In Fig. 7, if the fibers in the lower portion of the piece near the face CDG happen to have, on the average, thicker walls than those in the upper portion, near the face A BFE, the lower part will shrink more than the upper part. The distance CD, orig inally equal to the distance AB, becomes smaller and the shape of the whole piece changes as shown in Fig. S.
The only way in which warping can be prevented is to have the timber thoroughly dried out before it is used, as after it is once thoroughly seasoned it will not warp unless it is allowed to absorb more mois ture. All wood which is to be used for fine work, where any warping after it is in place will spoil the appearance of the entire job, must be so seasoned, either in the open air or in a specially pre pared kiln.
The wood of the conifers which is very regular in its structure shrinks more evenly and warps less than does the wood of the broad leaved trees with its more complex and irregular structure. Sap wood, also, as a rule shrinks more than does heartwood.
Checks. Another defect which is caused by the drying out of the timber and the con sequent shrinkage of the cell walls is what is known as checking. In any log of wood there is always opportunity for shrinkage in two directions, along the radial lines following the direction of the medullary • rays, and around the circum ference of the log following the direction of the annual rings. If the wood shrinks in both directions at the same rate, the result will be only a decrease in the volume of the log, but if it shrinks more rapidly around the circumference of the log than along the radial lines, the log must develop cracks around the outside as shown in Fig. 9. Such cracks are called checks. In timber which has been prepared for the market they show themselves in the form of cracks which extend along the faces of solid squared timbers and boards, seriously impairing their strength. Fig. 10 shows checks as they would appear in a square post or column.
Conversion of Timber into Lumber. Lumber may be found in lumber yards in certain shapes ready for use, having been cut from the logs and relieved of their outside covering of bark. The cutting up of the logs is done in the mills by machinery and there are various methods in use for transforming the logs into boards, planks, and heavy timbers. The method of cutting the log determines the appearance of the wood when finished and also affects it in other ways. If the log is to be squared off so as to form only one heavy beam or post, a good rule to follow is to divide the diameter into three equal parts and then to draw perpendiculars to this diameter at the division points one on each side of the center, as shown at A and B in Fig. 11. The points C and D in which these perpendiculars to the diameter cut the circumference of the log, together with the points E and F in which the diameter cuts the circumference of the log, will be the four corners of the timber. The lines joining these points will give an outline of the timber, which will be rectangular and will be found to be the largest and best timber which can be cut from the log. Another good rule is to divide the diameter of the log into four equal parts and to proceed in the same way as described above, using the outside quarter points from which to draw the perpendiculars as shown in Fig. 12. This method will give the
outlines of a stiffer beam than the one described above, but there will be more waste from the log and the beam will not be on the whole as strong as the other.
In Fig. 13 are shown several different methods of cutting planks from a log.
First it is divided into quarters, and the planks are cut out as shown in the figure, there being four ways in which the work may be done. All of the four methods shown may be said to give what is called quarter-sawed lumber since the log is first cut into quarters, but that shown at A is the best. All of the planks are cut radiating from the center of the log and there will be no split ting or warping, but the method is very expensive, as all of the planks have to be squared up afterward and there is much waste as a result. A fairly good method is that shown at B where the planks are nearly along radial lines and may be much more easily and cheaply cut out than can those shown at A. The method shown at C is a com mon one and leads to fairly good results, although only the plank nearest the center is on a radial line. It is practically as good a method as that shown at B and is much more simple. The method shown at D is not so good as the others, as planks cut out in this way are very liable to warp and twist. If the silver grain, caused by cutting of the medullary rays is desired, the planks must be cut as shown at A, B, or C.
Plank:; are sometimes simply sliced from the log as shown in Fig. 1.1, without first dividing it into quarters, but this is the worst possible way of cutting them, as the natural tendency of the timber to shrink causes the planks to curl up as shown in Fig. 15. It is almost impossible to flatten them out again, and they can not be used in that condition.
There is another method of cutting up a log which has been introduced more recently than the others, and which is known as the "rotary cut." It consists in placing the log on a movable carriage which keeps it whirling rapidly about its longitudinal axis, at the same time bring ing it up against a long stationary knife which catches the log and peels off strips around the circumference of any desired thickness. This method is used extensively in the preparation of wood to be used as veneers, and in the case of many kinds of wood the figure is brought out to better advantage in this way than is possible with any other method.
Irancy Lumber. When a log of wood has been sawed up into boards, each board is apt to have along the edge a strip of the bark which was originally on the outside of the log, and the edges will not he square with the face of the board, owing to the cylindrical shape of the log. Such boards should be squared up by having the rough edges to which the bark adheres trimmed off. But sometimes the bark alone is stripped off, leaving the boards with the edges not square with the face. Such boards are said to be waney, and eery often specifications state that no wancy lumber shall be employed on the work. The pieces which are cut off when waney boards are trimmed in order to square them up are called "edging" and are used to make laths.
Slabs. The pieces known as slabs are those which are left over after a log has been sawed up into boards. In cross section they are of the shape of a half moon, and are covered with bark. They are useless except for laths or fuel.