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Laser Technology

Producing better laser-cut parts

By Russ Olexa

January 2010 - Warped, uneven or unleveled steel can cause problems when cut with a laser.

Trapped internal stress can cause steel to warp when it’s cut, which can cause out-of-tolerance parts or even damage the laser’s cutting head when it contacts a piece of cut steel protruding from the metal skeleton.

This is a common problem laser processors experience. To remedy it, steel coils should have their trapped internal stress relieved through a stretcher-leveling process that allows the strain to be reduced from the sheet material. This prevents long laser-cut parts from warping when the material is heated through the cutting process and the steel’s memory tries to re-establish its original shape.

Make it flat
Steel isn’t naturally flat. When it’s rolled, cooled and coiled at the mill, it wants to stay in its original shape and molecular composition.

A 100,000-psi yield strength, 72-in. steel coil stock can have a 2-in. to 12-in. crossbow with wavy edges and excessive crown if it’s not properly coiled at the mill. If a customer demands flat, laser-cut parts, this material would cause problems.

To get the quality that’s needed for consistent cutting, these shape imperfections and the material’s trapped internal stresses need to be eliminated.

Steel used to be, and sometimes still is, flattened and stress-relieved using a process called temper passing, along with a roller leveler and a cut-to-length line.

Temper passing is a cold reducing process that compresses the coil-fed steel stock using high pressure between two large-diameter work rolls. The high compressive force elongates the steel. This, in turn, helps to reduce trapped internal stress. But it also decreases the material’s thickness by 1.5 percent to 2 percent, which can be a concern for some parts.

To eliminate this issue, as well as others, another process can be used called stretcher leveling. The equipment for this is produced by Red Bud Industries, Red Bud, Ill.

Leveling steel
Red Bud has developed stretcher levelers capable of producing panel-flat blanks at material thicknesses up to 3/4 in. The company’s newest machine is the fourth generation of its stretcher leveling systems. Previous machines were limited to a maximum thickness of 0.135 in., 0.312 in. and 0.5 in.

Dean Linders, vice president of marketing and sales at Red Bud, says the stretcher leveler stretches the material sufficiently lengthwise to exceed the yield point in all the fibers of the strip from the top to bottom and from edge to edge, equalizing internal trapped stresses throughout the material. Also, the machine will remove edge wave and center buckle from the material. Additionally, the material’s thickness won’t be reduced through stretcher leveling. 

A temper mill can also change the steel’s mechanical properties if it’s worked too hard. When material is stretched, it only has to be processed just a little past its elastic limit. It won’t come back to where it was, and it’s hardly measurable, according to Linders.

A temper mill can also roll dirt or scale onto the material’s surface. Steel surface scale is harder than the steel, says Linders, so it can be rolled into the surface of the strip. This can cause issues in subsequent operations. The stretcher leveler eliminates the introduction of inclusions in the material’s surface.

Importance of quality
JTV Mfg. Inc., Sutherland, Iowa, is a job shop with five lasers. Three of them have 5-ft.-by-10-ft. beds, and the others have 6.5-ft.-by-12.5-ft. beds. The company also has a full range of metal fabrication equipment, including robotic welding and CNC machining centers.

Rick Sohn, purchasing agent for JTV Mfg., buys the company’s steel from McNeilus Steel, Dodge Center, Minn., which has two Red Bud stretcher levelers.

"I feel the stretcher-leveled steel gives us a better-quality part," he says. "With this steel, the laser cuts better because the head isn’t always correcting for the steel’s waviness in the Z-axis. For instance, if you cut a part that’s 8 ft. long, and you cut one side of it, and then you cut the other side, and the first cut side of the part pops up out of the steel skeleton, now your laser is trying to cut through this bow in the material. So it would have a dramatic effect on the laser cut if the material isn’t flat. It also affects the tolerances of the material and the finished part. The laser beam’s angle is affected also because when the material shifts, it changes the cut angle if the material is no longer flat."

JTV Mfg. makes parts for companies such as John Deere and Caterpillar, as well as agricultural implement and equipment manufacturers. These include ones that build sky lifts or grain wagons. JTV Mfg. can cut up to 6-in.-thick steel using its oxy-fuel plasma system. The majority of its laser-cut parts are either from 16-gauge steel or 1/2-in.-thick plate.

"Our production manager keeps a pretty good eye on the material that we cut and the quality of it after cutting," says Sohn. "Other steel that we had would warp after cutting due to picking up the memory set from the mill’s rolling. At times, he could tell that if the material was improperly temper passed, it would still have the memory set from the mill. With the stretcher-leveled material, we hardly ever have problems with it. That’s because all the waviness has been pulled out through the stretching process.

"I’ve had 1/2-in.-thick steel parts that weren’t stretcher leveled," he continues. "These parts are probably 2 ft. by 3.5 ft., and you can put one on top of the other, and I can stick my pinkie between them. So there’s a big difference between steel that has been stretcher leveled and steel that hasn’t been."

"In our opinion, [stretcher-leveled steel] is the most beneficial when the parts hit the press brakes or weld fixtures," says Rian Vos, co-owner of JTV Mfg. "Any steel can look flat going on the machine to be thermal cut, [but] the key is being flat when it comes off. There’s nothing more frustrating than chasing bend angles on inconsistent material or muscling parts into a weld fixture."

Speeding up
Jones Metal Products Inc., Mankato, Minn., has been in business since 1942 as a full-service metal fabrication job shop. It’s family-owned with about 100 employees. The typical range of steel thicknesses the company works with varies from 16 gauge to 1/2 in. thick.

"We tried some stretcher-leveled steel from McNeilus and found that when we laser cut parts, it stays flat--it doesn’t start warping once you begin cutting it," says Les Bruns, purchasing manager at Jones?Metal Products. "This is a big plus for us. We get a much tighter-tolerance part with the sheet staying flat on the laser bed.

"We can also cut faster because we don’t have to worry about the sheet warping up [from the laser’s heat] and causing a possible collision with the laser’s cutting head. The laser can handle the warpage in the sheet, but it has to slow down for it, as the head has to travel along the Z-axis to compensate for any material changes in height."

Before using stretcher-leveled sheet, Bruns says Jones Metal Products was having problems.

"We would start cutting a piece, and by the time we had it cut out, it would start warping," he says. "Then, as the cutting head tries to cross the area where the warped part is sticking up, it will knock the laser head off and stop the machine, causing downtime and work for our maintenance people who have to reinstall the head and get the laser working properly again."

Asked if the stretcher-leveled material adds a lot of cost to the steel price, Bruns says, "No, it really doesn’t. In fact, just about all the material that we buy today is stretcher leveled." FFJ

 

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