Executive Summary
- Automated metal cored process achieves deposition rate of 18 lbs/hr.
- Semi-automatic fabrication productivity improved 40 percent
- Reduced material handling, shop floor re-organization enhance worker safety
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| To create a flexible and more efficient work environment, Skyline Steel created skid-mounted welding stations featuring drums of Tri-Mark metal cored wire and Miller power sources with boom-mounted feeders; note the lack of cable clutter. |
Exposed architectural steel is a staple in the design of infrastructure projects throughout the world, but it offers a unique challenge: all fabrication flaws are fully exposed for the world to see.
The city of Phoenix’s Valley METRO transit authority opened to the public its new METRO Light Rail system on December 27, 2008 without a hitch. Each of the 28 Stations along its 20.3 Mile track has canopies and enclosures supported by exposed architectural steel columns ranging in height from 23- to 30-ft. The challenge? The Valley METRO required all welds — including four running the entire height of the column — to be uniform, continuous and aesthetically pleasing while meeting quality standards.
After the company awarded the contract failed to produce columns that met aesthetic and quality requirements, project contractor Archer Western turned to Skyline Steel, Inc. of Gilbert, Ariz. (a Phoenix suburb). Well known for its full-service structural steel fabrication and erection capabilities on commercial buildings and carports, Archer Western hoped that Skyline Steel, called in at the 11th hour, could deliver 850 columns — involving approximately 8.5 miles of welding — in time to prevent construction delays.
Skyline Steel president Rick Dancer, a man who knows how to take intelligent risks, knew his company could accomplish the job on time and within budget by automating many of the fabrication processes and optimizing workflow. Dancer gathered a team that included Dan Henry, shop foreman and automation guru at Skyline Steel, Praxair Productivity Specialist Jim Kelley and Territory Manager Joel Ashmore from his welding supply distributor and Alex Demirjian, a district manager for Miller Electric Mfg. Co. and Hobart Brothers Company.
Together, the team assembled an automated welding system that ultimately enabled welding at 18 inches per minute (IPM) and depositing 18 lbs. of weld metal per hour. Further, Henry’s reconfiguration of work flow, enhancements to semi-automatic welding processes and automating other fabrication steps improved productivity by an estimated 40 percent and reduced rejects to zero percent on the transit project.
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| Skyline Steel’s automated system welds the light rail columns traveling at speeds of 18 inches per minute (IPM) while depositing 18 lbs. of weld metal per hour—and achieving zero rejects on the transit project. |
We’re producing at approximately double of what we expected in our original survey when we were estimating this project. It’s been very successful for us,” says Dancer. He notes that, “If Praxair, Hobart and Miller weren’t as supportive as they have been, we wouldn’t have been successful with this project. Going forward, they have earned my confidence and will always get the first look at any of our future ventures.”
Fabrication Requirements
The transit authority’s specifications called for a 3/8-in. wall 4-in. x 6-in. rectangular steel tube be sandwiched between two wider pieces of ½-in. plate steel and connected to an identical assembly by an 8-in.-wide section of 3/8-in. flat bar steel and “dog bone” connectors to create the column.
As noted, the weld bead profiles along the length of the beam needed to be incredibly consistent for cosmetic reasons. In addition, they needed to meet AWS D1.1 Structural Steel Welding codes and 100 percent of the beams would be subject to ultrasonic testing (UT) for weld quality. Further, the beams would have to hold certain straightness tolerances. Lastly, each rectangular tube would have dozens of holes drilled into it at 4-in. intervals for aesthetic purposes.
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| A single CNC machine drills holes (for architectural styling) in this rectangular steel tube and cuts the tube to length. |
Selecting an Automated Welding Process
To keep construction on schedule, Skyline Steel had less than three months to reach full-scale production. The tight deadline occurred because the company awarded the contract could not fulfill its requirements. This company attempted to use a semi-automatic welding process. Because weld lengths exceed 20 ft., operators needed to reposition themselves several times. As a result, the weld beads did not consistently meet the aesthetic requirements of the transit authority, and the frequent starts and stops led to porosity and inclusions in the weld.
“The earlier fabricator failed because it was depending on the human factor,” says Dancer. “Fortunately, we were able to identify that this had to be an automated scenario so that we could control quality on a consistent level.”
