The necessity for this action may be plainly perceived if we apply the rip saw for cross-cutting and the cross-cut saw for ripping. Suppose, for example, we place the saw shown in [Fig. 2750] to cut across the grain of the piece of timber, and as its tooth met the wood it would indent it as at g, [Fig. 2751], and as this is in line with the grain, the tooth would wedge in the piece and the piece cut could not be dislodged without first tearing the fibres apart at each end. Or suppose we take the cross-cut saw and apply it for ripping (as cutting lengthways of the grain is called) and if we indented the surface with a single tooth it would leave a mark as at f, [Fig. 2751], which is lengthways of the fibre, so that the tooth would here again wedge between the fibres and not cut them. The next tooth would make a mark parallel to f, but on the other side of the saw slot or kerf as it is called, still leaving the fibre unsevered at its ends where it should be severed first.
In order that the saw may not rub against the sides of the slot or kerf, and thus be hard to move or drive, it is necessary that the kerf be wider than the thickness of the saw blade, and to accomplish this the teeth are bent sideways, each alternate tooth being bent in an opposite direction, as shown in the front view of the teeth in [Fig. 2753]. This bending is called the set of the saw, and should be sufficient to make the kerf about two-thirds wider than the thickness of the saw blade.
While preserving the feature of severing the fibre before attempting to dislodge it from its place, we may at the same time give the teeth of rip saws more or less sharpness by fleaming their faces.
In [Fig. 2754], for example, the throat face is filed square across or at a right angle to the length of the saw, but the back face a is at an angle, making the points of the teeth sharper, and therefore enabling them to cut more freely. The result of this fleam would be that the tooth, instead of cutting equal and level all the way across as in [Fig. 2751] at e, would cut at the corner first and only across its full width as it entered deeper into the wood; we have, in fact, placed the leading part of the cutting edge more at the extreme point and less in front of the tooth.
In [Fig. 2755] the throat or front face of the saw is given fleam, as shown by the line b, which is not at a right angle to the saw length, and as a result the cutting edge is carried still more advanced at the point and more towards the side of the tooth and we have, therefore, to a certain extent, qualified it as a cross-cut saw.
We might give the face b so much angle as to carry the leading part of the cutting edge to the side of the saw, thus giving it the characteristics of a cross cut.
In [Fig. 2756], both the throat face b and the back face a are given fleam, making the points extremely sharp, and showing the leading part of the cutting edge towards the side, the corner leading still more.
Fig. 2757.