The Teeth Of Ripping-Saws. Fig. 60 shows a plan, elevation, and section of three teeth as they are usually made for a ripping-saw. The following paragraphs present a consideration of the action of an individual tooth.

All wood is fibrous, and any tool which is to produce a cut along the length of the fibers, as the saw kerf ab, Fig. 59, must, at each period of action, take something from the ends of such fibers as may lie in the path of the proposed opening. In fulfilling this condition, the action of a ripping-saw's tooth is not unlike the action of a chisel when used as shown by Fig. 149. Each tooth in its turn removes its share from the fiber ends over which it passes, just as the chisel at every change of position takes its slice and lengthens the cut. The cutting edge of a saw tooth, however, is bounded by a more obtuse angle than that of a chisel, and as a cutting tool is inferior. Thus, if one of the three teeth shown by Fig. 60 is applied to a saw kerf in the position it would occupy as part of a complete saw, it will appear as represented by Fig. 61, its motion being in the direction of the arrow. It is defective as a cutting tool, because of the position of the line ab, the advancing face of the tooth. This defect is more clearly illustrated by Fig. 62; this shows how a chisel would look if its edge were made to cut in the same manner as that of a saw tooth. But the fact is that a great discrepancy exists between the form of the saw tooth and that of the chisel, for it has been demonstrated that a chisel, to give good results, must be at least as acute as is indicated by the dotted line a; and it would seem that the former might be improved by bringing it more nearly to the outline of the latter. Suppose this be attempted, and that the face of the tooth indicated by the line cb, Fig. 60, be changed to cb'. Such a change must result either in removing material from the tooth, and thereby weakening it, or in changing the line ed to a position cd'. In other words, if the tooth is not weakened, the space between it and the next will be reduced. Again, if to make the advancing face still more acute, the line cb" is accepted, and the tooth is not made smaller (that is, weakened), there will be no space between it and the next tooth. Having no spaces, there can be no teeth, and consequently the attempted change is impossible. It will thus be seen that the angle of the advancing face of the ripping-saw tooth cannot, unless it is weakened, be much more acute than is shown by Fig. 60 and Fig. 61.

Fig. 61

Fig. 62

Fig. 63

The form of the tooth may be wholly changed, however, to the outline shown by Fig. 63, and some advantage may thus be gained in respect of the cutting angle; but such a tooth, while suitable for machine-saws of considerable size, is too complicated for small saws.

Nothing remains, then, as a possible means of improving the cutting edge of the saw tooth, except a modification of the angle bcd, Fig. 60. If it could be shown that there is an excess of strength in the tooth, above what is needed to perform its work, the angle might be changed to b'cd, or even to b"cd, and the value of the tooth as a cutting tool be increased. Moreover, it does not at first seem unreasonable to attempt such a change, for it is evident that the cutting wedge of the chisel (which we have regarded as the typical cutting tool), while much more acute than the angle bcd, is yet strong enough to be entirely satisfactory.

A more careful comparison of the saw and chisel, however, discloses the following facts: first, a saw tooth must be softer than a chisel in order that it may be set and filed, and being softer, is therefore weaker in its substance; secondly, the width of the saw tooth is less than half the width of the narrowest chisel made, and, in this respect also, it is at a disadvantage; and, thirdly, in using a chisel the operator's attention is given entirely to its one cutting edge, and if at any time it is likely to receive too much strain, it is at once relieved; while each saw tooth, on the contrary, forms but a small part of a tool that receives little attention and much vigorous handling while it is being driven through straight grain, crooked grain, or hard knots, as the case may be. From a consideration of these points, it seems clear that the cutting-angle of a saw tooth must be less acute than that of a chisel. But the degree of acuteness can be determined only by use. Fig. 60 shows the form which years of experience have proved the most practicable for general work, and while some bench-workers do file their saws "under," producing a tooth similar to dcb', as many more go to the other extreme and use a tooth similar to dcf. The typical form given is easily kept in order, and, when in that condition, will cut freely and well.