Chapter XXXIV.—WOOD-WORKING.
Pattern-making.—Of the different kinds of wood serviceable to the pattern-maker, pine is, for many reasons, usually employed. It should be of the best quality, straight-grained, and free from knots; it is then easy to work in any direction, possessing at the same time sufficient strength for all but the most delicate kinds of work, and having besides the quality of cheapness to recommend it. Care taken in its selection at the lumber-yard will be amply repaid in the workshop. When it is straight-grained, the marks left by the saw will show an even roughness throughout the whole length of the plank; and the rougher the appearance, the softer the plank. That which is sawn comparatively smooth will be found hard and troublesome to work. If the plank has an uneven appearance—that is to say, if it is rough in some parts and smooth in others—the grain is crooked. Such timber is known to the trade as cat-faced. In planing it the grain tears up, and a nice smooth surface cannot be obtained. Before purchasing timber, it is well to note what convenience the yard possesses for storing. Lumber on the pile, though it be out in all weathers, does not deteriorate, but becomes seasoned; nevertheless its value is much increased if it has an extemporised roof to protect it from the sun and rain. But as it is not convenient to visit the pile for every customer, quantities are usually taken down to await sale, and for such a shelter must be provided, otherwise it will be impossible to insure that the lumber is dry, sound, and fit for pattern-making. It is obvious that the foregoing remarks on the storage of lumber apply to all woods.
The superiority of pine for pattern-making is not, however, maintained when we come to fine delicate patterns or patterns requiring great durability. When patterns for fine work, from which a great many castings are to be made, are required, a fine pattern wherefrom to cast an iron pattern is improvised, because, if pine were employed, it would not only become rapidly worn out, but would soon warp and become useless. It is true that a pine pattern will straighten more easily than one made of a hard wood; but its sphere of usefulness in fine patterns is, for the above reasons, somewhat limited. Iron patterns are very desirable on account of their durability, and because they leave the sand easily and cleanly, and because they not only do not warp but are also less liable than wooden ones to give way to the sand, while the latter is being rammed around them by the moulder, a defect that is often experienced with light patterns, especially if they are made of pine. Iron patterns, however, are expensive things to make, and therefore it is that mahogany is extensively employed for fine or durable pattern work. Other woods are sometimes employed, because they stand the rough usage of the moulding shop better and retain the sharp corners, which, if pine be used, in time become rounded impairing the appearance of the casting. Mahogany is not liable to warp, nor subject to decay; and it is exceedingly durable, and is for these reasons the most desirable of all woods employed in pattern-making, providing that first cost is not a primary consideration. There are various kinds of this beautiful wood: that known as South American mahogany is chiefly used for patterns.
Next to mahogany we may rank cherry, which is a very durable wood, but more liable to twist or warp than mahogany, and it is a little more harsh to the tool edge. If, however, it is stored in the workshop for a length of time before being used, reliable patterns may be made from it. In addition to these woods, walnut, beech, and teak are sometimes employed in pattern-making.
The one property in all timber to be specially guarded against is its tendency to warp, bend, expand, and contract, according to the amount of humidity in the atmosphere. Under ordinary conditions, we shall be right in supposing a moisture to be constantly given off from all the exposed surfaces of timber; therefore planks stored in the shop should be placed in a rack so contrived that they do not touch one another, so that the air may circulate between the planks, and dry all surfaces as nearly alike as possible. If a plank newly planed be lying on the bench on its flat side, the moisture will be given off freely from the upper surface, but will, on the under surface, be confined between the bench and the plank: the result being that a plank, planed straight, and left lying as described, will be found, even in an hour, to be curved, from the contraction of the upper surface due to its extra exposure; therefore it is obvious that lumber newly planed should be stored on end or placed on edge. Lumber expands and contracts with considerable force across the grain; hence if a piece, even of a dry plank, be rigidly held and confined at the edges, it will shrink and break in two, often with a loud report. There is no appreciable alteration lengthwise in timber from the above causes; and if two pieces be glued together so that the grain of one crosses that of the other, they can never safely be relied upon to hold. Hence they had better be screwed so that there will be a little liberty for the operation or play of the above forces, while the screws retain their hold. The shrinkage, expansion, and warping of timber may perhaps be better understood by the following considerations: The pores of wood run lengthwise, or with its grain, and hence the moisture contained in these passes off more readily endwise or from any surface on which the pores terminate.
The Shrinkage of Timber.—The direction in which timber shrinks in seasoning or drying is shown in the following figures, which are extracted from a lecture delivered by Dr. Anderson before the Society of Arts in London, England. The shrinkage of timber lengthwise of the grain is very slight, its shrinkage in a direction across or at a right angle to the length of the grain being much greater and depending upon the part of the log from which it is cut.
