Part 6 (2/2)

The size of saws is indicated by the length of the blade in inches.

The coa.r.s.eness of the tooth is indicated by the number of ”points”

to the inch. ”Points” should not be confused with teeth as there is always one more point per inch than there are teeth. For example, a five point rip-saw has five points to the inch but only four full teeth, Fig. 87. Rip-saws run from 4 to 7 points per inch; crosscut-saws from 6 to 12 points per inch.

In general, saws are of two kinds, rip-saws and crosscut-saws.

The _rip-saw_, Fig. 87, may be thought of as a series of chisels set in two parallel rows which overlap each other, for each tooth is filed to a sharp edge which, at each stroke, chisels off a small particle from the end of the wood fibers.

The shape of the teeth is the result of experience in uniting a number of factors: as, strength of the individual tooth, the acuteness of the cutting angle, and the ease of sharpening. The steel of a saw is softer than that of a chisel, in order that it may be filed and set.

Hence it is weaker and the edge cannot be so acute. A typical form of tooth is shown in Fig. 87, in which A is an edge view, B the side view, and C a cross section. The angle of each tooth covers 60, one side, the ”face”, being at right angles to the line of the teeth. The cutting edge runs at right angles to the sides of the blade.

This arrangement works with entire success along the grain, but if a rip-saw is used to cut across the grain, since there is no provision for cutting thru the fibers, each tooth catches in them and tears them out, thus leaving a rough and jagged surface.

In the _crosscut-saw_, therefore, the teeth are filed to points, and the cutting edge is on the forward side of each alternate tooth.

In Fig. 87. A' is the edge view, B' is the side view and C' is a cross-section. In a properly filed crosscut-saw a needle will slide between these two rows of teeth from one end of the saw to the other.

[Ill.u.s.tration: Fig. 88. Rip-Sawing on a Horse.]

In action the points, especially their forward edges, cut or score the fibres of wood, and then the triangular elevation of wood left between the two rows of points is crumbled off by friction as the saw pa.s.ses through. Thus it drops farther and farther into the cut. A crosscut-saw may be thought of as a series of knife points, arranged in two parallel rows. Ordinarily the angle of the ”face” of each tooth with the line of the teeth is about 65, and slightly steeper than the back of the tooth. The angle of the cutting edge of each tooth may be filed more acute when the saw is to be used for soft wood only.

A crosscut-saw when used to rip a board, works slowly, for there is no chisel action to cut out the fibres between the points, but the cut, tho slow, is smooth. In cutting diagonally across a piece of wood, especially soft wood, a rip-saw cuts faster, but a crosscut, smoother.

In ripping a board, allowance should always be made for planing to the line afterward. In starting a cut with the rip-saw, the weight of the saw should be borne by the right hand so that the teeth may pa.s.s over the edge of the wood as lightly as possible. The left thumb acts as a guide. If the saw be handled thus, and the angle with the board be quite acute, it is not necessary to start with a back stroke. When the kerf is well started, the whole weight of the saw may be applied. An easy light stroke is better than a furious one. The line should be followed carefully, but if the saw runs from the line it may be brought back by taking short strokes near the point of the saw and twisting the blade slightly in the desired direction. If the saw binds and buckles because of the springing together of the wood, the kerf may be wedged open with a screwdriver or a bit of waste wood. A drop of oil rubbed across each side of the saw will make it work more easily.

Care should be taken in finis.h.i.+ng a cut to hold up firmly the part of the wood which is being sawn off so that it will not split off or splinter.

[Ill.u.s.tration: Fig. 89. Rip-sawing with Wood Held in Bench-Vise.]

Sawing may be done either on a saw-horse, Fig. 88, or at a bench. For big, rough work, the former is the common way, the worker holding the material in place with one knee, because this method enables him to exert his greatest strength. A convenient way for rip-sawing a small piece of wood is to insert it in the vise, Fig. 89, with the broad side of the board parallel to the vise screw, and the board inclined away from the worker who stands upright. The start is easy, the sawdust does not cover the line, and the board is not in danger of splitting. The board, however, has to be reversed after it is sawn part way thru, in order to finish the saw cut.

The _back-saw_ or _tenon-saw_, Fig. 90, is a fine crosscut-saw, with a rib of steel along the back, which gives to it its name. Since it is intended for small accurate work, the teeth have little or no set.

In sawing, the wood may be held either in the vise or on the bench-hook. To help start the saw and at the same time to keep the edges of the cut sharp, it is well to make a little groove with the knife, on the waste side of the line to be followed, cutting the side of the groove next to the line at right angles to the surface. The saw drops directly into this groove, Fig. 91. In starting the saw cut, the saw should be guided by holding the thumb of the left hand against the side of the saw just above the teeth. Until the kerf is well started, the saw should be held so that the teeth just touch the wood. It is better not to attempt to start the saw level, i.e., with the teeth resting clear across the wood, but the handle should be raised so that the start is made only at the farther edge of the wood. Then as the saw is gradually lowered, the kerf will extend quite across the wood.

Fig. 92. When the back-saw is used for ripping, the wood is held in the vise, end up. Begin sawing as in crosscutting, that is, at the farther corner with the handle end of the saw up, and gradually drop the handle. Watch the lines on both the front and back sides, and if necessary, reverse the piece to follow them.

[Ill.u.s.tration: Fig. 90. Using the Back-Saw with Bench-Hook.]

[Ill.u.s.tration: Fig. 91. Starting a Saw Cut in a Trough Cut With Knife.]

[Ill.u.s.tration: Fig. 92. Direction of the Back-Saw.]

[Ill.u.s.tration: Fig. 93. Dovetail-saw.]

[Ill.u.s.tration: Fig. 94. Compa.s.s-Saw.]

The _dovetail-saw_, Fig. 93, is a small back-saw for delicate work.

The _compa.s.s-saw_, Fig. 94, is narrow, pointed, thick, to prevent buckling, and with a wide set to the teeth, to help in following the curves. The teeth are a cross between the rip and crosscut teeth. It is used in sawing curves.

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