July 2008 Edition

WELDING

Light Rail Requires Heavy-duty Welding

By creating an automated welding system, a welding shop created 8.5 miles of continuous welds and doubled its profits

With exposed architectural steel, it’s as important to look good as to be good. This gives rise to new welding challenges. Quality, uniformity, and appearance are important when the welds will be seen by millions of people.

When the Phoenix, AZ, Valley Metro transit authority decided to build its light-rail lines with steel and fabric canopies at the terminals, the authority wanted every detail of the canopies as aesthetically pleasing as possible, right down to the welds.

The project called for a 3/8"×4"×6" rectangular tube sandwiched between two 1/2"×8" wide plates. Two sandwiches would then be linked by a 6" plate. Once assembled, four canopy arms needed to be added before the entire assembly was painted and installed in the Phoenix light-rail terminals.

The challenge in the manufacturing was that the welds could not be ground, yet needed to be uniform, without starts and stops, for lengths up to 30'. The project required 850 canopies with a total of more than 8.5 miles of symmetrical, continuous welds.

What the city received from its original welding sub-contractor were submerged arc welds that did not adequately penetrate the joints, and semi-automatic welds that contained numerous inconsistencies where the welders repositioned themselves to continue along the length of the steel beams.

When Phoenix’s city planners decided that the welds on the first set of canopies were not good enough, the project’s general contractor, Archer Western, turned to Skyline Steel, Phoenix, a shop that was certified by the city.

Automation Was the Answer

"The city’s dilemma was that they needed thousands of linear feet of continuous flat welds," Rick Dancer, president of Skyline, said. "We would need to automate the process to avoid any discontinuities in the welds."

“This machine does the work of 10 guys, and the welds are perfect.”

He also knew that if the company was going to use automation, it needed assistance. After initial research into automated welding systems, Skyline turned to its representatives at welding gas supplier Praxair. Together the two companies came up with an automated welding system to meet the challenge.

After a week of effort, the team produced an acceptable weld. The previous welding company took nine months and still did not produce the required weld.

"Once we understood the needed end result, we got together with our suppliers and acquired the equipment and consumables to do some development work in our Phoenix laboratory," Jim Kelly, the Praxair productivity specialist, said.

Putting Together the System

The team developed a welding system consisting of different supplies, including metal-cored wire for a high deposition, aesthetically-appealing, and spatter-free weld; a proprietary gas blend developed for use in automated welding systems, which provided a stable arc; and consumables with fixed recess tips and a gas diffuser plate built into the nozzle, to produce a calmer gas flow, reducing weld defects.

Skyline also chose the Dimension 1000 from Miller Electric Mfg. Co., Appleton, WI, as the power source for its welding system.

The Power Source

Skyline chose the power source for its reliability – especially at high temperatures – and for its high duty-cycle rating.

"We’re running a high amperage project with a large diameter wire, so we needed a machine that would have the power available to run continuously at those current levels," Kelly said.

Rated to 1,000 amps at 100 percent duty cycle, the Miller power source handled the current required for the project. The welding company tested a unit rated at 450 amps, which just barely handled the current requirements.

"We were bumping up against its duty cycle rating," Dan Henry, shop foreman, said. "It might have been up to the task, but we didn’t want to take any chances."

Instead, looking toward the long-term versatility and production capacity of its equipment, the firm chose the Dimension, knowing it could handle the canopy project as well as future projects.

Another reason Skyline chose Miller was the company rates all of its welders to 104°F, which ensures availability of stated current levels, even in the Arizona heat. Since there is no specific standard that states a power supply’s temperature rating, most manufacturers rate their equipment to 86°F. Above this temperature, there may be a drastic drop in the duty cycle, affecting the capability.

The Miller power supply offered direct current, constant voltage capabilities for gas metal arc welding and flux-cored arc welding, as well as constant current capabilities for shielded metal arc welding and carbon arc gouging. It also features Fan-on-Demand, which reduces the amount of airborne contaminants that enter the machine, and sealed circuit boards, which protect its electronic circuitry from contamination. By operating only when the welding arc is on, Fan-on-Demand also reduces power consumption.

Skyline also chose the Dimension based on Miller’s reputation for quality and attentive customer service.

The Finishing Touch

Bringing together the wire, power source, consumables, and gas was the automated linear welding system, consisting of a manipulator arm that travels at a distance that produces a continuous weld the entire length of the joint.

Torch-mounted feelers sense the joint and adjust the torch as needed, while an operator observes through a custom-made welding shade. A programming pendant lets the operator remotely change the welding parameters if necessary.

The arm also holds the power supply and the wire, making the welding connections as sound as possible to eliminate potential welding flaws caused by poor connections in the welding cables when traveling such distances.

Almost immediately, the system was able to produce the continuous, flawless welds as required by the project.

Man vs. Machine

To compare the difference between the automated system and the human element, the company had one of its welders perform the same weld as the automated system.

“We’re producing at about twice the rate . . . we’re also profiting at twice the amount.”

"It wasn’t pretty," Henry said. "It took a long time and we still had the problem with starts and stops. This machine does the work of 10 guys, and the welds are perfect."

Moreover, the automated system exceeded everyone’s expectations by averaging a travel speed of 18 ipm and depositing 18 lb per hour of finished weld.

"We’re producing at about twice the rate that we projected in order to make this profitable," Dancer said. "Since we’re producing at twice the rate we expected, we’re also profiting at twice the amount we expected."

Part of that productivity is attributable to Skyline’s perfect record in ultrasonic tests and reduced clean-up.

"We test every column we manufacture and none have failed ultrasonic testing," Henry said. "Plus, there’s considerably less time spent in non-value added post-weld operations, such as chipping slag or grinding spatter."

Skyline exceeded its client’s expectations.

"This project was way behind," Henry said. "Archer Western – the project’s general contractor – literally came to us in the fourth quarter with two minutes left on the clock. We came through in the clutch and they’re happy with the results."

"Skyline helped us tremendously," Douglas Gabriel, of Archer Western, said. "The process they implemented to get the canopies installed on time and to the required specifications was a major factor in our ability to complete the project on time." Miller Electric Mfg. Co.

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