Robots in manufacturing have evolved greatly since the early 1960s when General Motors brought the Unimate, the first industrial robot, into its die casting plant in New Jersey. Its role was to take on spot welding jobs that would otherwise be dangerous for humans.
However, it wasn’t until the 1980s that robotics in manufacturing really began to take off. As a big part of that surge, Yaskawa introduced the Motoman ERC control system in 1988, which brought control to 12 axes. By 1998, the Motoman XRC system increased to 27 axes and could synchronize up to four robots.
Despite these significant leaps in innovation, one question remains: Are there any welding operations that robots can’t do? It’s a topic Welding Productivity discussed with Zane Michael, director of thermal business development at Yaskawa America Inc., Motoman Robotics Division.
Mind the gap
The most common welding process performed by a robot is MIG, but TIG welding, resistance spot welding and resistance seam welding are also prevalent, particularly in automotive manufacturing. These are environments where the tolerances are tightly controlled and the weld joints are clearly established. Perhaps more importantly, they are environments where there are minimal gaps between the materials being welded.
To give some context, Michael describes a welder MIG welding in an open root joint where there is a gap between two plates where the welder deftly holds the wire in the center of the gap and moves it back and forth – with the wire at the leading edge of the puddle. The welder watches the puddle carefully, checking its fluidity, and making subtle adjustments in technique to ensure the quality of the weld. It’s a process that takes skill and experience.
This is not something robots are capable of experiencing. “Robots are not good at welding gaps period,” Michael explains. “A robot is very repeatable. It’s designed to do the same thing time and time again.”
Fit-up audit
When Michael visits a potential customer, he’s essentially taking an audit of “acceptable parts” – as in acceptable for a robotic welding process. He checks the joint fit-up for gaps, but he also looks for ways to refine the manufacturing processes so the gaps are minimized. And if they can’t be minimized, then he often suggests that robotic welding be reconsidered altogether.
“Solve your gap problems first with your part tolerances and manufacturing processes – whatever is causing a variation in the gap between the parts needs to be fixed,” he says. “For a robot, we like a consistent, tight fit-up between the parts we’re welding. That’s my No. 1 rule.”
What about part size? Everyone is familiar with welding robots that zoom around skeletal automotive frames, but what are the limitations in regard to larger parts?
Although it might be a more costly venture, part size is not a barrier to robotic welding. Michael provides an example of a customer that utilizes robots on 53-ft.-long trailers where the robots go down the entire length of the trailer. The robot is on a servo track, and the weld goes from one end to the other. However, it’s important to note that the customer has invested in tooling that makes sure the materials have tight fit-up so the weld joint is optimal.
“The customer worked on their material tolerances,” he says. “Their tooling holds the parts together so they can accurately weld it – no gaps to deal with.”
A laser sensor affixed near the torch assists the robot in keeping the weld on track. The sensing technology finds the path of the joint before the weld is made, shifting the program path to the exact location of the weld joint.
“Robots are good at finding a weld joint,” Michael explains. “Now imagine you’re welding this 53-ft. weld down the side of the trailer and there is a 0.010-in. to 0.020-in. gap. The appearance of the weld will change. It will probably burn through the part. The robot doesn’t know what to do with that. So that’s why it’s just a good rule of thumb – no gaps.”
Position requirements
In the hands of a human, a weld gun can be utilized in almost any position because the welder can watch the puddle and make adjustments as needed to ensure the weld is filling the joint. A robotic welder can use touch sensing or laser sensing technology to program the weld path, but it doesn’t have flexibility in regard to position, which means the positioner has to move the weld joints into a horizontal or flat position because the robotic welder can’t “see” what the puddle is doing like the manual welder.
“If I’m welding out of position,” Michael says, “I’ve got gravity working against me. As a welder, I can adjust my technique ever so slightly to control that weld puddle where I want it to be to produce a quality weld. I can’t make those variations with a robot.”
On larger equipment where a lot of heat is involved, distortion becomes an issue, as well. However, thanks to finding and tracking technology, robotic welders can make slight adjustments to stay on the right path as the part distorts during the welding process. While this sensing technology increases cycle time and adds more money to a robotic welding solution, it can be worth it.
Understanding WPS
Welding procedure specifications (WPS) determine the details and parameters of the welding process that a welder will use to tackle a specific part. And the WPS won’t change for a robotic welding solution.
Michael explains this with an apple pie recipe analogy: If you get your hands on an apple pie that you consider perfect, you’d follow the recipe exactly to make one for yourself. The same amount of sugar, butter, flour, apples and time in the oven must be the same. You can’t make it faster or with different amounts of ingredients and expect it to taste the same.
But that WPS will change for manual welding. Most welders paid to work 8 hours a day are only going to have an arc going for about 20 percent of that time. A robot, by contrast, in an environment where there are no gaps and the weld joint fit-ups are good, has an efficiency around 80 to 85 percent, Michael says.
“A lot of people think a robot will weld faster by traveling faster,” he notes. “While a robot can make a given weld faster than a manual welder, there is a WPS ‘recipe’ to follow, so the weld travel speed will be one of the critical variables to follow.
A robot will get from one joint to the next joint much faster than Bob the welder; Bob’s got to raise his hood, he’s got to move his chair around – he’s got to get comfortable. Meanwhile, the robot has half that weld joint done.”
