The more commonly designated rolling-mill parts include rolls, housings, roller table, manipulator, guides, guards, passes, and collars. Several of these parts have been mentioned.

Fig. 47 shows the outline of a pair of rolls with examples of typical passes between the rolls. A and C are open passes, i. e., a fin of metal may be squeezed out from the sides of the billet if it is pressed hard enough. To avoid the forming of a side fin, the closed pass B is used. F, F, are collars on the rolls. C is called a diamond pass, and D is a bearing to support the upper roll because of its great reduction of diameter at the middle.

Fig 47. - Forms of Passes in Rolls.

Fig. 48. - Example of Rolling an Angle Bar.

Fig. 48 shows six passes necessary for rolling a billet into angle bar of unequal legs. Successive changes in cross-section from the billet to the finished shape must be gradual, as here shown. The hot material could not stand a radical change between successive shapes, as it would tear or distort. To avoid stretching one leg of the bar more than the other in rolling, the passes must be so made that the two ends of the legs will lie in a line parallel to the axis of the roll. Both rolls run at the same number of revolutions per minute, but the upper roll is slightly larger in diameter than the lower roll so that the rolled material will peel from the upper roll. To make the material peel from the lower roll when the first end passes through, a guard is fitted, as in Fig. 49. To prevent material from being engaged by the roll collars, and to direct it into the right passes, guides are fitted as shown in Fig. 49. Fig. 45 shows guides leading to the lower roll, and guards may be seen along the piece marked B in that figure.

Fig. 49.

Finishing rolls, which do not exert much pressure, and plain rolls for plates and sheets, are made of cast iron with the roll surface chilled to harden it. Blooming and roughing rolls are made of cast steel to withstand the enormous shocks of heavy work. Structural rolls are made of cast steel.

Boll surfaces are first turned to shape in large lathes and are then ground by special appliances to a true surface.