On the contrary, those parts which arc intermediate between the bulges, feel tight and tense under the fingers, because they are stretched in their positions and rendered comparatively straight, by the strong edges of the bulged or convex parts: the flat portions, arc the hinges upon which the bulged parts move, and such flat parts arc sensibly too small for their respective localities, the others being too large.
Now, therefore, in prescribing the rule for the avoidance of these errors, it is simply to treat every part alike, so that none may be stretched beyond its proper size so as to become bulged, and thereby to distort the whole plate. When the mischief has occurred, the remedy is to extend all the too-small parts, or the hinges of the bulges to their true size, so as to put every part of the plate into equal tension, by allowing the bulged or too-large parts room to expand. Uniform blows should be therefore directed upon all the straight or too-small parts of the plate, the force and number of the blows being determined by the respective magnitudes of the errors, and the rigidity of the plate.
In flattening plates, the greater part of the work is done with solid blows upon a true and nearly flat anvil; the face of the hammer is slightly round, and its weight and the force of the blows are determined by the strength of the plate, the slighter plate requiring more delicate blows, and being more difficult to manage. In the commencement, the rectangular plate is hammered all over with great regularity in parallel lines beginning from one edge; it is generally turned over and similarly treated on the other side. Circular plates are hammered in circular lines beginning from the center, that is supposing the plates of metal to be soft, and in about the ordinary condition in which they are left by the laminating rollers; as the equable hammering gives a general rigidity, which serves as a foundation for the correctional treatment finally pursued. With a steel plate hardened in the fire, and which is already far more rigid than the soft plate, it is necessary to begin at once upon the reduction of the errors and distortions, which usually occur in the hardening and tempering.
The hammer should be made to fall on one spot with the uniformity of a tilt-hammer, the work being moved about beneath it. As however the regularity of a machine is not to be expected from the hand, it is scarcely to be looked for that the work shall be at once flat. Whilst the errors are tolerably conspicuous or considerable, the man accustomed to the work will still keep the hammer in constant motion, and will so shift the work, as to bring the tight parts alone beneath its blows, hammering with little apparent concern just around the margins of the loose parts, or at the foot of every rise. As the plate becomes more nearly flat, it is necessary to proceed more cautiously, and to hold the plate occasionally between the eye and the light to learn the exact parts to be enlarged, the straight-edge is also then resorted to.
In many works, especially in saws which require very great truth, the elasticity is also examined; this is frequently done by holding the opposite edges of the plate between the fingers and thumbs, and bending them at various parts. As previously explained, all the portions which are technically called tight, or those lines upon which the loose enlarged parts appear to move as on hinges, are strictly the parts to be extended by gentle hammering. For instance, supposing that in the plate, fig. 288, there were only one central buckle a, the whole exterior portion would require to be stretched, beginning from the base of the bulge; but it must be remembered the extreme edges of the plate will yield with greater facility than the more central parts, and therefore require somewhat fewer blows, as the blows are all given as nearly as possible of the same intensity, and the number of them is the source of variation.
If, as it is more to be expected, there are two or more loose parts, such as a, and b c, the more quiescent part between them must be first hammered, as working upon any loose or bulged part only magnifies the evil. Where the intermediate space is narrow as at d, less blows will be needed, and such tight parts will soon, and sometimes very suddenly, become loose from the two bulges melting into one. It should be rather the general aim, to throw the several small errors into a large one, by getting the plate into one regular sweep; dealing the blows principally between the dotted lines, not carelessly so as to increase the general departure from the plane surface, but with an acute discrimination to lead all the defects in the same direction, by making the plate as it were a part of a very great cylinder, as at e or f, fig. 289, but with as little curvature as possible.
When this is accomplished, and that the work is free from loose parts, it is hammered on the rounding side, in lines parallel with the axis of the imaginary cylinder; so that in r, the lines would be parallel with the edge from which the rise commences, and in f, or the plate which is bent diagonally, the lines of blows would be necessarily oblique, although as regards the curvature, the same as in e. The reason why any reduction of curvature should at all result from this treatment, (action and re-action being alike,) is due to the greater roundness of the hammer than the anvil; the rounder hammer effects the change more rapidly, but also the more indents the work.
In a circular saw, the general aim is first to throw the minor errors into one regular concavity, which may be supposed to extend to b, b, in the imaginary section, fig. 290, and then the margin a, b, would be hammered in a proportional degree, to enlarge it until it just allowed the interior sufficient room to expand to the plane surface