Team overcomes robotized bending challenge
March 7, 2013
A high-product-mix manufacturer chooses robotized bending not because it lacked a skilled workforce, but because it wanted to make the best use of the talented workforce it already had.
They look simple enough—just half-round stainless steel shells designed to encapsulate boiler tubes, shielding them from their harsh environs. But the tube shields produced by Metso’s Fairmont, W.Va., metal fabrication operation are anything but simple.
Metso makes a variety of products for the power generation, waste-to-energy, pulp and paper, mining, and other industrial sectors. It’s a global organization with thousands of employees and dozens of locations, including one in West Virginia. The company’s 52,000-square-foot Fairmont Service Center produces sheet metal parts for various Metso product lines. In all, it churns out 7.5 million pounds of fabricated metal products a year, and of that, about 3 million lbs. are tube shields.
That’s a lot of tube shields, and to make them press brake operators would spend their days standing by the press brake. “We would stroke press brakes all day, every day,” said Chuck Kuretza, Metso’s plant manager. “We often stroked these brakes 65 to 70 times an hour.”
The part volume seemed more than adequate to justify a robotic bending system, but the decision wasn’t straightforward. The first challenge was the part’s shape—a long, narrow half-round. The flat blank transformed into a round shell; how would a robot maneuver this part between bends, let alone grasp it at the end to stack it?
Considering all these challenges, why did Metso even consider press brake automation at all? As Kuretza explained, safety considerations topped the list. “Safety is a big focus for our company, and these parts are narrow. Our press brake operators were using special tools [to manipulate parts]” to avoid getting too close to the pinch point.
He added that manual press brake bending didn’t make the best use of available talent—an unusual statement these days, when so many shops are adopting automation because (at least in part) they can’t find the qualified people they need to meet demand. At Metso’s 26-employee fabrication operation in Fairmont, this isn’t the case. “To even get in this shop, you have to be a good machinist or welder,” Kuretza said.
He added that the company finds good workers by paying an extremely competitive wage, far above average for the area. Combine that with good benefits and a good work environment—including clear air thanks to comprehensive weld fume collection—and talent comes knocking.
“We train machinists and welders to run the other, more production-oriented machines,” Kuretza said. “They’re not always happy standing in front of the press brake, performing repetitive work all day.”
Managers knew increased throughput could be a big benefit of bending automation. With the right setup, including considerations made for blank and part stacking, it could also run on its own with minimal, if any, operator intervention, both during the day and a lights-out shift at night. But they also knew that a robot sometimes can’t account for part variability. Metso’s managers had their doubts.
Still, one overriding factor drove the management team toward robotic bending, sources said. It wasn’t because they lacked a skilled workforce or wanted to reduce the head count. Instead, it was to make the best use of the talented workforce they already had.
One tube shield may look like another, but they’re not identical. Volumes were more than sufficient to justify automation, but these tube shells come in different diameters, thicknesses, and lengths. Essentially, the shop custom-fabricates each order. Would a robot make sense for such a high-variety operation?
To find out, Metso identified the core products that represented most of the shop’s part volume. Metso’s managers called these products its “prime movers.” These represent about 40 percent of all the tube shields the company produces, including those with 2.0 and 2.5 in. outside diameters made of 10-, 11-, and 12-gauge material (see Figure 1).
The shop’s 0.5-in.-capacity shear cuts blanks within ±1⁄32 in. along a 48-in.-long sheet, so blank size variability wasn’t a problem. But those blanks were of varying widths and lengths, and formed to various radii. Such variability isn’t usually considered low-hanging fruit for automation. But automation would free the shop’s workers to perform lower-volume, less repetitive tasks. And most important, it would foster a work environment with happier, safer employees. As sources explained, this benefit was just too good to pass up.
Conventional air forming in a V die wouldn’t do. Instead, the shop opted for a custom-machined radius die and a round top punch. The die had to be machined with just the right geometry so that when the tool descended, it would form a radius within an extremely narrow window, ±0.005 in.
“The tolerance is tight for this kind of application,” said Drag Djuric, applications engineer at Accurpress Tooling Systems in Surrey, British Columbia. “We needed to make custom tooling because of the springback. They almost wanted this perfect.”
This wasn’t coining, in which the punch penetrates into the thickness of the workpiece, forcing it into the desired shape. Instead, this was a multihit bottoming operation. The tool descended to the top of the material thickness, but no farther, so the tooling engineers still had to deal with springback that would occur after each hit. To attain a specific radius, the die had to be ever-so-slightly bigger, the punch ever-so-slightly smaller.
