Exercising control

Resistance welding companies strive to boost process reliability, repeatability

Practical Welding Today November/December 2003
November 20, 2003
By: Lincoln Brunner

Three-phase, mid-frequency inverter
This three-phase, mid-frequency inverter is a dual-gun projection welding machine with a pneumatically operated lift table for the workpiece. Although mid-frequency technology isn't new, North American buyers are increasingly turning toward their benefits, equipmentmakers say.

Getting control over the resistance welding process might seem sometimes like catching a frog with boxing gloves on.

However, as end user quality demands increase and other welding processes encroach on resistance welding's traditional turf, the need to control the process and provide repeatability and flexibility never has been greater.

Control Freaks

Just ask Larry Moss, president of Automation Intl. Inc., Danville, Ill. He and his team have devoted huge amounts of time to helping customers gain control over their resistance welding operations, both locally and remotely.

"It can show you in graphic detail what that weld accomplishes—wave forms, pressure, [and] currents," Moss said of his company's butt-welding controls. "It can look down to the millisecond and look at what happened in that weld."

Automation Intl. has 30 railroad customers welding rail in the field with its controls, which collect thousands of bits of data on each weld and store them for future reference.

"The computer not only is making the weld and controlling the weld being made, but it's gathering all this data and sending it back in real time to headquarters," Moss said.

In today's just-in-time (JIT) environment, real time is what it's all about, according to Dimitrios Cecil, founder of Computer Integrated Welding, Troy, Mich.

Three years ago, Cecil and his team were invited to conduct a design of experiment (DOE) on a DaimlerChrysler resistance welding operation that was woefully out of spec—routinely expelling the metal that was supposed to form the weld nugget by letting the electrodes slip and slide across the sheet metal.

Using its patented real-time monitoring and control methods, CIW showed the DaimlerChrysler people what was happening and how to correct it. The key was monitoring and correcting for variables during production rather than guessing at how long to weld a certain material and with what level of weld current based on averages, Cecil said.

"We go with real time—real-time fit-up, real-time weld," Cecil said. "[We] terminate the weld, [and] when the gun opens 0.0020 inch, we tell the robot, 'Move.' So the robot is a slave to the gun. The weld gun is indexing based upon the opening condition of the next linear interpolation. Instead of estimated, it's actual."

Thomas Zambelli, head of business development for Dengensha America Corp., Bedford, Ohio—observing the preponderance of robotic welding among his customers—said his company has worked with robot manufacturers to integrate welding control with the robotic control unit.

"In the past, you'd have had a separate [unit] for your welding parameters, and then your robot parameters would be programmed separately. We've workedto combine that into one unit. That's where we've focused a lot of effort here in the recent development."

For automakers, relying on after-the-fact destructive testing rather than pre-welding high-tech monitoring and control costs money with redundant welds—from five cents to 12 cents per weld and up to 1,000 redundant welds per vehicle. For a supplier, it can cost perhaps $16,000 to buy back a vehicle that has a defective assembly that was discovered only after it became part of a finished product.

"It's costing them $16,000 to buy a car that has a project weld nut missing that costs about 80 cents," Cecil said.

"[Testing afterward] doesn't fit into the JIT package, because when they find out, there's no inventory or warehouse for them to go retrieve it from. It goes from their site via truck to an assembly point site, and then right into the vehicle."

While customers are asking for more feedback from their welding equipment as the welding process is being completed, some shops actually are going lower-tech for more jobs, one equipment company exec said.

"We have, in many instances, redesigned welders to allow better cell layout, as they are going back to less automation in some plants because of quick changeover of product mix," said Jim Dally of Standard Resistance Welder Co., a division of Jim Dally & Associates, Winston, Ga.

Those customers also are moving to cell-type manufacturing for the flexibility it provides, said Sandy Lee, vice president of sales for Resistance Welder Corp., Bay City, Mich.

"Customers are wanting more and more flexible-type automation so they can change models fast," Lee said. "Back in the '60s, '70s, and '80s, everything we built was high-speed automation, and you automated everything. They're going more to cell-type manufacturing. It's a little more labor-intensive, but it gives them the ability to change over faster and run different models faster."

Lee noted the recent rise of faster and more consistent servo-driven welding guns on spot welding lines, which are becoming popular in the automotive industry. He also has observed a push for tighter tolerances, and his company—which builds virtually 100 percent custom machinery—has responded by helping them to decide reasonable tolerances and the equipment needed to achieve them.

"It's a quality issue," he said.

Zambelli said automotive customers are ever pressing for efficiency, and servomotors are part of the mix.

"We not only try to supply a more efficient welding machine to our customers, but we're also doing a lot of automatic feeding equipment for fasteners being welded on automotive stampings," he said. "So our thrust and push is toward making the automotive manufacturers more efficient by using servo guns, because you can take one gun and use that in several different areas."

