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A107
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A107 SAW A, First roughing rolls. B, Second See also:ditto. C, See also:Guide rolls for ovals or diamonds. D, Ditto for rounds or squares. E, See also:Driving pinions. See also:Engine, 30 in. X 22 in. See also:cylinder, See also:direct-coupled to rolls. Runs from too to 18o revolutions per See also:minute to suit See also:work. The See also:shears are used for cutting the smaller sections, the hot saw for cutting the See also:merchant See also:iron. F F + F t I F F' E Y I E ii A, B, Three-high sets. C, See also:Works either three-high or two-high; a, being a dummy See also:roll. D, Two-high set (guide rolls). E, Coupling pieces. F, Housings. G, Pinions. The See also: The plate-mills proper are those which roll from 4 in. to about 2 in. thick. See also:Armour plate-mills are a See also:special design for massive plates and See also:sheet-mills are for thin plates or sheets having a less thickness than '-, in. Armour plate-mills are of two- high reversing type usually, with forged steel rolls. They are of immense proportions, the rollers ranging from to to 14 ft. in length, by from 3 to 4 ft. in See also:diameter. In sheet-mills, on the other See also:hand, the rolls seldom exceed 30 in. in diameter, and they are chilled. The See also:size of sheet-mills has within the last few years been consider-ably increased (since the introduction of steel sheets), and all new mills are made from 28 to 30 in. diameter. The mills are of the two-high type and are almost the only instance of the retention in See also:present practice of the non-reversing mill. It is found more convenient in this See also:case than the reversing or the three-high mills, because two men roll two pieces at once, one handing over a sheet just rolled to his See also:fellow just as the latter has entered a sheet between the rolls on his See also:side. See also:Strip-mills are a smaller but similar type, used for rolling the thin narrow strips required for the hoops of barrels, ties for See also:cotton See also:bales, &c. The details of these mills cannot be discussed here, nor the numerous arguments in favour of the two systems. See also:English practice retains the two-high reversing mill for all heavy work, the exceptions being those just noted. See also:American practice retains the three-high mill. Grooved Rolls.—In the mills designed for rolling various sectional forms the same distinction between two-high and three-high re-mains, but new problems arise. By " sectional forms " is meant all those which are not plates and sheets, such as bars of See also:round and square See also:section, angles, channels, rails and allied sections (fig. t), for the See also:production of which grooved rolls are required. The shapes and proportions of these grooves are such that reduction is effected very gradually. When See also:metal is squeezed or hammered, one effect is to spread it laterally, since the metal cannot be appreciably squeezed in on itself. But the lateral See also:extension is very much less than 470 the See also:longitudinal. The most marked effect of reduction in thickness is extension in length. But as there is some lateral extension, three courses are open: one is to See also:gauge the exact amount of width required for extension; another is to turn a See also:bar over at intervals in See also:order to exercise pressure on the portions extended laterally and obliterate them (open passes) ; and a third is to allow the extensions to take the See also:form of fin to be cut off subsequently (closed passes). The first is generally impracticable. The second can be illustrated by diagrams representing roll sections. The work of reduction is generally divided between three sets of rolls. The first are the cogging-, or blooming-rolls, as they are termed in America, in which ingots are reduced to blooms with dimensions suitable for rolling the various sections. In these an See also:ingot of say 14 in. square may be reduced to a See also:bloom of 6 in. square. 
