ROLLING MILLS are machines employed to convert masses of metal into bars, plates and commercial shapes. They consist of rolls in pairs, driven in opposite directions at nearly the same surface speed, and variously grooved and molded, between which the metal is passed while hot. In successive rollings by passing through a series of openings between the rolls known as 'passes? the metal suffers gradual reduction in thickness and a consequent in crease in length or breadth, or both, as the case may be. For example, a 15-inch square ingot is reduced to eight inches square by from 12 to 15 passes through the rolls. In very power ful mills such a reduction is sometimes achieved in six or seven passes. See Fig. 1.
The rolls, which are the vital part of the roll ing mill, are made of cast steel or a tough cast iron, depending upon the product the mill turns out. For plates and sheets cast iron is used in many mills, the surface of the rolls being *chilled,* rendering them very dense and hard, and capable of a high polish which is im parted to the sheets. If the rolls are unusually long, for making large sheets, they are of forged steel. Steel rolls have an advantage in that their deflection under a given load is only about half as much as with cast iron; they are practically unbreakable; and they hold their corners better against crumbling under the pressure. On the other hand, steel rolls cannot be chilled; their temper is soon removed by the heat of the material passing through them; and they are apt to develop a roughness of the surface, and their corners soon lose their sharpness. In proportioning a roll for heavy work it is the rule to make the length not more than twice the smallest diameter at the deepest groove. It is not uncommon to find rolls for light work up to four times the smallest diam eter. The roll is formed so as to have a journal at each end, and this is regularly half the diameter of the body of the roll. The outer extremities of the iournal are formed into cross-shaped iwobblers* by which they are driven. As used in the various mills rolls
are arranged according to one of two general plans: as rolls* where the axis of one pair is movable, so as to allow of its being brought closer to the other or vice-versa; and the fixed rolls, whose axes are set in place permanently. In the former type the reduction of an ingot is accomplished by changing the distance between the rolls each time the metal passes; in the second the gradual reduction is made by running the ingot through a series of passes, each smaller than the preceding. The rolls are held in position by housings or frames of great strength, and of suck construction that a broken roll may be quickly removed and a new one put in place.
Rolling mills are of four principal designs: (1) the pull-over" mill, in which there are two rolls set one above the other, the bar being rolled in the passes between the rolls as it goes forward, and is then passed back to its first position over the top roll, to receive its second reduction as it again goes forward work being done on the bar only on the for ward trip; (2) the reversing" mill, two rolls high, but the motion reversible, so that after the bar has gone through on its forward trip the motion of the rolls is reversed, and the bar returns to the former position through another pass — work being done on the bar in both its directions of travel; (3) the high" mill, in which there are three rolls, one above another, the bar going forward between the bottom and middle rolls, and returning be tween the middle and top rolls; (4) the (rock ing mill," consisting of two pairs of rolls set in a housing shaped in the sector of a circle in such fashion that the housing may be rocked to and fro, bringing alternately one pair of rolls and then the other into position. The bar is run through one pair as it goes forward, the mill is rocked back and the bar returns through the other pair, the power working continuously in one direction. See Fig. 2.