Fig. 2780.
“For a pattern to be moulded horizontally, [Fig. 2780] shows the best style in which it can be made. Its diameters are turned parallel; the required draft is given by making the rim of the flange a little thinner than at the hub, and by making the end faces of the hub and the core prints slightly rounding. If the hub is very small, as, say, a half-inch or less, and the flange does not much exceed it, the pattern may be made solid, as shown in [Fig. 2778]; but if the hub be small and the flange large, it should be made as shown in [Fig. 2776].
“To construct the pattern shown in [Fig. 2773], we proceed as follows: From a piece of plank we saw off a piece of wood a little larger and thicker than the required flange, and turn it up between the lathe centres, using a pattern makers’ contraction rule, which has its measurements larger than the actual standard ones in the proportion of one-eighth inch per foot: so that a foot on the contraction rule is 121⁄8 standard inches, and an inch is 11⁄96 standard inches. The reason for this is, that when the metal is poured into the mould, it is expanded by heat; and as it cools it contracts, and a casting is, therefore, when cold, always smaller than the size of the mould in which it was made. Brass castings are generally said to be smaller than the patterns in the proportion of one-eighth inch per foot, and cast-iron castings one-tenth inch per foot; and so, to avoid frequent calculations and possible errors, the contraction rule has the necessary allowance in every division of the foot and of the inch. It is not, however, to be supposed that the possession of such a rule renders it possible for the pattern-maker to discard all further considerations upon the contraction of the casting; because there are others continually stepping in. Such, for example, is the fact that the contraction will not be equal all over, but will be the greatest in those parts where the casting contains the greatest body of metal.
“In the smaller sizes of patterns, such as those of 6 inches and less in diameter, there is another and a more important matter requiring attention, which is, that after a moulder has imbedded the pattern in the sand, and has rammed the sand closely around it, it is held firmly by the sand and must be loosened before it can be extracted from the mould. To loosen it, the moulder drives into the exposed surface of the pattern a pointed piece of steel wire, which he then strikes on all sides, causing the pattern to compress the sand away from the sides of the pattern in all directions; and as a result, the mould is larger than the pattern. In many kinds of work, this fact may be and is disregarded, but where accuracy is concerned, it is of great importance, especially in the matter of our example (brasses for journals), for they can be chipped and filed to fit their places much more rapidly than they can be planed, and it is necessary to have the castings as nearly of the correct conformation as possible. In cases where it is necessary to have the castings of the correct size without any work done to them, the shake of the pattern in the sand is of the utmost importance. If it is required to cast a piece of iron 3 inches long and 1 inch square, supposing the pattern were made to correct measure by the contraction rule, the moulder, by rapping the pattern (as the loosening it in the mould is termed) would, by increasing the size of the mould above that of the pattern, cause the casting to be larger than the pattern; that is to say, it would be longer and broader, and therefore, in those two directions, considerably above the proper size, since even the pattern was too large to the amount allowed for contraction. The depth, however, would be of correct size, because the loosening process or rapping does not drive the pattern any deeper in the mould. It follows that, to obtain a casting of as nearly the correct size as possible, the pattern must be made less in width and in length than the proper size, to the amount of the rapping; and to insure that the moulder shall always put the pattern in the sand with the same side uppermost, the word “top” should be painted on the face intended to lie uppermost in the mould. The amount to be allowed for the rapping depends upon the size of the pattern, and somewhat upon the moulder, since some moulders rap the patterns more than others; hence, where a great number of castings of accurate size are required, it is best to have two or three castings made, and alter the pattern as the average casting indicates. For castings of about 1 inch in size, the patterns may be made 1⁄32 inch too narrow and the same amount too short; but for sizes above 6 inches, allowance for rapping may be disregarded.
“In patterns for small cast gears, the rapping is of the utmost consequence. Suppose, for instance, we have six rollers of 2 inches diameter requiring to be connected together by pinions, and to have contact one with the other all along the rollers; if we disregard the allowance for rapping, the pinions will be too thick, and we shall require to file them down, entailing a great deal of labor and time, besides the rapid destruction of files.
Fig. 2781.