Spot Welding: Still in the Driver's Seat
As car manufacturers explore more exotic, lighter-weight materials, robot manufacturers are building better tooling and more efficient resistance spot welding systems
BY WINN HARDIN
Although the auto industry has used robotic resistance spot welding for years, demands for greater repeatability, faster production cycles, and the ability to weld lighter-weight, more exotic materials are driving equipment development.
Spot welding is one of the most mature applications in robotics. The speed, precision, efficiency, and resulting cost reductions afforded by automated resistance spot welding are well documented and accepted, particularly in the automotive industry. However, industry requires that even the most mature solutions continue to evolve.
End users, including experts from the Big Three automakers, seek ever more speed and economy from their robotic applications. These engineers want 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, and 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.
Fig. 1 - Automakers are looking for robots with greater repeatability and weld requirements down to±12 mm.
"Eliminating labor was the justification in the 1980s, 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, Tex. "It's the repeatability of the robot that's prized today, with weld requirements down to±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 automakers use automated spot welding today." - Fig. 1.
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).
Fig. 2 - Big Three automakers say new lines of servo guns, such as this one from FANUC Robotics, help to increase the density of welding machines and maximize plant floor space while performing better on lighter-weight, exotic metals. (Photo courtesy of FANUC Robotics.)
"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," he continued. "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" - Fig. 2.
At the forefront of this new technology are servo guns, which 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 DaimlerChrysler. "That's what the industry would like to see."
These demands are being answered. Industrial automation and welding systems manufacturer ComauPICO, Royal Oak, Mich., has been redesigning its weld guns for two years, moving away from copper cast models to aluminum machined models, according to Hans Nickesch, director of advanced manufacturing.
"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 approach allows 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, Mich.
"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 vs. a dedicated (welded or permanently attached) system that could keep you down for hours," said Munro.
Another approach to addressing fast and furious production schedules is to use single-source control for multiple robots. Motoman, Inc., West Carrollton, Ohio, has developed the ability to control up to four robots from a single controller and teach pendant. This lets the robots work closely together without concerns about them colliding. It also provides a safer work environment with all robots under the sole control of a single pendant. A highly prized byproduct of using one controller for several robots is that integration costs are reduced.
To further support a dense robot population in a confined space, special robots have been developed. Shorter robots can be positioned close to the body while larger robots reach over them. These robots are created from modifications to a few different castings. Spare parts, like motors and reducers, are common to the larger-volume standard robots, thereby keeping the cost of the robot low. According to Motoman, one Japanese automotive plant was able to reduce by half its number of spot stations by taking advantage of the multiple robot controller with compact robot arms.
Dressing for Success: Going beyond the Weld Gun
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."
Fig. 3 - Motomanšs ES165 robot offers a 165-kg payload and an internal harness that eliminates the need for the supports and swivels that are usually necessary with external dress-out packages.
Motoman has introduced manipulators specifically for spot welding - the ES165 and ES200, with 165- and 200-kg payloads, respectively - Fig. 3. These robots have utilities (air, water, and power) routed in cable harnesses through the arm and out to the robot wrist. The standard cable harness supports either servo-controlled or pneumatic guns. The internal harness eliminates the need for supports and swivels associated with external "dress-out" packages.
"The internal harness for robot motors has been providing years of reliable service with mean time between failure criteria of 24,000 hours," said Chris Anderson, Motoman Inc. "Integrating the welding harness provides similar results and greatly reduces downtime associated with external cables. They wear quicker due to greater flexing and rubbing on surfaces. Quick connectors facilitate easy changing and it can be scheduled as preventive maintenance with the main robot motor harness."
Other advantages include reducing teaching time by 20% or more because off-line programs can be used directly without touch-up due to cable interference, the cables are integrated in the slim arm profile, allowing better access into confined spaces, and OEMs like the fact the internal spot harness is covered in the robot manufacturer's warranty.
Fig. 4 - The torch barrel of the EA1400 robot is mounted to an impact sensor in line with the wrist instead of being offset mounted. This results in improved torch access and longer life than with a standard torch cable.
Motoman has extended the concept of an integrated harness to arc welding with the EA1400 robot - Fig. 4. The torch cable is routed through a hollow upper arm and wrist assembly. This design protects the cable from rubbing against parts and fixtures and eliminates sharp bends in the cable that can interfere with wire feeding. The torch barrel is mounted to an impact sensor in line with the wrist instead of offset mounting with a standard robot. The result is improved torch access and 25 times more life than a standard torch cable arrangement.
Another consideration when building a welding cell is the size of the robot and weight of the weld gun. While welding cycle times or multihead 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, Mich.; ABB, New Berlin, Wis.; and, ComauPICO among others, companies are also improving standard-capacity robots so they are 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 Michael Sharpe, engineering manager for materials joining, FANUC Robotics, Rochester Hills, Mich.
"We've been able to do that using high-efficiency motors," said Sharpe, "so we don't have to overdesign 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 toward 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 vs. 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.
The concept of using a robot to manipulate parts for pedestal-type welding machines has been widely used by many of the Japanese automotive parts suppliers. Once a part is tack welded in a fixture, it can be "passed" between stations by multiple robots. Operations may include arc welding by other robots or part identification stamping or inspection. The result is that finished conforming parts are placed in bins or conveyors while nonconforming parts are separated. Part changeover is as easy as adding a new gripper, while capital equipment, like robots, continues to be reused. One manufacturer used automatic tool changers to have the weld line change over when dies were changed on the press line (several different parts were run each day).
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 midweld.
How to Stay Competitive
As manufacturers continue to explore more exotic, lightweight 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 vanguard of technology includes carbon composite, modular weld guns that use higher-frequency transformers with less iron and, therefore, less weight. These and other new technologies all are fighting for a place in modern spot welding applications.
WINN HARDIN is an editor at large for the Robotic Industries Association (RIA), Ann Arbor, Mich., (734) 994-6088. BRIAN HUSE, RIA, also contributed to this article.
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