With the development of natural-gas and crude-oil burners for commercial heating, several good furnaces have been designed in which a large quantity of metal can be melted at one time. Fig. 181 shows a furnace of this character in section. This type has tandem melting chambers with burners at the end, which may be used separately or both together. The waste gases from the bath of liquid metal are used to heat up a fresh charge in the other chamber. The metal is charged and poured from the openings at the top of the furnace. Each chamber may be revolved separately, to empty the furnace when the charge is melted. Fig. 182 shows the general arrangement of the oil feed pump and blower for these melting furnaces. The flame plays directly on to the metal. The oil pressure should remain constant at about 5 pounds per square inch. But the air pressure is regulated to vary the intensity of the heat as desired.
The pouring ladle must be well heated before using. This is done with a special gas burner, or, when crucibles are used, they are often heated by means of a small fire in an ordinary furnace.
Different sizes of furnaces are built to melt from 250 to 2000 pounds of metal at a heat. Twelve or fourteen heats a day can be run. The saving is approximately 50 per cent in time, and is also very considerable in expense, over ordinary crucible furnaces of equal capacity.
When the castings are taken from the sand they should be rapped smartly to free all loose sand, then, if machining is to be done on them, they should be plunged, while hot, into water. This softens the castings. This method is used also to blow out cores from small work.
Since brass does not burn into the sand as much as iron, the small castings in many shops are brushed clean, before being cut from the gates, by means of a circular scratch brush mounted on a spindle similar to a polishing wheel. A sprue-trimmer, shown in Fig. 183, is part of the equipment of a brass foundry. These machines are made to operate by foot as shown, or by power. With them the castings are cut neatly and quickly from the runners.
A good method of cleaning brass and bronze is by pickling. Make a mixture of 2 parts common nitric acid and 1 part sulphuric acid, in a stone jar. Place the piece to be cleaned in a stone dipping basket. Fig. 184, and dip once into the acid, then wash off in clean water, and dry in sawdust.
In many cases, brass chips and filings are turned back to the foundry to be remelted. The smallest portions of steel or iron in these would prevent their being used in this way, as they make extremely hard spots in the castings.
Fig. 185 shows a magnetic separator which effectively removes all steel and iron chips. The brass chips and sweepings from the machine shop are placed in the hopper of this machine. They are caused to be spread out on one side of a slowly revolving brass covered drum. Inside of this brass shell are strong magnets which hold to their surfaces the steel and iron chips, while the brass chips drop off into a tote box. A stiff brush at the back of the cylinder removes the iron chips, and they drop into a separate box.