Fig. 2116.
An inspection of hammered saw plates, however, will show that the marks (which are scarcely visible, having a merely dulled surface), are usually about one-half wider than the thickness of the plate, and about four or five times as long as they are wide. Obviously, also, the direction of the effects of a blow follow the direction in which the hammer travels. If, for example, the long cross-face falls vertically its effects will extend equally all around the hammer mark, as at a in [Fig. 2115], but if the hammer moved laterally to the left while falling its blows would have more effect on the left-hand side of the mark as at b, or if it moved away from the operator its effects would extend most in front as at c, the amount increasing with the force of the blow, and it may be remarked that quick blows are not used, because they would produce indentations or chops; hence, the force of the blow is regulated by the weight of the hammer rather than by the velocity it travels at. On account of the oval shape of the blow delivered by the long cross-face and by the twist hammers, the dog-head hammer, shown in [Fig. 2116], is used to regulate the tension of the plate or saw, the effects of its blow when delivered vertically being circular, as at a, in [Fig. 2117]; obviously, however, if in falling it moved vertically in the direction of arrow c the effects would extend as at b. But while the dog-head is used entirely for regulating the tension, it may also be used for the same purposes as either the long cross-face or the twist hammer, because the smith operates to equalize the tension at the same time that he is taking down the lumps; hence he changes from one hammer to the other in an instant, and if after regulating the tension with the dog-head he should happen to require to do some smithing, before regulating the tension in another, he would go right on with the dog-head and do the intermediate smithing without changing to the smithing hammer. Or, in some cases, he may use the long cross-face to produce a similar effect to that of the dog-head, by letting the blows cross each other, thus distributing the hammer’s effects more equally than if the blows all lay in one direction.
Fig. 2117.
In circular saws, which usually run at high velocity, there is generated a centrifugal force that is sufficient to actually stretch the saw and make it of larger diameter. As the outer edge of the saw runs at a greater velocity than the eye it stretches most, and therefore the equality of tension throughout the saw is destroyed, the outer surface becoming loose and causing the saw to wabble as it revolves, or to run to one side if one side of the timber happens to be harder than the other, as in the case of meeting the edge of a knot.
The amount of looseness obviously depends upon the amount the saw expands from the centrifugal force, and this clearly depends upon the speed the saw is to run at; so the saw straightener requires to know at what speed the saw is to run, and, knowing this, he gives it more tension at the outside than at the eye; or, in other words, while the eye is the loosest, the tension gradually increases towards the circumference, the amount of increase being such that when the saw is running the centrifugal force, and consequent stretching of the saw, will equalize the tension and cause the saw to run steadily.