With semi-automatic welding ruled out, many structural steel fabricators would automatically turn to the submerged arc welding process and a longitudinal seam welder. In this application, however, the sub arc process had two drawbacks. To start, the joint created by the rounded corner of the steel tube set flush against the steel plate had a deep, narrow configuration (a “flare-bevel” joint), and the sub arc process typically performs best with wider angles that accommodate a larger-diameter wire. Welding the joint with sub arc could risk incomplete penetration. Achieving 100-percent penetration was possible with higher input, but more heat also creates more warping, increasing the amount of time necessary to remove camber.
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| Dan Henry, shop foreman, shows off the fixture he developed to hold the tube; the two protruding “horns” slip inside the tube, while the base also rotates and locks in place. |
In addition, Henry wasn’t a fan of the sub arc process because it requires cleaning up the flux afterward, which ran contrary to his goal of improving throughput by eliminating unnecessary steps. Further, this particular job required precise placement of the weld bead, and, of course, the flux obscures the weld arc. If the weld placement began drifting off center, there was no way correct it in time to prevent a scrapped column.
Systems Components
Understanding that the semi-automatic and sub arc welding wouldn’t meet requirements, Skyline, ultimately purchased a welding system consisting of the following:
• A Miller Dimension™ 1000, a CC/CV DC power source a rated output of 1000 amps/44 volts a 100 percent duty cycle.
• Hobart Brothers’ 1/16-in. diameter Tri-Mark® Metalloy® 76 metal-cored wire.
• Arcmatic automation and guidance system.
The Dimension 1000 provides more than enough of the power needed to weld the 1/16-in. metal cored wire at full production capacity while achieving the penetration and deposition goals set forth by Skyline. With its high output and multiprocess flexibility (MIG, flux cored, metal cored, sub arc, DC TIG and air carbon arc gouging), Skyline Steel can easily adapt this welder for future structural projects.
Metal-cored wires provide outstanding mechanical properties in structural applications (yield strengths of 78,000+ psi, Charpy V-notch of 72 ft-lbs. at 0 degrees F), excellent deposition rates for high productivity (18 lbs./hr in Skyline Steel’s case) and a penetration profile suitable for the flare-bevel joint. Their bead wetting action is better than solid wires, minimizing cold lap on heavier steel sections.
Metal cored wires help Skyline Steel accomplish production goals because they produce and spatter- and slag-free welds. Other than a light wire brush after welding, Skyline Steel operators performed no other post-weld clean-up tasks. Metal cored wire’s deoxidizers, along with the distributor’s proprietary gas blend, Stargon® VS, also eliminated pre-weld preparation, as it cut through minor mill scale or mill oil. These and any other light contaminants basically become immaterial to the quality of the weld (and these welds were cut, bent and basically destructive tested and UT tested to meet the engineering requirements of the job).
Some manufacturers avoid metal cored wires because of their perceived high costs. However, Praxair’s Kelly states that, “Examining the purchase price of filler metal, a welder or any single system component is exactly the wrong thing to do. Instead, we showed Skyline what metal cored wire would do to enhance the company’s productivity. As a result, Skyline realized that the ‘delivered cost’ per pound of deposited weld metal was much less expensive with metal cored wire than with other alternatives.”
“Its my job to turn hours into minutes,” says Henry. “Our new system accomplishes just that by increasing productivity while greatly lowering labor costs.”
“Our new automated system requires one person to run it, but it accomplishes the work of up to 10 operators and does it all without the inconsistency of the human factor,” says Dancer. Henry confirmed this fact while validating the automated system. He ran tests with semi-automatic flux cored welding, and, compared to the automated system, “The results weren’t pretty.”
Getting to the Point of Welding
Welding the column seams with travels speeds of 18 ipm at deposition rates of 18 lbs./hr would mean nothing if a fully prepared, tack welded beam didn’t arrive at the automated weld station in an efficient manner. To this end, Henry configured work flow and added CNC drills, saws, copers/bevellers and material handling systems.
Column fabrication starts with tubing being offloaded from a flatbed truck directly onto a roller table. Henry knew that this task would be much easier and more accurate if Skyline used an automated drill press rather than relying on magnetic drills and/or torches to create each hole and cut the tube to length.
