Press brake versus the plate roll

This heavy-walled cylinder was formed with no flat section, using a prebender followed by a high-tonnage press brake.

First things first: What do you want your forming operation to accomplish? Do you want to form a range of parts? Do you want high production? Do you want a good operator to be able to produce good parts? See the diagram in Figure 1. Let's say you can pick only two: Which is most important? Press brakes lean more toward a range of products (the top circle in the figure), while plate rolls lean more toward high production. If you have a good operator, both can make accurate parts.

Say you have a large, sweeping radius spanning the width of the workpiece. What investment is best for the job, a plate roll 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. Think of your current capabilities: What process will increase your competitive force?

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

The variables abound, and it can be difficult to know where to begin. 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 (see Figure 2).

Any forming project needs to start with the material properties, including the yield and tensile strength, the radius being formed, and the length of the part. 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. Whether they are forming on a press brake or plate roller, machine operators know the headaches that arise when a new batch of material hits the floor and they find it's on the opposite end of the specified thickness range.

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 springback. Except for certain press brake bottoming or coining setups, forming large radii can amplify the effects of springback 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.

Whether you’re forming on the press brake or plate roll, the aim is to maintain a parallel line of pressure wherever the tool or roll contacts the workpiece. Unfortunately, physics works against this ideal, resulting in deflection. Both press brakes and plate rolls 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 rolls 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 precrown before load during the forming cycle. Other crowning systems use hydraulics (see Figure 3).

FIGURE 1 When deciding what you want your forming operation to accomplish, think of these three factors.

In plate rolls, the crowning is in the rolls (see Figure 4). 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 rolls are designed for optimal crowning for a specific thickness range, usually about 75% of the machine's nominal capacity. So a machine with 1-in. nominal rolling capacity has optimal crowning for 0.75-in. plate, but might have excessive crowning (that is, too great of a bulge in the middle of the roll) for 0.25-in.-thick material. Excessive crowning squeezes the workpiece too hard in the middle and can produce an hourglass shape. Conversely, insufficient crowning can lead to barreling, where the cylinder diameter is greater in the middle than on the ends. The same effect can form radius parts with a canoe shape, bulging in the center and tighter on the ends.

To correct for this, certain press brakes and plate rolls now offer dynamic crowning systems that use sensors to detect the pressure and apply 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 (see Figure 5).

Both press brakes and plate rolls 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.

Of course, mar-free bending requires the right procedures and careful tool handling. Precision-ground rollers are hardened, but they still can be damaged, so operators need to be aware of what they are sending through the rollers—especially when rolling narrow pieces, where the machine concentrates all its pressure on a very small area.

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 first bent on a separate machine called a prebender, 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 forming, sometimes called nosing, can be performed with the correct machine and tooling (see Figures 6 and 7).

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 plate 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 prebending 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 (see Figure 8).

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.

FIGURE 2 Both press brake and roll bending can form large-radius bends. Which is best depends on the application requirements and a fab shop's variety and volume of jobs.

Welding brings up another issue: Welding power sources can cause extreme electrical damage to a nearby plate roll's control system. Make sure the welding ground is on the part, not on the machine. If electrical damage is even a possibility, it might make sense to invest in an upgraded shielded electronic system, which protects the electronics of the plate roller.

Press brakes are ubiquitous for a reason: They’re extraordinarily versatile, and a wide range of machines are available. They can of course bend a variety of angles, be they open, acute, or 90 degrees. But they can also form large-radius parts and, with the appropriate tooling, even cylinders and other complex shapes.

Some applications require special tools to create large-radii bends. For thinner-gauge applications, a round or wide half-moon punch matched with a flexible urethane die can literally "wrap" sheet metal around the punch shape, creating a large, sweeping radius in just a few hits.

But a brake also can form wide radii and cylinders through conventional air bending, where material is positioned against the backgauge and a radius punch descends to a V die. But instead of descending far into the die space to bend the work to a specific angle, the punch simply "bumps" the material slightly into the die opening. Following each stroke, the material is advanced, then bumped in increments—which is why it's sometimes called incremental bending—until the intended curve is achieved.

