Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 86

The coarseness and fineness of saws are estimated by the number of teeth points in an inch. Figs. 223 to 237 show (to full scale) a variety of sizes. By the saw-maker’s term “pitch” is meant 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 in the most perfect manner a saw with such teeth would require that the action at each end should be the same; hence, these are the forms of teeth generally met in the ordinary two-handled saw used for the cross-cutting of timber and some varieties of soft stone. The forms, however, of teeth to cut in both directions are sometimes more varied. In cross-cutting timber with a one-handled saw, the action is either tension or thrust—one of these only. The only reason why both are not adopted seems to be that very different muscular motions and postures of the body would then be introduced, and experience may have shown that these are more fatiguing than the alternate pressure and relaxation which take 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, and not separate the fibres. In this case the back of the tooth may be sloped away, as shown later, 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 thus they are distinguished in the tool trade; some of these names are: Peg tooth, M tooth, half-moon tooth, gullet tooth, briar tooth; also upright pitch, flat pitch, slight pitch. The customary shape has the face of the tooth 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 handsaw pitch. (The substance of this paragraph and of the five that precede it is derived from Kigg’s Cantor Lectures.)

Fig. 238.—Stright Back Hand Saw.

Fig. 239.—Hand Saw with Nibbed Back.

Fig. 240.—Skew Back or Hollow Back Hand Saw.

Fig. 241.—Saw with Rule Edge.

Fig. 242.—Perforated Saw.

Fig. 243.—One-man Two-handled Saw.