This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The saw is a tool for cutting and dividing substances, chiefly wood, and consisting of a thin plate or blade of steel with a series of sharp teeth on one edge, which remove successive portions of the material by cutting or tearing. Some representative examples of handsaws are illustrated below: Fig. 292 is a panel and ripping saw; Fig. 293, a grafter saw , Fig. 294, a tenon saw; Fig. 295, a dovetail saw; Fig. 29G, an iron bow saw; Fig. 297, a frame turning saw.
The saw is essentially a tool for use across or at right angles to the fibres of the wood, although custom and convenience have arranged it for use along the fibres, still not when those fibres are straight and parallel. If in the growth of timber there was not any discontinuity in the straight lines of the fibres, then all longitudinal separation would be accomplished by axes or chisels. It is because this rectilineal continuity is interrupted by branches and other incidents of growth that the saw is used for ripping purposes. Were not some tool substituted for the wedge-like action of the axe, timber could not as a general rule be obtained from the log with flat surfaces. Hence the ripping saw, a tool which is intermediate between an axe and a saw proper. To study the saw as a tool fulfilling its own proper and undisturbed duties, it must be regarded in the character of a cross-cut saw. In this character it is called upon to meet the two opposing elements - cohesion and elasticity of fibre.
To deal with the treatment of fibrous wood at right angles to the length of the fibre is then clearly an operation in which considerations must enter, differing in many respects from those that decide action in direction of the grain. The object now is, as it were, to divide with the least expenditure of power a string which connects two ends of a tensioned bow. If a blow be given in the middle of a bow-string, the elasticity imparted by the bow to the string renders the blow inoperative. The amount of this elasticity is very apparent when one notes the distance it can project an arrow. Indeed, any one who has struck a tensioned. cord or a spring is well aware that the recoil throws back the instrument, and by so much abstracts from the intensity of the blow. To separate the string in this experiment even the pressure of a knife blade is insufficient; for a heavy pressure, as manifested by the bending of the string, is borne before separation takes place. It may be taken for granted that in thus severing the string, the power expended has been employed in two ways; first in bending the string; second in separating it.
If the string be supported and prevented from bending, and the same cutting edge be applied, and the power be measured by weights or a spring balance, it will be seen how much of the former wa3 expended in the useless act of bending the string, and therefore quite lost in the separating of it.
If the cutting instrument were a short narrow edge, or almost a sharpened point, and drawn forward, each fibre would be partially cut. A repetition of this action in the same line would still further deepen the cut. But a cutting edge requires support from a back, i.e. from the thicknessing of the metal, otherwise it would yield. Further, a cutting edge held at right angles to the surface of the fibres may not be the most effective position. Let any one draw the point of a knife across the grain of a smooth pine plank, holding the blade first at right angles to the surface, and, secondly, inclining forward, he will observe that by the first operation the fibres are roughly scratched; by the second they are smoothly divided.
Hence, even where the edge has deepened, this back support or metal strengthening must follow. It cannot do so upon this knife contrivance, because the sharp edge has not prepared a broad way for the thick back, which being of a wedge-like character should be acted upon by impact and not by such tension or thrust as in this case is only available. Therefore simple cutting is insufficient for the purpose of separating the fibres, but it has been suggestive.
If now something must enter the cut thicker than the edge, then it is clear that the edge alone is insufficient for the required purpose, and an edge, as a cutting edge alone, cannot be used for the separation of the fibres cross-wise. Longitudinally it may be, and is used, but in reality what appears to be thus used is a wedge, and not a cutting edge, for in a true cut the draw principle must enter. The axe and chisel do not work upon the cutting "edge," but upon the driven "wedge" principle. They are driven by impact, and not drawn by tension or thrust by pressure.
The consideration now suggested is not simply how to cross-cut the fibres, but, further, how to permit the material on which the edge is formed to follow without involving an inadmissible wedge action. It may be done as in a class of saws called metal saws, viz. making the " edge " the thickest part of the metal of the saw. This however, ignores the true principle of the saw, and introduces the file. It may, in passing, be well to remark that in marble cutting, where the apparent saw is only a blade of metal without teeth, this want of metal teeth is supplied by sharp sand, each grain of which becomes in turn a tooth, all acting in the manner of a file, and not a saw proper. A former method of cutting diamonds was similar to this. Two thin iron wires were twisted, and formed the string of a bow. These were used as a saw, the movable teeth being formed of diamond dust. A similar remark applies to a butcher's saw; its metal teeth really act as files.