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.
A few words upon the handles of single-handed saws. Whatever may be the other conditions required in handles, the large majority of saw-handles have the curved hooked projections a and b, Fig. 299; these are connected with the pressure of the sawyer on the teeth. If, in sawing, the hand bears upon the upper hook a, then an increased pressure is given to the forward teeth; if upon the hook b, the pressure on the forward teeth is released, and consequent ease in sawing results, also a pressure may be given to the back teeth. The angle at which direct thrust ought to act upon the line of teeth in the saws is obviously very different. Each material may be said to have its own proper angle. Provision may be made by 2 set screws above a and b for varying the intersection of the line of thrust with the line of teeth. It will be further noticed that in the handle of the "one-man saw," Fig. 301, the upper hook is wanting, and this because under any circumstances the weight of the saw is more than sufficient, and therefore it is not requisite that any resolved portion of the workman's energy should be compounded with this. Not so with the other hook; that is retained in order that thus the weight of the saw may be taken from the work.
For these reasons the line of direct thrust is nearly parallel with that of the teeth. We seem to be guilty of much inconsistency in the placing as well as in the formation of saw handles.
A brief recapitulation of what has been said may suitably close this far from exhausted branch of the subject.
There have been considered : -
The effect of impact transverse to fibre.
The effect of thrust transverse to fibre.
The passing of a cutting edge transverse to fibre.
The reduction of length of cutting edge transverse to fibre.
The introduction of combined vertical with horizontal cut.
The rounding off the back of cutting edge.
The pressures required in sawing.
Tension compared with thrust.
The angular position of handle.
The resolution of forces operating.
Now may be considered the circumstances which influence the form and position both of the teeth and the edges to be put upon them, in the case of hand-saws operating either by thrust alone, or by thrust and tension combined (as in the 2-handled cross-cutting saws used by 2 men, or in the whip and frame saw used in saw pits). Unless specially mentioned the thrust hand-saw for cross-cutting will be the only one considered.
It may be well at the outset to explain that the coarseness and fineness of saws are estimated by the number of teeth points in an inch. The sawmaker uses the term "pitch," but not in the sense as employed in wheels and screws. By pitch he "means the inclination of the face of the teeth up which the shaving ascends." Clearly if the saw is to cut when drawn in both directions, the slope of the teeth from the points must be the same on both sides; indeed, this may be considered the primitive form of saw teeth, and derived as the saw is said to have been from the backbone of a fish, it is the form that would be suggested. To use a saw with such teeth in the most perfect manner would require that the action at each end should be the same; hence, these are the forms of teeth generally met in the ordinary 2-handled saw used for the cross-cutting of timber. The teeth of these saws are generally wide spaced, and the angle included in their point is from 40° to 60°. The forms, however, of teeth, to cut in both directions, are sometimes more varied, especially when the material is not of uniform non-fibrous character.
When this equality of tension in both directions cannot be had, and the workman is required to cross-cut the timber by a one-handled saw, it is clear that he must consider the action as that of tension or thrust alone - one of these only. The sole reason why both are not adopted seems to be that were it so, very different muscular motions and postures of the body would be introduced, and probably experience has shown that these are more fatiguing than the alternate pressure and relaxation which takes place in the ordinary process of hand-sawing. Now, if the cut is in the thrust only, then the form of the back of the tooth must be the very reverse of that of the front, for it ought to slide past the wood, because not required to separate the fibres. In this case the back of the tooth may be sloped away, or it may be shaped otherwise. The faces of the teeth are no longer bound to be formed in reference to an equality at the back. Indeed, with the liberty thus accorded, there has arisen an amount of fancy in the forms of teeth, which fancy has developed into prejudice and fashion. Names dependent either upon uses or forms are given to these, and they are distinguished by such names in the trade.
Peg tooth, M tooth, half-moon tooth, gullet tooth, briar tooth; also "upright pitch," "flat pitch," "slight pitch." Of these varieties, custom has selected for most general use in England the one in which the face of the tooth is at right angles to the line of the teeth. The backs of the teeth are, therefore, sloped according to the distance between the teeth and the coarseness or fineness of the saw. This is called ordinary, or hand-saw pitch.
A consideration of the action of the saw in cross-cutting timber settles the cutting edge, and so suggests the mode of sharpening. Taking our ordinary cross-cutting single-handed saw as the type, the forward thrust is intended to separate the fibres, and this not in the way of driving a wedge, but in the actual removal of a small piece by two parallel cuts. For example, if O O. Fig. 300, be a fibre, then the action of the saw must be to cut clean out the piece a, b, so making a space a, b, wider than the steel of which the saw is made. The cleaner the cuts a d,bc are the better. Now this clean cut is to be made by the teeth advancing toward the fibre. If they come on in axe fashion, then the separation is accomplished by the direct thrust of a sharp edge, in fact, by a direct wedge-like action. Now a wedge-like action may be the best for separating fibre adhering to fibre, but it is an action quite out of place in the cross-cutting of a single fibre, in which cohesion has to be destroyed. There is needed a cutting action, i.e. a drawing of an edge, however sharp, across the mark for separation; this drawing action is very important. Admit for the present that such action is essential, then the saw tooth as constructed does not supply it.
Clearly the sharp edge must somehow or other be drawn and pressed as drawn across the fibre. Two ways of accomplishing this present themselves. The effect on the action of the workman is very different in these cases. In the first we must press the saw upon the fibre, and at the same time thrust it lengthwise. Now in soft timber, and with a saw having teeth only moderately sharp, this pressure will tend rather to force the fibres into closer contact, to squeeze them amongst each other, to solidify the timber, and increase the difficulty in cutting. Two actions are here, pressure and thrust. In the second case the pressure must be very light indeed; if otherwise, the point of the tooth will gather up more fibres than the strength of the workman can separate; indeed, as a rule, in the cross-cutting of broad timber, with all the saw teeth in action, pressure is not required, the average weight of the saw-blade sufficing for the picking up of the fibres. It is probably from the delicate and skilful handling which a tooth thus constructed requires, that hand-saws are not more generally constructed with teeth of this form. In addition to these there is the penetrating tooth, as the points of the peg tooth and others.
Whatever may bo the form of the teeth, the small piece ab, c d, Fig. 300, has to be removed so as to leave the ends from which it is taken as smooth and clean cut as possible, therefore the cutting edge must be on the outside of the tooth. This being so, it follows that the act of severing a fibre will be attended with compression whose effect is to shorten it. Thus condensed it is forced up into the space between the teeth. If now this space is not so formed as to allow the condensed piece to drop freely away so soon as the tooth passes from the timber, then the saw will become choked, and its proper action will necessarily cease. In large saws this is provided for in the shape of the "gums" in which the teeth may be said to be set. What in America are called "gums" are frequently in England called " throats." Saws cannot work easily unless as much care is bestowed upon the " throats " or " gums " as is given to the teeth.