As an example of a complex piece of loam work, let us consider the molding of a modern marine-engine cylinder, as shown in section, Fig. 123. The example given is that of a double-ported low-pressure cylinder of a triple-expansion type. In this case a full wooden pattern should be built, with core boxes for the various dry-sand cores that enter into the construction of the mold.
The limits of this article prevent a detailed discussion of this subject; we will, therefore, confine ourselves mainly with an explanation of the drawing, Fig. 123. The heavy building plate has a spindle opening somewhat to one side of its middle to be under the center of the cylinder. Upon this building plate the foundation of the mold is swept, carrying the seat for the cope ring, the bottom face of the flange, and the seat for the main cylinder core. The cope ring 1 is made wide enough on one side to carry that part of the mold forming the steam chest. The main cylinder core 2, the construction of which has already been explained, is next swept-up and lifted away, finished, and baked.
Now the cope ring is seated, and the mold built and struck off for the bottom of the steam chest on a level with the bottom face of flange. Then the pattern may be set. Its position is accurately determined by the main cylinder print and the smaller prints of the steam chest which are bedded into the loam in accordance with measurements along a radial line marked off on the loam surface. With the pattern well oiled, the cope is built to the height of the upper flange of the cylinder; the entire back of the steam-chest core print being left open. The top of the steam chest is lifted off with the drawback 3, which joints at the middle of the upper steam nozzle, and carries that part of the mold to the level of the main cope joint. The two steam nozzles and the exhaust nozzle may be made with separate cores as explained in D, Fig. 122. By using the drawback, the entire top of the chest core print is left open for convenience in setting the chest and port cores.
The top of the cylinder is jacketed, and through it pass the stuffing-box and manhole openings. The flanges of these two openings connect and in the pattern are left loose. The whole top surface is so irregular that it requires three levels of sticker plates to mold it, aside from two small cover plates over flanges.
To the main cover 4 4 4 4 with its various lengths of fingers, is bolted a crab 5 5 5 to carry the loam below the flanges of the stuffing box and manhole; and below this again are hung the dry-sand cores, 8 8 8, forming the jacketed part of the cylinder head. On top of the main cover is fastened a separate plate, 6, to shape the top of the upper steam inlet. And at 7 a plate with wrought-iron bars cast along its edge carries the loam back of the steam-chest flange. The small cover plates, 9 and 10, allow the flanges to be drawn for the parts which they mold.
The pattern is made in many parts so as to properly draw from the mold. When this has been done, all mold surfaces are carefully blackened and slicked before baking.
While the mold proper is being built, the dry-sand cores should be made up by the core makers, with the necessary rods, hangers, vent cinders, etc., as described under Core Making.
The manhole core, 11, is made with a stop-off piece in the box to give the proper angle at the bottom of the core. It is hung to the cover and clears the main core by 1/3 inch. The stuffing-box core rests in a print in the main cylinder core, and is held by a taper print in the cover plate 10.
The jacket cores are hung as shown. The openings made in the loam above the crab, to allow the hook bolts to be drawn up tight, are stopped off with green sand as previously described. The inlet cores 1212, the exhaust core, 13, and the lightening cores, 14 14 14, are all bolted directly through the steam-chest core, 15, to horizontal bars which are long enough to bear against the sides of the mold at the back. The upper inlet core, 12, is kept from lifting under the pouring strain by being bolted to the body of the main cylinder core. Stud chaplets are also set between the inlet and exhaust cores to ensure correct thickness of metal at these points.
The vent is taken off from the main cylinder core through the stuffing-box core at the top. Sometimes a small ladle-ful of metal is poured through this opening, when the piece is being poured, to ensure lighting these gases. The vent for the series of port cores is taken off by ramming a cinder bed up the entire back of the steam-chest core, allowing the gases to escape at the top. For safety, also, vents are taken from the bottom of the port and chest cores by the usual pipe vent.
The provision for pouring this mold requires especial attention. Notice the construction of the main basin, 16. The long runner 17, leading to the bottom gate, is left open on one side when the mold is built, so that it may be easily finished and kept free from dirt. Its open side is closed by cover cores when the mold is rammed up.
Ten or twelve small gates like 18 are connected with the pouring basin, by semicircular channels, but are so placed that no metal shall fall on a core. With the basin arranged as shown, the bottom part of the mold is first flooded with iron. When this has been done, the metal is poured in faster, so that hot iron is well distributed around the shell of the casting through the small top gates. Should the mold be poured at first from these top gates, the fall of the iron through the full height of the cylinder to the lower flange might result in cutting the loam on that surface.
Molds of this size are usually rammed in a pit so as to bring the pouring basin conveniently near the floor. The portion above the floor level is, of course, rammed inside a casing, as described in the previous example.
To guard against uneven cooling strains in this intricate casting, the clamping pressure on the mold is relieved when the metal has solidified, but the sand is not removed from around the brickwork for several days. This allows very gradual even cooling.
It will be noticed that the piston does not work directly upon the inner walls of this type of cylinder. A separate hollow shell or lining is cast of strong tough iron. This has outside annular ribs at top and bottom and middle, which are turned to fit correspondingly projecting ribs seen on the inside of the casting just under consideration. An air space is thus left between the lining and main casting which forms a jacket around the bore of the cylinder.