Practical Carriage Building
Compiled by M.T. Richardson, Vol.1. 1891
The effect of different bevels in the teeth of saws is to cut to different depths. A tooth of the bevel shown at B, Fig. 49 will cut into the wood deeper than a tooth of the level shown at A. It follows then that with very soft woods in which cutting may be done very rapidly, a tooth calculated to cut deep into the wood is best, while for hard woods less bevel must be employed. The bevel of the cutting edge of the tooth should be the same for all saws, but the bevel of the point which is governed by the angle at which the file is held is to be varied as circumstances require.
The philosophy of some saws cutting better than others is easily explained and may be illustrated in a very familiar manner. Suppose it is desired to cut across a board with a knife or other sharp pointed instrument. If the knife is held as shown in Fig. 48, the point will cut deeper and at the same time easier than if it is held as shown in Fig. 49. Hence if follows that the cutting edge of the tooth in a saw should incline forward after the manner of the knife shown in Fig. 48, or in other words that its shape should be as indicated in Fig. 50 rather than of the kind shown in Fig. 51. If the bevel of the cutting edge of the tooth-the bevel of the point in relation to the material to be cut- and the pitch or inclination of the cutting edge be carefully considered and properly combined, a perfect tooth will be obtained.
Fig. 52 shows the teeth of a saw not at all exaggerated as it is sometimes found in use among men who have neither genius nor knowledge in filing. Such a saw will undoubtedly scratch its way thorough a piece of timber, and in course of time a log or board may be cut in two by it; but how much better will a saw cut which is filed in a scientific manner.
It holds to reason that the teeth of a saw, both in their size and in the manner in which they are filed, should be adapted to the wood they are intended to cut. A tooth appropriate for very soft wood is not the best for very hard wood, and a tooth that cuts hard wood to advantage will not work very satisfactorily upon quite soft wood. Hence the necessity of considering the nature of the material to be cut before selecting a saw and before undertaking to put it in order.
The first thing to be done in filing a cross-cut hand saw is to see that the teeth are perfectly straight upon the edge, or, if anything, a little higher in the middle than at the end.
This is very important, for if any tooth is shorter than the next, it will not cut at all, and is therefore worse than useless, because it gives the next tooth more than its fair share of the work to do, rendering the cut imperfectly made. A cross-cut saw for very soft wood should have about six teeth to the inch. The bevel to be observed in filing is shown in Fig. 53. The teeth should have a full set, and the sides should be jointed thoroughly. About setting, however, something will be said further on. In this figure, as well as in the others in this paper, the point of the saw is supposed to be on the left hand side of the filer. A good sized file, say four and a half or five inches in length should be used.
There is no difference in the angle between a large and a small file. The only point therefore to be considered is the cut of the file-whether it is fine or coarse. A fine cut, with sharp corners, is much to be preferred in this class of work. By soft wood, in this connection, is meant such as basswood, pine, and cedar. Fig. 54 shows the shape of teeth to a scale somewhat larger than natural size. It also shows the bevel of the point of the tooth in a saw adapted to soft woods. The bevel on the point is consequent upon the bevel of the file and the bevel of the back of the tooth.
Fig. 55 shows a cross-cut saw with eight teeth to the inch, which is adapted to medium woods, and Fig. 57 shows one with ten teeth to the inch, adapted to hard woods. The position of the file to be observed in filing is shown in each engraving. Figs. 56 and 58 show the shape of the teeth upon an enlarged scale, and sections through the points of the teeth in the saws shown in the two preceding cuts respectively. By medium woods is meant such as black walnut, cherry, chestnut, &e. hard woods include such as hickory, oak, ash, maple, beech, &e. By observing the directions above given it will be seen that the bevel on the pitch or cutting edge of the tooth is the same in all; but the bevel of the point looking the length of the saw is quite a different thing. The angle of the pitch of the cutting edge of the tooth has been considered already.
As already stated in the examples here given, the end of the saw is supposed to be to the left of the filer; or, in other words, the point of the saw is supposed to be in the direction toward which the point of the file is directed in the cuts. Some may object to this, contending that the saw should be filed toward the handle, because in so doing it prevents a feather edge on the teeth of the saw. A feather edge cannot be considered a serious objection, since every saw, after filing, should be laid down fiat—first on one side, then the other—and jointed or planed down until every tooth is exactly in line. For this purpose a long whetstone or fine- cut file with straight edges should be used. Such an operation takes off all of the feather, makes a finer edge, and affords more cutting surface. If a saw is newly set and lot jointed as above recommended after filing, all the cutting surfaces will be at the extreme point of the teeth, which only scratch and do not cut. The operation of such a tool may be likened to marking across a board with the point of a pin. In Fig. 59 A represents the point of a tooth as it is left after filing, while B represents the same tooth after it has been jointed. It will be seen by comparison that the operation of jointing gives the tooth a real knife edge.