The number of rolled products now common in the iron and steel trade, consisting of blooms, billets, structural shapes, plates, rails, bars, rods, etc., necessitates rolling mills of a variety of types and sizes for their production.

The largest mills, known as blooming, cogging and slabbing mills, roll ingots into blooms, slabs, or large billets. Some of these mills, as Fig. 44, have roll passes for rolling the larger structural shapes immediately after other passes of the rolls have broken the ingot down into a bloom of the required size. This saves much expense in reheating. Figs. 40, 43 and 44 are examples of the largest mills.

Mills next in size are in greater variety because of the greater diversity of products rolled by them. They receive blooms, slabs, and billets from the heavier mills, and roll them into a variety of forms such as small billets, structural shapes, railroad rails, sheet bars and plates.

The small type of mills, known as merchant and sheet mills, receive sheet bars and small billets from the second group of mills. Merchant mills roll billets into small rods and bars of a great variety of sizes and cross-sections, known as merchant bar and familiarly seen as stock in the blacksmith shop. Sheet mills roll sheet bars into thin sheets commonly seen as sheet iron.

* The thumb projection on this rail projects horizontally when rolled. The last pass through the rolls bends it up as shown.

Besides the foregoing classification of mills according to size and work they do, they are also classified according to design as follows:

(1) The reversible mill is one in which the two rolls are reversible and the material rolled is fed between them alternately from opposite sides of the mill. Figs. 40, 43, and 44 are examples of this class.

(2) The three-high mill is one with three rolls, which are not reversed. Fig. 45 is an example of this class.

(3) The universal mill is one which is provided with a pair of vertical rolls in addition to a pair of horizontal rolls. These four rolls press on all sides of a square piece of material as it passes through the mill. Figs. 40 and 44 are examples.

(4) The pull-over mill is a small two-roll mill which is not reversible. This mill is used for rolling thin sheet metals. When a slab passes between the rolls, it is pulled back over the top roll to be passed through again. A type of this mill is shown in Fig. 55.

(5) The continuous mill is a succession of small two-roll mills placed near together in line. They are used to roll merchant bar. In rolling small bars the metal loses its heat so rapidly that it is necessary to roll it quickly. The continuous mill accomplishes this by passing the material from one set of rolls to the next, and in this way a bar may be in the process of rolling between as many as ten pairs of rolls at once.

(6) The looping mill is another arrangement of several small two-roll mills to operate on small material quickly. These mills are placed edge to edge along a line and a red-hot rod a hundred or more feet long passes from the first mill in a U-shaped loop through the next, and so on through each successive mill in a snake-like curve until it is coiled up after passing through the last mill. This, material is used mostly for drawing into wire, which will be described later. As in the continuous mill, the material is passing through several pairs of rolls at the same time.