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Plate Bending: Plate Rolling Machine and Press Brake

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 PLATE ROLLING MACHINE vs PRESS BRAKE

The reason behind the versatility and many applications of sheet metals relies on their durability and their ability to be crafted into numerous finish products.

One process that steel plates undergo is their fabrication. Steel plate rolling can be done either on plate rolling machines—bending rolls that incorporate three or four rollers to form curved shapes—or “bumped” with a radius die on a press brake. Both methods have their strengths and weaknesses. Even if they share a common purpose and functionality, they still have their differences that can be suitable for various conditions.

How do Plate Rolling Machine and Press Brake Work?

Plate rolling machine

Plate rolling machine/bending rolls transform flat sheet metal and steel plates into cylindrical or radius parts.

Plate rolling machines are designed to perform one function: They transform flat sheet metal into cylindrical or radius parts. Plate rolls have the advantage of being able to form a complete cylinder (360 degrees) within the radius specifications of a particular machine.

Plate rolling machines are especially efficient at producing large-diameter cylinders. The machine can be equipped with side and top supports to prevent the material from collapsing during the rolling process.

Plate rolling machines can accommodate certain cone geometries when the rolls are tilted so they are not parallel. Alternatively, the plate roll can be fitted with conical-shaped rollers to accomplish the same task.

Press brakes

Press brakes are more versatile than plate rolls. They can produce simple cylinders or complex shapes out of steel plates and sheet metals.

With the appropriate tooling, press brakes can form simple cylinders or complex shapes. Cylinders are formed by “bumping” the plate. A radius punch on the ram presses the material into a matching concave V die to create the desired radius and arc length. After each stroke, the material is incrementally advanced and pressed until the curve is complete.

The primary advantage of a press brake is its versatility. In addition to forming cylinders, it also can form sharp, 90-degree bends and all sorts of other angles that are considered normal press brake work.

Not all long parts are cylinders, however. Many applications call for long parts that are half- or quarter-cylinders, such as arches, arcs, ship bottoms, and plow blades. Press brakes with deep throats are well-suited for these types of applications.

Cones also can be formed with a press brake using precut blanks or special tooling.

The Difference Between Plate Bending Machine and Press Brake

  1. Everything else being equal (same plate thickness, same plate radius, same grade of steel), press brakes can usually “nose” the plate closer to the end of the plate thereby minimizing or eliminating any flatness. Plate rolling machines can also prebend plates to minimize straight tails but usually not as well as press brakes.
  2. Press brakes lean more toward a range of products, while plate rolls lean more toward high production. If you have a good operator, both can make accurate parts.
  3. Plate rollers can usually roll 360-degree cylinders better than press brakes can.  The ends of the curved cylinders can end up being obstructed by the press brake die or by the machine itself.  What press brakes do with cylinders of large radii is that they form the parts in sections and then weld them together.
  4. Plate rolls have their limitations. Steel plates that have very thick walls and small diameters will not work on plate rolls. The part diameter also prohibits the use of a top roll that is large enough to handle the pressures required. Deflection will be evident along the length of the top roll.
  5. Press brakes are more effective in bending and rolling diverse applications of steel plates. Different kinds of bends and varieties of parts can take advantage of the capabilities of press brakes. The press brakes can also form cylinders out of materials with varying levels of thickness.
  6. Press brakes are more versatile machines, although they may require special tooling to form some types of parts. Plate rolls are faster and better suited to high‐volume production.

Capabilities and Limitations

Plate rolling machines still have limitations. They cannot form cylinders with very thick walls and very small diameters. The part diameter does not allow for the use of a top roll that is large enough to handle the pressures required. There would be more deflection along the length of the top roll than could be compensated for. In these instances, a press brake is the best choice for the job.

Depending on the radius, however, it may not be possible to form a complete 360-degree cylinder with a press brake because of frame and tooling obstructions. In the cases of a very large radius, parts would need to be formed in sections and welded together.

Bending Basics OF Plate Bending Machine and Press Brake

Say you have a large, sweeping radius spanning the width of the workpiece. What investment is best for the job, a plate roller or a press brake? It depends on the part geometry and the material grade and thickness, of course. Part volume and budget also come into play.

Making the right choice requires knowing exactly what plate rollers and press brakes can accomplish. Press brakes can do more than just 90-degree bends, and plate rolling machines can do more than roll cylinders all day.