The grooves form rectangular sections (See also:box passes). The See also:top roll being raised, the ingot is passed through the largest groove; then the roll is lowered and it is passed through a second See also:time. Then it is turned round through 90° and re-rolled. Afterwards the same processes are gone through till the last groove is reached. There is a great difference between, say, a plate and a See also:rail, but the cogging-rolls have to be so designed as to produce blooms for varied forms. There are three See also:principal forms: the box just noticed, the See also:gothic and the See also:diamond (fig. 4), all open passes. For plates, B A, Box Pass. B, Gothic Pass. C, Diamond Pass. See also:provision is made in " slabbing " rolls for roughing out, first in a box pass, and then in a broad See also:flat groove, alternating with the square groove for correction of the edges. Gothic passes and diamond passes produce blooms which are subsequently used for various shapes having little resemblance to each other. These shapes are See also:simple, and little difficulty arises in the work of See also:drawing down. The rolls make 40 to 50 revolutions per minute; the difference in the See also:area of the See also:cross section (See also:draught) between adjacent grooves is from 20 to 25%. The formative rolls for finished sections are of two classes: roughing and finishing. The roughing-rolls approximate much more closely to the finished sections than the cogging-rolls, but the aim is to make them do See also:duty for a wide range of sections, in order to See also:change them as seldom as possible. Thus the gothic pass (fig. 4) will serve alike for rolling square or round bars. Finishing rolls must be changed for every different section, except when slight See also:differences in thicknesses only are made in the webbed portion of a rolled section. With the exception of rounds, sections are usually roughed and finished in closed passes—that is, the bar is wholly enclosed by the rolls. The groove in the lower roll is flanked by collars slightly deeper than the enclosed bar. These enter into grooves turned on the upper roll, and between them the bar is confined (fig. 5). It passes through a See also:succession of these grooves, See also:Perry & Son Ltd., See also:Bilston.) being diminished in area and extended at each pass. A certain amount of fin is squeezed out, and this is obliterated in the succeeding pass, and more formed, until in the finishing pass the amount of reduction is very slight, a See also:surface finish being the principal result. Since but a slight amount of lateral extension occurs, it follows that the reduction wholly or mainly in the See also:vertical See also:plane is the most favourable See also:condition. Rounds, squares and flats are whollyreduced in this way and offer no difficulty. The most unfavourable section is the See also:joist or girder, the channels, tees and rails follow, and after these the various angles. In rolling a channel or a girder section (See also:figs. 6, 7, 8), a square bloom is taken, and passed in succession through closed passes. The first produce shallow grooves in the opposite faces, gradually deepening until the insides of the flanges assume a definite slope. The See also:angle of slope becomes gradually lessened, and the thicknesses of See also:web and flanges, and also the See also:radius in the corners, are reduced. At the same time the width over the flanges is being gradually increased. While this is going on, the See also:fibres of the flanges are being strained, because the rolls run at a higher See also:speed at their peripheries than next the See also:body. The metal is being violently thrust and See also:drawn in different ways, so that while See also:economy has to be studied by reducing the number of passes as much as possible, undue stress must be avoided by making the reductions as easy as is practicable. These things cannot be put into a See also:formula, but the roll-turners work by experience and empirical rules gathered by See also:long practice. In order to avoid these deep groovings, and also severe lateral thrusts on the rolls, angle sections are always rolled with the slope of the flanges approximately equalized; so too are zeds (fig. 1, No. 32). The reduction is then effected with the minimum of stress to the metal. See also:Variations are readily made in the thicknesses of rolled sections without changing the rolls, by simply varying the distance between their centres. This is effected by the See also:adjustment of the top roll (fig. 5). Differences in thickness are made in igths of an See also:inch, up to a maximum'of about z in. Another detail of design in closed passes is so to shape the rolls as to make any pass obliterate the fin produced in the previous groove. Sometimes sections are turned over to effect this, but often the bodies of the rolls are turned of suitable diameters to produce the result. See also:Guards are required to prevent the bars from becoming wrapped round the rolls (" collaring "). With the same See also:object the upper roll is always made larger in diameter than the lower. Its speed is therefore slightly greater than that of the lower one. This stretches the plate or bar very slightly on the upper side, and so imparts a downward See also:movement to it towards the See also:floor, which is what is required. The difference is diameter varies with circumstances, ranging from 1-th to about i in. Besides the See also:standard types of mills noticed, the two-high and three-high, there are special mills. The merchant mill simply denotes either one of the above types used for the production of flat bars. The continuous mills are special designs for rolling small rods to be drawn into See also:wire. In these there are several pairs of rolls placed in See also:series, so that the See also:billet is rolled from one stand to others in succession without re-See also:heating. There are a number of different designs, one of which is the Belgian looping mill, so called because the See also:rod is See also:bent backward and forward in the form of the See also:letter S in its passage through adjacent sets of rolls. In another design a flying shear is employed, which automatically cuts off billets from the bar while the latter is travelling at the See also:rate of 6 or 8 ft. per second. (J. G. Additional information and CommentsThere are no comments yet for this article.
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