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.
For the purpose of separating a bundle of fibres, the "edge" cannot be the edge with which we are familiar in axes and chisels. Such an edge drawn across will cut fibres on a surface only; this is insufficient, for a saw is required to cut fibres below a surface.
The tearing also of upper fibres from lower ones is not consistent with true work. To actually cut or separate these is the question to be considered, and the simple answer is another question. Can a narrow chisel be introduced which shall remove the piece of fibre whose continuity has been destroyed by cutting edges previously alluded to ? If so, then an opening or way will have been found along which the back or strengthening part of the cutting edge can be moved. If, however, we look at the work of a single cutting edge, we notice that, although the continuity of the fibre is destroyed, yet the separated ends are still interlaced amongst the other fibres. To obtain a piece removable as by a small narrow chisel, it will be requisite to make a second cut parallel to the first. This being done, there is the short piece, retained in position by adhesion only, which must by some contrivance be removed, for it is in the way, and the room it occupies is that in which the back of the cutting edge must move.
To slide, as it were, a narrow chisel along and cut it out is more simple in suggestion than in execution.
There is another defect upon the application of what at first seems sufficient in principle, but only wanting in physical strength - it is the absence of any guide. To draw a pointed cutting edge along the same deepening line needs a very steady hand and eye. This consideration of the problem requires that some guide principle must enter.
To increase the number of cutting edges, and form as it were a linear sequence of them, may give a partial guidance, and if the introduction of our chisel suggestion be impracticable, then another device must be sought. Instead of the 2 parallel cutters, it will be possible to make these externally parallel but internally oblique to the line of cut, in other words to sharpen them as an adze is sharpened and not as an axe, and in doing so one obstacle will be removed, it is true, but a blemish which was non-existent will appear. The combining obliquity of the dividing edges will so press upon the intervening piece of fibre as to press it downwards into and upon the lower fibres, thus solidifying, and, in so far as this is done, increasing the difficulty of progressing through the timber.
Note the mode of operating, as shown by Fig. 298. The portions of wood a b d and e c d have been removed by the gradual penetration of the oblique arms - not only have they been cut, but they have been carried forward and backward and removed, leaving a clear space behind them of the width a e. But how with regard to the portion within the oblique arms? That part would either be left as an impeding hillock, or it would have to be removed by the introduction of such a plan as making rough the insides of these oblique arms. If we consider the nature of the material left, it will be admitted to consist of parlicles of woody fibre adhering to each other only by the glutinous or gummy matter of the timber, and not cohering. If the breadth a e is not too large, the whole of the heap would be rubbed away by the power exerted by the workman. There will therefore be not only economy in power, but economy also in material in narrowing a e. If attention be given to the form of the pieces bent from the plane of the metal of which this cutting instrument is made, it will ho observed that the active portion has 3 edges, of which the lower or horizontal one only is operative, for the tool rides upon the fibres, divides them, and when the dividing has been accomplished, the sloping parts will remove the hillock.
To act thus, the lower edges would require to be sharpened at a and e so as to clear a gate for the metal to follow. The action of the tool as described would require a downward pressure, in order to cause the cutting segments to penetrate vertically. The resistance to this downward entrance is the breadth of the " tooth," for it rides upon a number of fibres and divides them by sliding over; the complete action requires not only downward pressure for the cut, but also horizontal pressure for the motion, the latter both in the advance and withdrawal of the tool. These 2 pressures being at right angles do not aid each other, and will employ both hands of the workman. It is very obvious that the compounding of these will give freedom to at least one hand.
For the present, assume that the 2 pressures to be compounded are equal, then the simple operation is to employ one pressure making (say) an angle of 45° with the horizontal line of thrust. Although this be done, yet if the saws be any length, clearly the angle will vary, and therefore the effect of the sawyer's labour will be counteracted, either as a consequence of excessive thrust or of excessive pressure at the beginning or ending of the stroke. In fact, not only the position in which the handle is fixed on the saw, but the very handling itself will require those adaptations which experience alone can give.
The effect of this will be to cause the forward points to penetrate, and cross-cut the fibres obliquely. The return action will be altogether lost unless the instrument is arranged accordingly, and sloped in the other direction.
If the tool becomes a single-handed one, and relies for its operation upon thrust or tension in one direction only (say thrust), then cutting edges on the back portions of the teeth are useless, and had better be removed.
The experiment worthy of trial is, can the whole power, or nearly the whole power, be converted into a tension or thrust for cutting purposes. To do this the cutting edge must be so formed as to be almost self-penetrating; then the cutting edge is no longer a horizontal edge, but it becomes oblique, on the advancing face, and formed thus there is no reason why it should not also be oblique on the back face, and so cut equally in both directions. The inclination of these faces to the path of the saw must be determined by the power - whether it is capable of separating as many fibres as the teeth ride between, and if these are formed to cut each way (as a single-handed tool) whether it could be done; because it necessitates a construction to which tension and thrust may be alternately applied. The nature of the wood, the power and skid of the workman, and the strength of the metal, must answer this suggestion.