This name is given to the machine shown in Fig. 224 to distinguish it from the plate straightening rolls. The machine consists of three solid-forged rolls supported parallel in heavy bearings. The lower rolls are driven by gearing and the upper roll revolves from contact with the plate rolled. The upper roll may be raised or lowered to suit the thickness of the plate and the curvature to which it is rolled. In case a sheet is rolled into a complete cylinder, it is removed by lifting one end of the upper roll and sliding the cylinder out. To do this, the yoke K is screwed down against the extension bar B until the other end of the roll and its bearing are lifted out of the way.

Plates are rolled cold, and are run back and forth until the curvature gradually increases to that of a template made as a guide.

The rolls cannot bend a plate for a short distance from each end, as shown by the diagram in Fig. 225. Suppose the plate to be moving in the direction of the arrow p. That part of the plate between its contact points with the rolls B and C is not bent, as bending takes place only after the plate passes the first contact point K with the roll B. When the plate reaches such a position that the end J has passed over the crest of the roll C and begins to drop, practically no further bending will take place between the end and the point K. In shop practice, this difficulty is obviated by placing a bar of half-round iron at M and rolling it against the plate.

Fig. 225.

The lower rolls have one or more longitudinal notches, as at GG, to grip the edge of the plate when started in the rolls, or to flange the ends of narrow strips of plating when needed.

Plate-bending rolls are also made to operate in a vertical position, and are known as vertical-bending rolls.