The processes denominated sharpening and setting a saw, consist, as the names imply, of two distinct operations: the first being that of filing the teeth until their extremities are sharp; the second, that of bending the teeth in an equal manner, and alternately to the right and left, so that when the eye is directed along the edge, the teeth of rectilinear saws may appear exactly in two lines, forming collectively an edge somewhat exceeding the thickness of the blade itself.
Circular saws require exactly similar treatment, if we consider the tangent of the circle to be substituted for the right line; and therefore the sharpening of straight saws will be first described, and those peculiarities alone which attach to the sharpening of circular saws will be then separately noticed.
Setting the teeth, which in practice is always subsequent to the sharpening, will also be placed subsequently in the section; the commencement of which will be devoted to the modes of holding the saw in the operation of sharpening, and the description of the files used.
In sharpening the saw it is mostly fixed perpendicularly, and with its teeth upwards, various modes being adopted according to circumstances. The tail-vice used by the saw-maker in sharpening the saw, measures from nine to twelve inches wide in the chops, and also nine to twelve inches high, or above the screw; proportions exceeding those of tail-vices used by mechanicians generally. Slips of wood, or clamps of sheet lead bent to the figure of the jaws of the vice, are interposed between the saw and the vice, so that the elasticity of the wood, or the inelasticity of the lead may give a firm hold, and prevent the disagreeable screeching noise that accompanies the action of the file when the saw is insecurely held; and the greater the noise the less the amount of work that is done.
The joiner employs a wooden vice resembling that of the saw-maker as to proportions, but it is fixed in the screw-chops of his work-bench.
In sharpening pit-saws, the sawyer seldom finds it necessary to remove the handles or frames. The long or whip-saw, and others not having frames, are supported in the sawing-horse, a trestle about five feet long and two feet high, with four or five uprights or wooden pegs, sawn half-way through to receive the back edge of the blade; the horse raises the edge of the saw about three feet from the ground. A more convenient mode is to have a jointed-horse, fig. 654 two halves of which open somewhat like the jaws of a pair of pliers; when the saw has been inserted, the legs of the horse are distended by the stretchers at the ends, and fix the blade.
The files used in sharpening saws arc triangular, round, half-round, and mill saw-files. The equilateral triangular files, commonly designated as three-square files, vary from about three to nine inches long; for small saws they are generally taper; for large, sometimes nearly parallel, when they are called blunts, a term applied to other nearly parallel files. The triangular file is exclusively used for the teeth of figs. 643 to 646, and more or less for all the rectilinear teeth. For small teeth, the double-cut Lancashire files arc the most used, on account of the keen-ness of their edges, and the common size is 4 1/2 inches long. The generality of other saw-files are single or float-cut, that kind of file tooth being considered to 'cut sweeter,' and do more work.
Round files from 5 to 8 inches long, arc used in saw-mills for the gullets of the teeth, figs. 650 to 668. and flat files for tin-tops; but the pit-sawyer and some others always employ half-round files, as the one instrument may be then applied to both purposes; these files arc always blunt or parallel.
Mill saw-files are in general thin, flat and parallel, from 6 to 14 inches long, float-cut on the sides, and with smooth, square edges. Sometimes, however, they have round and cutting edges, and are of taper figure.
The five ordinary modes of sharpening saws will be explained and illustrated by enlarged diagrams in three views, which denote the ways in which the teeth are bevelled and set; but a few general observations that apply to each mode will be first given.
In general, the angles of the points of the saw-teeth are more acute, the softer the material to be sawn, agreeably to common usage in cutting tools; and the angles of the points, and those at which the files are applied, are necessarily the same. Thus in sharpening saws for metal, the file is generally held at 90 degrees, both in the horizontal and vertical angle, as will be shown; for very hard woods at from 90 to 80 degrees, and for very soft woods at from 70 to 60 degrees, or even more acutely. The vertical angle is about half the horizontal.
In general the horizontal angle of the file is alone important, (that is, considering the saw-blade vertical and with the teeth upward,) although to assist the action of the file it is customary to depress the handle a little below the point of the file, and only to file on those teeth which are bent from the operator. When the tooth that is bent towards the individual is filed, it vibrates with much noise, and is disposed to strip off the teeth from the file, instead of being itself reduced.
To insure the action of each tooth, the edge of the saw should be quite straight; it is therefore occasionally topped, by laying the file divested of its handle, lengthways upon the teeth, and passing it along once or twice, to reduce these few points which may be above the general level. The file is pressed hard at the two ends of the saw, where the blade is less worn, and is applied lightly in passing the middle; the file should be held perfectly square, to reduce the edges alike. The new point of each tooth is then made to fall as nearly as possible upon the center of the little facet, thus exposed by the process of topping or ranging the teeth; and the faces or fronts of the teeth are always filed before the tops of the same.