When a mold is poured, the intense heat of the iron burns out those properties in the sand which give it its bond, making it necessary that a certain proportion of new sand shall be mixed with the heap sand and used as facing, as has been explained in earlier paragraphs.
The facing sand should be mixed daily for the molders by one or more of the laborers, at a place convenient to the storage sheds and molding floors. A hard smooth floor of clay or of iron plates is a great advantage.
Fig. 132. Bed Gage.
The proportions of the different sands are measured by the shovelful, bucketful, or barrowful, and the sands are spread over each other in flat layers, sufficient water being sprinkled on to temper the pile. The sand then is cut through once with the shovel, is put through a No. 2 sieve, all lumps being broken up, and the refuse thrown out is put through a No. 4 sieve, and finally is thrown in a pile ready for use.
When this work is done by hand, the ordinary screen sieve commonly employed by masons is used for the riddling, and a round foundry riddle for the final sifting. To reduce the labor of this, the riddle is slid back and forth on a pair of parallel bars supported conveniently above the storage pile.
The two classes of labor-saving machines used in mixing facings, core sand, etc., are those which mix by riddling; and those which mix by a combined breaking and stirring action. There are many varieties on the market, the illustrations shown being typical.
The rotary sieve shown in Fig. 133 is made with wire screen on the revolving sides, and is driven by belt or connected motor. Sand shoveled into the central opening is sifted in a pile on the floor, or directly into a barrow. The rubber hammer on top automatically raps each face of the sieve as it revolves, knocking the meshes free of sand.
Fig. 134 shows one of the latest labor savers in this line. Here a foundry riddle is supported in a metal ring attached to the piston of the machine. It is made to vibrate rapidly by means of cornpressed air or steam. These shakers are made with portable tripod, as shown; they are also made stationary or are fastened on a post by means of a swivel joint, to be swung over a wheelbarrow or over a molding machine, and out of the way again when not in use.
Fig. 134. Sand Shaker.
Fig. 135. Pulley-Driven Centrifugal Mixer.
Fig. 135 shows a centrifugal mixer. Inside of the umbrella casing, a horizontal plate about 12 inches in diameter and carrying a number of vertical steel pins about 6 inches long is fastened to the top of a short upright shaft driven by a belt running inside of the casing shown at the base of the machine. The machine runs about 1,500 revolutions per minute; and sand shoveled into the hopper is very evenly broken up by the pins and thrown against the steel hood, breaking and shattering any lumps of clay or loam and making a very uniform mixture. The hopper may easily be removed to clean the plate. The machine is used for the final mixing.
Fig. 136 shows a foundry grinder or facing mill. It is the type of mill used for mixing loam. Either the pan or the rollers are attached to the driving shaft and made to revolve, crushing and mixing whatever is shoveled into the pan. Frequently, a stout blade, something like a plowshare, is fixed between the rollers, and prevents the mixture caking to the bottom of the pan. The faces of the rolls are of very hard or of chilled cast iron to withstand wear. When the loam mixture is sufficiently ground, it must be shoveled from the pan and delivered to the molders or stored temporarily. It will set if stored too long. This is the type of mill used in the steel foundries, for grinding the facing materials. The various sands are dumped into the pan at one side, and, when ground sufficiently, are shoveled directly from the pan into a centrifugal mixer. This prepares them for use.
After a casting has solidified in the mold, the flash should be removed, leaving the casting in the sand. For light bench work and snap-flask work, the mold is lifted bodily and the sand dumped on the pile; the bottom boards are piled in one place, and the cases are piled in another ready for the next day's work. As the molds are dumped, the castings are removed from the sand and piled at the edge of the gangway. When all eastings have been removed from the sand, the gates are broken and thrown in a \ pile by themselves. When cold enough to handle, the castings are removed to the , cleaning room, and the gates and sprues are sent to the scrap pile. With heavier floor work, the clamps are removed as soon as the casting has set; the flash is rapped with a sledge hammer and is stripped off the mold, leaving the castings to cool gradually in the sand. Sometimes a sharp blow is given on top of the runner while it is still red; this breaks it off before the flask is shaken out. At a red heat, cast iron is very weak and can easily be broken.
