Robots Facilitate High-speed Welding

People agree that a weld speed of 100 in./min is considered high speed. But the definition is subtle and can vary from one application to another. In some cases, a speed of 80 in./min is considered fast, while in other cases a speed of 50 in. or 60 in./min is also considered high speed. Whatever the definition, high speed welding tends to lose its pizzazz, because sacrifices are often made in accuracy, repeatability, and consistency with higher scrap rates resulting from the operation.

There are several initiatives underway to provide higher speed without these attendant disadvantages. For example, a fairly recent development from The Lincoln Electric Co. (Cleveland, OH) is the Tandem MIG process, which offers travel speeds double that of conventional Gas Metal Arc Welding (GMAW) processes. This process is a dual wire, high-speed welding process designed for either robotic or hard automation welding applications. The process yields high travel speeds exceeding 250 in./min, which is more than double that of conventional, single wire MIG processes, with deposition rates in excess of 35 lbs./hr.

The Tandem MIG process utilizes the programmable Waveform Control Technology of Lincoln Electric Tandem MIG 450 power sources to coordinate two separately generated MIG welding arcs in unison. The machines offer 450 Amperes of welding power at 100% duty cycle per arc. The process can be used with both solid and metal-cored wire from 0.035 to 1/16-in. diameter. The independent torch configuration permits selection of two different diameter wires-one on the lead torch and a separate diameter on the trail torch. Typical applications for the Tandem MIG include automotive wheels and frames, cylinders, gussets and stiffeners, tanks and pipe and tubular components.

The process lends itself best to robotic automation, according to Tim Morehead, marketing manager for The Lincoln Electric Co. 'The high speeds are possible with robotic automation, because it is more flexible and intelligent than hard automation.' Morehead says, 'With the robot, one now has six axes of motion for travel. Typically, in hard automation, you will have a dedicated cam follower or some travel mechanism that is dedicated and will always travel the same path each and every time. As one is welding, one can follow the joint with a vision camera, which can follow the weld at speeds up to the top end of possible welding speeds, like 150 in./min.'

Tandem MIG software enables two independent arcs to operate in close proximity, which means effective control of the heat input, bead wetting, and travel characteristics. In addition, the process power sources produce unique output waveforms engineered for weld puddle control and arc interaction. Independent control of both the lead and trail wires of the tandem process allows for precise procedure optimization. Tandem MIG is tolerant of common fit-up problems and resists burnthrough.

The Tandem MIG provides seamless integration for performing common robotic software functions including individual arc fault detection, wire/stick detection, bi-directional welding, and single arc welding for tacking, seam tracking, and touch sensing. But, for hard automation, the system is designed for supervised and precise control of welding parameters. These parameters are set by PLC programming to prevent tampering by unauthorized personnel. It is useful for applications that require preprogrammed welding parameter changes during the welding cycle or changes in parameters to accommodate flexible automation.

'For high-speed welding, robots win by default, because you cannot do it any other way and still achieve all the benefits of high-speed robotic welding,' says Morehead. 'In order to achieve high-speed welding, one is typically running fairly high amperages, so that if you miss the joint or if you slow down, you stand a very good chance of burning through. As you move towards higher speeds, everything becomes just like a race car. Everything becomes more critical. A little flick of the wrist or a little movement this way or that, and you would be making scrap or bad parts. So, as you go faster, it is more critical to be accurate and you have to control each and every one of your variables.'

One customer who is using the Tandem MIG process is Titan International Inc. (Quincy, IL). The company is a leading global supplier of complete tire and wheel assemblies for off-highway vehicles. The company's primary markets include agriculture, consumer, earthmoving/construction, and military applications.

Titan International uses several combinations of hard automation and robotics in Tandem MIG welding for wheels 16 in. x 46 in. high. 'Basically, we wanted to increase our production and pieces per hour as far as our weld quality would allow us to do,' says Rich Cook, welding supervisor. 'Lincoln helped us to get set up with one gun running two wires, which gave us a speed of 100 in./min on our 30-in. wheels. We were able to maintain that speed with hardly any splatter and we also got the penetration and pieces per hour that we needed. With the Tandem process, I was able to get twice as much work done in one day.'

