The boys now took up the systematic study of tools, as Ralph suggested that they had spent time enough on toys and curiosities.
A cutting tool must be constructed with reference to the material it is to cut. In the machine shop, we find the angle of the cutting edge large—often 80 degrees—while a razor has a cutting edge of about 5 degrees. All cutting tools are wedges, whether saws, chisels, planes, axes, or knives, and the angle depends on the hardness of the material in which it is to work. The action of the tool may be a chisel action, a knife action, or both. In the rip saw, the teeth are really a series of chisel edges cut in one piece of steel, while in a cross cut saw we have a knife action for cutting the fibres, followed by a chisel action for removing the wood.
The side view of a rip saw is shown at a ([Fig. 101]), the end view at b.
The chisel-like edges are bent outward to right and left alternately. This is called the "set" of the teeth and its purpose is to make the cut wider than the body of the saw, to prevent friction. As the saw teeth pass through the wood, the fibres spring back against the saw blade or body, and the friction makes the work almost impossible without "set" to the teeth. All woodworking saws must be set, and special tools called "saw sets" are sold for the purpose of bending out the teeth.
Fig. 101. Teeth of rip saw
The rip or slitting saw should only be used for cutting with the grain. When used across the grain, the action is exactly like that of a narrow chisel, and it will tear the fibres instead of cutting them.
The teeth of a cross cut saw are shown in [Fig. 102]. At a is the side view, and at b the end view. The teeth are set and filed to a knife edge. This gives two parallel lines of knife-like teeth which cut the fibres in two parallel lines, while the body of the tooth cuts out the wood in the form of sawdust. All woodworking saws belong to one of these two classes, and the cutting angles of the teeth are shown in [Fig. 103].