Truck Manufacturer Improves Aluminum MIG Welding With XR Wire Feed Systems
This leading manufacturer of aluminum vehicles increases its competitive edge with the new XR-Edge feeder system.
Oshkosh Truck Corporation's welders make spot welds on aluminum truck cabs that look like rivets. This capability depends not only on the skills of the operators, but also on precise control of aluminum wire feed speed and wire handling. The company's new push-pull type wire feed systems have greatly improved that aspect of aluminum MIG welding by almost eliminating burnback, wire stubbing, weld porosity and erratic feeding.
"The cosmetic appearance of the weld is very important to our customers," says Larry Clark, Process Engineer at Oshkosh Truck (Oshkosh, Wisconsin).
"We want a very consistent spot that looks just like a rivet; the 0.090 in. aluminum skin appears as if it was riveted to the body when it was actually welded."
Oshkosh Truck works around-the-clock building heavy military and commercial trucks from the ground up - from bending and stamping steel and aluminum sheet, to fabrication and assembly of chassis and bodies, to painting. Much of the work performed in the company's main manufacturing facility (North Plant) is welding or welding-related. Of Oshkosh Truck's 1,300 employees, approximately 100 are welders. The company's annual steel and aluminum (grades 5052 and 6061) use includes more than 100,000 linear ft. of sheet and plate ranging from 0.06 in. to 0.5 in. thick, 18,000 linear ft. of angle and tubing, and up to 1,500 ft. of strip stock. Miller Electric's new XR-Edge™ wire feed system plays a key role in welding a variety of aluminum-bodied vehicles, including concrete mixing trucks and aircraft rescue and fire fighting trucks. In the manufacture of all vehicles, the company conforms to the extremely rigorous quality standards required by the military for its vehicles.
"A lot of companies have separate welding codes, one for specialty products and a less demanding code for commercial products," says Joe Campbell, Sr., Owner of Machinery & Welder Corporation (West Allis, Wisconsin), Oshkosh's welding supply distributor. "Oshkosh Truck welds to one quality standard, the same strict code they use for building military vehicles. That's very rare, and it's also why their products are in demand."
Recently, the manufacturer won a contract to produce a 7-ton off-road, 15-ton on-road, 6 X 6 military transport vehicle called the MTVR. Oshkosh will produce as many as 8,100 vehicles, with full rate scheduled to begin in 2001.
In addition to the 10 new wire feed systems with 30-foot guns, Oshkosh Truck also acquired 15 XMT® 304 inverter-based power sources. Future purchases are planned for in the following year, as the company moves toward a completely upgraded welding operation.
Maneuverability Prompts Change in Feeder Choice
For aluminum welding, Oshkosh Truck previously used 10-foot guns fed by push-only type feeders. This limited the range of motion and maneuverability around the truck cabs. Often, because the gun cable couldn't reach, operators had to move the weldment to access tight spaces. Considering that they needed to use an overhead lift to move weldments such as truck cabs, it's easy to see how short gun cables slowed the fabrication process.
"We also had to be aware of how the gun was oriented, as far as avoiding stiff bends that would contribute to feeding problems," Clark notes. "We did have the feeders mounted on booms for some additional reach, but we had to put the auxiliary equipment on trunions, or on a cart and heel it around where we could always keep the side being worked on turned toward the equipment."
"A push-only type wire feeder often creates problems when running aluminum wire," says Bill Lancour, Sales and Service Representative, Machinery & Welder. "Because of its low columnar strength, any significant bend in the cable causes the wire to buckle, which may lead to a bird's nest at the drive roll."
To prevent bird's nesting, the XR-Edge wire feeder features a true push-pull design. The push feeder motor provides continuous torque to the wire. The gun motor controls the actual wire feed speed at the arc. Each motor works in concert to provide accurate and positive wire feed speed.
"I haven't had any bird's nests at all," says Mike DeBehnke, a welder at Oshkosh Truck. He adds that "We used to have trouble because wire speed wasn't constant. I would get a lot of drag. But with the new wire feeders, we get really consistent, nice-looking spot welds even though we're using 30-foot guns. Unless I miss a pre-punched hole in these aluminum skins, I don't have to replace a spot weld. I did 220 spots the other day without replacing a single one, and they all looked uniform."
Oshkosh welder Don Fredrick, likes the enhanced maneuverability he gets with a 30-foot gun for skip welding 3/8 in. floor plate to the 1/8 in. tubing of a truck cab's frame. The XR-Edge maintains constant tension, so the wire feeds consistently, even when the cable has three or four right angle bends in it. "I can turn the gun in any direction and flip back and forth without any problems –– no matter how I hold it, the wire feeds smoothly," he says. "Also, I can get the gun into tight spots and corners. We encounter tight spots all the time on truck cabs because we run most of our aluminum welds vertical up for better penetration."
