Shrinkage, or the contraction of castings in cooling, is provided for by adding from one-tenth to one-eighth of an inch to each foot in the dimensions of patterns. This is a simple matter, and is accomplished by employing a shrink rule in laying down pattern-drawings from the figured dimensions of the finished work; such rules are about one-hundredth part longer than the standard scale.
This matter of shrinkage is indeed the only condition in pattern-making which is governed by anything near a constant rule, and even shrinkage requires sometimes to be varied to suit special cases. For small patterns whose dimensions do not exceed one foot in any direction, rapping will generally make up for shrinkage, and no allowance is required in the patterns, but pattern-makers are so partial to the rule of shrinkage, as the only constant one in their work, that they are averse to admitting exceptions, and usually keep to the shrink rule for all pieces, whether large or small.
Inherent or cooling strains in castings is much more intricate than shrinkage: it is, in fact, one of the most uncertain and obscure matters that pattern-makers and moulders have to contend with. Inherent strains may weaken castings, or cause them to break while cooling, or sometimes even after they are finished; and in many kinds of works such strains must be carefully guarded against, both in the preparation of designs and the arrangement of patterns, especially for wheels and pulleys with spokes, and for struts or braces with both ends fixed. The main difficulty resulting from cooling strains, however, is that of castings being warped and sprung; this difficulty is continually present in the foundry and machine-shop, and there is perhaps no problem in the whole range of mechanical manipulation of which there exists more diversity of opinion and practice than of means to prevent the springing of castings. This being the case, an apprentice can hardly hope for much information here. There is no doubt of springing and strains in castings being the result of constant causes that might be fully understood if it were not for the ever-changing conditions which exist in casting, both as to the form of pieces, the temperature and quality of metal, mode of cooling, and so on.
Castings are of course sprung by the action of unequal strains, caused by one part cooling or 'setting' sooner than another. That far all is clear, but the next step takes us into the dark. What are the various conditions which induce irregular cooling, and how is it to be avoided?
Irregularity of cooling may be the result of unequal conducting power in different parts of a mould or cores, or it may be from the varying dimensions of the castings, which contain heat as their thickness, and give it off in the same ratio. As a rule, the drag or bottom side of a casting cools first, especially if a mould rests on the ground, and there is not much sand between the castings and the earth; this is a common cause of unequal cooling, especially in large flat pieces. Air being a bad conductor of heat, and the sand usually thin on the cope or top side, the result is that the top of moulds remain quite hot, while at the bottom the earth, being a good conductor, carries off the heat and cools that side first, so that the iron 'sets' first on the bottom, afterwards cooling and contracting on the top, so that castings are warped and left with inherent strains.
These are but a few of many influences which tend to irregular cooling, and are described with a view of giving a clue from which other causes may be traced out. The want of uniformity in sections which tends to irregular cooling can often be avoided without much loss by a disposition of the metal with reference to cooling strains. This, so far as the extra metal required to give uniformity to or to balance the different sides of a casting, is a waste which engineers are sometimes loth to consent to, and often neglect in designs for moulded parts; yet, as before said, the difficulty of irregular cooling can in a great degree be counteracted by a proper distribution of the metal, without wasting, if the matter is properly understood. No one is prepared to make designs for castings who has not studied the subject of cooling strains as thoroughly as possible, from practical examples as well as by theoretical deductions.
Draught, or the taper required to allow patterns to be drawn readily, is another of those indefinite conditions in pattern-making that must be constantly decided by judgment and experience. It is not uncommon to find rules for the draught of patterns laid down in books, but it would be difficult to find such rules applied. The draught may be one-sixteenth of an inch to each foot of depth, or it may be one inch to a foot of depth, or there may be no draught whatever. Any rule, considered aside from specified conditions, will only confuse a learner. The only plan to understand the proper amount of draught for patterns is to study the matter in connection with patterns and foundry operations.
Patterns that are deep, and for castings that require to be parallel or square when finished, are made with the least possible amount of draught. If a pattern is a plain form, that affords facilities for lifting or drawing, it may be drawn without taper if its sides are smooth and well finished. Pieces that are shallow and moulded often should, as a matter of convenience, have as much taper as possible; and as the quantity of draught can be as the depth of a pattern, we frequently see them made with a taper that exceeds one inch to the foot of depth.
Moulders generally rap patterns as much as they will stand, often more than they will stand; and in providing for draught it is necessary to take these customs into account. There is no use in making provision to save rapping unless the rapping is to be omitted.
Rapping plates, draw-irons, and other details of pattern-making are soon understood by observation. Perhaps the most useful suggestion which can be given in reference to draw-irons is to say they should be set on the under or bottom side of patterns, instead of on the top, where they are generally placed. A draw-plate set in this way, with a hole bored through the pattern so as to insert draw-irons from the top, cannot pull off, which it is apt to do if set on the top side. Every pattern no matter how small, should be ironed, unless it is some trifling piece, with dowel-pins, draw and rapping plates. If a system of draw-irons is not rigidly carried out, moulders will not trouble themselves to take care of patterns.