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Fabricating with Lasers

 

 

The flexibility of lasers becomes evident in a variety of sometimes surprising applications

 

By Bruce Morey
Contributing Editor 

 

As lasers become more powerful, better-tuned, and more versatile, engineers are finding new ways to optimize their use. From job shops looking for the ultimate multiuse tool to welders looking to extract every photon from their investment, newer lasers and specialized equipment to use them are delivering value.

For a job shop, flexibility means the ability to perform a variety of operations.

"Lasers are expensive devices, relatively speaking, and using them in a versatile way helps the investment decision," says Neil Ball, president of Directed Light (San Jose, CA) a parts supplier, contract fabricator, and specialty integrator of lasers. "For instance, having one laser that can cut, weld, and drill. We find that for short runs or prototype work, the laser's ease of programming allows us to adapt to a customer's specification quickly, without costly tooling changeouts or redesigns. Lasers can also process a variety of materials, from ferrous to plastics, or ceramics to silicon."

A machine pioneered by Directed Light proves his point. A laser workstation built on a Bridgeport milling machine base, it's a four-axis machine that uses a 400-W pulsed Nd:YAG laser. According to Ball, it can be made equally adept at cutting, welding, drilling, or marking by swapping out the tooling head. The machine accepts standard G-Code from CAM packages, and uses standard tool offsets to compensate for the beam diameter.

"Using the Bridgeport base to host a noncontact machining head made a lot of sense to us, because it was open, vibration free, and versatile," Ball says. "I consider this arrangement very well-suited for job shops, like ours, or making prototypes, or even a tool-and-die maker's workstation. It allows a lot of different concepts to be executed on one platform."

Job shops are not the only places where lasers have found a niche—lasers continue to expand their usefulness in welding. The flexibility of lasers in welding is attained by manipulating beam quality to achieve the best possible results. A dimensionless number, M2, measures a laser beam's quality. A laser beam that matches the best theoretical beam in terms of focusability will have an M2 equal to one. Comparing one laser to another, in general, the smaller the M2, the more focusable the beam. Laser engineers are also finding the best M2 for an application can vary—the smallest isn't always the best.

Current diffusion-cooled "Slab" CO2 lasers deliver beam quality near the practical limit of M2 equal to 1.1, according to Chris Dackson, powertrain industry specialist for Rofin-Sinar Inc. (Plymouth, MI). "For applications like synchro-ring welding, where very narrow welds are required and distortion is a big issue, the high beam quality of diffusion-cooled lasers is necessary," says Dackson. "Sheetmetal components like clutch housings are often machined with tight fits, which allows us to take advantage of the small spot sizes available."

According to Dackson, however, high beam quality and small spot sizes are not always ideal for powertrain welding. Rofin-Sinar has developed a diffusion-cooled laser that delivers a beam quality measured in M2 of approximately 2.3 to 2.5. Due to the shape of the intensity profile, it's referred to as a donut-mode. "Although high-quality beams can always be adjusted down with mirrors, it's better to deliver the right beam quality and avoid these types of optical gymnastics. The correct beam quality will allow delivery of the right spot size for the application," asserts Dackson.

For example, he reports that larger spot sizes are more useful when you're welding slip-fit joints rather than press-fit joints. In one study Rofin-Sinar conducted for a customer, a CO2 laser delivering a relatively low beam quality, with an M2 equal to 5.5, demonstrated acceptable weld width of 1.5 mm with a weld depth of 4.8 mm. "We achieved acceptable welds even when a gap of 0.6 mm was present," says Dackson, explaining that the advantage of a slipfit joint is the lower piece-part price, which can offset the higher cost of the holding fixture.

Dackson reports that their latest application is replacing bolts with laser welds in hypoid ring gears, eliminating tapping and drilling operations, and reducing the assembly's weight. Using a press-fit arrangement, this application welds with a 6-kW diffusion-cooled "Slab" laser with a donut-mode option.

Higher power and better beam quality from solid-state lasers has led Eric Stiles, laser division manager of Fraunhofer USA's Center for Coatings and Laser Applications (Plymouth, MI), to investigate robotic remote laser welding equipment that is coming on the market. In the remote-welding process, galvo-mirrors steer the laser beam, rapidly repositioning the beam on the workpiece. "Traverse time between welds is reduced to almost nothing, making the process attractive for parts with a high weld density, or large spacing between welds," says Stiles. Until a few years ago, CO2 lasers were the only practical lasers for remote welding. "Older solid-state lasers were limited in beam quality," Stiles explains. "For example, lamp-pumped Nd:YAG lasers typically had a working distance of only 6–8" (152–203 mm), and a small working field of 4 x 4" (102 x 102 mm)."

