The number of robotic cells in U.S. industry has doubled since 1992, from 46,000 to 92,000. Between 15 and 17 percent of installed robots about 15,000 are used for arc welding.
The U.S. is second only to Japan (with about 460,000 robots in use) in worldwide robot usage. We can see from these numbers that there is enormous potential for growth in the number of robots used in the U.S.
One trend that is pushing companies toward more automation is the declining number of trained welders. Some production managers are automating processes so they will not lose business as a result of long lead times and slow delivery. (A rule-of-thumb is that a welding robot can do the work of three or more manual welders.)
The following are some common arc welding gun, torch, peripheral, and other considerations for robotic welding.
What Is the Basic Equipment Needed ?
The general rule for welding power supplies is to use a machine that can deliver 20 percent more power than the amperage needed to weld. The parameters for robotic welding may be higher than for manual welding to accommodate a higher duty cycle and a possibly faster wire feeding.
The wire should be fed from a drum or large spool to minimize wire supply changing. Normally, the wire feeder for robotic welding is mounted on the robot arm, separate from the power supply.
For robotic welding, a control interface between the robot controller and the power supply and wire feeder is needed. Some power supplies are available with a built-in interface; sometimes the interface is optional.
Can Manual Guns Be Used with Robots ?
While some might think that a robotic GMAW torch is nothing more than a manual gun attached to a robot arm, there are significant differences. Some robotic guns have integrated wire incher controls, and all but the most basic models have emergency-stop capability to prevent damage to the robot arm and the welding gun in the event of a collision.
Robotic gun design should take into account the need for automatic cleaning. An air line may be incorporated for blowing spatter out of the nozzle, and the internal nozzle structure should allow complete reamer cleaning up to the gas ports.
Because a robotic torch probably will be running at a much higher duty cycle than a manual torch, those who are shopping for equipment must decide on an air- or water-cooled torch. Manual air-cooled guns usually are rated at a 60 percent duty cycle, while water-cooled models are rated at 100 percent duty cycle.
If robot arc-on time is going to exceed 60 percent, a fabricator may have to use a larger air-cooled robotic torch than the manual gun previously used for the application or switch to a water-cooled model.
Generally, the price of the welding torch is a relatively small part of the cost of a robotic workcell. A robotic torch should be chosen with the emphasis on the capabilities needed for a fabricator's particular applications rather than cost considerations.
How Are Neck-Change Torches Used ?
Some torches allow for a quick exchange of the entire swan neck, a feature that can be included in the robot programming. Nozzle cleaning and contact tip replacement can then be completed offline while the robot welds with the replacement neck.
The swan neck alignment can be checked and adjusted on an alignment jig before the neck is used again, which ensures that the replacement necks give precisely the same tool center point (TCP). This allows the robot program to position the torch nozzle in the same place so welds are made where they are intended.
Several manufacturers offer neck-change torches, and a patented mechanism on one model cuts the welding wire inside the torch as the neck is removed and closes water line valves in water-cooled versions. Manual neck changes can be made within five seconds on this model.
Neck-change torches have been available since 1987, and there are approximately 5,000 in use in the U.S., most supplied by three major welding gun manufacturers. As these torches represent about a third of all operating robotic GMAW guns, quick neck changing is an option that many fabricators find useful.
Whenever possible, cable assembly liner changes are made without removing the gun from the robot. Some models have quick-release components in the rear cable assembly for this purpose.
Because the gun is not removed from the robot, the TCP is unchanged, and reprogramming is avoided. However, liners usually are changed every two to four weeks as preventive maintenance or as necessary to change wire sizes.
The latest robotic GMAW gun development extends the neck-change approach to a completely modular torch system. This allows all the parts that wear, including complete cable assemblies, to be replaced without removing the torch body from the robot. The advantage, again, is avoiding reprogramming every time a component is changed.
A modular system also allows switching from a water- to an air-cooled torch without changing the TCP. As with the quick-neck-change system, removable torch necks can be realigned and water and/or gas flow checked away from the robot.
Similarly, cable assemblies can be refurbished or liners changed while the robot continues to weld with a replacement assembly.
How Is the Torch Mounted ?
The welding torch should not be attached directly to the robot. Rather, it should be attached to a safety torch mount equipped with a cutoff switch that will stop the robot in the event of a collision, preventing damage to the robot arm or welding torch. This automatic shutoff also can prevent injury to anyone who inadvertently enters the working area of the weld cell while the robot is operating.
What Are Peripherals ?
Peripherals are devices that help to keep the robot welding or provide some additional capability. They are mounted in the robot cell and controlled by the robot programer. The main peripherals needed are designed for spatter removal and include a reamer-cleaner for the torch nozzle and an air-blast and antispatter fluid delivery system.
A reamer cleaner is programmed to ream accumulated spatter periodically from the inside of the torch nozzle. Robot torches equipped with air-blast capability allow compressed air to be blown through the nozzle immediately after reaming, followed by a measured amount of antispatter fluid to coat the inside of the nozzle and prevent spatter adhesion.
Another peripheral a fabricator may need is an automatic wire cutter, which is designed to cut the welding wire to a predetermined stick-out length. This helps when precise arc initiation is needed, such as in aluminum welding, or in applications in which wire touch sensing is used for joint finding. There also are devices for holding the wire securely in the torch when touch sensing.
How Are Workpieces Positioned ?
One starting point for positioning the workpiece for robotic welding may be the fixtures already used for manual welding. However, loading and unloading stationary jigs in the robot cell can be time-consuming and impractical.
A revolving table positioner usually is a better choice, despite a higher initial cost. Robotic jigs also may need to be more accurate than those used for manual welding.
With a revolving table, the robot works on one workpiece on one side of the positioner while a finished piece is unloaded and a new piece to be welded is loaded on the other side. Obviously, this speeds up the process and keeps the robot welding as much of the time as possible.
Another, more expensive option pairs a welding robot with a material handling robot for jigless welding. This allows continuous welding and may enable access to otherwise hard-to-reach welds. This can be cost-effective when the same robot cell is used for a wide variety of different workpieces because it eliminates the need for extra jigs.
What Are the Fume Concerns ?
Managers and operators should never forget that robots are dangerous, and welding produces toxic fumes. Companies investing in robots should get a suitable safety enclosure equipped with appropriate fume extraction for the whole robotic cell.
One benefit of robotic welding is that an operator's exposure to dangerous fumes generated at the arc is eliminated. Some enclosures have switches that cut off power to the robot when the doors are opened so that an operator cannot enter when the robot is moving or welding.
Welding screens and curtains often are necessary to protect against welder flashburn. The Robotic Industries Association (RIA) publishes standards that managers should review if they are considering adopting automated processes in their shops.
The initial cost and effort involved in installing a fully equipped robotic welding workcell may initially seem too great for a small company, despite the long-term benefits in efficiency and productivity. However, robots are becoming increasingly affordable and cost-effective, so what was a daunting prospect a year or two ago may now be a realistic option. Unless the welding industry can reverse the decade-long decline in the number of trained welders, more automation is an inevitable trend as the industry enters the new millennium.
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