While individual preferences vary, most operators making the 1 to 2 in. skip welds set wire feed speed to about 400 in./min. and 25 volts. Operators making spot welds use wire feed speeds of 700 in./min. or greater and voltage settings of approximately 27 volts. The spot welding timer, located on the XR-Edge's side panel, is set for an arc-on time of 1.15 seconds.
Better Arc Starts
Because aluminum wire is relatively soft, it can slip through the drive rolls, resulting in burnbacks and arc stumbling. Oshkosh Truck, like many other companies, tried to solve this problem by using knurled drive rolls. Unfortunately, the knurls deformed and defaced the wire, producing shavings. Eventually these wire particles built up in the drive roll grooves and contact tip. This caused burnbacks and arc stumbling, the very problems Oshkosh Truck wanted to avoid.
Clark adds that "In the past, we had a drive roll groove that was supposed to accommodate anything from a .030 to 1/16 in. diameter wire, so naturally it was never a perfect fit for the wire - the groove could never get a really positive grip."
"On the old system we used, we had a lot of trouble with burnback. On bad days, I'd go through 10 or 15 contact tips," noted DeBehnke.
The XR-Edge appealed to Oshkosh Truck welding operators because it features smooth drive rolls with a unique V-groove that positively engages the wire without deforming it. In addition, its all gear driven drive rolls feed the wire more consistently than systems employing a single driven roll/idler roll configuration.
"The XR-Edge system is so good," states DeBehnke, "that the original contact tip lasted for over two months. The only reason the tip failed was because I ran out of wire."
Achieving good arc starts on aluminum is also challenging because surface oxides on the material can delay arc starts. Therefore, the ability of the operator to fine-tune the speed at which the wire approaches the weldment is an advantage.
"Oshkosh Truck operators now enjoy increased flexibility to change run-in speed," says Rod Behnke, Sales & Technical Representative, Machinery & Welder. "Some operators like to have their run-in speed match wire feed speed. Some like to slow it down. It all relates to arc starting and how the operator holds his torch and his preferences. The wire feeder adapts to the operator more than vice versa."
"My run-in speed is about 30 percent of my welding speed," says DeBehnke. "This run-in gives me a nice, slow start and I get a better looking weld. It gives me time to get the arc started instead of bang! right there. I like being able to adjust run-in speed to my type of welding."
Matching the power source to the wire feeder also enhances arc starts, particularly given the start-stop demands of spot welding. The XR-Edge's unique motor ramp control adjusts how long the wire drive motors take to ramp up to full speed. Motor ramp control can prevent long, flaring arc starts, burnbacks or wire stubbing.
"An inverter-based power source like the XMT 304 responds very rapidly - which is especially important for spot welding - so we kept the ramp control at the factory setting, which is maximum speed," Clark says. "The arc starting characteristics are better, and that avoids burnbacks."
The XMT power source is also very small, adds Clark. He says that "Space is an issue on our welding floor, and the equipment can't take up a lot of room."
Good Shielding Gas Coverage
Years ago, Oshkosh Truck used pure argon gas for welding, but often had to fight problems with less-than- optimal penetration. Ultimately, the company improved this situation by experimenting with an argon-helium mixture. Helium added heat to the arc; this improved penetration and increased the gas' ionization potential, resulting in a quicker arc and enhancing the consistency of the puddle. Oshkosh Truck settled on a 75 percent argon/25 percent helium gas mixture.
Still, good coverage of the shielding gas mixture wasn't all that it could be, Clark admits. Because they were being judged by the appearance of the welds they made, Oshkosh Truck welders were dissatisfied with any blackened or porous welds caused by inadequate shielding gas coverage.
This situation has improved because the XR-Edge sends the shielding gas mixture not only through the nozzle, but through the contact tip with the wire. This ensures complete gas coverage, enhancing cleaning action, reducing the potential for porosity, and minimizing post-weld clean-up.
"Good shielding gas coverage is probably the most important factor in our ability to make the spot weld look like a rivet –– the welds are clean and smooth, without any feathering," Clark states. "The operators are happier with the spots they produce on the XR-Edge versus our older technology. Previously, perhaps every eighth spot would fail, requiring the operator to grind it off and weld it again. If it failed after the cab had gone out for painting, they would have to ruin a perfectly good paint job to weld another spot on."
Technology Matching Talent
For more than 80 years, Oshkosh Truck has maintained its reputation as a world-class manufacturer of specialty trucks and truck bodies, earned in large part through the strict quality standards to which it adheres, and in particular, to the skilled professionals it employs.
In its manufacturing processes, the company strives to use only the most advanced techniques and tools, and its welding equipment plays an integral role in maintaining the quality of its end product.
"The welders at Oshkosh are pretty accomplished," says DeBehnke. "They've been welding 10 or 15 years. Now that we've got the new wire feeders and inverters, the quality of the welding equipment supports our talent."
2002 Miller Electric Mfg. Co