Pattern Making dates back to the time when the first article was made from molten metal for the use of man. The pattern must precede the making of its metal counterpart, and is therefore the first subject to be treated in the working of metal.
The pattern maker is essentially a worker in wood, though, where many eastings are to be made from the same pattern, the final or working pattern is made of metal. These metal patterns are very serviceable, and leave the sand more easily and cleanly than those made of wood. Metal patterns are always necessary when the work is of a delicate or very light character. In all such cases, however, the first pattern - from which the metal pattern is to be moulded - is made of wood, allowance being made for double shrinkage, and, when necessary, for double finish. The necessity for this will be clearly explained farther on.
The pattern maker should possess a practical knowledge of the properties of metals, First of all, he must understand the shrinkage of metals, that is to say, how much smaller the cold casting will be than the molten mass as it flows into the mould; he should know what the strength of the metal is; he should be familiar with the relative rapidity of cooling, so that internal stresses in the body of the completed casting may be avoided as much as possible; he also should know enough about the practical work of the moulder to decide upon the peculiarities of construction of the pattern for any given piece; and he must be sufficiently skilled as a draftsman to lay out, without the assistance of the designer, the drawings of the piece to be made. It is very true, however, that there are many good pattern makers who do not possess all of these qualifications.
The last-mentioned qualification is one of the most important. The drawings furnished the pattern maker are usually on a small scale.
In order to work to the best advantage, he must reproduce a part or all of them at full size, as working drawings. To do this in such a way that the lines and curves of the finished pattern shall be graceful and artistic in appearance, will require the same nicety and precision of workmanship that are demanded in the drafting room, and it is essential that the pattern maker have the same complete knowledge of the principles involved. To the extent, then, of being able, when necessary, to make a full-sized drawing of the article to be made, the pattern maker must be a draftsman.
In large establishments, where all the work comes to the pattern shop in the form of carefully executed drawings, the pattern maker is the means of putting the ideas of others into tangible shape. In smaller places, where no draftsman is employed, the pattern maker will be called upon to work out the designs for which he is to make his patterns, and he thus becomes the real designer.
Finally, the pattern maker is seldom required to make two patterns that are identically the same. His work, therefore, is varied, and he must be prepared to apply to the solution of new problems that arise such principles as he may already have learned.
As patterns are subjected to more or less rough usage, and are alternately wet and dry, it follows that the ideal material is one whose hardness is such that it will withstand the wear and tear of handling and at the same time be impervious to the effects of moisture. Such material is to be found in the metals, but, as the cost of working these into the proper shape is considerable, some kind of wood is usually substituted.
If, then, wood is to be used, another qualification is to be added - namely, it should be easily worked. The best wood for the purpose is undoubtedly white pine. Care should be exercised in the inspection of the wood, to see that it is clear, straight-grained, and free from knots.
The straightness of the grain can be determined by the appearance of the sawn face. This should present an even roughness over the whole surface. The wood should be seasoned in the open air, but preferably sheltered by a roof, and should be piled so that the air will have free access to all parts of the plank. In the natural process of air drying, the moisture slowly works out to the surface and evaporates until the wood is dry or "seasoned." Such stock is firmer, stronger, more elastic, and less affected by heat and cold and by moisture and dryness, than kiln-dried lumber. In kiln drying, the outside surfaces and ends of the boards are dried more rapidly than the inside, producing strains that cause the wood to bend and warp while the pattern is in the process of construction. For this reason it is better to "build up" the larger pieces of a pattern by gluing together three or more (never tow) pieces of thinner stock. When the patterns are of moderate size the stock to be glued may vary from | inch to 1 inch or even 1¼ inches in thickness, in proportion to the size required. Stock of 2 inches thickness or over can seldom be found sufficiently seasoned; and, if forced by kiln-drying, it will be cheeked and strained to an extent that will render it useless for pattern work.
While pine is in general the ideal wood for pattern work, it is soft and weak, so that, if small and strong patterns are desired, a harder wood is usually employed. Mahogany is much used for this purpose. Like pine, it is not liable to warp, and, when straight-grained, it is worked with comparative ease. There are many varieties of this beautiful wood, varying greatly in firmness of texture. The soft bay wood, often sold as genuine mahogany, should be avoided for patterns, being but little harder than pine. Cherry is also extensively used, but is not so easily worked to a smooth surface as mahogany, and is more liable than the latter to warp and to be affected by moisture. Black walnut, beech, and maple are used to some extent. Black walnut is stronger than cherry, but, like beech and maple, is likely to warp.
It may be stated then, that, in the United States, white pine is the material commonly employed for pattern making. Lumber 1 inch, 1¼ inches, and 1½ inches thick will be found convenient in the construction of such patterns as are most commonly called for. It will be a great saving of time and labor, after the lumber has been carefully selected, to have it taken to the planing mill and dressed on two sides to the following thicknesses:
One-inch, dressed on two sides to 7/8 inch;
One and one-quarter-inch, dressed on two sides to 1 1/8 inch;
One and one-linlf-inch, dressed on two sides to 1 1/8 inch; and, if such can be found well seasoned, a small quantity of two-inch, dressed to l¾ inches.
In addition to these sizes there should be a moderate amount of 1-inch resawed and dressed to $ inch or to inch; and the same amount of l¼-inch resawed and dressed to ½ inch. The two last thicknesses are used for gluing and building up the rims of pulleys, gear wheels, and other light work where strength and durability are required.