Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 83

Taking the ordinary cross-cutting single-handed saw, the forward thrust is intended to separate the fibres, by removing a small piece by two parallel cuts. For example, if O O (Fig. 216) 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 parallel cuts a d, b c are, the better. Now this clean cut is to be made by the tooth advancing towards the fibre. If the tooth come on in axe fashion, there is a direct thrust of a sharp edge; and though a wedge-like action may be the best for separating fibre adhering to fibre, it is quite out of place in the cross-cutting of a, single fibre, in which cohesion has to be destroyed. There must be a cutting action, that is, the drawing of an edge across the mark for separation; besides this drawing action there also is pressure. Now, in soft timber, and with a saw having teeth only moderately sharp, this pressure tends 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 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, and when all the saw teeth are in action, pressure is not required, the average weight of the saw-blade sufficing for the picking up of the fibres. Whatever may be the form of the teeth, the small piece a, b, c, d (Fig. 216), 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, and thus condensed it will be forced up into the space between the teeth. This space must be so formed as to allow the condensed piece to drop freely away as soon as the tooth passes from the timber, or the saw will become choked, and its proper action cease. In large saws this is provided for in the shape of what in America are called “gums” and in England “throats,” in which the teeth may be said to be set. Saws cannot work easily unless as much care is bestowed upon the throats or gums as is given to the teeth.

Fig. 216.—Wood Fibre Divided by Saw.

Fig. 217.—Hand Saw Handle.

Fig. 218.—Hand Saw Handle.

Fig. 219.—Hand Saw Handle.

Fig. 220.—Hand Saw Handle.

Fig. 221.—Hand Saw Handle.