Patterns and castings are so intimately connected that it would be difficult to treat of them separately without continually confounding them together; it is therefore proposed to speak of pattern-making and moulding under one head.
Every operation in a pattern-shop has reference to some operation in the foundry, and patterns considered separately from moulding operations would be incomprehensible to any but the skilled. Next to designing and draughting, pattern-making is the most intellectual of what may be termed engineering processes—the department that must include an exercise of the greatest amount of personal judgment on the part of the workman, and at the same time demands a high grade of hand skill.
For other kinds of work there are drawings furnished, and the plans are dictated by the engineering department of machinery-building establishments, but pattern-makers make their own plans for constructing their work, and have even to reproduce the drawings of the fitting-shop to work from. Nearly everything pertaining to patterns is left to be decided by the pattern-maker, who, from the same drawings, and through the exercise of his judgment alone, may make patterns that are durable and expensive, or temporary and cheap, as the probable extent of their use may determine.
The expense of patterns should be divided among and charged to the machines for which the patterns are employed, but there can be no constant rules for assessing or dividing this cost. A pattern may be employed but once, or it may be used for years; it is continually liable to be superseded by changes and improvements that cannot be predicted beforehand; and in preparing patterns, the question continually arises of how much ought to be expended on them—a matter that should be determined between the engineer and the pattern-maker, but is generally left to the pattern-maker alone, for the reason that but few mechanical engineers understand pattern-making so well as to dictate plans of construction.
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To point out some of the leading points or conditions to be taken into account in pattern-making, and which must be understood in order to manage this department, I will refer to them in consecutive order.
First.—Durability, plans of construction and cost, which all amount to the same thing. To determine this point, there is to be considered 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 as an expensive one.
Second.—The manner of moulding and its expense, so far as determined by the patterns, which 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.
Third.—The soundness of such parts as are to be planed, bored, and turned in finishing; this is also a matter that is 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.
Fourth.—Cores, where used, how vented, how supported in the mould, and I will add how made, because cores that are of an 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 to ensure sound castings.
Fifth.—Shrinkage; the allowance that has to be made for the contraction of castings in cooling, in other words, the difference between the size of a pattern and the size of the casting. This is 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.
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Sixth.—Inherent, or cooling strains, that 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.
Seventh.—Draught, the bevel or inclination on the sides of patterns to allow them to be withdrawn from the moulds without dragging or breaking the sand.
Eighth.—Rapping plates, draw plates, and lifting irons for drawing the patterns out of the moulds; fallow and match boards, with other details that are peculiar to patterns, and have no counterparts, neither in names nor uses, outside the foundry.
This makes a statement in brief of what comprehends a knowledge of pattern-making, and what must be understood not only by pattern-makers, but also by mechanical engineers who undertake to design machinery or manage its construction successfully.
As to the manner of cutting out or planing up the lumber for patterns, and the manner of framing them together, it is useless to devote space to the subject here; one hour's practical observation in a pattern-shop, and another hour spent in examining different kinds of patterns, is worth more to the apprentice than a whole volume written to explain how these last-named operations are performed. A pattern, unless finished with paint or opaque varnish, will show the manner in which the wood is disposed in framing the parts together.
I will now proceed to review these conditions or principles in pattern-making and casting in a more detailed way, furnishing as far as possible reasons for different modes of constructing patterns, and the various plans of moulding and casting.
In regard to the character or quality of wood patterns, they can be made, as already stated, at greater or less expense, and if necessary, capable of almost any degree of endurance. The writer has examined patterns which had been used more than two hundred times, and were apparently good for an equal amount of use. Such patterns are expensive in their first cost, but are the cheapest in the end, if they are to be employed for a large number of castings. Patterns for special pieces, or such as are to be used for a few times only, do not require to be strong nor expensive, yet with patterns, as with everything else pertaining to machinery, the safest plan is to err on the side of strength.
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For pulleys, gear wheels, or other standard parts of machinery which are not likely to be modified or changed, iron patterns are preferable; patterns for gear wheels and pulleys, when made of wood, aside from their liability to spring and warp, cannot be made sufficiently strong to withstand foundry use; besides, the greatest accuracy that can be attained, even by metal patterns, is far from producing true castings, especially for tooth wheels. The more perfect patterns are, the less rapping is required in drawing them; and the less rapping done, the more perfect castings will be.
The most perfect castings for gear wheels and pulleys and other pieces which can be so moulded, are made by drawing the patterns through templates without rapping. These templates are simply plates of metal perforated so that the pattern can be forced through them by screws or levers, leaving the sand intact. Such templates are expensive to begin with, because of the accurate fitting that is required, especially around the teeth of wheels, and the mechanism that is required in drawing the patterns, but when a large number of pieces are to be made from one pattern, such as gear wheels and pulleys, the saving of labour will soon pay for the templates and machinery required, to say nothing of the saving of metal, which often amounts to ten per cent., and the increased value of the castings because of their accuracy.
Mr Ransome of Ipswich, England, where this system of template moulding originated, has invented a process of fitting templates for gear wheels and other kinds of casting by pouring melted white metal around to mould the fit instead of cutting it through the templates; this effects a great saving in expense, and answers in many cases quite as well as the old plan.
The expense of forming pattern-moulds may be considered as divided between the foundry and pattern-shop. What a pattern-maker saves a moulder may lose, and what a pattern-maker spends a moulder may save; in other words, there is a point beyond which saving expense in patterns is balanced by extra labour and waste in moulding—a fact that is not generally realised because of inaccurate records of both pattern and foundry work. What is lost or saved by judicious or careless management in the matter of patterns and moulding can only be known to those who are well skilled in both moulding and pattern-making. A moulder may cut all the fillets in a mould with a trowel; he may stop off, fill [94] up, and print in, to save pattern-work, but it is only expedient to do so when it costs very much less than to prepare proper patterns, because patching and cutting in moulds seldom improves them.
The reader may notice how everything pertaining to patterns and moulding resolves itself into a matter of judgment on the part of workmen, and how difficult it would be to apply general rules.
The arrangement of patterns with reference to having certain parts of castings solid and clean is an important matter, yet one that is comparatively easy to understand. Supposing the iron in a mould to be in a melted state, and to contain, as it always must, loose sand and 'scruff,' and that the weight of the dirt is to melted iron as the weight of cork is to water, it is easy to see where this dirt would lodge, and where it would be found in the castings. The top of a mould or cope, as it is called, 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 bottom or drag side.
Expedients to avoid dirt in such castings as are to be finished all over or on two sides are various. Careful moulding to avoid loose sand and washing 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. Moulds for sinking heads are formed by moulders as a rule, but are sometimes provided for by the patterns.
The quality of castings is governed by a great many things besides what have been named, 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—things which any skilled foundryman will take pleasure in explaining in answer to courteous and proper questions.
Cores are employed mainly for what may be termed the displacement of metal in moulds. There is no clear line of distinction between cores and moulds, as founding is now conducted; cores 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. 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 [95] is called. 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.
An apprentice may get a clear idea of this venting process by inspecting tubular core barrels, such as are employed in moulding pipes or hollow columns, or by examining ordinary cores about a foundry. Provision of some kind to 'carry off the vent,' as it is termed by moulders, will be found in every case. 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.
A learner will no doubt wonder why sand is used for ............