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Precision sheet metal bending, bump by bump

The right tools and setup make the craft of bump bending more efficient

Figure 1
The arc length is the measured inside surface of the bumped radius. Illustration courtesy of ASMA LLC.

A smooth, wide radius in thick, high-strength plate seems simple enough, but to actually form it is anything but. A bump bend is really dozens of bends, bumped by the brake punch a few degrees at a time. Each bend line has all the variables that go into a conventional bend. If an error occurs, it stacks up throughout the bumped radius, giving you a defective piece that either needs to be reworked or scrapped.

Building tools large enough to handle these massive bends in one or just a few hits usually isn’t cost-effective, and sometimes it’s just not practical; the required tonnage and springback variation from batch to batch are just too great. Depending on the bend characteristics, you could make the form in a plate roll. But quite often, bumping on a high-tonnage press brake remains the most practical, flexible option.

Many operators use templates to ensure they’re bumping a piece to the correct radius and angle. It’s tedious work, yet if technicians prepare properly and have the right tools, bump bending can become much more predictable and efficient.

Arc Length, Radius Pitch, and Die Width

In his new online book, TheArtOfPressBrake (www.theartofpressbrake.com), Steve Benson, president of press brake training firm ASMA LLC, Salem, Ore., describes how you can calculate the bump bending variables. You start by determining the arc length, as measured on the inside surface of the radius (see Figure 1). “There are many different ways this length can be calculated,” Benson writes, “and one of the easiest is: Arc length = 6.28 × Inside radius × (Bend angle complementary/360).”

The radius pitch is the distance between bumps (steps) used to bump-bend the angle (see Figure 2). The greater the number of steps, the smoother the outside radius will be. For a smooth outside radius in a 90-degree bump bend, you may choose to bump the metal only 2 degrees every hit. This means that after 45 steps, you will have created a 90-degree bump bend (45 steps × 2 degrees each step = 90 degrees). To obtain the radius pitch, divide the number of steps by the arc length. Determining the radius pitch is critical. Although a narrow pitch can create an extremely smooth outside bend radius, it also makes an operation more time-consuming and costly.

“[A narrow pitch] multiplies any small error that might occur from the machine, material, or tooling,” said Marten Weidgraaf, general manager at Elgin, Ill.-based Ursviken Inc. “If an already bent face rests inside the die, it aggravates an easy bending calculation. Such a condition also develops offset forces to tools that the machine must manage.”

Next comes the die width. As Benson writes, during bump bending the punch descends into the die by just a few degrees for each bump. An optimum die opening is double the radius pitch. This narrow V opening allows the part to sit flat on both die shoulders. Ideally, if the right tools are available, the die width governs the radius pitch. The wider the die, the wider the radius pitch, and the more “choppy” the bump bend becomes.

If the die width were wider than double the radius pitch, previously formed sections would sink slightly inside the die opening. This alters the bend characteristics and can shift the plate edge sitting against the backgauge upward, which can change the resulting bend angle.

In addition, Benson writes that it’s best practice to use a punch tip radius large enough so it doesn’t leave a deep bend line with each bump, which in turn will create a rougher outside surface. He recommends the punch radius be more than 63 percent of the thickness of mild steel; the punch-nose radius can be larger if you are working with other materials like high-strength plate, for which operators might use a punch-nose radius several times the material thickness (see “How an air bend turns sharp,” available at thefabricator.com).

Finally, you need to determine the depth of penetration, which for a smooth bump bend shouldn’t be much deeper than the pinch point, where the punch nose firmly holds the material. “As a starting point for test bends,” Benson writes, “the depth of penetration can be expressed as Depth of penetration = (Die width/2) + Material thickness – 0.02.

Figure 2
The less distance there is between two bump lines, the smoother the outside bend radius. Illustration courtesy of ASMA LLC.

Note that Benson describes this as only a “starting point for test bends.” Determining optimum settings for a bump bend, especially the depth of penetration, is very much a trial-and-error affair. For instance, the first bump may call for a little more punch penetration than the second, and from there the punch depth may vary slightly from step to step, depending on the nature of the bend, and the material thickness, hardness, and springback.

When it comes to die width and punch penetration, Benson adds a caveat about die width: “Watch your tonnage loads.” Despite the only slight punch penetration, forming tonnages ramp up quickly, especially in thick or hard material.

Hard materials with significant springback complicate matters too. Springback requires overbending, so to bump 2 degrees will require that the punch penetrate farther. Just how far? Again, it’s complicated. If you have a narrow die width, changing the level of punch penetration becomes extremely sensitive. A tiny variation in punch position can change your bend angle dramatically—a challenge when you’re bumping a few degrees at a time.

Moreover, a narrow die width usually means a narrow radius pitch and numerous steps along the arc length of the bump bend. Minute errors early in the sequence can stack up to significant angular errors after dozens of bumps.

Bending software has progressed to the point where the act of programming isn’t as complicated as it once was. But determining the initial variables, including the depth of penetration of the punch, still can involve trial and error.

“The variation in springback from batch to batch may be too big to be dependent completely on software,” said Mikael Linderot, technical sales engineer of Ursviken Technology AB in Vasteras, Sweden.

Modern press brakes can perform adaptive forming, with angle measurement devices that can correct variances in-process, but they work best for standard radius bends, not necessarily bump bends. Each individual “bump” is in essence an extremely wide radius bend, just a few degrees complementary, and measuring that creates challenges. As Linderot explained, measurement systems in adaptive forming start to work when a bend angle reaches between 9 and 25 degrees complementary, depending on the specific technology used. “The devices also need flat faces to measure against,” he said, adding that during a bump bend, that’s not possible.