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| At Skyline Steel, flatbed trucks offload steel directly onto roller tables that feed the company’s CNC equipment. Here, Henry, calls up information on the light rail columns. |
Once the rectangular steel tube enters the shop it is processed through a FICEP-brand CNC horizontal drill press that drills each hole uniformly on each side of the tube. After the tube moves through the drill press, a saw at the end of the machine cuts it to length. An operator then transfers the tubing via roller tables to a fit-up station to be tacked together with the other tube and connecting steel plates. [As a side note, the CNC machine has increased production speed and made the job safer by taking hand tools out of the hands of the workforce.]
Rolling, Rotating Fit-Up Station
Skyline’s fit-up station is basically a rotating spit mounted on two wheeled horses that makes it easy to quickly move the assembly around the shop from station-to-station without using fork trucks or cranes.
Henry designed this fit-up station to rotate so that his welders only weld in the flat position. This allows Skyline to use solid wire and make tack welds that penetrate deeply into the joint, a method that makes it almost impossible to tell a tack was ever there once the final weld has passed over it. It also allows an operator to flip the column to weld in different spots to help dissipate heat and prevent warping.
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| This operator tack welds the tube and plate in place, readying the column for the automated welding station. |
To weld the column, the tacked beam moves on its wheel-mounted horses to a set of tracks that run parallel with the automated system’s tracks. The column is locked in place and the automated welding torch is placed at one end of the column. Torch-mounted feelers sense the joint and adjust the torch as needed. An operator observes through a welding shade built into a hood that surrounds the torch, and he has the ability to remotely change torch position and welding parameters if necessary as the automated welding system rolls down the track.
Once the first weld is complete (it’s completed in a single pass), the column is rotated on its axis and welded on the alternate side to dissipate heat. This cycle takes place twice, completing the column in four welding passes.
An overhead crane loads the column into a camber machine that creates a perfectly straight column in less than 10 minutes. After straightening, the columns move to a semi-automatic welding station for final fit-up where the dog bones and base plates are welded on. From there, Skyline’s QC personnel inspect and test each column before sending them out to be galvanized and painted.
Organized Semi-Automatic Welding Stations
For tack and finish welding, Skyline relies on a variety of skid-mounted welding systems. Existing power sources include Miller Electric’s Dimension 452 and XMT Series inverters; both are matched with boom-mounted 70 Series wire feeders. All Miller welders deliver their rated welding output at 104o F, where some competitive welders need to be de-rated at temperatures higher than 86o F (which, in Arizona, occur daily during the summer).
Payout drums of filler wire sit on each skid and significantly reduce the amount of time spent changing out filler wire. Shielding gas is piped to each power drop and bulk cylinders deliver the gas to a gas mixer to create the proprietary gas blend mix that eliminates much of the pre weld preparation. This not only improves the accessibility of the equipment but, when matched with the boom-mounted feeders, substantially reduces the amount of dangerous and obtrusive clutter on the shop floor. Some systems, due to their location, will have a cylinder mounted to the skid for greater mobility.
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| Skyline Steel’s automated system enables the operator to observe the arc through a special hood and make adjustments to ensure proper weld bead placement |
Deadline Met; Safer, More Productive Workplace Created
The workflow created by Skyline emphasizes speed, safety and productivity. Adding equipment that allowed them to automate has substantially increased production rates. Relying on wheeled fixtures, skids, boom-mounted feeders, roller tables and bulk gas distribution systems has taken a large amount of equipment and clutter off of the shop floor and made it safer for workers to do their jobs. And what Skyline saves in labor on this project, it can reinvest in labor-hours elsewhere to further expand its business. The bottom line is that this system allows them to do more with less without sacrificing an ounce of quality.
“I’d say we improved our productivity by 40 percent,” says Henry. “Our failure rate and re-work time is down to zero. We bought the system, implemented it into our shop and now everything is working far better than what I expected. We’re producing at twice the rate that we thought we would be able to and had to be able to in order to have a chance of having any profitability on this project.”
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| Part of the team that helped Skyline Steel improve welding productivity includes (L-R) Nick Peterson, Welding Engineer for Miller Electric Mfg. Co., Praxair Productivity Specialist Jim Kelley, Praxair Territory Manager Joel Ashmore, Dan Henry, shop foreman and automation guru at Skyline Steel, and Alex Demirjian, a district manager for Miller Electric Mfg. and Hobart Brothers Company. |