be “three high,” as shown in fig. 36, with the upper and the lower
roll moving constantly to the right and the middle roll constantly
to the left, so that the piece first passes to the right between
the middle and lower rolls, and then to the left between the middle
and upper rolls. The advantage of the
“reversing” system is that it avoids lifting
the piece from below to above the middle
roll, and again lowering it, which is rather
difficult because the white-hot piece cannot
be guided directly by hand, but must be
moved by means of hooks, tongs, or even
complex mechanism. The advantage of the
three-high mill is that, because each of its
moving parts is always moving in the same
direction, it may be driven by a relatively
small and hence cheap engine, the power delivered by which
between the passes is taken up by a powerful fly-wheel, to be
given up to the rolls during the next pass. (See also Rolling Mill.)
130. Advantages and Applicability of Rolling.—Rolling uses very much less power than drawing, because the friction against the fixed die in the latter process is very great. For much the same reason rolling proceeds much faster than drawing, and on both these accounts it is incomparably the cheaper of the two. It is also very much cheaper than forging, in large part because it works so quickly. The piece travels through the rolls very rapidly, so that the reduction takes place over its whole length in a very few seconds, whereas in forging, whether under hammer or press, after one part of the piece has been compressed the piece must next be raised, moved forward, and placed so that the hammer or press may compress the next part of its length. This moving is expensive, because it has to be done, or at least guided, by hand, and it takes up much time, during which both heat and iron are wasting. Thus it comes about that rolling is so very much cheaper than either forging or drawing that these latter processes are used only when rolling is impracticable. The conditions under which it is impracticable are (1) when the piece has either an extremely large or an extremely small cross section, and (2) when its cross section varies materially in different parts of its length. The number of great shafts for marine engines, reaching a diameter of 2218 in. in the case of the “Lusitania,” is so small that it would be wasteful to instal for their manufacture the great and costly rolling mill needed to reduce them from the gigantic ingots from which they must be made, with its succession of decreasing passes, and its mechanism for rotating the piece between passes and for transferring it from pass to pass. Great armour plates can indeed be made by rolling, because in making such flat plates the ingot is simply rolled back and forth between a pair of plain cylindrical rolls, like BB of fig. 35, instead of being transferred from one grooved pass to another and smaller one. Moreover, a single pair of rolls suffices for armour plates of any width or thickness, whereas if shafts of different diameters were to be rolled, a special final groove would be needed for each different diameter, and, as there is room for only a few large grooves in a single set of rolls, this would imply not only providing but installing a separate set of rolls for almost every diameter of shaft. Finally the quantity of armour plate needed is so enormous that it justifies the expense of installing a great rolling mill. Krupp’s armour-plate mill, with rolls 4 ft. in diameter and 12 ft. long, can roll an ingot 4 ft. thick.
Pieces of very small cross section, like wire, are more conveniently made by drawing through a die than by rolling, essentially because a single draft reduces the cross section of a wire much more than a single pass between rolls can. This in turn is because the direct pull of the pincers on the protruding end of the wire is much stronger than the forward-drawing pull due to the friction of the cold rolls on the wire, which is necessarily cold because of its small section.
Pieces which vary materially in cross section from point to point in their length cannot well be made by rolling, because the cross section of the piece as it emerges from the rolls is necessarily that of the aperture between the rolls from which it is emerging, and this aperture is naturally of constant size because the rolls are cylindrical. Of course, by making the rolls eccentric, and by varying the depth and shape of the different parts of a given groove cut in their surface, the cross section of the piece made in this groove may vary somewhat from point to point. But this and other methods of varying the cross section have been used but little, and they do not seem capable of wide application.
The fact that rolling is so much cheaper than forging has led engineers to design their pieces so that they can be made by rolling, i.e. to make them straight and of uniform cross section. It is for this reason, for instance, that railroad rails are of constant uniform section throughout their length, instead of having those parts of their length which come between the supporting ties deeper and stronger than the parts which rest on the ties. When, as in the case of eye bars, it is imperative that one part should differ materially in section from the rest, this part may be locally thickened or thinned, or a special part may here be welded on. When we come to pieces of very irregular shape, such as crank-shafts, anchors, trunnions, &c., we must resort to forging, except for purposes for which unforged castings are good enough.
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| Fig. 37.—Steam Hammer. |
A, Round bar to be hammered. |
131. Forging proceeds by beating or squeezing the piece under treatment from its initial into its final shape, as for instance by hammering a square ingot or bloom first on one corner and then on another until it is reduced to a cylindrical shape as shown at A in fig. 37. As the ingot is reduced in section, it is of course lengthened proportionally. Much as in the smith’s forge the object forged rests on a massive anvil and anvil block, B and C, and is struck by the tup D of the hammer. This tup is raised and driven down by steam pressure applied below or above the piston E of the steam cylinder mounted aloft, and connected with the tup by means of the strong piston-rod F. The demand for very large forgings, especially for guns and armour plate, led to the building of enormous steam hammers. The falling parts of the largest of these, that at Bethlehem, Pa., weigh 125 tons.
The first cost of a hammer of moderate size is much less than that of a hydraulic press of like capacity, as is readily understood when we stop to reflect what powerful pressure, if gradually applied, would be needed to drive the nail which a light blow from our hand hammer forces easily into the woodwork. Nevertheless the press uses much less power than the hammer, because much of the force of the latter is dissipated in setting up useless—indeed harmful, and at times destructive—vibrations in the foundations and the surrounding earth and buildings. Moreover, the effect of the sharp blow of the hammer is relatively superficial, and does not penetrate to the interior of a large piece as the slowly applied pressure of the hydraulic press does. Because of these facts the great hammers have given place to enormous forging presses, the 125–ton Bethlehem hammer, for instance, to a 14,000-ton hydraulic press, moved by water under a pressure of
