Feel the HEAT, See the Light: Solving Problems and Improving Productivity in Robotic GMAW Welding
by Brian J. Doyle
, US Sales Manager
Panasonic Factory Solutions Company Posted 05/19/2008
For quality-minded manufacturers, robotic GMAW welding (commonly called ‘‘MIG Welding’‘ in the US) is a difficult process to keep under control. To get the desired result, a rather temperamental and dynamic process must be applied to parts with dimensional tolerance and fit-up variations from upstream operations. It is no wonder that many attempts to improve welding quality yield disappointing results, and many attempts to improve welding productivity are seen as contradictory to the quality effort – the faster you go, the smaller your process window. Between varying parts and bad targeting caused by worn tips and differing wire cast, it can feel like you are chasing your tail. One new process, called HEAT (High-Efficiency Advanced Tip), delivers the best of both worlds. It can squeeze the process variation out of GMAW by making it more stable. It also makes the process window larger and more accommodating of part variations in joint position and gap. All while raising productivity 30% or more, depending on the application.
School of Hard Knocks
To explain how HEAT works, please humor me as I turn the clock back to the mid-80s. As a young engineer, I proudly took one of the new manual pulse machines into a famous farm implement manufacturer’s shop for a demonstration. The application was hydraulic oil tanks, made of medium-to-heavy gage sheet metal, perhaps 3-4 mm thick mild steel. The manual welders were using old CV (constant voltage) machines and welding in a choked-down, low voltage spray arc. I was armed with the latest transistor-controlled, synergic pulse and expected an easy sell. The old timers in the tank department smiled at me condescendingly and then proceeded to clean my clock for the rest of the afternoon. In the end they saw me off politely saying, ‘‘We like your new machine, son, but we just can’t weld that slowly here.’‘
What had gone wrong? These guys were on piece work. They had found out – by trial and error – that the most productive way to weld that thickness of material was to turn the wire feed all the way up, choke the voltage to a volt or two below spray, hold 2’‘ of stickout, and pour a molten river of steel down the seam that pasted over any fit-up problems quicker than you can say ‘‘Let’s haze the college boy’‘.
Why it Works Manually
As you can see in Figure 1, holding a longer stickout (the distance between the contact tip and the work, sometimes called ESO) changes the wire burn-off rate. The longer wire softens the arc by adding slope to the circuit. This means you can increase the wire feed speed by 30 - 40% and still have the same amperage. With that same amperage, you won’t burn through. And with more wire, you can bridge gaps and travel faster without undercut. I didn’t stand a chance.
Not Suitable for a Robot, or is it?
It was long assumed that welding with this long stickout technique couldn’t be done with a robot, as the wire cast and helix would cause targeting problems and the weld would miss the joint. But the HEAT process uses a special tip that has a ceramic insulator built into it, providing that extra stickout distance of 12-15mm that the process requires. You program the robot with the normal apparent tip to work distance, and the extra stickout comes from the tip. The actual transfer of electricity is provided by a collet inside the tip. The collet grips the wire at a precise point, just above the insulator (Figure 2). This results in a straightening effect along with the improved conductivity. Both targeting and electrical stability of the welding process are dramatically improved (Figures 3 and 4). The net effect is a more stable welding process with less variation, a wider process window, and a productivity gain from the faster wire speed.
The final innovation only became possible recently – the use of a servo wire feeder and a bus interface between the welder and robot controller. With the HEAT process, this allows the robot controller to script a precise dance of wire feed ramp-up, welding current ramp-up, and robot path lift-start, permitting the process to start reliably every time despite the long stickout. Special waveforms are also in the controller optimizing for the long stickout effect once the arc is established.
The HEAT process can be used several ways. Figure 5 simulates a flair groove weld between two pieces of tubing such as you would find in a car seat or a car head restraint. If the wire is out of the joint as little as 1 mm, standard GMAW welding often burns through the favored tube. With the HEAT process, the softer arc and extra wire fill in the groove without burning away the favored tube. The benefit is a wider process window for joint placement.
Another potential application (Figure 6) shows a heavier weld of 3 mm material in a 3 o’clock position slightly downhill with a gap. This simulates an automotive engine cradle or other underbody part like a frame or control arm. In this case the welding speed has increased to over 60 inches per minute and the gap is covered without undercut. The extra wire from the HEAT process has again opened a larger window for joint fit-up while simultaneously improving productivity.
Is it time to change the tip?
Thanks to the collet tip, the inherent stability of the current transfer takes the guesswork out of maintaining the process. With conventional tips, the tip starts degrading as soon as it is put in and the welding current can drop 20 amps in the first ten minutes of use. This results in operators guessing when to change the tip, or worse – when to start touching up points after the tip begins to slot out. With the HEAT process the contact, and hence the amperage, stays constant over the life of the collet (typically twice the life of a conventional tip). When the collet can no longer grip the wire, the performance drops and it must be replaced. This is a predictable event.
Overall, the HEAT process is more stable, more productive, and more tolerant of part variations than conventional GMAW welding.
The process is not for everyone. It loses its advantage once you get considerably beyond 6 mm material, since welding heavier plates needs penetration more than extra fill.
So if you’re welding medium-to-heavy gage sheet metal and you want to turn up your production speeds, call Panasonic for more information about HEAT. See the light, and put the teach pendant back in its holder where it belongs.
Brian Doyle is the Welding and Robotics National Sales Manager at Panasonic Factory Solutions Company of America. In addition to 12 years with Panasonic’s Welding & Robotics, he’s spent time at Genesis Systems, Praxair, Cloos International, Reis Robotics, and Union Carbide in the Linde Division. Doyle holds a Bachelors of Science in Chemical Engineering from the University of Notre Dame.