Practical Carriage Building
Compiled by M.T. Richardson, Vol.1. 1891
Wood Saw - Its use and Management
The saw, critically considered, is a series of knives set on a line. Each tooth is a knife which cuts and carries away a small portion of material. Each individual tooth is kept from cutting too deeply by the teeth which immediately precede and follow it. When the saw is properly constructed, each tooth cuts its own allotted portion and carries the chip or slice of material along to the edge and drops it on the outside. Saws used for special purposes or employed upon specific materials, require to be constructed upon different principles, as well as managed in different ways.
The cross-cut saw, embracing those which require two men to operate them, and those commonly used for cutting off boards and small timbers by hand, is perhaps in most general use of the various forms in which the saw is made. The rip-saws of the mills probably do a larger amount of cutting, but mechanics generally have more to do with the class of saws here named. To understand the cross-cut saw, first consider just what it is expected to perform, and this can best be done by examining the end of a log or timber which is to be cut. In Fig. 43 is presented the end view of such a piece of wood, showing the ends of the fibers. The grain, as it is frequently called, consists more or less of minute fibers and threads, which must be severed in the process of cutting. As the material is non-elastic and unyielding, the fibers must be cut; they cannot be pushed to one side. Since, from the shape and construction of the saw, it is necessary to leave a small space between the two parts of the timber, technically known as the kerf, the saw must be made to cut the wood in a way to provide the channel in which it runs. This it does by cutting the fibers which constitute the structure of the wood in two places; that is, it cuts them at points corresponding to the sides of the kerf. The intervening wood or portions of fiber are rasped or scraped away by the teeth of the saw as it is worked back and forth in the process of cutting still deeper. This action of the saw is clearly illustrated in Fig. 44. The question of the shape of the teeth and the pitch or hovel which they should have in order to best accomplish this result, next comes up. Fig. 45 shows a section through the cut made by a saw.
The bottom of the cut should be square, as shown in the illustration. It will not do for it to be in shape like the groove formed in the edge of a saw by the teeth; or, in other words, to be the reverse of the shape shown in Fig. 46. The saw, to operate in the best manner, must be managed in such a way that those portions of wood lying between the cuts made along the sides of the kerf will crumble away readily and be removed from the channel of the saw without any appreciable labor.
Now, the sharper each tooth is, that is, the more bevel it has on the point, the deeper it will cut. But, for reasons just explained, it must not be allowed to cut deeper into the wood, than will readily crumble out across the groove from the point of one tooth to the point of the other. A section through a saw is as shown in Fig. 46, and when it is properly filed the angular groove will be so true that a fine needle will slide along the entire length from one end to the other without falling off. The cutting, as has been explained, is all done by the outside edge of the tooth. The actual operation of the teeth is as shown in Fig. 44, already referred to. The wood is cut by the point of the tooth along the sides of the kerf, and the portion between the cuts crumbles out.