Some tube shields have edges that are offset slightly, creating a “hood” that can mate securely on top of another tube shield. It’s like a dogleg offset, only this one offsets in a curved fashion. For this, Metso uses another custom tool set with the same die height as the tool set that performs the first three bends. This allows the workpiece to undergo the first three hits with one tool set, and then slide across the bed to the other tool set for the final bend, which forms the hood sections.
With new tooling on a new Accurpress brake, the bend sequence looks simple enough. The operation requires hits in three places along the blank width. It forms the two sides first and then bottoms the center to form the final half-round shape—not an ideal geometry for a robot. So how can a robot maneuver and grip the sheet when it’s being formed into a half-round section?
“Having the right gripper overcomes a lot of problems.”
So said Allen Guernsey, a Yaskawa Motoman robotic systems integrator with Mid-West Machine Products Inc., Golden, Colo. A contract fabricator itself, Mid-West has made good use of robotic bending over the years. Several years ago Guernsey began using the experience he gained on Mid-West’s shop floor (which has five robotized bending cells) to help other fabricators integrate automated bending cells of their own.
For Metso’s application, Guernsey knew part gripping would be a primary challenge. “So we used the correct suction cups to allow for the curvature of the part,” he said.
Mid-West tested various suction cup designs, finally going with a flexible cup with a bellow, which can expand to conform to a round shape (see Figure 2). Most important, the gripper can handle all the variation in the tube shield part family, so the robot need not change out grippers between tube shield runs.
Guernsey added that those in the food industry have used these kinds of suction cups to handle eggs, an apt metaphor for this bending challenge. An egg has a curved surface that must be handled just right, or it will break. Like an egg, the workpiece also has a curved surface. If handled incorrectly, the part wouldn’t shatter like an egg, but it certainly would be bent out of tolerance.
The next challenge came with part positioning. “With these parts, if you push too hard [against the backgauge], you actually get slightly different bend angles and flange lengths,” Guernsey said. The excess force can alter the part position ever so slightly, enough to cause the part to form out of tolerance.
To overcome this, Mid-West integrated feedback sensors within the backgauge and the robot gripper assembly. These sensors serve two purposes. First, they eliminate the need for a squaring table, which bending robots typically need to ensure they have the blank oriented correctly in their grippers. The sensors allow the robot to lift a part from the stack and slide it against the backgauge, which communicates positioning information back to the robot. The robot then makes necessary adjustments before bending.
Second, the sensors allow the robot to control the pressure it applies when pushing the part against the press brake’s 6-axis backgauge. Once the sensors detect that pressure has reached 10 PSI, they send that information to the robot, which then knows not to apply any more pressure.
“This system really gives the robot a human touch,” Guernsey said. “Just like a person, the robot pushes with 10 pounds of force, and then releases. Like a human finger, the robot gripper retracts and gets out of the way when it’s not needed.”
This subtle action gives the robot motion its almost human feel. After the suction cups place the blank on the radius die, the gripper rotates downward to reveal fingers the robot uses to gently push the blank against the backgauge for the first hit. After this, the suction cups maneuver the half-formed blank to form the second bend. Once the backgauge engages, the fingers again gently hold the part in place until the ram descends and bending commences.
At this point, the part profile looks like a U with a flat bottom, and the second bend, and the fingers position the workpiece for the final bend and again hold the part against the backgauge until the ram engages. If hood forms are required, the robot then slides the workpiece over to the adjacent tool set for the final bend. Once parts are formed, the robot’s suction cups pick and place the curved part into nearby boxes (see Figure 3).
The shop implemented the robot cell in July 2012 using a tooling set designed to handle about 40 percent of the tube shield product line. According to Kuretza, labor costs associated with the product have plummeted more than 75 percent, and throughput is through the roof.
“If they run the brake throughout the day and overnight, we calculated that they can produce more than 1,056 tube shields a day,” said Todd Lang, sales engineer at Fox Machinery Associates Inc., the Bridgeport, Pa., machine distributor that worked with Accurpress and Mid-West on the project.
This year Metso plans to invest in more tooling so that the automation can handle virtually all the tube shields produced in the facility. Not only has tube shield throughput increased, so has production for the entire operation. Cross-trained technicians now spend their days in the rest of the facility. One day they may be shepherding a short run for a special order through several operations; the next day they may work with various parts at a welding or machining station.
The robotized bending cell has paid off in a direct financial sense. But it has also made employees happier, safer, and more productive. That’s not a bad ROI either.