Beam quality in newer solid-state lasers, like disk-lasers or fiber-lasers, provides the longer working distances needed for remote laser welding. New robotic remote systems with disk or fiber lasers offer working distances of 0.5 m or greater, and the possibility for large work envelopes. Remote welding heads for attaching to the end of robot arms are available from companies like Utica Enterprises (Shelby Twp, MI), HighYAG Laser Technologie GmbH (Stahnsdorf, Germany), Trumpf Inc. (Plymouth, MI), or ScanLab (Naperville, IL).

"The use of standard industrial robots, which can only be used with solid state lasers for remote welding, offers a cost savings versus the large gantry-style systems [used with] CO2 remote welders," observes Stiles.

Another dimension of flexibility is routing laser beams along multiple fiber-optic cables via an optical switch, sometimes termed multiplexing. Lasers can then be time-shared or energy-shared. Time-sharing delivers all of the energy into a single beam for a given cycle time, while energy-sharing divides the beam for simultaneous use. "Our time-sharing switches operate at less than 50 msec, and are especially good if you have a process where there is a lot of cycle time involved in manipulating, fixturing, verifying, or testing the part while the laser sits idle," says Tom Kugler, applications engineering manager for GSI Group, Laser Division (Northville, MI). Workcells with multiple robots are another application he sees for time-sharing. Each fiber can be set up to perform a different process, for example cutting and welding at different times. Although careful process engineering is required to use this technique, at 10% of the cost of a laser, a multiplexer is more attractive than buying more lasers.

"Energy-sharing is especially useful if you need to operate simultaneously on the same part in different places," says Kugler. "This either reduces distortion or increases production rates. For instance, on precision parts like fuel injectors we can minimize the distortion caused by the pull towards the heat-affected-zone by simultaneously welding 180 or 120° opposed. This is where tolerances are in the micron range, and simultaneous welding is the only solution."

Trumpf Inc. (Plymouth, MI), expands the flexibility of laser fabrication to a new level by sharing lasers with its Lasernetwork device. This either connects a single laser to multiple machining stations, or connects multiple laser devices to multiple machining stations. The challenge it solves is the communication between them—machining centers can request power as needed, optimizing availability and uptime. Just as importantly, redundancy can be built into the system to ensure critical operations always have laser power, through either reserve lasers or redirecting less-critical lasers. A computer control system tracks requests and usage, and provides safety interlocks as needed.

"Lasernetwork is well-suited for users with multiple lasers in the same facility, and especially for high production, such as in the automotive industry," says David Havrilla, product manager of high power lasers. According to Trumpf, information about the laser unit and the exact light path is transmitted in parallel with each laser-light cable via a code cable. The time lag for switching laser power from one laser-light cable to another is about 70 msec. Havrilla reports that Lasernetwork can either time-share or energy-share.

A special machine that is constrained to fabricate only tubes may seem the antithesis of flexibility, but that is not the case when it's equipped with a laser. Lasers provide an unusual kind of flexibility for fabrication. Lasers embedded in specially designed machines for loading and cutting tube stock permit quick setups, fast production rates, and straightforward changeover to new parts, when compared to traditional methods.

"The laser can slit, mitre-cut, recess, cut-over corners, and cut any shape on thin or thick-wall tubing," says Jim Rogowski, product manager 2D lasers for Trumpf Inc. (Farmington, CT), manufacturers of the TruLaser Tube product line. "It's not limited to any material type or tensile strength." Equipped with a fast-axial-flow CO2 laser that delivers up to 3200 W, TruLaser Tube machines are tuned for cutting the thinner materials typically found in tubes and profiles. As an example of the processing envelopes achievable, the TruLaser Tube 5000 can cut a wall diameter of 0.25" (6.4 mm), a maximum outer wall diam of 6" (152 mm), an input tube length to 256" (6502 mm) weighing up to 20 kg/m, and produce finished parts to 118" (3-m) long. An option allows production of parts to 236" (6-m) long. He cites fast production rates with cutting speeds greater than 300 ipm (7.6 m) in thin tubes for mild steels as thick as 3/8" (9.5 mm).

