CHAPTER XVIII.

SHRINKAGE IN CASTINGS.

To allow for the shrinkage in castings, the pattern is not infrequently made in form and size to meet the requirements of any known case. Suppose, for example, that the surface of a large casting is found to be hollow, then that surface upon the pattern will be made sufficiently rounding to allow for the shrinkage, thus giving a casting with the desired flat surface. In large bodies of metal the shrinkage is always sufficient to demand an allowance therefor by the pattern maker; and it always takes place in the largest mass of the metal. The directions of this shrinkage are thus given for particular forms by Mr. ALFRED C. WATKINS:

SOLID CYLINDERS.

In the case of a shaft, or other solid cylinder, it will be noticed that the surface of the casting at the ends will be slightly depressed. This is occasioned by the surface of the cylinder being cooled by the walls of the mold first, and setting, while the central portion yet remains fluid or soft. In a few moments more the central portion cools, and in shrinking draws in the ends of the cylinder, the outer crust acting as a prop or stay to the atoms of metal adjacent to it. If this theory be correct, the depression should take the form of an inverted cone, owing to the gradual checking of the shrinkage as it approaches the outer crust. In practice this will be found the case-the obtuseness of the angle being greater or less, according to the nature of the iron to shrink.

GLOBES.

In the case of solid globular castings, the heart or central point within will usually be found hollow or porous, owing to the following causes: The walls of the mold cooling off the outer surface, causes it to set immediately; the interior, cooling from the exterior inward, endeavors to shrink away from the outer crust, which resists its so doing; hence, the interior is kept to a greater diameter than is natural, and there being but so much metal in the entire mass, the atoms are drawn away from the central point toward all directions, to supply the demand made by the metal in shrinking.

DISKS.

In the case of flat round disks or plates, they will usually be found hollow on the top side, although in some cases the hollow is on the bottom side. This is owing to the following causes: The top and bottom faces, together with the outside edge, become set first through contact with the mold, leaving the center yet soft. When the center shrinks a severe strain is put on the plate by an effort to reduce its diameter, which the outer edge resists. Now, if the cop be thin, the heat will radiate rapidly in that direction, causing the outer or top side to set first; the under side, setting later, will drag the top side over with it, causing it to round up on top and dish in the bottom. Or if the pattern be not perfectly true in every direction, the strains first spoken of will cause any curved portion to become more exaggerated. If the pattern be perfectly true, cop and drag of the same thickness, and both rammed evenly, there is no reason why the plate should not come out perfectly true, the strains being all self-contained in the same plane and balanced. If the plate, however, have an ogee molding projecting downward around the edge, it will likely be depressed on the top surface when cast. This is due to all the surfaces being set alike and at the same instant, excepting the metal within the corners, which, containing the most metal in a mass, will shrink last of all. When this does shrink, its tendency is to pull over the top side of the molding toward the plate, which being soft, although set, will be forced downward at the edges, giving a chance for the strains within the plate, as above described, to aid in the distortion.

ROUND AND SQUARE BARS.

These strains are similar in both, and are already treated of under solid cylinders. There is another feature, not before spoken of, which is rather curious. If two bars of the same dimensions and mixture of iron be heated to the same temperature, the one allowed to cool in the mold, the other plunged while hot into water, the latter will bo found to have shrunk the most. This is due to the particles about the surface having been enabled, by the softness of the interior metal, to get closer to each other than they could have done if the material had cooled slowly.

RECTANGULAR TUBES.

These are usually cast with a core, which has a tendency to retain the shape of the casting; still the flat sides will show a tendency to bulge up slightly at the middle. This is due to much of the same

causes the outer surface is cooled instantly by the wall of the mold, and is set; the inner surface is not cooled quite so rapidly, owing to the core being of harder material, and not so good a conductor of heat. When this does cool it will pull inward the outer skin of the casting, forming a slight curve; each side acting for itself, will produce the same effects.

GUTTER, OR U-SHAPED CASTINGS.

These are usually made thinner at the edges than at the middle, because the pattern has been made with draught. When castings of this shape are taken from the mold, they will be found rounded over in the direction of their length, the legs being on the curved side. This is explained by the mold cooling and setting the legs first; then when the back or round shrinks, it pulls upward the two ends of the casting.

WEDGE-SHAPED CASTINGS.

In parallel castings of any length, having a cross section similar to a wedge-or similar to a "knife" in paper mill work—the thick side will invariably be found concave and the thin edge curved. This is due to the same causes as explained above. The thin edge is set as soon as cast; the thick edge, cooling later, shrinks and draws the ends of the casting upward, and with them the thin edge, which acts as a pillar to resist further shrinkage.

RIBS ON PLATES.

All ribs have a tendency to curve a plate, if they be thicker or of the same thickness as the plate, owing to the fact that whatever shrinkage strain they possess is below the general plane of the shrinkage of the plate itself. If the ribs be thinner than the plate, they will cool first; and by resisting the shrinkage of the bottom of the plate, cause it to curve upwards, or dish on top.

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GENERAL LAWS REGARDING SHRINKAGES.

The most metal in a mass always shrinks last; hence if a casting be composed of irregular thickness, it will be liable to be broken by the forces contained within itself. It is, therefore, especially necessary that columus and castings supporting or resisting great pressures, should be so designed as to prevent this great error. Moldings on columns are often so badly designed with regard to this matter, that the columns are excessively weak where they should be the

strongest. As a rule, moldings should seldom be cast on a column, but rather bolted on. Much of the irregularity of flat castings and those of irregular shapes, could be remedied by a proper attention to cooling the castings while in the mold. To be sure, this is done to a certain extent, though few molders know why they do so. They know that by removing the sand from a particular casting, it will straighten in the shrinking. This is but the result of experience, not of thought, or any attempt to know why they so act. It is useful to know, also, that all shrinkage takes place while the casting is changing from a red to a black heat.

Engine beams, connecting rods

In large cylinders, say 70 in. diameter, 10 ft. stroke, the contraction of diameter is about....

"at top.

"at bottom

" in 16 in.

In Thin brass..

"Thick brass

"Zinc

"Lead

"Copper « Tin

TO CALCULATE STRENGTH OF PIPES OR OTHER THIN

CYLINDERS.

RULE:-Multiply the inside diameter of the pipe or cylinder in inches by the pressure in lbs. per square inch that is to act inside of it, and divide the product by 10,000. To this result add a sufficiency to insure a good casting, and to enable the pipe to stand handling; and this will give the total thickness.

NOTE: The amount to be added varies with the diameter of the pipe.

EXAMPLE:-What must be the thickness of a 25 inch cylinder for a steam engine, so that it may stand 60 lbs. per square inch?

25+60 1500 10000= or of an inch; add to this
inch+32. Add another for reboring.

MOLESWORTH'S RULE for calculating the necessary thickness of metal for cylinders or pipes, is as follows:

RULE:-Multiply the inside diameter of the pipe or cylinder by the pressure in lbs. per square inch it is to bear, and divide the product by 4000. The last product to be increased one half.

It is to be noted, however, that the rules for calculating the necessary thickness of a cylinder to withstand a given pressure, do not give the thickness that the pattern maker requires, because the number of times allowed for reboring the cylinder, its situation as to its being subjected to oxidation, and other similar considerations, have caused the existence in actual practice of greater thicknesses than those given by any of the rules; and in a general way specific kinds of cylinders are made to conform in thickness to that which practico has demonstrated to suit the requirements of the duty; this latter term including more than mere strength.