Working with large sections of plate and high-strength steel poses a challenge few are able to take on
May 6, 2011
Bending heavy plate and high-strength steel used to require multiple workers, a large amount of bending experience, and lots of time. That's not the case anymore.
The view of a 1,000-ton press brake from 100 feet way and that of a 100-ton press brake from 10 ft. away is somewhat similar. However, the jobs done on those press brakes differ greatly. Bending the big stuff is a much larger endeavor.
Bruce Sine, project manager, Greiner Industries Inc., Mount Joy, Pa., has been working with his company's 40-ft.-long, 2,750-ton-capacity press brake (see Figure 1) for the last two years. The company had experience with large workpieces, bending them on its 14-ft., 750-ton press brake, but nothing like the jobs that come in for bending work on its giant Baykal brake. Sine said some of the workpieces can weigh as much as 15,000 pounds.
"Handling is an issue that you have to be careful about. It's not forgiving if you even have a 2,000-lb. plate moving around," he said. "It moves you wherever it wants to."
Seemingly the big jobs beget more large orders, according to Sine. When the company had only 14-ft. press brakes, people routinely asked them to bend parts that were 20 to 25 ft. long. Now with a 40-ft. press brake, Greiner gets requests to bend stuff up to 50 ft. long.
The company purchased a 26-ft., 1,100-ton Baykal press brake one year ago—ordered through Fab-Line Machinery Inc., St. Charles, Ill., like the larger press brake—to assist with some of the longer-plate jobs. In anticipation of perhaps taking on more of those longer jobs in the future, Greiner prepared a foundation adjacent to its newest press brake and had designs prepared for another 36-ft. brake. The possibility of installing a tandem press brake capable of bending 60-ft. lengths of metal is indicative of the success the metal fabricator has found working with huge workpieces. In fact, its bending department is housed in a 56,000-sq.-ft. facility built two years ago just for bending and forming activities.
"We keep growing," Sine added. "We used to be in the main shop, but we got too busy and too big."
The industry segments generating a lot of this high-tonnage bending activity aren't likely to slow down in the coming months, particularly as the economy continues to pick up. Agricultural and construction equipment-makers are still sending their heavy-duty equipment to economic hotbeds all over the globe. Vehicle and trailer manufacturers need new high-strength, low-weight components bent for new-generation, gas-saving designs. Given the increase in oil prices, the domestic oil and gas industry soon will take off, if it hasn't already.
That leaves the select few that specialize in this high-tonnage bending in an enviable position. But what used to be an exclusive club isn't so exclusive anymore. Technological advancements have helped to make big-time bending an operation that more shops can execute successfully—not just the ones with the most experienced press brake operators.
If a shop is bending large metal sections, it needs some sort of material handling assistance, usually in the form of a crane or even a lift truck in certain situations. Sine said that Greiner's bending and forming facility has six cranes, one with a 10-ton capacity, three with 25 tons, and two with 15 tons. Company management is actually looking to buy another crane to add even more flexibility for moving the large workpieces.
Additionally, shops likely have some sort of front-of-brake assistance—ranging from a simple table (see Figure 2) or front supports (see Figure 3) to more sophisticated equipment (see Figure 4). It's a necessity when trying to make the correct bend this huge on an often irregularly shaped workpiece.
"Typically, you would bend the sheet, take it out of the machine, check your angle, put it back in, re-hit, take it out, check again, and rehit it. But if you have a plate that's 20 ft. long and weighs 1,000 lbs., you've got to do this with a crane and a helper," said Paul LeTang, bending product sales manager, LVD Strippit, Akron, N.Y.
The front-of-brake tools can help position the part in the right place, but something needs to stop it. That's what heavy-duty backgauges do. These reference points help to ensure the material is in the right position for the first hit. Although they are called "heavy-duty," backgauges still can be knocked out of position with sloppy material placement. A programmable pushing assistance on advanced front-of-brake material movement devices can prevent this.
Another tool that more fabricators are considering for their press brakes is a laser line alignment tool (see Figure 5). The laser generator, placed in the ram, projects a laser light that then can be used as a reference point for matching opposing marks on each end of the plate, aligning the material for the first hit.
As any fabricator of heavy parts will tell you, however, high-tonnage bending is not a one-hit operation. By training, operators are taught to sneak up on the bend angle because if the part is overbent, it's scrapped—an expensive nightmare. Springback—the tendency of a formed metal part to try to return to its original shape because of its elastic recovery—ensures that multiple hits will be needed with both simple cold-rolled steel and exotic alloys (see Figure 6).
"What [fabricators] don't understand is if they would trust what the control has available to them, it would be a lot easier," said Todd Kirchoff, manager, direct sales, Cincinnati Incorporated, Harrison, Ohio.
