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NEWS ma i7 ui I%/O/// I%///////%//y I'ml /i%/, A rrn dN p m, r II 746 carried on by different men in distinct shops, yet the foregoing principles and methods apply to all alike. See also:Work is done in See also:green, i.e. moist See also:sand, in dry sand (the moulds being dried before being used), and in plastic See also:loam (which is subsequently dried). See also:Hand and See also:machine moulding are practised in each, the last-named excepted. The See also:differences in working are those due to the various characteristics of the different metals and See also:alloys, which involve differences in the sand mixtures used, in the dimensions of the pouring channels, of the temperature at which the See also:metal or alloy must be poured, of the fluxing and cleansing of the metal, and other details of a See also:practical See also:character. Hence the practice which is suitable for one See also:department must be modified in others. Many castings in See also:steel would inevitably fracture if poured into moulds prepared for See also:iron, many iron castings would fracture if poured into moulds suitable for See also:brass, and neither brass nor steel would fill a See also:mould having ingates proportioned suitably for iron. A See also:special See also:kind of casting is that into " chill moulds," adopted in a considerable number of iron castings, such as the railway wheels in the See also:United States, See also:ordinary See also:tramway wheels, the rolls of iron and steel See also:rolling See also:mills, the bores of See also:cast See also:wheel hubs, &c. The chill ranges in See also:depth from 4 in. to i in., and is produced by pouring a special mixture of mottled, or strong, iron against a See also:cold iron See also:surface, the parts of the casting which are not required to be chilled being surrounded by an ordinary mould of sand. 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 See also:account of by the See also:pattern-maker and moulder. A pattern and mould are made larger than the See also:size of the casting required by the exact amount that the metal will shrink in cooling from the molten to the cold See also:state. This amount varies from s in. in 15 in., in thin iron castings, to s in. in 12 in. in heavy ones. It ranges from ag in. to $g in. per See also:foot in steel, brass and See also:aluminium. Its variable amount has to be See also:borne in mind in making See also:light and heavy castings, and castings with or without cores, for massive cores retard shrinkage. It is also a fruitful cause of fracture in badly proportioned castings, particularly of those in steel. Brass is less liable to suffer in this respect than iron, and iron much less than steel. (J. G. Additional information and CommentsThere are no comments yet for this article.
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