HERE is as much special knowledge required in the choice of tools as in that of a piece of furniture, a scientific instrument, or a machine. And not only so, but the ability to keep them in work-ing order involves knowledge which can only come after considerable experience. Hence, we propose to discuss not only the points of good tools, but the condition to be observed in order that they shall be maintained in the high-est state of efficiency of which they are capable. Taking the saws in the first place –sawing being preliminary to all other operations—there are these important points to be borne in mind : buckle, adaptability, forms of teeth, set, temper, handles. The various common saws are termed “hand,” “tenon,” “dovetail,” ” compass,” ” keyhole,” and ” bow.” Our remarks will have reference to second-hand articles equally with those bought brand new from a shop, and some of the points enumerated seldom apply to new tools. ” Buckle ” in a saw means that the plate is not in an equal condition of tension throughout, but that some portions are denser and more strained than others. The visible signs of buckle, when very bad, are bending of the blade, indentation, wind, irregular reflection of the light, and the inability to keep the saw straight in its cut. Holding a hand-saw by the handle, and vibrating the blade sharply, the presence of buckle will be detected by a flapping sonorous ring, sharp and pronounced, indicating that the saw plate is not in a condition of equal tension throughout. Buckle does not necessarily mean ruin to the saw, but it is difficult to remove, save by a skilful operator. Hence the best plan is to take the tool to a saw-sharpener, who will remove or minimize the evil by a process of hammering, effected on the self-same principle as the ” raising ” of works in sheet metal. As a matter of economy no one should buy a buckled saw second-hand, but it often happens that careless amateurs will buckle good saws by straining and forcing them to their work, and the attempt to hammer it out usually makes matters worse.
Of vital importance also is the adaptability, or otherwise, of a saw for its work. By this is not altogether meant the class of saw used, since the functions of the different kinds are obvious enough—but rather the shapes and set-ting of the teeth, matters so all important that they will be treated in detail. The sizes and shapes of saw teeth are vastly modified according to the work they have to do, the difference consisting mainly in dimensions, ” rake,” “set,” and “pitch.” Size is partly dependent upon the class of saw, but varies much in the same type. Shape is governed by rake, being adapted to the work for which the saw is designed, while ” set ” governs the clearance of the saw blade in its cut, and on this depends the ease, therefore, with which it will pass through a piece of timber. The size of the tooth saw. The rake varies from these being the two extremes, A being adapted for ” cross cutting,” of working across the grain, and, when of small size, for metal sawing ; while B is only properly adapted for “ripping,” or sawing down with the grain in soft woods. Minimum of set given, and B quite the maximum; the first being for metal saw-ing, and for hard and dry woods, and consisting only in a thinning down of the blade behind the teeth ; the second representing the maximum for cross-cutting the soft woods, and consisting of a bending of alternate teeth to right and left. Now, in any case, the aim is to strike the happy mean both of rake and set for the particular class of work which any saw has to do, and to assist the amateur in this matter we here give the shapes of some typical teeth to actual sizes and from a study of these we may see that the general inferences to be drawn are : That the softer the wood the greater should be the amount of set, and of rake, and of ” pitch ” of teeth, or distance from tooth to tooth, while with the harder woods the opposite conditions obtain. The increased set is necessary in the softer woods, because their fibres do not lie so firm and dense as those of the hard woods ; they tend, therefore, to press closely and cause increased friction against the saw blade; neither does their dust get away so freely from the cut, as the finer, drier dust from the hard woods. For this last reason also the pitch, or distance between teeth, is greater for soft than hard woods in order to give greater “spacing,” since a saw with small tooth-space would become choked up with the fibrous, fluffy dust, which ought to get away immediately it is formed. For the same reason, also, a saw which will cut a piece of dry, well-seasoned wood with facility, will be found to operate with difficulty on the wood of the same kind when wet. Further, the rake, which in Fig. 15, C, acts so efficiently in penetrating and removing the material by its wedge-like action, is too penetra-tive for hard woods, hitching into the grain and jarring the hands of the work-man. A saw having much rake is un-suitable for any cross cutting, for which work teeth having little or no rake and an excess of set are properly suited. Similar principles apply to the tenon and dovetail as to the larger hand-saws ; but as a matter of practice, the typical tooth forms are often so modified that a kind of compromise is made, and a single handsaw or a single tenon saw is made to do duty on all kinds of wood and in all directions of the grain. Fig. 16 shows such teeth, which, though not ripping soft wood with such facility as Fig. 15, C, nor hard wood like Fig. 15, A, nor cross-cutting soft or hard woods like B, D, will yet operate on all but excessively wet stuff. In each of these figures the appropriate amount of set is shown in plan below the corresponding tooth profile. Wood which is very wet, as
some firewood logs and thick planking, can only be cut with a saw set specially
t ; I
Fick. 16.—HAND-SAW TEETH.