Incremental bending starts with knowing the bend angle and the arc length of the entire bend, from one tangent point to the other. Then the operator determines how many steps, or hits, he wants over the entire bend. The more hits he has, the narrower the pitch (the space between the hits), and the smoother the resulting curved form will be.

That said, narrow pitches in an incremental bend amplify errors. If a 90-degree incremental bend has 45 steps every 2 degrees, and if each one of those bends is a little off, what begins as a small error can snowball into a major defect. This is one reason consistent process variables—tooling, machine repeatability, material thickness, and more—are so important.

Die selection is entirely different from conventional air bending, where the radius forms as a percentage of the die opening and the punch's depth of penetration determines the bend angle. Bumping usually occurs over an acute die that's double the width of the pitch, though die selection can vary with the application. Regardless, the wider your pitch, the large the die opening, and the "choppier" the incremental bend becomes, with distinct bend lines evident on the outside radius.

That pitch is set in the program, which moves the backgauge. In many applications, operators push the plate against the backgauge, which in turn pushes the plate forward with every bump. That said, a press brake operator can use an array of gauging strategies to bump half- or quarter-cylinders as well as various complex forms, all readily formable on a press brake with a deep throat (that is, the space behind the tooling).

Unlike plate rolls, press brakes with the right tooling, tonnage, and bed length can form both exceedingly thick and thin materials and an incredible variety of shapes—even cylinders. In fact, many brakes can form small-diameter cylinders completely with no special tools required. A cylinder is bumped to nearly 360 degrees, allowing enough space for the punch to make the final bump. If the press brake has sufficient open height to accommodate the cylinder diameter, the ram lifts the punch so the operator can remove the workpiece, which can then move on to a fixture that pushes the cylinder ends together before the final longitudinal seam is welded.

Of course, this works only for cylinders of a certain diameter and thickness. Depending on the application, tooling and frame obstructions might not make it possible for a press brake to form a complete 360-degree cylinder. In these cases, parts may need to be formed in individual sections and welded together.

Press brakes with the right tooling and gauging configurations can even form cones and conical sections. Seeing a brake in action bending a conical section or a cylinder exemplifies both its main strength and its main weakness. Its main strength is, again, its flexibility. A brake is the Swiss Army knife of forming. It can form a conical section followed by another part that requires a few 90-degree bends, followed by a panel with a narrow edge flange. It can then bump incremental bends on the edge of a plate, even in between two straight flanges or other formed features—something that would be impossible for a plate roll to do. To provide clearance during the bend sequence for forming various part geometries, a brake can have segmented tools across the bed. That's another benefit that plate rolling can't provide.

FIGURE 3 A press brake with improper preload crowning can produce parts with a bow.

The fact that a brake can form a cone section exemplifies is flexibility, but its slow speed when doing so reveal its weakness. Even a seemingly simple incremental bend can be slow-going and an extremely complex affair. Most automatic angle measurement and compensation devices—dimension-measuring lasers and other sensors designed to work with conventional air bending—cannot detect problems in the ever-so-slight "angles" created with each incremental bump the punch makes in the material. And no matter how narrow the pitch, the brake can't roll; it still needs to bump the workpiece, leaving bend lines on it. The right tooling can make these lines extremely subtle, sometimes nearly invisible on the bend's outside surface, but they’re still there.

All this said, certain production environments do make good use of a brake's incremental bending capabilities. For instance, certain specialized press brakes—large tandem machines with special loading, tooling, and gauging systems—can form cylinder after cylinder after cylinder extraordinarily efficiently. But the entire system is designed around a product or product family. Programs are set, materials are consistent; front-, back-, and even sidegauging keep the workpiece steady; and all these elements work together to create an efficient, repeatable process (see Figure 9).

Of course, this isn't the norm in the typical job shop or high-product-mix manufacturer. If a brake forms large cylinder section after large cylinder section, tying up the overhead crane to manipulate the piece, then sits idle as the operator spends time setting up the next batch of jobs (which, of course, are entirely different), the process might be worth scrutinizing. It could be a serious bottleneck. And if it is, the right plate roll might be able to help.