The most effective way to clean small castings is in a rattling barrel. Fig. 137 shows a modern set of dustless barrels. The shell of the barrel is 5/8-inch boiler plate riveted to cast-iron heads, with a door arranged to be entirely removed for packing and (lumping. The bearings are hollow, and from one end the dust is drawn off through a galvanized-iron pipe. This pipe connects with an air-tight wooden chamber, as shown in Fig. 138, varying in size with the number of barrels connected with it. In this chamber hang a number of cloth-covered screens. An exhaust fan is connected to this chamber at the opposite end from the inlet pipe. When the fan is in operation, a strong current of air is drawn through the barrels and through the chamber. The dust, entering the chamber, settles on the screens, so that but little dust escapes to the outside air. When necessary, the exhaust is stopped, and, by means of a crank on the outside of the dust chamber, the screens are shaken and the dust drops off, when it can be removed through a trap into an ash can or wheelbarrow. The driving shaft carrying the pinion revolves all the time, and any barrel may be thrown over into gear or drawn out of gear by the operation of a hand lever. The barrels should run about 25 revolutions per minute. Each barrel should be packed as full as possible with several shovelfuls of gates, shot iron, or hardened stars thrown in with the castings. The cleaning is accomplished in from 20 minutes to half an hour by the scouring action of castings, scrap, etc., rubbing against each other. Castings up to 50 or 100 pounds can be rattled, but only those of a similar character as to design or weight should be packed in together, otherwise the lighter castings will be broken by the heavier. When removed from the barrel, the work should show a smooth clean surface of an even gray color.
From the rattlers, castings go to the grinding room, where projecting gates or other slight roughness is removed on the emery wheel.
Heavy castings are cleaned by hand, by pickling, or by sandblasting.
When cleaning by hand, the worst of the sand is rapped off by light hammering, the remainder scraped off with old files and with steel-wire brushes such as that shown in Fig. 139. Some shops rub off finally with broken pieces of coarse emery wheel. Risers and fins are removed with cold chisels. The pneumatic chisel, shown in Fig. 140, is used as a timesaver. Where work is light enough to handle, small fins are removed by emery wheels; medium coarse wheels will cut faster on cast iron than fine ones, and will hold their shape better.
It is when castings must be cleaned by hand, that the value of a good facing dust shows itself. With the proper facing, the sand parts readily from the casting leaving a fine-looking smooth surface. With poor facings, on the other hand, the iron burns into the sand, making it hard to clean, and leaves a rough surface on the work.
Pickling is a method of cleaning resorted to where there is much machining to be done on a casting. The work is placed in a pile on a suitable platform, and dilute sulphuric acid is thrown over it during one day, frequently enough to keep it well wet. The platform should be arranged to drain the acid back into the vat. Acid is diluted from 1:8 to 1:10. After about 12 hours' bath with acid, the castings are washed clean with hot water. The acid acts on the hard skin of oxide of iron which forms when the iron strikes the damp sand, and it eats through this skin to the iron itself. The washing water should be hot enough to warm the castings sufficiently for them to dry rapidly without rusting. The acid must be thoroughly washed off, or it will continue to eat into the iron and cause a white powder - sulphate of iron - to form on the surface.
An excellent arrangement for a pickling department is to have the trough arranged on skids which allow it to be rocked endwise.
This drains into the pickling vat when acid is being thrown on, and into the gutter when the castings are being washed. Sheet lead is the best protective covering for small pickling troughs, but it is expensive and not durable enough to stand for heavy work.
For castings of such shape and size that they cannot well be rattled, but are too small to be cleaned by hand, the sand blast has been used to advantage in many shops.
Fig. 141 gives an idea of the arrangement of the cleaning stall. Castings are placed on the wooden grating. By means of compressed air a sharp silica sand is forced through a strong rubber hose and is directed against the castings by a hardened-steel nozzle. The operator wears a helmet supplied with fresh air by an air hose, to protect his eyes and lungs from the clouds of fine dust. An exhaust hood is arranged also to take off as much of the dust as possible.
The manual labor of this method is practically reduced to nothing, aside from handling the castings. The system, however, requires the installation of a rather considerable equipment, which has debarred its use in many foundries.
In removing cores, the bars become very much bent. In such shape they were formerly scrapped, or refitted to suit new cores with a hammer and block of iron. Fig. 142 shows a very practical power machine which delivers the bars perfectly straight. The machine consists of a pair of rolls, with different sizes of grooves turned in them, which pull a rod through a flaring mouthpiece and deliver it through a corresponding eye on the opposite side. The machines are made in different sizes, and take rods from 1/4-inch to 3/4-inch diameter.