Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 82

For the purpose of separating a bundle of fibres, an edge drawn across will cut the surface fibres only; this is insufficient, for a saw is required to separate fibres below a surface. This separation must be a cutting (not a tearing) action. Looking at the work of a single cutting edge, it will be noticed that, although the continuity of the fibre is destroyed, 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 be removed, for 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; for instance, the absence of any guide would cause great difficulty. To draw a pointed cutting edge along the same deepening line needs a very steady hand and eye. To increase the number of cutting edges, and form, as it were, a linear sequence of them, may give a partial guidance. Instead of having two parallel cutters these cutters may be externally parallel but internally oblique to the line of cut, as shown in the sectional and exaggerated view of saw teeth (Fig. 214), in which 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 a portion within the oblique arms is left, this consisting of particles 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, and both power and material are economised by narrowing A E. The resultant saw kerf is shown in section by Fig. 215. The active portion of a saw has three edges, of which the lower or horizontal one only is operative, for the tool rides upon the fibres and divides them, and the sloping parts remove the hillock. To act thus, the lower edges would be required to be sharpened at A and E, so as to clear a way for the metal to follow. The resistance to the downward pressure, required to cause the cutting segments to penetrate vertically, 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 two pressures being at right angles do not aid each other, and will require the use of the workman’s two hands. The compounding of these pressures will give freedom to at least one hand. For the present, assume that the two 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. 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.

Fig. 214.—Diagram showing Principle of Saw’s Cutting Action.

Fig. 215.—Saw Kerf.