Fig. 3080.
The concave saw shown in [Fig. 3080], is employed for barrel heads. The three pieces for a barrel head are clamped together and fed in a circular path, so that the saw cuts out the head at the same time that it bevels the edge.
The advantage of the circular saw lies mainly in the rapidity of its action, whether used for ripping or cross-cutting purposes. In order, however, that it may perform a maximum of duty, it is necessary that the teeth be of the proper shape for the work, that they have the proper amount of set, that they be kept sharp, and that the tension of the saw is uniform throughout when running at its working speed.
The centrifugal force created by the great speed of a circular saw is found to be sufficient to cause it to stretch and expand in diameter. This causes the saw to run unsteadily unless it is hammered in such a way as to have it rim bound when at rest, leaving the stretching caused by the centrifugal force to expand the saw and make its tension equal throughout. The saw obviously stretches least at the eye, and the most at its circumference, because the velocity of the circumference is the greatest, and the amount of stretch from the centrifugal force is therefore the greatest.
It is obvious that the amount of centrifugal force created will depend upon the speed of the saw, and it therefore follows that the hammering must be regulated to suit the speed at which the saw is to run when doing cutting duty, and in this the saw hammerer is guided solely by experience.
A circular saw may have its tension altered and impaired from several causes as follows:
1. From the saw becoming heated, which may occur from the arbor running hot in its bearings, or from the work not being fed in proper line with the saw.
2. From the reduction in diameter of the saw by frequent resharpening of the saw, this reduction diminishing the amount of centrifugal force generated by the saw, and therefore acting to cause the saw to become loose at the eye.
3. From the saw teeth being allowed to get too dull before being sharpened, which may cause the saw teeth to heat, and thus destroy the tension.
4. From stiffening the plate at the throats of the teeth when gumming the saw, an effect that is aggravated by using a dull punch.