On the other hand, if the eye is tight and the circumference loose the saw will flop from side to side as it runs, and the remedy is to stretch it round about the eye, letting the blows fall wider apart as the outer edge of the saw is approached. The combinations of tight and loose places may be so numerous in circular saws that as the smith proceeds in testing with the straight-edge he marks them, drawing a circular mark, as at g, in [Fig. 2119], 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 [Fig. 2120] a plate with a kink 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 would be delivered on one side, radiating from the top of the convexity, as on the left, and on the other as shown radiating from the other end of the concavity, as on the right, the smithing hammer being used. This would induce a tight place at a which would 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. 2121]. 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. 2122], 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. 2123], with tight places at h, j, k, and l, for which the hammer blows would be delivered as marked, and on both sides of the plate. Another complicated case is given in [Fig. 2124], g g being two loose places, with tight places between them and on each side. In this case, the hammering with the long cross-face would induce tight places at d and e, requiring hammer blows as denoted by the marks.
Fig. 2125.
The saw or plate straightener’s anvil or block is about 12 by 18 inches on its face, which must be very smooth and is slightly convex, because it is necessary that the plate should be solid on the block, directly beneath the part of its surface which is being hammered, otherwise the effect of the blows will be entirely altered. If, for instance, a, in [Fig. 2125], represents the straightening block, and b a plate resting thereon, then the blows struck upon the plate anywhere save over the very edges of the anvil will have but little effect, because of the spring and rebound of the plate; and the effect of the blow will be distributed over a large area of the metal, tending to spring it rather than give it a permanent set. If the blow is a quick one, it may indeed indent the plate without having any straightening effect. On the other hand, by stretching the skin on the upper side of the plate, it will actually, under a succession of blows, become more bent. In fact, to use a straightening block, so large in proportion to the size of the plate that the latter cannot be adjusted so that the part of the plate struck lies solid on the block, renders all the principles above explained almost valueless, and is a process of pounding, in a promiscuous way, productive of hammer marks, and altogether fatal to the production of true work.
Fig. 2126.
To straighten the plate shown in [Fig. 2125], we place it upon the anvil, as shown in [Fig. 2126], striking blows as denoted at a, and placing but a very small portion of the plate over the anvil at first; and as it is straightened, we pass it gradually farther over the anvil, taking care that it is not, at any part of the process, placed so far over the anvil as to drum, which will always take place if the part of the plate struck does not bed, under the force of the blow, well upon the anvil.
