3-roll or 4-roll plate rolling machine?

The fourth roll clamps the plate against the top roll, and that changes everything: the plate does not slip, both ends are pre-bent without removing it, and the cycle is faster and more repeatable. It is the configuration of choice for series production, pressure vessels and parts where every cylinder must come out identical to the previous one. It also makes cone rolling and single-operator work easier.

The 3-roll plate roll is simpler, more economical and more than enough for many jobs. It is the reasonable choice when volume is low, part variety is high and the flat edge is not critical, because it can be cropped or finished in a press. It gives up some precision and speed, in exchange for a lower investment.

In practice the choice comes down to one question: how much do the flat edge and repeatability weigh in your production? If you make series parts, with dimensional demands or pressure vessels, the 4-roll pays for itself; if you make varied, low-volume parts, the 3-roll is the sensible investment. The rest of the variables —material, thickness, width and diameter— are defined the same way in both cases, and are developed in the article on how to specify a plate rolling machine.

Neither the 3-roll nor the 4-roll is «better» in the abstract: each solves a type of production. The 4-roll wins on precision, pre-bending and series work; the 3-roll wins on simplicity and cost for varied work. Defining the part and the volume well is what tips the balance one way or the other.