A good starting point, though, is knowing exactly how each machine forms a large radius in the workpiece and the controls, tools, and roll configurations that make it all possible.

Material Strength, Thickness, and Radius

The higher the tensile strength and the tighter the radius, the more pressure you need to form. More pressure usually means more deflection, which in turn will change your machine requirements.

Also, don’t forget about the material property variances, including the minimum and maximum thickness of a sheet or plate, as well as variances in material yield and tensile strength. All these have an effect on a formed part.

Material property variation spurs challenges in any metal forming operation, but it can really step to the forefront on large radii. This has to do with how that large radius is formed and the effects of spring back. Except for certain press brake bottoming or coining setups, forming large radii can amplify the effects of spring back and other process variables that change with the material characteristics. The more consistent your material, including its thickness and strength, the more consistent forming will be.

Deflection and Crowning

Both press brakes and plate rolling machines have crowning methods that account for machine deflection. When the machine deflects, the forming pressure it exerts isn’t constant from one end of the machine to the other.

Both press brakes and plate rolling machines are most rigid at their side frames and least rigid in the middle. If a machine had no method of crowning, the workpiece would force the middle of the bending area to bow.

Crowning counteracts this effect. In press brakes, this occurs using devices such as strategically placed wedges below the press brake bed that change the crown before load during the forming cycle.

In plate rolling machines, the crowning is in the rolls. A crowned roll has a diameter that’s slightly larger in the middle, and that subtle “bulge” counteracts the deflection.

Because crowning is built into the rolls themselves, plate rolling machines are designed for optimal crowning for a specific thickness range, usually about 75% of the machine’s nominal capacity.

To correct this, certain press brakes and plate rolling machines now offer dynamic crowning systems that use sensors to detect the pressure and apply for the needed compensation. For press brakes, this involves moving the wedges or similar mechanisms below the bed just the right amount in just the right place. On plate rolls, dynamic crowning systems allow for either the manual or automatic adjustment of roll pressure.

Mar-free Bending

Both press brakes and plate rolling machines can work with cosmetically critical material. In the press brake arena, urethane punches and dies, as well as urethane tape, can help a press brake create mar-free bends. And in the plate rolling world, plate rolls can be ordered with polished, precision-ground rollers that are simple to clean and won’t collect mill scale as frequently as conventional rolls.

Minimum Flanges and the Unbent Flat

Bending

The press brake world deals with minimum flange lengths; a machine with conventional tooling can’t form all the way to the material edge. The minimum flange length is usually determined as a percentage of the die opening. Essentially, the plate needs to be able to sit securely on the die throughout the forming cycle. That said, incremental bending (more on this later) often uses acute dies with narrow die openings, so minimum flange requirements usually aren’t an issue. Also, special tool sets—like flexible urethane dies with built-in stops for gauging or rotating “wing” dies—can allow you to form a radius nearly or even all the way to the edge in a single hit.

In some cases, plates are the first bent on a separate machine called a pretender, which forms to the edges of the material with virtually no flat, before they’re brought to the press brake for incremental bending. It’s a technology with a long history in the heavy-wall pipe industry that’s now spilling over into other sectors. This type of form, sometimes called nosing, can be performed with the correct machine and tooling.

Rolling

In plate rolling, you have the unbent flat section at the leading and trailing edges. It’s usually barely noticeable, especially in sheet metal and plates rolled to a large diameter. But they’re there, and they’re unavoidable because the pinch rolls need a place to hold the material.

An operation called pre-bending minimizes the flat sections on the plate’s leading and trailing edges. In a typical setup, the operator performs the prebend to the leading and trailing edge, usually leaving an unbent flat 1.5 to 2.5x the material thickness, depending on the application and material.

For critical-dimension cylinder rolling, an operation might opt to roll the cylinder, weld the longitudinal seam, grind it down, then reroll to eliminate the unbent flat. But in most cases, that small unbent flat section remains.

How to Choose?

Efficiency and safety get to the crux of the matter, and efficiency in particular hinges on a fabricator’s product mix. Consider again the part with the large, sweeping radius.

If all a fabricator wants to produce is cylinders—and a lot of them—then a plate rolling machine is an ideal choice. It is a good machine for a tank or vessel production shop or a rolling house that is doing 20 to 30 large-diameter cylinders per day.

If a job shop has a more diversified workload, then it needs the flexibility to do different types of jobs. In that scenario, a press brake is a way to go.