There are also advances in robotic Gas Metal Arc Welding (GMAW), according to Chris Anderson, market segment manager, welding, at Motoman Inc. (West Carrollton, OH). One such advance includes single wire GMAW, which incorporates a high-speed pulse wire achieving speeds of 4 m/min.

One example of GMAW robotic welding comes from Tower Automotive (Grand Rapids, MI). The company produces upper and lower body structurals, chassis and suspension stampings, assemblies and modules for passenger cars, SUVs, as well as light, medium, and heavy trucks throughout the world. The company has 25 manufacturing facilities in North America and facilities in Europe, South America, and the Pacific Rim. The company produces components like axles, anti-cradles, chassis, and sheet metal for OEM automakers. With an installed base of 1,500 robots, the company carries out the same kind of GMAW with robots from the major robot-makers-ABB, FANUC, Motoman, Kawasaki, KUKA, Nachi Robotics, and many others.

'One of the keys to successful robotic welding is the quality of incoming parts,' explains Jeff Noruk, manager and leader of advanced technology, welding, and robotics, Tower Automotive, at its Milwaukee facility. 'And, even if you have good parts, but you don't fixture them consistently, you could get into big trouble. But, with good parts and consistent fixturing, you would be in good shape. One of the biggest challenges to high-speed robotic welding is to eliminate variation in producing parts. The higher the welding speed, the higher the likelihood of losing consistency.'

The view is further amplified by Dean C. Phillips, manager of welding engineering, ITW Hobart Brothers (Troy, OH). 'One of the biggest advantages of robotic welding is repeatability,' he says. 'The robot doesn't forget its program and can produce parts with repeatable accuracy and consistency. But that's only as good as part fit-up and fixturing. This is an issue because in welding, you need to have a consistent joint. You're putting in the electrode at a certain rate. If the joint opens up, you don't have enough electrode at the same travel speed to fill the joint and get the reinforcement that you need. Sometimes, reducing the travel speed will reduce scrap and bad parts, but there are also many other issues connected with high-speed welding.'

One of the biggest advantages of using robots for high-speed welding is the capability of welding complex shapes at high speeds, according to Dennis Harwig, manager of the arc welding and automation team at the Edison Welding Institute (Columbus, OH). 'The main advantage of the robot is that you can deal with Cartesian coordinates and be able to weld complex shapes at high speeds,' he explains. 'Whereas, a human welder is limited to about 20 in./min.'

EWI is one of the prime organizations that helps its member companies to provide solutions to robotic and other welding problems. Its family of arc welding encompasses six major processes with dozens of variations of electrodes/filler metals, shielding gases, arc power systems, controls, automation, and robotics. Manual, automatic, and robotic platforms allow EWI to match manufacturers' existing or planned setup.

By maintaining close relationships with manufacturers, EWI is frequently able to borrow special apparatus when necessary. Its welding and automation apparatus, augmented with sophisticated data acquisition and analytical equipment, allow precise measurement of what is happening during any welding operation.

EWI engineers often support member companies in the implementation of automated systems with guidance and assistance in:

Plant layout

• Selection of optimum robotic and mechanized work cells
• Development of welding parameters to rapidly reach required production levels
• Tooling for welding automation, and
• Development of specialized systems.

Advantages of Robotic Welding

• Part repeatability and consistency
• Accuracy and speed
• Programmability
• Reduction in scrap and bad parts
• Reduction in overall cost
• Tireless, continuous operation
• Quality of finished parts
• Accurate  and automatic monitoring of welding parameters
• Capability of welding complex shapes at higher speeds than manual welding
• Real Return on Investment (ROI)
• Reduction in cost through volume production

James F. Manji is a free-lance writer in Brunswick, OH.