330 IRON train not reversed at each successive passage, the bar is lifted by hand or otherwise to the top of the top roller, which then guides it back to the side from which it started, after which it is put through the next groove, and so on ; this causes loss of time, and prevents the bar being rolled so many times before it cools ; accordingly for thin plates or rods of small section the three-high train is always u&ed (or some more complex analogous combination of rollers). As a rule the speed of rotation of the rolls is less the heavier the plate, &c., rolled ; for very thick armour plates (of which sizes up to 20 and 30 tons are sometimes rolled) a comparatively small number of re volutions per minute suffices ; for thin rod a much larger number up to several hundreds ; from 80 to 100 revolutions is about the usual speed for rolling rails and similar heavy bars, 50 or 60 being employed for very thick ones, 120 to 150 for lighter bars, and con siderably more rapid speeds for very thin bars, rods, and wire. Space will not permit of descriptions of the details of various kinds of rolling-mill plant, &c., introduced in different works, and more especially of numerous American improvements in this direction, particularly those of Messrs Fritz : for descriptions of these, the technological journals, &c., of the last few years must be consulted. For rolling tires a peculiar arrangement of comparatively small rolls is employed ; the ingot after having a hole punched through it (or cast in a thick ring if of molten " ingot metal ") is subjected to a kind of rolling action between two surfaces, one inside the ring and the other outside, so that the circumference of the ring is gradually extended and its thickness diminished, whilst the flange is simultaneously formed. When thin rods of rectangular section are required (e.g. , nail rod), they are often made by rolling out a bar into a long wide thin plate, which is then passed through a " slitting mill" consisting of a pair of steel rollers with deep grooves, the projecting portions of the one fitting into the grooves of the other, but not reaching to the bottom of the grooves. These projecting portions act as revolving shears, so that the plate is "slit" as it passes into thin rods, the width of which is regulated by the width of the grooves ; after pass ing through the machine these are straightened by hand. in order to cut puddle bars into lengths for piling, powerful shears are employed. Fig. 49 represents one form of double shears Fig. 49. worked by a reciprocating lever actuated by an eccentric ; fig. 50 represents a much more powerful form of guillotine shears. A massive plate B with a terminal shearing edge AA works slowly up Fig. 50. and down in guides by means of eccentrics C, C, C on the shaft D ; this cutting edge passes just in front of a similar one permanently fixed to the guide posts. The plate to be sheared is supported on a table aa in front of the shears, running backward and forward upon a little railway ; whilst the upper cutting edge is elevated, the plate is quickly run into position, and as the cutting edge descends is sheared across, the cut-off portion dropping down ; as the cutter ascends again the plate is turned round or pushed forward, so as to bring under the cutter the portion to be sheared at the next down stroke. The same kind of machine serves for cutting boiler plate .to any required dimensions, and for shearing purposes generally. Substituting a solid steel plunger for the cutting edge, the machine becomes a punching machine, the plate to be punched being sup ported on a travelling table, and the spot where the rivet-hole, &c., are to be pierced being brought under the end of the punch whilst the latter is raised. A powerful fly-wheel is requisite whenever the plate to be sheared or punched is massive. For cutting crop ends olf hot or cold railway bars after rolling so as to reduce them to a Fig. 51. uniform length, an arrangement analogous to a travelling table is used, urged forward by an eccentric, a toothed wheel and screw, or a chain and axle,-&c , fig. 51 ; on this the bar FF is placed against stops B, B, and the table then moved forward (as represented in the figure by means of the handle G actuating an eecentrie on the axis AA) so as to bring the bar gradually against the pair of rapidly revolving circular (usually toothless) saws C, C worked by a band and pulley E, and thrown out of gear by shifting the band to the "idle pulley" D when required; to prevent the saws from becoming overheated, the lower portions dip into water troughs as they revolve. A single circular saw revolving 5000 times a minute (with a cir cumferential velocity of about 300 miles per hour) has been used for this purpose. For obliterating file marks, reducing to roughly plane surfaces, cutting slots, smoothing, and many other purposes for which grindstones, &c., were formerly employed, emery wheels made of pulverized emery cemented together with calcium silicate are often used with great advantage. Cold Rolling. In the ordinary processes of rolling iron plates and rods, &c., the metal is at a high temperature so as to be softened by the heat ; when cold iron is similarly rolled the compression thereby produced gives a much greater stiffness and elasticity : thus Fairbairn found that an increase in strength in bars from 60,746 Ib per square inch before rolling to 88,230 after was thus effected. Whipple (of the U. S. navy) finds that in sheet iron the tenacity is thus increased by 60 and even 100 per cent, of the original value, and Thurston has recently found analogous results both as to increase in tenacity and power to resist strains and in the modulus of elasticity. The process is largely in use in America, being carried out by rolling the bars, &c., out to a little larger dimensions than the finished metal is intended to have ; they are then " pickled " in dilute hydrochloric acid to dissolve off the film of oxide from the surface, passed through lime water to neutralize the adherent acid, and finally rolled many times in succession between grooves of great smoothness, so cut as only to compress portions and not the whole of the circumference at each passage through the rolls. For a description of the appliances used in cold rolling as carried out at the works of Jones & Laughling, U. S., who make a speciality of this business, see Engineering, vol. xxvi. p. 347 (1878). For straightening and planishing circular steel bars such as shaft ing rods, &c., a peculiar machine is sometimes used, consisting of a pair of revolving disks with bevelled faces, which compress the rod slightly between them, at the same time rotating it and traversing it fonvards (and, by reversal, backwards) by one continuous move ment, so that all parts of the surface are equally operated on, and the bar leaves the machine circular, straight and bright-surfaced.
Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/346
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