- Created: 2018-10-08
Cutting grated material with oxyfuel has always come with a degree of difficulty, but a new plasma process offers a better way
by Jimmy Myers, senior editor
Oxyfuel technology has long been used for cutting grated material, but the process, unfortunately, has many drawbacks. When cutting an interrupted surface, welding wire is fed under the oxyfuel torch to act as a pre-heat for the material.
However, for this method to work, a large and expensive rotating bevel head is required to keep the wire in front of where the torch is moving. And with cut speeds of around 17 ipm on an average grated material, the prolonged exposure to heat makes cut quality an issue.
Plasma too has issues with cutting interrupted surfaces. Dave Maxham, vice president of sales and marketing at Soitaab USA Inc., explains that cutting interrupted surfaces, such as grating, with plasma involves a problem that engineers have long worked to alleviate.
“You get a really lousy cut with plasma because if you just try to run the plasma torch across the grate, when it’s over a bar, you’re fine,” he says, “but when it gets to an open space, the arc jumps around looking for contact and starts cutting everywhere. It’s all over the place.”
To remedy problems with oxyfuel and plasma, Soitaab introduced the Plasma Hotwire grate cutting system last year, which it demonstrated at Fabtech in early November. After placing a few machines in Europe shortly after the show, the company installed the first one in the United States for a company that works exclusively in grated material.
“They’re loving it,” Maxham says of the company’s feedback. “It’s exceeding their expectations. I’m sure that’s why they purchased two more so quickly.”
All in the wire
Engineers set out more than a year ago to achieve faster cutting times on interrupted surfaces with plasma, including steel and aluminum materials, but with high-quality results. With the patented plasma cutting solution, welding wire is automatically fed underneath the torch tip, acting as the anode to the torch’s cathode. This completes the electrical circuit, yet there is no direct connection to the workpiece.
The wire essentially tricks the plasma arc, simulating a continuous surface under the torch. The grate beneath the torch is cut like it is a flat plate. There is no loss of plasma arc, even as the torch moves over the openings in the interrupted surface, which means there is no slowdown of the torch and no pre-heating is required.
Soitaab touts the Hotwire technology as providing 5 to 10 times faster cutting over oxyfuel. Feedback Maxham received from one of the company’s customers boasts speeds 10 to 15 times faster than their oxyfuel method.
“This is called indirect plasma cutting because you’re basically completing that electrical circuit without any kind of direct connection to the workpiece,” Maxham explains. “It doesn’t matter what the grate configuration is or where the holes are. By using that welding wire to complete that circuit, anything underneath it is going to get cut. Because we control the height (of the torch) using a capacitive sensor, you get a really nice, clean cut with the plasma.”
Capacitive height sensing technology that controls the height of the torch when cutting uneven surfaces has been in use for years. Over time, manufacturers began to prefer and measure arc voltage between the electrode and the plate for torch height control. For example, over a wavy material surface, as the plate gets closer, the arc voltage changes and it raises the torch a bit, so that operators can keep the torch at a perfect height to get a consistent, quality cut throughout the entire length of the material. However, that doesn’t work with an interrupted surface.
“With grated material, the arc voltage doesn’t have anything to finish the circuit to measure, most of the time,” Maxham says. “With capacitive height control, the height is only adjusted as the torch head moves over a bearing bar in the grated material. The capacitive sensing rides over the material, raising and lowering the head to the perfect cut height, offering a consistent, quality cut from beginning to end.”
As said, oxyfuel solutions also utilize welding wire that’s automatically fed under the torch, but for a much different purpose. Maxham explains that the older method doesn’t complete the electrical circuit, but as the wire melts, it tricks the torch into thinking it is pre-heating.
The drawback is that to keep fooling the torch into thinking the metal is pre-heated, the welding wire has to be in front of where the torch is going. This requires a complicated head that rotates, constantly keeping the wire feed in front of the torch.
“Your mechanics are astronomically more complicated because you have this big rotating bevel head on your machine,” he says.
With Soitaab’s solution, it doesn’t matter where the wire is introduced from, and it doesn’t have to be in front of where the torch is going.
“You’ve completed that circuit so you don’t need any of the big, expensive, hard-to-manage infinite rotation technology,” he says. “It’s a much simpler mechanism.”
Other advantages include almost no vibrations during the cutting process and retooling that is quick and easy. Furthermore, by removing the Hotwire feeder, the machine can be used for cutting standard flat plates.
Plasma cutting uses more consumables, more gas and more electricity than oxyfuel, but the perk is the speed of the cuts. To give customers a better idea of what to expect with Soitaab’s plasma cutting solution in terms of cost per cut, Soitaab created a variety of tables based on material type and thickness. More often than not, the savings are fairly drastic, especially considering volume jobs.
Soitaab’s “plasma Hotwire advantage” fact sheet shows an example of a specific part. When using Hotwire technology, it costs around $10 per hour to cut the part. For oxyfuel, the cost per hour is $8. Labor, regardless of the system being used, is approximately $25 per hour.
The big difference is that when cutting the same material type and thickness, the Hotwire cut only takes 1 min. and 57 sec. whereas the same cut using oxyfuel takes 6 min. and 12 sec. That equates to a total of $1.14 for the Hotwire cut and $3.41 for the oxyfuel cut.
Currently, the Hotwire technology works with materials up to 80 mm thick, which Maxham believes is plenty for manufacturers working with grated materials. Some of the common uses for grated material include shipbuilding and other marine structures, such as decking or docks. Stairwells in an office or in apartment buildings as well as in mezzanines are other applications that employ grated material.
Soitaab’s first Hotwire systems were ordered with a 24-ft. length for handling the grated sheets. But because the system is so fast, customers are starting to order it with a 48-ft. cut area to have two work zones. Typically, customers install the Hotwire technology on Soitaab’s Lineatech and plasma Nova machines.
“It pays for itself pretty quickly,” Maxham says, adding that because it cuts through material so fast, bottlenecks will occur without a more efficient material handling system. “It absolutely produces way more than what people are used to. To address that, the majority of machines we’ve sold and or quoted have the double length to offer a bigger cutting zone as well as a load/unload zone.”