Forming&Bending Metal Sheet Material
Forming&Bending Metal Sheet Material in One Pressbrake or Plate Roller
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.
Deflection and Crowning Table for bending metal sheet profiles
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 hydraulic anti-deflection table type.
Mar-free Bending Tools for Press brakes
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 Incremental Bend on a Press Brake
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.
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
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.