This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Now, while the dog-head is used entirely for regulating the tension, it may also bo used for the same purposes as either the long cross-faced 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.
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 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 wobble 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.
In circular saws the combinations of tight and loose places may be so numerous that as the smith proceeds in testing with the straight-edge he marks them, drawing a circular mark, as at g, in Fig. 134, to denote loose, and the zig-zag marks to indicate tight places.
To cite some practical examples of the principles here laid down, suppose we have in Figs. 135 and 136 a plate with a knick or bend in the edge, and as this would stiffen the plate there, it would be called a tight place. To take this out, the hammer marks could be delivered on one side radiating from the top of the convexity as in Fig. 135, and on the other as shown radiating from the other end of the concavity as in Fig. 136, the smithing hammer being used. This would induce a tight place at a, Fig. 135, which could be removed by dog-head blows delivered on both sides of the plate. Suppose we had a plate with a loose place, as at g in Fig. 137, we may take it out by long cross-face blows, as at a and b, delivered on both sides of the plate, or we might run the dog-head on both sides of the plate, both at a and at b, the effect being in either case to stretch out the metal on both sides of the loose place g, and pull it out. In doing this, however, we shall have caused tight places at e and f, which we remove with dog-head blows, as shown. If a plate had a simple bend in it, as in Fig. 13S, hammer blows would first be delivered on one side, as at a, and on the other side, as at b.
A much more complicated case would be a loose place at g, in Fig. 139, with tight places at h, i, k, I, for which the hammer blows would be delivered as marked, and on both sides of the plate. Another complicated case is given in Fig. 140, g being two loose places, with tight places between them and on each side. In this case, the hammering with the long crossface would induce tight places at d and e, requiring hammer blows as denoted by the narks.
Rose had some examples to illustrate how plainly bending a plate will show its Sight and loose places. With a rectangular piece of plate that is loose in the middle, the straight-edge lies flat on it; but if you try to bend the middle of the plate downwards with your hands, you will see that it goes down instantly, the straight-edge showing a large hollow in the middle, as in Fig. 141, the same thing occurring with the straightedge tried on both sides of the plate. Another piece is tight in the middle, and when you try to bend its middle downwards in precisely the same way, it comes upwards, and the straight-edge shows it to be round as in Fig. 142. In the first case the middle actually moves, because it is loose; in the second place the edges move, because they are loose.
With two circular saws, one tight and one loose at the centre, the same thing occurs; for if you bend the loose one down, it goes down, leaving a wide space between the eye of the saw and the straight-edge; while if you try to bend the middle of the tight one down it refuses to go there, but goes at the outside, leaving the straight-edge resting on the middle. Here, again, then, the part that is loose moves the most. These examples are simple cases, but they impart a general knowledge of the principles involved in the skilful use of the hammer.