The exercise equipment and agricultural-implements industries, according to Rogowski, are two fields that have embraced tube-processing machines. Challenged by peaks and valleys in production, operators in both industries need to quickly changeover part production on a moment's notice.

"The challenge in using tube laser processing to its fullest is recognizing that it is 3-D machining," says Rogowski. He explains that a deep understanding of how to design a process in 3-D is vital to exploiting the possibilities of the machine. Each TruLaser Tube machine is sold with a 3-D processing-design package called TruTops Tube. The software imports CAD models and unfolds the model in its virtual world for the process designer to program cuts, holes, and diameters. The software then rolls it back up for review, and develops a CNC code for the machine. Rogowski reports that TruTops Tube can import all popular CAD formats, such as Catia V5 or SolidWorks.

The BLM Group USA (Wixom MI) also sells laser tube cutters. Since introducing their first model—a simple three-axis version—in 1988, they have advanced their product offerings to multiaxis capability with fully automated loading of tube stock, and the option to load automatically into a tube bender for automatic part fabrication. Today, the current tube cutter model LT712D cuts rounds to 6" diam, squares up to 4.75" (121 mm) and rectangular stock to 3 x 6" (76–152 mm), handling a tube weight of 15 kg/m and bundles of tubes as long as 27' (8.2 m). In the standard configuration, it comes equipped with a Rofin-Sinar DC 025 diffusioncooled CO2 laser rated at 2500 W.

Jeff Arendas, national product manager, Lasertube Systems, explains that automated laser tube cutting consolidates up to six separate manual operations, including sawing, punching, drilling, notching, machining, and deburring. "This machine lets you go from raw stock to a finished part in one operation," says Arendas. "Parts typically run in between 15 and 60 sec, and a changeover takes 3–5 min."

Arendas compares the current market in North America for laser tube cutters with the state of 2-D sheetmetal cutting 15 or 20 years ago. Early adopters have seen the benefits of abandoning hard tooling and manual processes in favor of the laser's flexibility and throughput. He relates how job shops are now receiving prints for parts that can only be manufactured with a laser tube cutter.

BLM Group's programming software, Artube, provides the ability to import 3-D designs directly from the customer's CAD data and automatically turn it into machine code. "The greatest return comes when you design the part with the laser process in mind," explains Arendas, "taking advantage of the laser's ability to combine parts or reduce downstream fabrication costs by employing self-fixturing features."

 

Laser Peening Boosts Fatigue Resistance

Laser shock-peening is a specialty niche for lasers. The laser does not heat the surface—it uses an ablative layer to generate a high-pressure shock wave that cold-works the metal when the ablative layer explodes. A transparent layer over the ablative material, typically water, forces the shock wave to penetrate into the metal. The resulting residual compressive stresses make it fatigue-resistant. The technique focuses energy more precisely than a comparable shotpeening device.

"Laser shock peening also penetrates much deeper than typical shot peening, to a depth of 0.040–0.080" (1–2 mm). Shot peening might reach a tenth of that depth," says David Lahrman, a director with LSP Technologies Inc. (Dublin, OH). Because it penetrates so deeply, applying laser peening to large or broad areas can induce part distortion. The primary application for this technology is at critical locations on the part where traditional shot peening is not providing sufficient benefit. Lahrman describes it as useful in extending the fatigue life of high-wear or critical-use parts by treating critical areas on those parts, such as turbine blades for aircraft engines. The laser employed in their LaserPeen process is a custom lamp-pumped Nd:YAG rod-laser that delivers up to 50 J in 8–30 nsec at a wavelength of 1064 nm, providing a power density of 5–10 GW/cm2. The continuous average power is 31 W.

Recognizing that applying an ablative layer (as a paint or tape) is a process bottleneck, in 2005 LSP Technologies introduced their robotic-based RapidCoater system to apply and then remove the layer. "This has reduced processing costs and increased the processing rate by more than three times," says Lahrman. He reports an average throughput speed for their laser shock peening of about 1 in2/min (645 mm2/min). Although the process was developed for (and used in) critical aerospace components, Lahrman sees future applications in medical implant devices or even automotive, if the cost is reduced further.

 

This article was first published in the November 2007 edition of Manufacturing Engineering magazine. 


Published Date : 11/1/2007

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