings for rolls. The mould for the barrel part of the roll is a heavy cylindrical casting of iron truly bored out to a size sufficient to allow of the roll being turned to the required size. ”

“ The matrix is rapidly dried on heating surfaces, and then accurately adjusted in a casting machine curved to the exact circumference of the main drum of the printing press, and fitted with a terra-cotta top to secure a casting of uniform thickness. On pouring stereotype metal into this mould, a curved plate was obtained, which, after undergoing a certain amount of trimming at two machines, could be taken to press and set to work within twenty-five minutes from the time at which the process began. ”

“ Is it not probable that in the process of casting, little drops of molten metal are sometimes splashed out of the stream, which immediately solidify and become coated with a skin of oxide, then falling back into the stream of rapidly cooling metal, they do not remelt, neither do they weld or amalgamate with the mass, owing to this protective coating, thus forming dangerous flaws in the casting? ”

“ The drying press is kept at a high temperature, usually by steam heat.
The matrix thus dried out to a thick, flexible cardboard is then ready for the casting of the stereotype, which is done by pouring molten stereotype metal against the face of the matrix placed in a casting-box designed for this purpose. A successive number of stereotypes can be cast for the same mat before it is injured by the hot metal. For job-work stereotyping the casting-box is flat, and the molten metal is either poured by hand or automatically pumped in the casting-box. ”

“ The possibilities of die-casting are numerous. By this method of manufacturing it is possible and practical to cast pieces that could not possibly be machined. It is an every-day occurrence to make castings with inserted parts of another metal, as, for instance, a zinc wheel with a steel hub. It is also possible to make babbitt bearings that are harder and better than can be made in any other way. Often there are two or more parts of a device that have formerly been made separately, machined and assembled, that can be die-cast as one piece. In such cases the saving in production is very great. ”

“ Unlike the modern Bessemer, open-hearth, and other steel products which are reworked, i.e., rolled, forged, etc., cast iron is a comparatively old alloy dating back over several centuries. It cannot be rolled, forged, or otherwise reshaped, so its final form must be given to it at once by pouring or “casting” the molten metal into a mold. Its castings serve exceedingly well in the hundreds of places for which they are adapted. They are comparatively cheap, can be readily duplicated in small or large quantities, and those from the softer grades of cast iron may be machined easily. ”

“ The cover of the casting-pan, which has a hole at each corner for the passage of the metal, is then clamped to it, and lifted by a movable crane and gently lowered into the metal-pot,—containing, it may be, a thousand pounds of liquid metal,—till the metal begins to flow slowly in at the corners. ”

“ The metal should be kept just above the melting point and at a uniform temperature. If the metal is worked too cold, the result will manifest itself in castings that are full of seams and creases, and it will be difficult to “fill” the thin places in the dies. ”

“ The mold is as shown, and the upper side of the metal is at all times exposed to the air. This makes it necessary to have all core prints on the under side, as this side will be the one in view when the casting is finished, and the upper side, as the casting lies in the mold, will be the inside or unexposed side. In case of curved work, reinforcing strips of sheet brass should be placed in the mold and imbedded in the casting, as shown in the sketch by the dotted lines.
A little practice will enable anyone to produce very neat cored castings. ”

“ According to Ansley (1968) , small castings using nonferrous metals (aluminum, magnesium, or copper alloys) may be produced using plaster molds with a surface finish as fine as 2-3 μm and an accuracy of +/-0.1 mm over small dimensions and +/-0.02 mm/cm across larger surfaces (a drift of 2 mm over a 1 m2 area) .
Traditionally, the plaster casting technique requires a split metal pattern in the shape of the object to be cast. This pattern is used to make a hollow mold into which molten metal is poured, eventually solidifying to make the desired part. ”

“ I soon had the metal mould ready for casting. It consisted of a base plate of cast iron, on the surface of which I placed a ring or hoop of iron turned to fully the diameter of the intended speculum, so as to anticipate the contraction of the alloy. The result of the very first trial of this simple metal mould was most satisfactory. It yielded me a very perfect casting: and it passed successively through the ordeal of the first rough grinding, and eventually through the processes of polishing, until in the end it exhibited a brilliancy that far exceeded that of the sand mould castings. ”

“ The purpose of chill-casting is to produce a surface hardness in the metal.
The shrinkage of metal is a fact which has to be taken account of by the pattern-maker and moulder. A pattern and mould are made larger than the size of the casting required by the exact amount that the metal will shrink in cooling from the molten to the cold state. ”

“ When, instead of the example now selected, the core of the piece to be cast must go beyond the mould of the external surface, as is the case with a pipe open at each end, the thing is more simple, because we may easily adjust and fix the core by its two ends.
In casting a retort, the metal is poured into the mould set upright. It is important to maintain this position in the two last examples of casting; for all the foreign matters which may soil the metal during its flow, as the sand, the charcoal, gases, scoriæ, being less dense than it, rise constantly to the surface. ”

“ But there is really no need for more than one casting process. In the twentieth century machine the matrices will be replaced by permanent [Pg 273] type from which, when ranged in the line, an impression will be made by hard pressure on a small bar of soft metal or plastic material. All the impressed bars having been set together in a casting box having the necessary curvature, the final stereo plate for printing from will be taken at once by pouring melted metal on the combined bars. ”

“ It is not, however, to be supposed that the possession of such a rule renders it possible for the pattern-maker to discard all further considerations upon the contraction of the casting; because there are others continually stepping in. Such, for example, is the fact that the contraction will not be equal all over, but will be the greatest in those parts where the casting contains the greatest body of metal. ”

“ The metal of which type is made consists of lead and antimony; the antimony hardens it and makes it take a sharper impression. The letters are first cut in steel, and from these “dies” the moulds are made in brass, by stamping, and in these the types are cast.
Stereotype consists of plates of metal taken, by casting, from a forme of type set up for the purpose: an impression was formerly carried on by plaster-of-Paris moulds, but lately what is termed the papier-maché process is adopted. ”

“ It does not pay to cast rough heavy work that can be made just as efficiently by sand casting. Generally speaking, the greatest saving can be effected by die-casting small pieces which have previously required a large amount of machining to produce. On large plain work the amount of metal required for the casting makes the cost excessive on account of the difference in cost of the metals. If, however, the large work must be finely finished by polishing, etc., it is oftentimes found of advantage to die-cast. Corners, especially those joining thick and thin sections, as at B, Fig. ”

“ These are subsequently replaced piece by piece in the encircling frame, and a core made within it, leaving a space of 1/4 in. or thereabouts into which the metal is poured. The advantage of this process is that the artist’s model is not destroyed as in the cire-perdue, and if a “waster” results, a second mould can be taken. A large statue occupies from one to three months in the moulding.
The extreme tenuity of objects which are hammered, drawn or rolled cannot for obvious reasons be attained by casting. ”