Considering all these challenges, technicians make good use of templates. They may need to bump a little, compare it to the template, bump a little more, measure to the template, then bump again, making sure not to overbend. They may need to turn the plate around to form a flange or bumped radius on another side. Previously bent flanges aren’t good gauge points, of course, so here they may rely on bend-line marks. Some brakes emit an infrared laser to help line up the punch with the intended bend line.

All this craft, combined with the fact that large plates aren’t easy to move, means that most of the cycle time in heavy bump bending consists of everything the technicians do between the bends: moving and measuring the workpiece and making process adjustments where necessary. This is where material handling and tooling strategies come into play.

Part Positioning

When possible, technicians push the plate against the backgauge, and the first bump commences toward the front of the arc length (see Figure 3). The backgauge then moves forward for every step until the last bump. This makes it easier for operators to remove the part and gives them a flat plate edge to gauge against.

Figure 3
When possible, bump bending occurs from back to front, with the backgauge moving incrementally forward for each hit. Illustration courtesy of ASMA LLC.

Of course, the operator can’t safely go behind the brake to hold the material steady. What if one bump causes the part to shift slightly against the backgauge? This throws off the part positioning, so that when the gauge moves forward for the next bump, the punch doesn’t hit where it should. A small positioning error early in the bump progression can throw off the final angle significantly.

Weidgraaf described one operation that uses a specialized backgauge. A conventional backgauge finger has a vertical backstop and a horizontal component that supports the material. Weidgraaf, however, described a 6-axis backgauge finger that doubles as a clamp. It’s essentially a backgauge finger with opposable thumbs that grips the plate from above and below to ensure the plate’s gauge position remains consistent throughout the bending sequence (see Figure 4).

The grippers also help position large workpieces. When a flat sheet is brought to the brake, the gauges grab the plate edge and pull it back to the programmed position, making operators’ jobs a lot easier and safer. No longer does a team of technicians have to struggle to position a large plate.

Variable Dies

Tool changeover also adds time between jobs. Say a job requires a bump bend followed by a conventional radius air bend. A smooth bump bend will require a narrow die width, while the radius bend, particularly in thick plate, will require a much wider opening. A variable die can be used for both bends. “A variable die means you can change your die opening between hits,” Linderot said.

Similarly, variable dies can help when bumping complex bends, like those with a wider radius at one end of the part and a shorter radius at the other end. The technician can set a short die width to bump with a narrow radius pitch, then set a wider die width to bump the wider radius, which can be formed smoothly with a wider radius pitch (that is, more space between the bumps).

Advanced Crowning

Yet another variable is deflection. All press brakes deflect under load, and it can become a big issue when you have extremely large workpieces. “Say you’re bump bending, and you have a constant error of only a fraction of a degree,” said Weidgraaf. “You will see that after the entire workpiece is formed, you have a bow or kink in it.”

Modern brakes have automatic crowning compensation systems to control this effect. As Weidgraaf explained, they’re more exact and certainly more efficient than shimming. Certain systems have mechanical compensation not just in the center of the bed, but also in specified increments across the entire workspace. Such technology, feeding information back to the CNC, gives technicians the ability to tweak forming along an extremely long bend line—a few thousandths here, a few thousandths there (see Figure 5).

The Time Between

When you analyze a bumping operation of large workpieces, you may find that the actual bumping really doesn’t take that long. What takes time is everything that happens between bending: moving and transporting large workpieces in and out of the press brake.

Workpiece supports can help. These include rollers that help position the plate onto the press brake bed, as well as supports that move up with the workpiece as it’s formed (see Figure 6). Workpiece supports can make an operation much more efficient not only because they free up an overhead crane, but also because they hold the plate in the formed position after each bump, ready for the operator to check with a template.

“If he needs to rehit it, he can pick up exactly the same bend line,” Weidgraaf said. “If you lay a large piece down flat, it becomes quite an art to lift it back up and position it to exactly the same bend line.”

Figure 4
Throughout the bend cycle, the gripper holds the plate without losing the reference point. A radius bend is shown, though the technology can be used for bump bends as well. Illustration courtesy of Ursviken Inc.

Linderot added that serious accidents can occur if using the crane for workpiece support. If operators aren’t careful, a brake can exert so much tonnage onto a workpiece that it can pull down and destroy an overhead crane trying to hold it.

In addition, he said that some applications may benefit from handling systems that actually rotate the part. Supports approach the part from the front and back of the tooling, lift the heavy workpiece off the die, and rotate it to the other side, with no overhead crane required (see Figure 7).

Adding Efficiency to Craft

Bump bending—particularly in large, thick workpieces—remains more art than science. Material characteristics vary from batch to batch. Precise backgauging (say, when you have bends on both edges of the part) sometimes just isn’t possible. But performing the basic calculations beforehand and having the right tools can make these challenging jobs less time-consuming and, most important, much safer.

About the Author
The Fabricator

Tim Heston

Senior Editor

2135 Point Blvd

Elgin, IL 60123

815-381-1314

Tim Heston, The Fabricator's senior editor, has covered the metal fabrication industry since 1998, starting his career at the American Welding Society's Welding Journal. Since then he has covered the full range of metal fabrication processes, from stamping, bending, and cutting to grinding and polishing. He joined The Fabricator's staff in October 2007.