What sort of springback values are fabricators dealing with? With cold-rolled steel plate, maybe the material kicks back 3 to 5 degrees. The high-strength steels can be more varied—perhaps between 10 and 20 degrees—but that's not the gospel truth.
"I know that they have measured in excess of 40 degrees of springback," said Gordan Baker, Pacific Press Technologies, Mount Carmel, Ill., recalling a shop that was making booms out of high-strength steel for telescoping cranes.
Typically, sheets reserved for one particular job likely came from the same coil and exhibit similar properties, LeTang said. That can help cut down on the trial-and-error process, because once the first part is produced accurately, the operator won't have to guess how the subsequent parts will react. However, because steel mills usually deliver small allotments of thick or high-strength material slabs at a time, plate characteristics in one delivery are likely different from those in a previous delivery.
Controls can help speed up the bending process as well.
"Most people buying a new machine in this tonnage range are very seriously looking at a higher level of control," LeTang said.
These controls ensure accuracy of ram depth and make corrections to the subsequent hits. For example, an optical protractor reads the bend angle, and the ram lowers to bend the part. The ram pulls up, allowing the springback to be released, and the protractor checks the angle again. After the control figures out the proper ram reversal position, the ram lowers once again and rehits the part, continuing the same process until the correct angle is achieved.
"It's like a typical servo drive that will be hunting for its home position, but instead of using encoders on the machine as a feedback loop, we are using the actual bend angle as the feedback device," said LeTang. "So the information going back to the controller is, in fact, the bend angle being received."
Even if the control technology doesn't automatically read the bend angle, it still can help produce accurate parts in a minimum amount of hits. The operator can update the library of bend programs with the ram reversal position, springback, and the tooling used for the bend. Once the information is uploaded to the control a "technology database" is created for that material type with its unique springback value. If that same material type is to be bent again, the control makes an adjustment for the springback, using the saved angle correction for that material, and proceeds with the bending.
"If you trust your control and you have the settings right, you should get it right after the second hit," Kirchoff said.
Press brake operators should trust the control, but not so much what they learned in school about tool selection—at least when it comes to the heavy stuff, Kirchoff added.
"For example, most operators were taught that you would use a V opening that is equivalent to eight times the metal thickness as a starting point. Well, for plate or high-strength steel, you may need something much larger than that," he said. "If you adhere to the traditional rule of thumb, you may end up cracking the material."
Reference charts are a good resource for determining which V-die opening and punch radius make sense for a specific type of material. Most machine tool manufacturers include this information with the press brakes they supply. For example, a quick look at Cincinnati's Press Brake Capacity Book reveals that for grade 60 of A572 high-strength, low-alloy plate for structural applications, the V-die opening should be 14 times the material thickness, not eight, and that the punch radius is equal to three times the metal thickness, not one.
"So if you don't know that, you will experience some cracking problems," Kirchoff said.
The mills also can provide this type of bending information. Being that they have access to the latest information about newer exotic alloys, they actually might have more up-to-date information than the machine tool builders.
Bending objects this large really doesn't lend itself to quick actions, and tooling changes also fall into this category. Manually removing a 20-ft.-long punch is both awkward and time-consuming. Having said that, some steps can be taken to speed up the process slightly.
Dies with adjustable V openings offer versatility without the pain of actually removing the tooling. Greiner's Sine said its largest press brake has adjustable bottom tooling, which can accommodate thin openings to 17-in. gaps, and it's worked out nicely for them.
"Being a job shop, you have to have that flexibility," he said.
Some high-tonnage press brakes have roller bearings in the ram so that punches can be slid in and out easily. It's still a big job to change out the upper tooling, but the press brake is up and running much sooner than it would be otherwise.
Pacific Press' Baker said he sees customers asking for ever-bigger press brakes—with requests for machines that can handle 40-ft. plates. Even now the industry is seeing brakes regularly built with throat depths and open heights around 60 in. and strokes greater than 24 in. The big bending business is burgeoning.
That's probably a necessity because the fabricators with press brakes that are 20 or 30 years old simply have a greater chance of struggling with accuracy when bending these heavy parts. The frames will last forever, but the control systems become obsolete eventually. Retrofitting may not be an option either, according to LVD Strippit's LeTang.
For instance, installing automated bend detection functionality on an older high-tonnage press might be impossible. LeTang said the press's hydraulic circuit, piping, pumps, valves, and cylinders are not capable of delivering the same performance as today's servo technology, which is necessary to deliver the precise hits that form the specified angle.
So the job of bending heavy plate hasn't changed much over the years, but the modern technology and material handling options have made the job a little easier. The stakes, however, are much larger. Fabricators literally can't afford to scrap these big parts and can't tolerate too much downtime because the press brake needs to be making parts, which also means making money. These are big jobs, and a select group of fabricators aren't afraid of doing them.