This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The subjoined remarks on the conditions to be considered in pattern-making are condensed from Richards' valuable manual on 'Workshop Manipulation,' which is more than once referred to as an indispensable companion for the intelligent worker in metals. He enumerates the following points: -
(1) Durability, choice of plan and cost. Consider the amount of use that the patterns are likely to serve, whether they are for standard or special machines, and the quality of the castings so far as affected by the patterns. A first-class pattern, framed to withstand moisture and rapping, may cost twice as much as another that has the same outline, yet the cheaper pattern may answer almost as well to form a few moulds.
(2) Manner of moulding, and expense, so far as determined by the patterns. These last may be parted so as to be "rammed up" on fallow boards or a level floor, or the patterns may be solid, and have to be bedded, as it is termed; pieces on the top may be made loose, or fastened on so as to "cope off;" patterns may be well finished so as to draw clean, or rough so that a mould may require a great deal of time to dress up after a pattern is removed.
(3) The soundness of such parts as are to be planed, bored, and turned in finishing. Determined mainly by how the patterns are arranged, by which is the top and which the bottom or drag side, the manner of drawing, and provisions for avoiding dirt and slag.
(4) Cores, where used, how vented, how supported in the mould, and how made. Cores of irregular form are often more expensive than external moulds, including the patterns; the expense of patterns is often greatly reduced, but is sometimes increased, by the use of cores, which may be employed to cheapen patterns, add to their durability, or ensure sound castings.
(5) Shrinkage. This is the allowance that has to be made for the contraction of castings in cooling, i. e. the difference between the sizes of the pattern and the casting - a simple matter apparently, which may be provided for in allowing a certain amount of shrinkage in all directions; but when the inequalities of shrinkage both as to time and degree are taken into account, the allowance to be made becomes a problem of no little complication.
(6) Inherent, or cooling strains. They may either spring and warp castings, or weaken them by maintained tension in certain parts - a condition that often requires a disposition of the metal quite different from what working strains demand.
For most ordinary purposes, patterns are made of wood; but in very heavy parts of machinery, such as pulleys and gear wheels, iron patterns are preferable. As there must be always a proportion of loose sand and "scruff" in a casting, it is important to arrange the pattern so that this part shall come in the least disadvantageous position. Thus the top of a mould or "cope" contains the dirt, while the bottom or "drag side" is generally clean and sound: the rule is to arrange patterns so that the surfaces to be finished will come on the drag side. Expedients to avoid dirt in such castings as are to be finished all over, or on 2 sides, are various. Careful moulding and washing, to remove loose sand, is the first requisite; sinking heads, that rise above the moulds, are commonly employed when castings are of a form which allows the dirt to collect at one point. The quality of castings is governed by many other conditions, such as the manner of " gating " or flowing the metal into the moulds, the temperature and quality of the iron, the temperature and character of the mould.
Cores are employed mainly for the displacement of metal in moulds; they may be of green sand, and made to surround the exterior of a piece, as well as to make perforations or to form recesses within it. The term "core," in its technical sense, means dried moulds, as distinguished from green sand: thus, wheels or other castings-are said to be " cast in cores " when the moulds are made in pieces and dried. Supporting and venting cores, and their expansion, are conditions to which especial attention is needed. When a core is surrounded with hot metal, it gives off, because of moisture and the burning of the " wash," a large amount of gas which must have free means of escape. In the arrangement of cores, therefore, attention must be had to some means of venting, which is generally attained by allowing them to project through the sides of the mould and communicate with the air outside. The venting of moulds is even more important than venting cores, because core vents only carry off gas generated within the core itself, while the gas from its exterior surface, and from the whole mould, has to find means of escaping rapidly from the flasks when the hot metal enters.
If it were not for the porous nature of sand moulds, they would be blown to pieces as soon as the hot metal entered them; both because of the mechanical expansion of the gas, and often from explosion by combustion. But for securing vent for gas, moulds could be made from plastic material, so as to produce fine castings with clear sharp outlines. The means of supporting cores consist of "prints" and "anchors." Prints are extensions of the cores, which project through the casting and into the sides of the mould, to bo held by the sand or flask. They have duplicates on the patterns, called "core prints," which should be of a different colour from the patterns. The amount of surface required to support cores is dependent upon their cubic contents, because the main force required is to hold them down, and not to bear their weight: the floating force of a core is as the difference between its weight and that of a solid mass of metal of the same size. When it is impossible, from the nature of castings, to have prints large enough to support the cores, this is effected by anchors, - pieces of iron that stand like braces between the cores and the flasks or pieces of iron imbedded in the sand to receive the strain of the anchors.
Cores expand when heated, and require an allowance in their dimensions the reverse from patterns, especially when the cores are made upon iron frames. For cylindrical cores less than 6 in. diam., or less than 2 ft. long, expansion need not be taken into account by pattern-makers, but for large cores careful calculation is required.
Shrinkage, or the contraction of castings in cooling, is provided for by adding 1/10 in. to 1/8 in. to each foot in the dimensions of patterns. This is accomplished by employing a shrink rule in laying down pattern-drawings from the figured dimensions of the finished work. Inherent or cooling strains is a much more intricate subject. They may weaken castings, or cause them to break while cooling, or sometimes even after they are finished; and must be carefully guarded against, both in the preparation of designs and the arrangements of patterns, especially for wheels and pulleys with spokes, and for struts or braces with both ends fixed. The main difficulty resulting is that of castings being warped and sprung by the action of unequal strains, caused by one part cooling or "setting" sooner than another. This may be the result of unequal conducting power in different parts of a mould or cores, or it may arise from the varying dimensions of the castings, which contain and give off heat in the same ratio as their thickness. 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 casting 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 mould remains quite hot, while at the bottom the earth, being a good conductor, carries of the heat and cools that side first, so that the iron "sets" first on the bottom, afterwards cooling and contracting on the top.
The draught, or the taper required to allow patterns to be drawn readily, is another indefinite condition in pattern-making : may be 1/16 in. to each foot of depth, or 1 in., or ihere may be no draught whatever. 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; a pattern in a plain form, that affords facilities for lifting or drawing, 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 1 in. to the foot of depth.