wide and regular; moreover, a saw set and sharpened with uniform regularity is capable of doing a much wider range of work than one having more set im-parted irregularly. This matter of setting and sharpening is of so much im-portance that we shall briefly describe the two processes. Supposing that we have a saw which has been so badly used that the teeth are all “cows and calves,” as the saying is (Fig. 17) which is very bad indeed.
FIG. 17.—Saw IN BAD ORDER.
The saw may be a very good one—good temper, elastic, free from buckle, and so on—but the teeth are spoiled, and we want to get these in working trim again. In the first place, “top” the teeth, that is, run a file all down the points, the saw being held in a vice (Fig. 18) the while, until all the points are onone level,
FIG. I8.—SAw VICE.
as indicated by the dotted line in Fig. 17. Then the teeth backs are to be filed until the points are renewed, and each
tooth is not only on a level with its fellows, but sharp. This filing does not, however, take place straight across at right angles with the blade, but dia-gonally, as shown by dotted lines in plan (Figs. 15 and 16) the purpose of this being to reduce the friction of the tooth points, and to impart a correspondingly keener and more incisive cutting edge, it being an axiom that any cutting edge pre-sented diagonally to material will cut with better facility than one presented at right angles thereto, because the former removes material in detail, begin-ning at one corner and working along to the other. The principle has its applica-tion in the diagonal bevel of a turning chisel, in the rounding of the edge of an axe or adze, in the “skew ” rebate plane, in the operation of paring with a chisel moved diagonally, in the diagonal slicing of a knife, and in the slight angle given to the cutting edge of a saw tooth. In the saw the teeth are bevelled in al-ternate directions, as shown in Figs. 15 and 16, which balances the action, and prevents the saw from running to one side in its cut. In practice all the teeth which slope in the same direction are sharpened in series, by which means the hand learns to preserve the precise angle alike for each, which would not be the ease if its direction were reversed at each tooth. The saw, therefore, being clamped in the vice, every alternate tooth comprising the set which Jeans away from the opera-tor is sharpened on the back. Then, the saw being reversed in the vice, the other half is also sharpened. With a saw in very bad order it is sometimes better to stop the teeth a second time after sharpen-ing and touch them up once more with the file. The smaller the saw the more difficulty is there in sharpening and setting, and it becomes very troublesome in the smaller tenon and dovetail saws, the shape of whose teeth will rapidly become unequal by carelessness. The “set” of saw teeth is variously given, and is of equal importance with correct sharpening. The best tools to use are the setting-block and hammer (Fig. 19). An automatic saw set is safer in the hands of an amateur, but work-men are not in love with such appli-ances, neither are they desirable from an educational point of view, since one of the chief uses of amateur handicraft is
the training of the eye and hands. For those who care about automatic saw sets
FIG. I9.—SETTING-BLOCK AND HAMMER
there are plenty in the market, most of them at reasonable prices. To use the setting-hammer and block, proceed as follows: Screw the block in a vice and lay the saw teeth upon the rounding ridge, the blade running, of course, longitudinally (Fig. 20). The amount of tip given to the saw blade in relation to the bloek determines the
FIG. 20.-RELATI VE POSITIONS OF S. H A MMER, AND BLOCK.
quantity of set, and, once decided on, the slope must be maintained constant throughout the whole operation of setting. The saw being held in the left hand, the right grasps the hammer, and a series of rapid and accurately directed blows strike the teeth over until their