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Laser Technology

Faster cutting

By Russ Olexa

April 2009 - What’s the easiest way to increase your laser’s speed? Try using a clean sheet of steel. It seems too simple to be true, but users have proved it works.

Ten years ago, The Material Works, Ltd. (TMW), a toll processor in Red Bud, Ill., began investigating ways to clean up stretcher/leveled hot-rolled black strip it offered customers who wanted to avoid the pervasive oil on hot-rolled, pickled and oiled strip because it hampered fabricating and finishing processes, and who also didn’t like the appearance of basic hot-rolled with its grit and mill scale covering the surface. The company wanted to supply customers something cleaner that would compete head-to-head with temper-pass strip in both flatness and appearance.

After experimentation, TMW found that large cylindrical rollers wrapped in 3M’s Scotch-Brite abrasive and rotating against the sheet left the surface smooth and practically white-glove clean.

Sometime later, TMW discovered samples from these development tests remained clean and rust-free, but untreated control samples developed normal surface rust. The rust-free samples had no protective oil or coating and hadn’t been wrapped or protected during this period. A few samples had been treated on only one side, and that side remained rust-free, whereas the untreated side had developed surface rust.

TMW was convinced it had something bigger than clean steel. Laboratory tests analyzed the paint adherence and corrosion resistance of the treated steel compared with conventional hot-rolled black, HRPO and cold-rolled steel of similar chemistry. Salt spray tests confirmed the treated steel offered corrosion resistance comparable to HRPO. Humidity chamber tests showed it took longer for rust to develop on the uncoated treated steel than on HRPO samples with a medium oil coating.

TMW further refined the surface-cleaning process, and in 2003 it had a production machine built to perform the treatment on a commercial basis. After patenting, the process was called Smooth, Clean Surface.

Today, SCS is produced under license from TMW at 10 worldwide locations and comes in both sheet and coil. As more fabricators used the SCS-processed steel, another property was revealed: SCS sheets laser and plasma cut faster than hot-rolled black or HRPO of the same thickness and chemistry. In some cases, the cutting-speed advantage reaches up to 35 percent.

The basis of rust resistance
Mill scale, the iron oxide that builds on the surface of steel during hot rolling, consists of three layers. Hematite and magnetite make up the outer layers, which are removed by the SCS treatment. Wustite, the innermost layer, is bonded directly to the steel substrate. SCS removes much of the wustite and polishes the remaining thinner wustite layer. The molecular structure of the remaining wustite layer is more stable and less prone to pick up free oxygen atoms.

By contrast, acid pickling removes all the wustite, exposing the substrate to the atmosphere and oxidation. The oil film added to acid-pickled strip is a barrier to oxidation. Through handling, the oil film can be wiped off, resulting in localized oxidation. Eventually, all the oil must be removed, usually by using multistage chemical wash solutions.

Faster laser cutting
Todd Berry, owner of Precision Laser Mfg. (PLM), East Peoria, Ill., was one of the first fabricators to observe and quantify the faster cutting speeds using SCS.

"When we received sheets of SCS-treated steel from TMW to cut parts, we were asked if one of their technicians could watch us cut it," Berry explains.

When the technician arrived, he explained that some fabricators had been adjusting their laser machine settings and were able to cut the SCS material faster.

"We were intrigued, so we made a few benchmark cuts then started to tune our 4-kW Balliu CO2 table laser for the SCS," Berry says. "We alternately varied power, nozzle diameter, focal length and assist gas pressure while we gradually increased cutting speed. When we got to where the cut quality started to deteriorate, we’d make some adjustments to bring quality back in line, then gradually increased speed again. It didn’t take much time before we honed in on what seemed to be an optimum set of parameters."

The result? "The SCS definitely cut faster, especially at the thinner gauges. At 12-gauge and 14-gauge, we saw a 25 percent increase over our best HRPO speeds, but that advantage lessens when you go to thicker materials. At 0.25 in., we see around a 10 percent advantage. That’s logical because what’s different about the SCS material is its surface. In thinner sheets, that surface effect is magnified relative to thick sheets."

Berry offers this explanation of the SCS surface effect: "Lasers work best on the cleanest, smoothest surface. On bare hot-rolled, the grit and mill scale makes that surface too irregular, which means an uneven focal length, not to mention more beam diffraction. On HRPO, you sometimes get oil splash, nearly always get an oil vapor, which fogs the laser lens, and the oil is like a magnet for dirt and debris, which can diffract the beam. The SCS surface, being as smooth and clean as it is, looks to a laser the same as a long, straight stretch of freshly paved interstate looks to a guy in a sports car."

