(fig. 73). This saw is used to make sharp curves, and consequently very little substance from the teeth to the back of the saw is allowable, or it will not go smoothly round the line of cut. The difficulty of giving to so weak a blade the necessary stiffness is overcome by the peculiar frame at the back. The saw blade is fastened into the projections from the handle, and the opposite end by small, easily detachable rivets.

Fig. 73.

The string at the back, when twisted, contracts in length, and pulls in the tops of the bowed ends of the frame, and the lower ends of these are pressed outward, the rail in the middle acting as a fulcrum; the blade is thus kept continually in tension.

The handles containing the extremities of the blade can be turned in their sockets, rendering the saw much handier than would otherwise be the case.

The blades of these saws are in various dimensions, and present the rather characteristic feature that they are 'free' from tooth to back, i.e., diminish in thickness, so that the section is almost triangular.

The teeth should be as fine as those of the tenon saw, or even as small as in the dovetail saw, and should be used very finely set.

Files are for wearing off the substance of work where chisels, gouges, or spokeshaves cannot be used, for sharpening tools, and various other purposes.

The mechanical principle involved in the construction of all files and rasps is very much the same as the saw, the only difference being that the space operated on by the file is generally larger, and this tool is only used for reducing the surface at a particular place, and not to rasp its way entirely through. The teeth are parallel oblique ridges raised by the blows of a sharp instrument, and sometimes other parallel oblique ridges in the opposite direction are added, giving numerous front cutting angles as well as two sets of inclined cutting edges.

These are 'double cut' files, and are generally in use. The teeth of most ordinary files are similar to those in fig. 74.

The action of the instrument which raises these ridges, however, has a tendency, as will be seen, to turn over the points slightly, and an ingenious sharpening process, by the wearing action of a shower of fine sand in hot water, has been recently introduced to give the ridges cleaner edges. The dotted lines show the effect of sand blasting.

The 10-in. half-round, the 8-in. square, and 8-in. round bastard (fig. 75) are the varieties which will be found to be all that is necessary.

Fig. 74.

Half-round.

Round.

Square.

Bastard. Fig. 75.

The hand-saw file might be used, but, unless the teacher is a very competent man indeed, he would do well to leave it for a skilled workman; only very few of even good workmen can satisfactorily sharpen a saw.

Second Cut Taper Saw File.

Fig. 76.

The hand-saw file may, however, have to be used in special circumstances