The saw is an indispensable tool, and in the case of most articles it is the first used. The blade is made of thin steel of various breadths, on one edge of which a series of sharp points form the teeth. The steel must be soft enough to be acted on by the file, and to admit of the teeth being slightly turned aside without breaking off.
The saw acts by tearing or cutting the fibres of the wood as the teeth of the blade pass over them. The teeth are, therefore, the characteristic part of the saw, and its efficiency depends on their form, size, and quality.
The shape and size of the teeth vary considerably in different kinds of saws. The form generally used in wood slojd is shown in Fig. 32. The form of the teeth is that of a scalene triangle, the base of which is formed by the blade. The shortest side froms an angle of 80°-90° with the base. In the frame saw (Fig. 37, B), the angle is 90°; in the bow-saw, the dove-tail saw, etc., it is S0°-85°.
Fig. 32. Teeth of a bow saw for ripping. 1/1.
The teeth of any given saw must always be alike in size and shape, and must always be set at the same angle. The shorter side of the teeth, being nearly at right angles with the blade, is the cutting side, and in working the saw this is the side which should enter the wood. When the saw is drawn back, the more sloping side of the teeth has very little effect upon the fibres, and the saw "goes empty."
Form and position of the teeth of a saw.
Fig. 33. Teeth of a bow-saw for cross-cutting, or wood-saw. 1/1.
The teeth of the bow-saw for cross cutting form an isosceles triangle of 50° between the teeth. A saw of this description cuts equally well backwards or forwards The space between the teeth must be great enough to leave room for the sawdust until the saw has carried the latter beyond the wood. Now, as the sawdust occupies more space than the wood from which it is produced, the teeth of the saw must be considerably longer than the depth of the cut made each time the saw passes through the wood, and the point only of the teeth must be allowed to cut the wood, to prevent hindrance to their action by an accumulation of sawdust. If the sawdust prevents the free passage of the saw, or if it clings about the teeth, it is either because the teeth are too small, or because too much pressure is laid on the saw.
Length of the teeth.
It is almost impossible to avoid considerable friction between the blade and the sides of the cut, and this friction is increased by the sawdust which accumulates at the sides of the blade. It is therefore necessary to give the blade a certain amount of " play ;" in other words, the breadth of the cut must be greater than the thickness of the blade. This is effected by bending the teeth alternately a little to the one side and to the other, or, as it is termed, by setting the saw.
Why the saw must be set.
Setting: is performed by means of the Saw-set, a steel blade 1/16 inch thick, the edges of which are indented by notches of various breadths. Some English Saw-sets are furnished with an adjustable slide rest. In setting a saw the blade is fastened into Saw Sharpening Clamps (Fig. 35) and these are screwed to the bench. One tooth after another is grasped by the notch of the Saw-set best adapted to the thickness of the tooth, and the blade of the Saw-set being held in such a way as to concealthe point of the tooth, the latter is then turned sharply aside. It must not, as is sometimes done, be twisted at the same time in the direction of the length of the blade, as this may cause it to break off. Great accuracy is required in the operation, and the setting should never be so extreme that the width of the cut is more than double the thickness of the teeth. If this width is exceeded the saw will not act easily.
Considerable practice and skill are required to set a saw. The points of the teeth should form a line exactly parallel to the length of the blade, but it often happens that some teeth project beyond this line and others fall within it. This fault may be remedied to some extent by drawing the blade between a couple of gouges, fixed points downwards in a piece of wood, with the convex sides facing one another.
Fig. 34. Saw-set. 1/3.
Fig. 35. Saw sharp- Unevenly ening clamps. One set saws. half loosely fastened to the other by means of wood screws. 1/6.
To what ex. tent a saw should be set.
The blade of the saw is placed between them, teeth upwards, and the points turned from the operator, or in the direction from d to c (Fig. 32), the handles are grasped with one hand to bring the gouges close together, and the blade of the saw is drawn forwards between them.
In consequence of the difficulty of setting a saw evenly and at a good angle, many different kinds of saw-sets and setting-tongs have been devised. The latter are intended to be adjustable for any desired inclination of the teeth. Some of these tools, however, are not practically useful, and those which are fully adapted for use are generally too expensive for ordinary purposes.
As indicated above, setting must not go beyond a fixed limit. Provided that the saw has free passage through the wood, the finer the cut it makes the better; and much less inclination of the teeth is necessary, in the case of dry timber, than in unseasoned or loose-fibred wood.