Deciding between a plate roll and a press brake isn't a binary choice, mainly because of the various types of machines available, including plate rolling. Plate rolling machines have different numbers of rolls and roll configurations, and each manipulates those rolls in different ways. Some of the most common plate roll machines are:

Three-roll, double-initial-pinch. These economical plate rolls tend to be simple to operate (see Figure 10). The side rolls are located at both the left and right sides of a fixed top roll, on the same axis. The side roll at the far end of the initial feeding point functions as a backgauge that helps square the plate, so the operator doesn't need an assistant to run the machine.

Double-initial-pinch machines make quick work of prebending when forming cylinders. In a single-initial-pinch machine, only the far roll can perform the initial pinch for the prebend, which means the operator needs to prebend one end of the workpiece and then remove, rotate, and adjust the plate so the same roll can perform the second prebend to the opposite edge.

In a double-initial-pinch machine, both bottom rolls move toward the fixed top roll in such a way that they perform the initial pinch for prebending. The operator simply loads the plate once. The far bottom roll performs the first prebend, after which the operator feeds the material through so that the near bottom roll can perform the second prebend on the opposite edge. Then the cylinder rolling can commence. The machine's side rolls also can tilt to allow for cone rolling.

Four-roll machines. These produce the most accurate, fastest bends. They securely hold the plate between the bottom and top rolls while the side rolls move vertically to create the intended bend (see Figure 11). They allow for quick prebending—again, no need to remove the material for the second prebend—and plate feeding can take place on either side. Each roll has an independent axis, which enhances accuracy and eliminates the need for an operator's assistant.

Cone rolling is relatively simple, with the side rolls tilting in order to establish the cone angle. The bottom roll also can be tilted to drive the major end of the cone.

Three-roll, variable-geometry. These machines can roll the widest range of material thicknesses and types in relation to the size of the top roll. They’re suitable for both thick and medium plate bending. The lower rolls move horizontally and the upper roll moves vertically (see Figure 12), resulting in a forming action that would be recognizable to anyone familiar with press brake operation.

FIGURE 4 Plate rolls are crowned for a specific range of material and bend radii. The setup on the top has excessive crowning (hourglass effect), the middle has insufficient crowning (barreling), and the bottom has optimal crowning.

Their press brake-like forming action is what makes these variable-geometry machines so flexible. Think of the lower rolls as a variable V die on a press brake. Moving the bottom rolls farther apart (a wider V) reduces the forming tonnage and allows the machine to form thick plate. Moving the rolls closer together (a narrow V) allows the machine to roll thin plate to small diameters. The top and side rolls can also tilt to simplify cone bending.

Some plate rolls come with features that help bridge the gap between press brakes and plate rolls. For instance, the system in Figure 13 has an upper beam, resembling a press brake ram, designed to support the top roll and apply the needed roll pressure to achieve certain radii that would otherwise require much larger rolls. It's able to produce parts at high speed that would otherwise need to be bumped on a press brake.

Like a press brake, a plate roll can tie up an overhead crane. To free the crane, some rolls come with CNC or NC side and overhead supports, which support a cylinder and prevent it from distortion from sagging under its own weight during rolling. Plate rolls can be integrated with motorized loading tables, infeed conveyors, and part ejectors, all of which can increase efficiency and, most important, ensure operator safety.

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. The press brake—being the flexible machine that it is—most likely could produce the part, but could it do it efficiently? If not, and if the piece fits a roll's thickness and radius-producing capacity, the plate roll might be the better choice.

Of course, all this depends on a shop's capacity level and current load. Managing any fabrication operation is a bit like chess. Thankfully, if a shop has the right press brake and plate roll for its product mix, both machines can be among the most effective pieces on the chessboard.

Kyle Jorgenson is CEO of Revolution Machine Tools.

But there's another, lesser-known option too: the press roll bending machine, a hybrid between the press brake and plate roll. The machine can have six rolls, three on top and three on the bottom, that work in concert to form a desired radius or radii.

In essence, these hybrid machines combine the stability and strength of the press brake ram with the plate roll's rolling capabilities. They help form parts that have thicker walls, tighter diameters, and are longer than a standard plate roll would be capable of.