the 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 gauge the exact amount of width required for extension; another is to turn a bar over at intervals in 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 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 ingot of say 14 in. square may be reduced to a bloom of 6 in. square. The grooves form rectangular sections (box passes). The top roll being raised, the ingot is passed through the largest groove; then the roll is lowered and it is passed through a second 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 rail, but the cogging-rolls have to be so designed as to produce blooms for varied forms. There are three principal forms: the box just noticed, the gothic and the diamond (fig. 4), all open passes. For plates,
Fig. 4.
A, Box Pass. B, Gothic Pass. C, Diamond Pass.
provision is made in "slabbing" rolls for roughing out, first in a box pass, and then in a broad 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 simple, and little difficulty arises in the work of drawing down. The rolls make 40 to 50 revolutions per minute; the difference in the area of the cross section (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 duty for a wide range of sections, in order to 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 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 succession of these grooves,
Fig. 5.—Pair of Rolls for producing Angle Sections. (Thomas Perry & Son Ltd., 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 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 vertical plane is the most favourable condition. Rounds, squares and flats are wholly reduced in this way and offer no difficulty. The most unfavourable section is the joist or girder, the channels, tees and rails follow, and after these the various angles. In rolling a channel or a girder section (figs. 6, 7, 8), a square bloom is taken, and passed in succession through closed passes. The first produce shallow grooves in
Fig. 6.—Reduction of Channel Section.
Fig. 7.—Reduction of Girder Section in Roughing Rolls.
Fig. 8.—Reduction of Girder Section in Finishing Rolls.
the opposite faces, gradually deepening until the insides of the flanges assume a definite slope. The angle of slope becomes gradually lessened, and the thicknesses of web and flanges, and also the 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 fibres of the flanges are being strained, because the rolls run at a higher speed at their peripheries than next the body. The metal is being violently thrust and drawn in different ways, so that while 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 formula, but the roll-turners work by experience and empirical rules gathered by long practice. In order to avoid these deep grooving, 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. 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 adjustment of the top roll (fig. 5). Differences in thickness are made in 116ths of an inch, up to a maximum of about 12 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. Guards are required to prevent the bars from becoming wrapped round the rolls ("collaring"). With the same 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 movement to it towards the floor, which is what is required. The difference in diameter varies with circumstances, ranging from 18th to about 1 in. Besides the 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 wire. In these there are several pairs of rolls placed in series, so that the billet is rolled from one stand to others in succession without re-heating. There are a number of different designs, one of which is the Belgian looping mill, so called because the rod is bent backward and forward in the form of the 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 rate of 6 or 8 ft. per second. (J. G. H.)
ROLLOCK, ROBERT (c. 1555-1599), the first principal of the university of Edinburgh, son of David Rollock of Powis, near Stirling, was born about 1555. He received his early education at the school of Stirling from Thomas Buchanan, a nephew of George Buchanan, and, after graduating at St Andrews, became a regent there in 1580. In 1583 he was
