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Roll Bending Basics: Minimizing the Unbend Flat

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plate rolling machine

Unbent flat in roll bending

Plate rolls need a place to grab the material during forming. That’s why, in any plate rolling situation, you have a narrow unbent flat section on the plate’s leading and trailing edges.

Choosing a plate rolling machine that matches the required roll geometry and thickness minimizes the flat portions at the beginning and end of the rolled sections. The rolls grab the lead-in plate edge and, once the plate is in the correct position, commence pre-bending to minimize the flat portion at the start of the roll. This operation takes considerably more power than rolling; typically a machine that can roll a 1.25-in. plate may only have enough power to prebend 1-in. plate.

To prebend, rolls act like a press brake in reverse. The bottom rolls rise, pushing the plate against the top roll to create the initial bend. After rolling, the rolls do the same thing at the plate’s trailing edge. The problem is that during each bending operation, the rolls must hold onto the plate somewhere, and these pinch points happen to be at the very leading and trailing edges of the rolled plate.

These edges remain flat, and the only way to eliminate them entirely is to reroll the section after welding or cut off the flat section, both of which hamper productivity. Rerolling also demands serious power from the rolling machine. For this reason, the best approach usually is to purchase a rolling machine that leaves a minimal amount of flat and can produce products that meet quality standards without rerolling.

How to choose the right bending rolls?

As always, the application dictates which machine is best. Consider a four-roll machine designed for a 1-in. plate. The machine will produce flat sections that are 1.5 to 2 times the material thickness. This means there will be 2 in. of flat on a cylinder of the 1-in.-thick plate, which is acceptable for most applications.

However, if the next job happens to be for a 0.25-in. plate, issues can arise with a fixed-geometry system. Its lower rolls can be adjusted only up and down, not side to side, so it still produces a 2-in.-wide flat section at the end of the rolled section. That’s eight times the material thickness on 0.25-in. material, which typically isn’t acceptable at all. In this case, a variable-geometry system would fit the bill, because it allows operators to position the lower rolls wherever needed to maintain the minimum width of flat at the leading and trailing edges of the rolled section for a wide range of material thicknesses.

From the original design principle of the machine: the pre-bending function of the CNC 4-roll plate bending machine, the 3-roll variable geometry plate bending machine, and the top roll variable geometry 3-roll bending machine has minimized as possible the unbend flat in the workpiece.

Works Cited