Page:The American Cyclopædia (1879) Volume IV.djvu/88

80 CASTING furnish sufficient metal for the casting of a cylinder of 24 tons, two large cupola furnaces are required, capable of melting 10 or 12 tons of iron per hour. A reservoir which will con- FIG. 17. Packing the Mould of a Steam Cylinder. tain 8 tons of the molten metal is erected at the side of the mould, at such an elevation that the metal will flow with a moderate current into the channels on the top ; or two such res- ervoirs may be placed, one on either side, the rest of the metal being poured from kettles suspended by cranes. The reservoirs are tap- ped from a hole in the lower part, in the same manner as the cupola is tapped, but the kettles are turned by an apparatus of wheel work adjusted to the bail. Each kettle may contain from 4 to 6 tons of molten iron, and the ap- paratus for pouring them must be of the most substantial character; for if any part of it should break, severe accidents would be almost sure to follow. The melted metal in the reser- voirs and kettles is kept covered with lumps of charcoal, which are raked off when the pouring is made. In the casting of cylin- ders, the shrinkage of the iron in cooling must always be particularly taken into considera- tion. This is quite uniform, and is one inch in 8 ft., or -fa linear measure. The circum- ference of a cylinder, therefore, having a di- ameter of 8 ft., will shrink on cooling 3-14159 inches ; and if it were cast over a perfectly in- compressible core, it would necessarily be rup- tured unless the core were removed before the shrinking began. Mr. Robert Cartwright has patented a process which has been used in casting the cylinders for the pneumatic piles for the great bridge over the Missouri river at Leavenworth, Kansas, with satisfactory results. It consists in making the core in sections and joining it together in such a way that it may be taken to pieces and removed immediately after the cast is made. It is designed especial- ly for cylinders open at both ends, but may be varied so as to be used when they are cast with .the bottom upward. Brass and bronze found- ing is very similar in its details to that of iron. The moulds for casting these alloys are made of the same material as for iron, although for the casting of fine articles of bronze a finer and more compact loam is used. The melting for large castings is usually done in a reverberatory furnace, charcoal being the best fuel. Cupolas are sometimes used, but at a great waste of the more oxidizable alloy. When the quantity is not large, black-lead crucibles are used, which are heated in a furnace placed beneath a plat- form raised one or two feet above the floor, for convenience of handling. The construction of a furnace for melting brass and bronze is shown in fig. 18, the first flue being repre- sented in section. The melting point of brass containing 33 per cent, of zinc is between 1,800 and 1,900 F. That which contains more zinc melts at a lower, and that which contains less at a higher temperature. The copper is usually melted first and the zinc added. Bronze con- taining about 10 per cent, of tin requires heat- ing some 200 higher; but bell metal, contain- ing 22 per cent, of tin, melts at about the same temperature as ordinary brass. Anthra- cite coal is used for heating the crucibles, and Fig. 18. Furnace for Melting Brass. the surface of the alloy is covered with char- coal, which of course is consumed and adds to the heat, at the same time that it prevents oxi- dation. The pouring is performed in the same manner as for iron. The screw propellers for first-class steamships are made of bronze to enable them to resist the action of salt water. The moulds are of loam, and for small wheels are made upon patterns, but for large ones are swept and modelled with tools. Some- times the hub of the wheel is swept with a templet, while the wings are separately mould- ed on a pattern. This is the most convenient, and perhaps the most perfect method, as the precise curve can more readily be given to the surface of the wings. The alloy for propeller wheels contains 10 per cent, of tin. The cast- ing of bells is usually performed in the following manner: The mould of a small bell is made with a pattern, the process being very simple