Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 52

So far as the actual cutting is concerned, the action of the knife is very similar to that of the chisel indicated in Fig. 130; but in the manner in which the force A is applied to the tool an important difference exists. In Fig. 132 the force A is again shown; but in this case it is obtained by the muscular resistance to the forces, B and C. Now, since C + A equals B (for it is a case of parallel forces, and the sum of the forces acting in one direction must equal the sum of those acting in the opposite direction), it is evident that B alone is greater than A. In the case of the chisel, the muscular effort is that required to produce the force A; but in the case of the knife, muscular effort equal to B + C has to be produced; and, of these forces, one alone is greater than A. Under these conditions, it is the advantage obtained by the sawing motion that enables the knife to compete with the chisel. When the thumb is extended along the back of the knife, there is virtually a strutted cantilever (Fig. 133), and the strain on the muscles is then not so great. In almost every case, however, the use of the knife results in the maximum of effort and the minimum of effect, and as a workshop tool it cannot compete with the chisel, though experience has shown it to have many advantages from an educational standpoint.

Fig. 133.—Thumb used as Strut in Cutting with Knife.

Figs. 134 to 136.—Firmer Chisels.