The harder the wood, the greater the number of teeth the saw should have.
We will now consider the cross-cut saw tooth in regard to rake or pitch; this being one of the most important features, too much care cannot be taken to have the correct amount of pitch for the duty required. To illustrate this. [Fig. 5] represents a board, across which we wish to make a deep mark or score with the point of a knife. Suppose we hold the knife nearly perpendicular as at B; it is evident that it will push harder and will not cut as smoothly as if it were inclined forward as at A. It follows then that the cutting edge of a cross-cut saw should incline forward as at [C, Fig. 6], rather than stand perpendicular as at [D, Fig. 7].
Fig. 6.
Fig. 7.
Too much hook or pitch and too heavy a set are very common faults, not only detrimental to good work but ruinous to the saw; in the first case, by having a large amount of pitch, the saw takes hold so keenly that frequently it “hangs up” suddenly in the thrust—the result, a kinked or broken blade; in the second, by having too much set, the strain caused by the additional and unnecessary amount of set is out of proportion to the strength of the blade, and it is broken in the same manner. The most general value of pitch used is 60 degrees, though this may be varied a little, more or less, to advantage, as occasion may demand.
In all cases the size of tooth depends largely upon the duty required; a long tooth has the demerit of being weak and liable to spring, but the merit of giving a greater clearance to the saw-dust. The throat space in front of each tooth must be large enough to contain the dust of that tooth from one stroke; the greater the feed the deeper the dust chamber required, or the more teeth. Where the teeth are fine the shape of the throat is of special interest.
The teeth of a hand-saw should be filed so true that on holding it up to the eye and looking along its edge, it will show a central groove down which a fine needle will slide freely the entire length. This groove must be angular in shape and equal on each side, or the saw is not filed properly and will not run true.