Getting In Early

When it comes to the state of sales and marketing, Tony Angelosante of American Industrial Equipment Corp., Union City, N.J. has his own ideas about how to tackle a lack of business.

"What we're doing to overcome [lack of business] is trying to reduce prices and get people to get off dead-center and place some orders," Angelosante said.

One key to getting resistance welding technology into the hands of manufacturers is getting face time with the designers of the products they're churning out, Moss said.

"One of our jobs is to prove to the [fabrication] industry that resistance welding is still very viable," Moss said. "One of the thing we've got to do is jump forward and get to the designers of these products. It's really a marketing thing. We can prove that resistance welding is economical, but if you have a designer designing a part and somebody tells him without any research, 'You've got to go to this new super glue,' that's where he's going to go.

"It's not a growing market; to some degree, it's mature," he said. "But people still are coming back and looking at it. That's what we have to keep doing."

Resistance welding equipmentmakers also are battling to stave off competition from metal clinching and foreign builders of resistance welding equipment, Lee said. If they don't educate the market, it's lights out.

"I don't think it's ever going to be what it was in the '90s," Lee said. "With customers going offshore and moving to Mexico and everything, it's going to reduce the economy as far as our business is concerned. We do a lot of work with the major appliance companies, and they're moving to Mexico like ants building a new home."

Marc Levesque, sales manager for Centerline Ltd., Windsor, Ont., sounded a somewhat more optimistic note.

"Within the next two years we should be back on track," said Levesque, who noted that the recent instability on his customers' balance sheets has forced Centerline to offer some creative financing options. "We are planning to increase our capacity to take advantage of the potential opportunity for increased business."

New Powers

In addition to the greater use of servo-driven welding guns, certainly one of the biggest trends on the equipment side of resistance welding is the burgeoning popularity of middle-frequency transformers. Mid-frequency technology is nothing new—AeroSpace Welding Inc. co-owner Craig Ittner said he has several from the post-World War II era—but North American buyers are turning their eyes more and more to their benefits.

"Although both technologies [servo drives and mid-frequency transformers] have been around for several years, just recently have we seen both used on large projects despite the added costs," Levesque said. "It appears that the benefits of these technologies have now surpassed the costs of implementation."

Although some North American resistance welding manufacturers have produced new, middle-frequency inverter welding machines for automotive applications and retrofitted mid-frequency inverter transformers and controls on existing machines, none appear to offer standard, catalogued models of mid-frequency inverters, according to Gerard Doneski, managing director of Lors Machinery, Union, N.J.

These welding OEMs appear to be lagging behind their European counterparts, which introduced standard models of three-phase, mid-frequency inverter resistance welding machines at international trade shows a decade ago. Doneski reported that one Italian welding manufacturer offers eight different standard models, ranging from 32 kilovolt amps (kVA) benchtop units to 240-kVA floor models at a full 50 percent duty cycle.

In addition to the mid-frequency inverter types, compact three-phase DC-rectified units are pushing past their traditional aircraft markets into mainstream precision sheet metal fabrication shops, he said.

"You can't weld aluminum with it," said Ittner, whose shop in Los Angeles maintains 25 resistance welding machines, mainly for aluminum applications in the aerospace market. "You can, but you can't do a decent job of it. I've never been able to see where you can get enough power to weld out of them."

Mid-frequency units use less electricity than traditional AC welding units because users don't need to increase the kVA rating for thicker material, as they do with AC units.

"In mid-frequency packages, it doesn't matter how much material you have—the gun is a lot more forgiving," Levesque said. "It can reduce the number of varieties of kVA you require. It doesn't matter what it sees in the throat. In AC power, you have to be careful on how deep the gun reaches into the material. Mid-frequencies are more forgiving that way."

Mid-frequency transformers also produce a constant and accurate welding current. In addition, they can offer long electrode life and good weld appearance. Though mid-frequency costs decidedly more than their traditional counterparts, their package size can allow overall gun weight to be reduced, which potentially can allow welders to use lighter-payload robots, Levesque said.

Lincoln Brunner is a Wisconsin-based freelance writer. He can be reached at lincbrunner@yahoo.com. Wisconsin-based correspondent Ken Musante (kmusante@airpost.net) contributed to this report.

Practical Welding Today would like to acknowledge the following sources used in this article:

American Industrial Equipment Corp., www.aiespotwelder.com
Automation Intl. Inc., www.automation-intl.com
Centerline Windsor Ltd., www.cntrline.com
Computer Integrated Welding, 248-733-9353
Dengensha America Corp., www.dengensha.com
Lors Machinery, www.lors.com
Resistance Welder Corp., 989-684-4030
Standard Resistance Welder Co., a division of Jim Dally & Associates, www.srwelder.com

Photo courtesy of Lors Machinery, Union, N.J.

Lincoln Brunner

Contributing Writer

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