Fig. 67.

Fig. 68 shows the shape of this sharpened tooth, and also the 'set' usually given. In actual sharpening, the file also removes a small portion of the back edge of the preceding tooth. This is not requisite but is unavoidable, and only has the effect of assisting to make the points of the teeth.

Now the points of the teeth serve to help the saw in entering the wood, and the points and chisel-like front edges are almost essential to the cutting of the teeth in the forward stroke.

The angle at which this front edge should be sharpened varies, but all saws, even for the hardest woods and to overcome the greatest resistance, should have this chisel edge. It may be taken generally, however, that for soft woods, and with the grain where the saw would cause much sawdust, which must be pushed out by the teeth, the angle may be less acute than for harder woods and across the grain, where fine-edged teeth are required.

Saws are made of many shapes to fulfil particular conditions; and neither makers nor workmen are at all agreed as to the particular shape, set, or sharpening of the teeth; but all thrust-saws for use by hand have some common characteristics.

The hole for the hand should be of such a shape as will easily admit all the fingers, and allow them to grasp the handle well and easily.

The handle should be fastened to the blade at such an angle that a line drawn from the point of the front cutting tooth 1 2 through the middle of the handle will cross the hole for the fingers at right angles. When held in the hand a saw should be balanced in any position, except that which it is held in when at work. In this position a much heavier weight will appear to depend from the handle, and this it is always desirable to obtain.

Fig. 68.

The riveting of the handle to the blade is a point of great importance.

The handle must be parallel to the plane of the saw edge, and the riveting must be very secure, as if it is at all loose, the defect will rapidly become much worse and spoil the saw as an effective tool.

If the blade is made to quiver, defective riveting may be discovered by a slight jarring sound coming from it. A good saw will give out a clear, full sound if the blade is bent and then suddenly released. The blades of all saws should be thin and of a dark colour. These dark-coloured saws are usually found to be well tempered, as are also those with a greasy appearance on the surfaces of the blade.

Thick saws are generally too soft, and though it is difficult to ascertain before purchase, too highly-tempered saws will not be found the best, as the points of the teeth are liable to break off on encountering any hard substance.

A very common test of the temper of a saw, however, is to bend it at least a foot from its natural position, but this is by no means reliable, and is harmful to the saw.