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Plasma cutting or laser cutting? How about both?

Combination machine expands plate cutting capabilities

The fiber excels at gauge material, but for thick plate, not so much—at least that was the common story. But just within the past few years, the story has changed.

“The traditional view is if you’re cutting thicker than 7 gauge, you don’t want to use the fiber, because it is just flat-out too slow,” said Paul Hardenburger, vice president of Rockford, Ill.-based MegaFab. “So as part of our development, we got a fiber laser, put it on one of our machines, and did a lot of test cutting in plate. We’re finding that we’re getting comparable, if not faster, cutting speeds than we got with the CO2 laser.”

He added that after adjusting the parameters, the company didn’t have problems getting thick parts out of the nest after cutting. “Though part profiles come in all shapes and sizes, of course, we haven’t had trouble. There’s also the technique of creating a nest in which the web itself loses integrity, which makes it easier to lift parts out of the nest as well.”

The speed equation for cutting also includes the pierce, which can be a significant factor when dealing with thick material. A laser may be able to cut effectively in plate, but if it takes forever to pierce through, the overall process time still turns out to be slower than the principal alternative, plasma cutting.

“If you’re laser cutting plate and it takes you 20 seconds to pierce that first hole, you’re adding to cycle time dramatically,” Hardenburger said. “So it was critical that we figure out how to pierce plate effectively, not only quickly but also in a controlled manner, so you can start cutting.”

According to Hardenburger, traditional plate-piercing techniques prolong the cutting cycle time, and they often don’t create the best conditions to start cutting. An operator might use a blast pierce, but that leaves a crater. Peck-piercing is another option, with a succession of laser pulses drilling through, but with that also comes a lot of heat. A really long pierce—between 20 and 25 seconds for 1-in. plate—may build up so much heat that it causes distortion.

“We’ve pierced 30-mm plate in less than a second,” Hardenburger said. “For most plate material, piercing occurs within a fraction of a second.” This saves time, of course, but it also reduces heat input.

To see the company’s proprietary piercing method in action, you wouldn’t see much: a quick pierce with molten material blown away by a side jet, and a fraction of a second later the machine starts cutting a contour. Somehow. It’s not a blast pierce or a peck pierce, but something entirely different. Most of the work occurs in the software. Certain aspects of the cutting head are unique, as well as the height of the head (though, of course, the company doesn’t delve into details). “It’s a complex process that’s made very simple by the CNC,” Hardenburger said.

After years of development work both in plate cutting and piercing, MegaFab recently introduced its Whitney PlateLASER. Machines with 6-kW IPG Photonics fiber lasers are on the market now, but lasers up to 12 kW will be available soon.

“Right now we’re [nitrogen] cutting 5-mm material at 10 meters per minute,” Hardenburger said. “Can we cut ¼-in. material that fast, or 3⁄8 in.? It’s really uncharted territory … and [piercing] is a huge piece of the equation. When you cut thick plate, you can’t just edge-start everything. You have to be able to pierce. So it starts with that, and then it moves into application development, and how fast we can go with a given laser power in a given material thickness.”

Many parts in agriculture, trucking, rail, mining, and other heavy industries may require intricate geometries that need the precision of laser cutting, especially if it means that parts can skip a secondary operation like manual grinding.

“About 25 years ago, when CO2 laser cutting jumped to the mainstream, people jumped onboard because they could get better edge quality than they could with a plasma,” Hardenburger said. “You could finish cutting and then put the part right into the weld fixture. The problem was, they were paying a penalty for this. Not all features on the part needed to be laser-cut. Consider agricultural equipment, where you have a lot of exposed edges. They’re part of the structure, but other than that, they’re not doing much.” They don’t require the laser’s precision, so they could be cut with the plasma, a lower-cost process.

Moreover, in thick plate the plasma far outpaces the laser. “A high-powered laser can cut through 1-in. plate at about 35 IPM,” Hardenburger said, “but a 400-amp high-definition plasma can cut at 90 IPM. That’s close to three times the speed.”

Having both laser and plasma cutting gives a fabricator more choices, and that was the thinking behind the PlateLASER’s combination capability. As an option, the machine can come with both a laser cutting head and a high-definition plasma cutting head on one machine. Such a machine is designed with the accuracy of the laser in mind; both the laser and plasma head motion are controlled by linear drives.

Historically, if some attributes of a part profile needed to be designed to laser cutting accuracy, engineers designed the entire profile to those tolerances, and understandably so, because the part was going to be made on a laser cutting machine anyway. But considering design criteria alone, the entire part profile or nest may not need to be that accurate.

“There’s a big legacy with parts having features designed for laser tolerances, and they don’t need to be,” Hardenburger said. “Traditionally, you were either going to laser-cut it or plasma-cut it. There’s no choice involved, so designers and engineers haven’t thought that way. But they now have that flexibility, which can lead to the lowest-cost manufactured part.”

About the Author
The Fabricator

Tim Heston

Senior Editor

2135 Point Blvd

Elgin, IL 60123

815-381-1314

Tim Heston, The Fabricator's senior editor, has covered the metal fabrication industry since 1998, starting his career at the American Welding Society's Welding Journal. Since then he has covered the full range of metal fabrication processes, from stamping, bending, and cutting to grinding and polishing. He joined The Fabricator's staff in October 2007.