Wood-working for Beginners: A Manual for Amateurs. Charles G. Wheeler
can often cut better with a draw-stroke, i.e., not merely pushing the tool straight ahead, but drawing it across sideways at the same time (Fig. 7). You can press the sharp edge of a knife or razor against your hand without cutting, but draw the edge across and you will be cut at once. Even a blade of grass will cut if you draw the edge quickly through your hand, as you doubtless know.
If you try to push a saw down into a piece of wood, as you push a knife down through a lump of butter, or as in chopping with a hatchet, that is, without pushing and pulling the saw back and forth, it will not enter the wood to any extent, but when you begin to work it back and forth it cuts (or tears) its way into the wood at once. You know how much better you can cut a slice of fresh bread when you saw the knife back and forth than when you merely push it straight down through the loaf. You may have noticed (and you may not) how much better your knife will cut, and that the cut will be cleaner, in doing some kinds of whittling, when you draw it through the wood from handle to point (Fig. 7), instead of pushing it straight through in the common way, and you will discover, if you try cutting various substances, that as a general rule the softer the material the greater the advantage in the draw-stroke.
Now put the sharpest edge-tool you can find under a powerful microscope, and you will see that the edge, instead of being so very smooth, is really quite ragged—a sort of saw-like edge. Then look at the structure of a piece of wood as roughly indicated in Fig. 6, and you will understand at once just what we do when we cut wood with an edge-tool. You see the microscopically small sticks or tubes or bundles of woody fibre of which the big stick is composed, and you also see the microscopically fine saw to cut them. Now if the edge of the tool is fine you can often do the work satisfactorily by simply pushing the tool straight through the wood, but do you not see that if you can draw or slide the tool either back or forth the edge, being saw-like, will do its work better?
This stroke cannot be used of course in chopping with the axe or hatchet, splitting kindling-wood, or splitting a stick with the grain with a knife or chisel. In these operations the main principle is that of the wedge, pure and simple, driven through by force, the keen edge merely starting the cut, after which the wedge does the rest of the work by bearing so hard against the wood at the sides of the cut that it forces it to split in advance of the cutting edge, as in riving a log by the use first of an axe, then of an iron wedge, and finally a large wooden wedge (Fig. 8).
Fig. 8.
Practical directions and suggestions about the different Tools and their Uses and the various Operations will be found alphabetically arranged in Part V.
CHAPTER III
WOOD
Before you can make anything successfully, you must have not merely wood, but the right kind of wood for the purpose. There are, also, "choice cuts" in lumber, as the butcher says of meat, and judicious selection of the stock often makes all the difference between a good job and a poor one; so let us examine a log and follow it through the sawmill.
Fig. 9.
You have, of course, seen the rings, or circular lines, on the ends of pieces of wood (Fig. 9). These are called the annual rings,[8] and each ring marks a new layer of wood added to the tree, for, as perhaps you may have learned, the trees we use for wood-working grow by adding new layers of wood on the outside. Examine the ends of pieces of wood of various kinds. In some pieces these rings will be very plain. In others they will be quite indistinct.
Notice that the wood nearest the bark, known as the sapwood, usually looks different from the inner wood, which is called the heart (Fig. 9).
In some trees you will see rays, or lines, radiating from the centre, and known as the medullary rays (Figs. 9 and 10), because they spring from the pith (Latin medulla). Sometimes these lines are too fine to be noticed.
You will see from Fig. 10 that the layers of wood are also shown in the lines of what we call the "grain" on the surface of a piece of wood cut lengthways, and that the lines of the grain are continuations of the annual rings. You will also notice at the ends of timber, after the seasoning has begun, cracks radiating from the centre, showing the natural lines of cleavage or separation.
Fig. 10.
Fig. 11.
The way the log is sawed is important, though you might naturally think that the only thing is to saw it any way that will give pieces of the required size and shape.
Why is green wood heavier and softer than dry wood, and the sapwood of green timber softer than the heart? Because of the sap or water contained. The amount of water is sometimes even as much as fifty per cent. of the weight of the wood, but the quantity depends upon the kind of tree, the season, etc. Now the more water the green log contains, the more it will shrink. It begins to dry and shrink as soon as the tree has been cut down. The sapwood shrinks more than the heart because it contains more water, and faster because, being on the outside, it is more exposed. The log shrinks most in the line of the annual rings, that is, around the tree. It shrinks much less in the line of the medullary rays, that is, across the tree. Shrinkage lengthways is too slight to be considered[9] (Fig. 11).
The result of all this unequal shrinking is that the log tends to split, or crack open, at the circumference (Fig. 12), the cracks running in toward the centre, in the line of the medullary rays. If the log is halved or quartered, so that the inner parts are exposed, the drying goes on more uniformly throughout, the cracking is not so bad, and the parts of the log will shrink somewhat as shown in Figs. 13 and 14.
Fig. 12.
Fig. 13.
Fig. 14.
The beams, joists, planks, or boards cut from a log have the same tendency to shrink unevenly that is found in the log itself. This causes them to be irregular in shape and to curl or warp more or less, according to the part of the log from which they are taken. A piece cut from the centre of a log will thus hold its shape better than if cut from one side (Fig. 15).
Fig. 15.
Fig. 16.
When a log is sawed into boards or planks (Fig. 16) the middle board shrinks but little in width and in thickness at the centre, but becomes thinner towards the edges. It does not curl, because it is cut through the centre of the log and has no more tendency to curl one way than the other. The outside board shrinks least in thickness and most in width, and all, except the middle one, shrink differently on one side from the other. They become convex toward the pith, or heart, and concave toward the outside. Different kinds of wood shrink and warp to different degrees. You can learn something about these matters by examining the stock in any lumber-yard.
Now to come to the practical application of our brief study of the log and the