Wherever possible, core boxes should be made with their widest opening exposed for packing the core, and designed so that the core may rest, while being baked, on the flat surface formed by striking off at this opening.
Core plates will sometimes become warped. When a core would be spoiled by resting it directly upon such a plate, the unevenness is overcome by sifting upon the plate a thin bed of molding sand and seating the core on this.
Fig. 65. Bedding a Crooked Cam.
All cores cannot be made with a flat surface for baking, as illustrated by a port core, the box for which is shown in Pattern-Making, Fig. 251. This core must be rolled over on a bed of sand. Using an oil mixture, ram the core carefully, bedding into it several wax vents. These should start near the end which will touch the main cylinder core and lead out of the end which will enter the chest core. To get this crooked core on a plate for baking, a wooden frame is roughly nailed together, which is large enough to slip over the core box when the loose pieces have been drawn off of the core, as shown in A, Fig. 65.
The space on top of the core is now filled with molding sand, rammed just enough to support the weight of the core. The edges of the frame project above the highest points of the core and form guides for striking off this sand and seating a core plate, as at B, Fig. 65. Box, frame, and plate are now firmly clamped and rolled over, and the frame and box removed, leaving the core well bedded on the plate ready for the oven, as at C.
In manufacturing plants quantities of cores are often required which cannot be baked on a flat plate. To save the time and material necessary to roll each core onto a bed of sand, metal boxes are made, Pattern-Making, Figs. 233 and 234, and the core is baked in one part of the box. Only one casting is required of the larger portion of the box. The smaller part is duplicated for every core required for the day's mold.
Mention has been made of the use of wires for strengthening small cores. In making larger ones, there is a greater weight of sand to cause strain in handling the core,and proportionately greater casting strain.
To resist these stresses a systematic network of rods is bedded in the core while being rammed, as shown in the sectional view, Fig. 66. Heavy bars aabb extend the length of the core to give the main stiffness.
Smaller cross-rods rest on these at the bottom and top, and with the small vertical rods tie the whole core together.
At even distances from each end lifting hooks c are placed.
Cross-rods through the lower eyes of these hooks bring all the strain of the lift on the long heavy core rods. The holes in the top of the cores where the lifting hooks are exposed, are stopped off when the core is in the mold, by moistening the sides of the holes with oil and filling up with green sand.
Cinders are packed in the middles of such cores. They aid in drying the core. They furnish good vent, and they allow the sand to give when the casting shrinks, thus relieving the strain on the metal itself.
For the largest class of cores for green-sand work, cast-iron core arbors are used, of which a very satisfactory type is shown in Fig. 67. This consists of a series of light rings, A, carried on a cast-iron beam, B. The rings are of about 1/2-inch metal cast in open sand and set about 8 inches on centers, and may be wedged to the beam. The beam has a hole at each end for lifting the core.
This skeleton is made up and tried in the box before the work of ramming the core is begun. It is then removed and given a coat of thick clay wash. A layer of core sand is first lightly rammed over the inside of the box, and the core arbor seated into this. The full thickness of core-sand facing is then firmly rammed, and the entire center filled with well-packed cinders. Vents through the facing at both ends provide for the escape of gases from these cinders.
Often, when but one or two large cores are wanted, the cost of making a box is saved by sweeping up the core. This is illustrated in the pipe core shown in Fig. 68.
The pattern-maker gets out 2 core boards and 1 sweep. The boards are made by simply nailing together 3 thicknesses of 7/8-inch stuff, with the grain of the middle piece crossing that of the others to prevent warping. The outer edges of the boards have the exact curve of the outside of the pipe pattern, and at the ends is tacked a half section of the core, shown at aa. One sweep docs for both boards. The curve is cut the exact half section of the core. The edge b equals the thickness of metal in the casting, and the stop c acts as a guide along the outer edge of the board.
Fig. 66. Pipe Core.
In making up this core, a thin layer of core sand is spread on the board and the outline of the core swept. On this the rods with their lifting hooks are bedded, and the vent cinders are carefully laid along the middle. The whole general shape is then rammed up in core sand larger than required, and by using the sweep it is brought to exact size. The core is then slicked off, blackened, and baked while still on the board. When both halves are dried, they are pasted together, the same as with smaller work. To prevent breaking the lower half when turning it over to paste, it is rolled over on a pile of heap sand.
For making stock cores, round or square, several styles of core machines have been put on the market within the last few years, of which the one illustrated in Fig. 69, is a good representative. This is arranged to be driven by hand or by power. The core sand is placed in the hopper, and by means of a horizontal worm at the bottom it is forced through a nozzle under just the right pressure to pack the core firmly. A clean-cut vent hole is left in the middle of each core. As the core is forced from the nozzle it is received on a corrugated sheet-steel plate, which is moved along to the next groove when the core has run to the full length of the plate.
The advantage of the machine is that with it an apprentice boy can produce a true, smooth, perfectly vented core, in very much less time than could possibly be done by hand-ramming.