Spot Welding Keeps Moving Forward

Spot welding is one of the most mature applications in robotics. Particularly in the automotive industry, the speed, precision, efficiency and resulting cost reductions afforded by automated spot welding are well documented and accepted. But industry requires that even the most mature solutions continue to evolve.

End users, including experts from the 'Big 3' automakers, seek ever more speed and economy from their robotic applications. These engineers seek more modular, lighter weight systems with increased cycle times and improved end of arm tooling (EOAT) - and vendors are answering the call as new automobile designs require more of their spot welding robots.

The Fast and the Furious
Fast paced design, furious production cycles, weight and cost reduction all are issues that drive automotive manufacturing today. Manufacturers cannot afford to sit still while consumers' tastes change. Robotic welding plays a key role in enabling car companies to keep pace with demand for new, more technologically advanced, higher quality product.

'Eliminating labor was justification in the '80s, and then car builders started driving their repeatability specs so tight that cars can't be manufactured by hand, they have to use machines,' explained Keith Crawford, general manager of EOA Systems (Carrollton, TX). 'It's the repeatability of the robot that's prized today, with weld requirements down to plus-minus 1 to 2 mm. You add to that cars becoming lighter. When it becomes lighter, you have to beef up welds to support the lighter structure, plus requirements that the cars do not rattle as much and the answer is to make more welds and make them more accurate so that you can create a more stable platform for the car. These are the justifications of why auto makers use automate spot welding today.'

As the demands on spot welding applications increase during the manufacture of automobiles, end users are looking at all of the new technologies to help make better products for their customers.

'We have a keen interest in servo guns because they potentially will allow us more control over the spot welding process if we can provide the technology on a cost effective basis,' said Steve Holland, director for controls, robotics and welding at General Motors (GM).

'One means to keep costs down is to increase the density of weld guns per station to reduce the floor space and hence the overall cost,' continued Holland. 'Advancements in both AC and DC (inverter-type) weld controllers and the coming of servo gun technology will all help expand welding applications for difficult materials, such as thin or exotic metals. These new technologies are much more capable of dealing with those materials.'

At the forefront of this new technology are servo guns, which typically are controlled by electronic motors rather than pneumatic or hydraulic cylinders, and can be more completely integrated and controlled through the robot interface software.

More than just servo weld guns, customers are calling for, 'the design of modular weld guns to include common parts for a variety of weld gun designs, allowing for continuous improvements to servo and hydraulic weld head designs,' commented Ken Mills, retired welding expert for Daimler-Chrysler. 'That's what the industry would like to see.'

And the industry is answering. Industrial automation and welding systems manufacturer, ComauPICO (Royal Oak, MI), has been redesigning its weld guns for two years, moving away from copper cast models to aluminum machined models, according to the director of advanced manufacturing, Hans Nickesch.

'Moving to aluminum guns eliminates a lot of variations, unknowns and manufacturing time. Of course it also makes it lighter too, allowing the gun to get from spot to spot faster. There's no value-added time between welds,' Nickesch said.

Just as important to ComauPICO is that machined guns can be made faster, eliminating the need to build new cast patterns after a few casts. Although machined aluminum weld guns have more parts than copper cast guns, the modular approaches allow users to quickly reconfigure a gun as the customer's product lines change.

Modular EOAT for the robot means less downtime, too, according to Frank Munro, president of international sales at Norgren Automotive (Mt. Clemens, MI).

'Compared to the alternative -- which has been to physically weld steel components and attaching grippers and clamps to the robot -- when you use modular components you can quickly fix a problem after a crash, and crashes do happen. Fixing a modular robot cell can be a matter of 10 minutes versus a dedicated [welded or permanently attached] system that could keep you down for hours,' said Munro.

Dressing for Success: Robots Find New Ways to Impress
Recent changes to spot welding solutions also extend beyond the weld gun. 'Robot companies are providing ways of integrating the dress - that's been a big plus for us,' said GM's principal engineer of welding technology, Joe Speranza. 'The robot companies have redesigned the robot with welding applications in mind.'

Although their weld guns work with many robot actuators, ComauPICO's Nickesch said, 'In our robot, all of the dressing goes through the wrist. Power goes through the arm of the robot, and reduces worries about damage and changing weld guns in the future since the power delivery system is integrated. With these designs, your weld gun choice isn't limited to the capabilities of the robot.'

