By CLARE GOLDSBERRY, associate editor

Eight monitoring packages from Livingston & Company Inc. mounted on a resistance welding cell.

Close-up view of the installed monitor packages.

Most resistance welding happens in a highspeed, high-volume automated setting. Variables in the welding process are always present, meaning that there will be some margin of error in most of the parameters, requiring fast, real-time monitoring to ensure quality components.

Monitoring each and every resistance-welded joint is critical to quality, particularly in a highly automated manufacturing environment. Real-time monitoring allows operators to respond quickly, which reduces scrap, improves overall quality and reduces cost-to-manufacture. Monitoring is not a solution to weld problems; however, there are a variety of parameters that, when monitored, can make improvements in both quality and productivity. The key to successful resistance weld monitoring is establishing permissible tolerances for each variable of the process, primarily current and the weld cycle.

While all computerized monitoring systems are set up to integrate with factory floor automation and robotics, there are different types of resistance weld monitors for different applications. Some monitors can track as many as 13 variables, such as the POD series from Livingston & Company Inc. (www.livco.com). Some of the parameters that the POD measures include set-down, the movement of the work piece during the welding process. "As you put the current on, the part melts into the steel and the electrodes move," said Matt Swayze, a Livingston & Company applications engineer. "Measuring how much they move is a good indicator as to how much set-down you have. You can closely tolerance that movement and archive that data." It also measures expansion, conduction angle, current, voltage, force, among others.

Resistance weld monitoring devices cover many welding applications, including those that require "gross" or "average readings" for broad-tolerance applications, Swayze said.

A monitor will actually record if a weld good or bad; for example, identifying a cold weld that didn't have enough energy to produce a good nugget size.

David Bacon, a teacher of resistance welding technology and consultant for Fortune 500 companies (www.updatetechnology.com) said that he believes the big challenge companies will have to face is coming up with a different method to measure the weld strength in resistance welding. "In aerospace, they weld samples and tensile test it," he said. "They will have to start to do this in the auto body shops [of the auto makers] because quantities and speed are tremendous in the automotive industry."

Bacon said that it's difficult to predict weld quality. Ultrasonic beam technology scans over the weld and spots the best signals and analyzes that set. "While it's somewhat of a lab odyssey, it can very accurately tell you what the weld size is and how well-fused it is, and can even find cracks — but [the ultrasonic beam companies] won't admit it. However, ultrasonics will eventually have to be the way they'll have to test resistance welds."

"The biggest deviation in resistance welding is the material," Swayze said. "Sometimes it's oily, and because it's stamped or drawn with these huge presses, stampings can vary and give you a problem with fitup, or how the two pieces of materials nest together. When you're doing quality control, typically to this day the best way to determine whether a weld penetrates and gives you a good nugget is through destructive testing, such as a ‘pull test.' "

The ability to store and archive data is critical to OEMs who are often concerned about liability issues, particularly with critical safety components in vehicle systems. Pinpointing the exact automation line, day and time that a suspect weld was done can often clear or confirm a suspected bad weld.

Tool wear is another measurement that is critical to optimum welds. When the weld tip starts to mushroom, a monitoring system that measures the displacement of every weld can be set to alert the operator when it's time to change the electrodes. "Tip dressing can be a solution," said Swayze. "In some cases the process of tip dressing can pull the tip off."

Swayze said that it's critical in high-volume production environments to flag defects as early in the process as possible. The further down the line in assembly the vehicle gets, for example, the more costly it is, particularly if you find a cold weld and have to pull the whole car off the line. "The idea is to verify in real time that you have good welds through continuous real-time monitoring," he said.

The benefits to resistance weld monitoring are obvious when it comes to creating a consistent process with narrow parameters to produce repeatable welds in a high-speed, automated manufacturing process. It's more than a quality issue, but a competitive one as well.