It is sometimes advisable to use cores even if it is quite possible to construct the pattern so that it would core its own holes. This is the case where it is desired that the faces of the casting and the holes shall be smooth and as true as possible without expensive machine work. The finished faceplate of an engine lathe, illustrated in Fig. 198, is a good example of such work.
It will be readily seen that the pattern for this casting could be put in the sand and withdrawn from the mold, leaving the sand standing where the holes are located. The trouble that arises from this method is due to the fact that, when the metal is poured and allowed to flow about the fragile projections that are left to form the holes, the sand washes away, so that the holes in the casting are irregular and much smaller than those in the pattern. For these reasons the holes should be cored, as the core sand is firm and better able to resist the washing action of the flowing metal.
Fig. 198. Typical Metal Faoeplate.
Where a large flat surface is to be given a finish, it is desirable that the metal should be as clean and free from sand and blowholes as possible. As the iron has a greater specific gravity than the sand of the mold, all particles of sand that may be washed away and all gases generated, rise to the surface of the molten metal. Those imprisoned by the cooling of the iron form the dirt and blowholes that disfigure the completed casting. In a casting such as the faceplate under consideration, it is desirable that the face should be upon the lower side when the metal is poured as it is to be planed smooth and should be clean iron. For the sake of convenience in setting the cores, the prints are put upon the face and make their impress in the sand of the drag.
The construction of the web, rim, and hub is to be very similar to that used in making the disk crank. Fig. 195. If the diameter is more than two feet, the grain of stock used in the web should be parallel to the radius. Each sector as it is fitted should be screwed to the wood faceplate, leaving a space of 1/16 inch between each to allow for the swelling and shrinking of the stock. The ribs are fitted and fastened in place after the lathe work is completed; one extra rib should be furnished the molder for mending up the mold. Leather fillets are to be used in the corners made by the ribs. Iron draw plates are to be fitted in both ends of the hub, at a, Fig. 199. The core box for the cores making the holes in the web is shown in Fig. 200.
Fig. 200. Core Box lor Faseplate Slots.
In molding, a threaded rod is passed through the cope mold, into the draw plate in the cope end of the hub. It is securely fastened above the cope flask, so that the pattern will be drawn from the nowel with the cope. By rapping on this draw iron, the pattern can be rapped so as to obtain a perfect draw from the nowel mold. After the cope mold has been turned over, the pattern is drawn as usual and any mending required to the mold is facilitated by the extra rib furnished.