Another consideration when building a welding cell is the size of the robot and weight of the weld gun. While welding cycle times or multi-head weld gun applications in automobile frame and aircraft body manufacture can be reduced by going to a large robot, including the new 500-kg robots from Kuka (Sterling Heights, MI), ABB (New Berlin, WI) and soon ComauPICO among others, companies are also improving standard capacity robots to be faster and more efficient.

'All the arms in the latest generation have been reduced in mass and overall body size, physical size and weight,' said FANUC Robotics' (Rochester Hills, MI) engineering manager for materials joining, Michael Sharpe.

'We've been able to do that using high efficiency motors,' said Sharpe, 'so we don't have to over design the arm to do a job. The advances in reducing the motor size at shoulder, arm and wrist, make the arm lighter. That in turn means we can reduce the weight of the arm and move it faster from weld to weld. We also get the weld gun closer to the part, all of which improves weld throughput by several percent, depending on the application.'

Putting Welding on a Pedestal
Other improvements to automated welding applications are helping speed throughput while increasing safety. A main trend that reduces the moving mass of the robot is to go to a stationary weld gun and have the robot bring the part to the weld gun.

'There's a distinct trend towards not moving the weld gun and, (instead), moving the part. It makes sense if you think about it. The robot has the capacity for x, y, z, and rotary axis movements. The deciding factor is often the size of the part versus the gun,' explained Norgren's Munro. 'A panel can be 30 kilograms, while a gun can be a couple hundred kilograms.'

The largest source of failure in a robot spot welding cell is the weld gun and the cabling (robot dress) used to operate the gun. The dynamic action of the robot motion can fatigue the robot dress causing downtime, while the spot welding gun is susceptible to damage from a crash. By putting the weld gun on a stationary pedestal, the robot dress is minimized as the tool required to hold the panel uses only air and signal power. The simpler robot dress makes for a more robust application and more cell uptime, explained Crawford of EOA Systems.

Other devices used to improve weld cell uptime include the EOA Systems Intelliflow Water Saver, which monitors the temperature and coolant flow rate to the entire cell including weld gun tips, transformer, SCR, shunts and cable, and alerts the operator when the system overheats or if a coolant line breaks. This approach can also reduce the number of 'closed' weld tips, which typically occur when a cell halts in mid-weld.

Safety First, Last and Always
When equipment design measures such as these are combined with personnel safeguarding measures such as passive sensors that detect human presence in the robotic cell and new safety standards developed by the Robotic Industries Association (RIA) and its member companies; the result is a safer, more efficient workplace with less downtime.

Safety and downtime are closely linked. 'A big issue with older robots was that they got lost when the cycle was interrupted, prompting workers to try and fix a problem without stopping the cell,' said Jeff Fryman, director of standards development at RIA. 'The robots had to be driven home and a lengthy restart process had to be started. A key element in safety is to remove the incentive for a worker to do something unsafe. We have to remove the incentive for the employee to bypass the designed safeguards and provide a robot system that doesn't have a lengthy restart process.'

Risk Assessment Software from RIA
The Robot Risk Assessment CD walks you through the stages of a risk assessment following the guidelines set in the ANSI/RIA R15.06 Robot Safety Standard. This easy to navigate program helps you conduct hazard analysis based on the application and the associated tasks the robot will be completing. It then leads you through the risk assessment where you will determine the severity of potential injuries and the frequency of exposure and likelihood of avoidance. Once the risk assessment is complete, the program provides for easy documentation.

RIA is working with the International Standards Organization (ISO) on the international robot safety standard (ISO 10218) and other safety issues to ensure up-to-date safety standards for the robot industry. Customized in-house training is available from the Association, and regional and national robot safety events are held each year to keep up with the changing marketplace.

How to Stay Competitive
As manufacturers continue to explore more exotic, light weight materials in their quest to capture the consumer's imagination, robotic integrators and suppliers will follow suit, building better tooling and more efficient automation systems for a highly competitive industry.

As we've seen, aluminum weld guns are gaining ground in the North American automobile industry. European advancements also bear watching, where the van guard of technology includes carbon composite, modular weld guns that use higher frequency transformers with less iron and, therefore, save weight. These and other new technologies all are fighting for a place in modern spot welding applications.