TMW married the PLM approach with observations from other laser operators to arrive at a set of instructions for faster laser cutting with SCS. But it didn’t always work well.

"The variability in laser systems, resonators, assist gas and other factors is just too great to come up with a one-size-fits-all instruction," says Alan Mueth, TMW’s vice president of technology and leader of the early SCS laser-cutting studies. "We concluded that more general guidelines or best practices for adjusting laser settings would be beneficial, so we went to the people we thought could best speak to the topic: the people who build the laser machines themselves."

SCS sheets of various thicknesses were shipped to Trumpf Inc., Farmington, Conn.; Bystronic Inc., Hauppauge, N.Y.; and Advanced Technology Sales &?Service Inc., Greensboro, N.C., a Mitsubishi Laser sales and service partner. Each location performed SCS speed-optimization trials using its own methods and experience. In all cases, the trials supported PLM’s findings for SCS--up to a 30 percent increase over hot-rolled black, HRPO and cold-rolled in lighter gauges, as well as single-digit percent speed increases as sheet thickness exceeded 0.25 in.

"I was surprised the SCS-treated material performed as well as it did because it doesn’t look like anything special," says Dan Bergeron, Trumpf’s laser application engineer who oversaw the SCS trial at Trumpf’s Laser Center. "HRPO and cold-rolled have a fairly uniform color and appearance, whereas the SCS sheets show the staining, blue edge and darker color characteristic of hot-rolled black. I understand why that is. The SCS retains a thin, polished layer of mill scale. Still, it can be deceiving. It’s only when you study the sheet closely that you see how smooth and remarkably clean it is. That’s why it performs better if you take the time to find its sweet spot in setting up your machine."

Based on the laser machine OEM trials, TMW transformed the SCS laser-optimization instructions into a set of more general guidelines for finding that sweet spot. One leading contract sheet metal fabricator and laser processor that adheres to those revised guidelines is Hawkeye Industries Inc., Tupelo, Miss. Hawkeye has a Trumpf 3040 laser cell and a TC6000L punch/laser combination machine.

"Our steel supplier brought in sheets of SCS?for us to try out," says Bryan Hawkins, founder of Hawkeye. "We needed better paint performance. Our painter reported getting a great finish with SCS, plus he was able to use a less aggressive wash cycle. But it was how SCS?lasers really got my attention. New lasers like ours are fast to start with, but SCS-treated sheets give us up to a 25 percent speed advantage over HRPO sheets. I’m convinced the SCS material gives us more accurate parts and less scrap, as well."

A fabricator using both SCS-processed sheets and tubes is Industrial Laser Solutions, a subsidiary of Tie Down Engineering, Atlanta. ILS operates three advanced laser-tube cutting cells, but at times it encountered problems with rust developing on purchased hot-rolled black and HRPO welded seam tubes. These tubes had to be sent out for cleaning, adding cost and cycle time.

Nancy Shadrix, materials buyer for ILS, suggested lasering tubes made from SCS-treated hot-rolled because the samples she experimented with seemed impervious to rust. An SCS trial was a success, and Shadrix started buying SCS tube, which eliminated the rusting problem. It also allowed her to consolidate hot-rolled black and HRPO purchases into a single SCS purchase.

Seeing the results in tube fabrication, Chuck MacKarvich, founder and president of Tie Down Engineering, wanted to replace hot-rolled sheet with SCS. But he needed to be certain it wouldn’t present issues in stamping, bending, welding and turret punching operations. Sloan MacKarvich, Chuck’s son and ILS leader, conducted SCS?trials in these processes and was pleased to see how well the material performed.

"We run four Mitsubishi ML 3015 SV flat-bed lasers," he says. "We saw speed increases with the SCS-treated material, but we also get a cleaner cut, even at tight tolerances. We’re confident that our lights-out laser operations will run more smoothly with the SCS sheets.

"There’s another bonus," he adds. "With the cleanliness of the SCS, we found our weld quality improved, and our welding cells became more efficient. Plus, our paint finish looks better on the SCS parts, so now we want to see if we can lean out our paint prep. Turns out there are more advantages to using steel this clean than just faster laser cutting." FFJ

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