Fig. 652.
Examine the teeth (Fig. 652) and you will see that they are pointed and sharp, somewhat like the point of your knife, and that they cut across the fibres much the same as your knife does when you hold it upright and draw it across a board.
Fig. 653.
You will notice, also, that the teeth are alternately bent outwards,—one tooth being bent out to one side, the next to the other side,—this spreading of the teeth (which is called the "set") making the saw wider at the points of the teeth than elsewhere. You will also notice that the sharp cutting edge of each tooth is on the outside. This set, and the way the teeth are sharpened, makes the cut wider than the thickness of the blade, thus giving the saw "clearance" and enabling it to slip back and forth easily and without "binding" (Fig. 653). As a practical matter of fact, however, it is nothing uncommon for a saw to bind in the cut, either from not sawing straight or from the wood closing on the saw (see Fig. 695). The teeth not only cut or break off the fibres in parallel lines at the points of the teeth, but also tear off and remove the bits of wood (i.e., the sawdust) between these parallel cuts.
The degree to which the teeth are set and the number of teeth to the inch depend upon the use to which the saw is to be put and the kind of wood to be used. Of course the finer the teeth the smoother the cut. Cross-cut saws are usually sharpened differently for soft and for hard wood, but little set being required for the latter, while the former needs a wider set to give the blade clearance, because the fibres of the looser-textured soft wood are bent aside by the tearing action of the saw teeth and are not so cleanly cut off as in the hard wood.
We have examined the teeth of the cross-cut saw and have seen that they cut across the grain of the wood very much as the point and edge of a knife, and that the fibres, being cut or broken or torn off in fine pieces, are removed from the kerf by the teeth. Now to saw in the direction of the grain, instead of across it, we use a saw based on a different principle. As we used little knives to cut across the grain, so we use little chisels to cut with the grain. Look at the teeth of the ripping-saw and you will see that they are little chisels sharp only at the end (Fig. 654), though not as acute as chisels for obvious reasons. These sharp ends, which are square (Fig. 655, showing set) or may be oblique, cut or tear off the fibres, and the front edges of the teeth push the pieces out of the cut. The teeth of the cross-cut saw are filed so that the front cutting-edge is drawn across the wood in the most effective way, much as you would draw the knife-point across, while the teeth of the ripping-saw are pointed forward at a more acute angle so that the cutting-edge is pushed through the wood, somewhat as you push a chisel.
Fig. 654.
