June 24, 2003 / Software and linear motors are driving laser cutters to higher levels of productivity.

Ron LaPelle's strategy for staying a step or two ahead of his competition is to raise the ante. So despite the recession, he added a 6-kW PlateLaser cutter from W.A. Whitney Co. (Rockford, IL) to the complement of seven lasers at the laser service bureau lie founded in 1987. The gamble has worked. Laser Engineering and Fabrication Inc.(LEF) (Tulsa, OK) not only can process jobs four to six times faster, but it also lets the company pursue jobs with a wider and thicker range of materials. It can cut carbon steel as thick as 1.25" (32 mm) and processes more aluminum, stainless steel, and high- temperature alloys such as Inconel and Hastelloy.

Consequently, the high-power laser has helped the company win contracts that used to go to shops with plasma cutters and other processes. Because the laser is more accurate and cleaner cutting, it eliminates secondary machining processes for cleaning surfaces, cutting edges to tolerance, and drilling holes. "Originally, these customers would have their plasma-cut plates machined, either at the same place that cut them or somewhere else," says John Baker, sales manager at LEF. Because the 6-kW laser eliminates these steps, he estimates it often does the same jobs between 5 and 10% cheaper, depending on type of cut and material.

Moreover, the laser saves time, not only by eliminating secondary operations but also by cutting faster than other lasers. The 6-kW laser shortens production times by 25 to 50% for large lots, which is an important selling point in today's world of just-intime production, according to Baker. The ability to process work faster also creates capacity. "If three one-day jobs are out there and you cut 25% of the time, you just gained another day," he explains. "Since our lead times are shorter, we're able to get some business that would have been turned away before."

The laser's speed has affected the lives of the operators, one for each shift. Unlike their counterparts on the other machines, they must dedicate themselves to tending the cutting operation. "If deburring is needed, we usually send it to a second process inhouse," says Rick Nacke, operations manager. The other laser operators do the deburring and some other tasks near their machines while the machines are cutting.

Nacke reports the speed does not compromise accuracy and repeatability because the builder fit the machine with linear motors. "A linear motor has no mechanics," says Jimmy Berry, applications specialist at Whitney. Because the slides ride on a magnetic field, "the motor's components are not in contact with one another. So whatever the motor can produce is transferred straight to the machine." There is no wear and backlash. Consequently, even with ig acceleration, position accuracy is +/-0.001" (0.03 mm), and processing accuracy is 0.005 to 0.015" (0.13-0.38 mm), depending on the alloy and material thickness. Repeatability is in the millionths of an inch.

ADJUSTMENTS ON THE FLY

Another productivity tool at the operators' disposal is the Material Parameter Library inside the machine's Intelligent Laser Control. All settings for processing a material are stored in the library. To program the machine, the operator either defines or imports the geometries to be cut, enters the material and its thickness, and inserts basic commands such as pierce, and cuts along toolpaths. The material library then adjusts for proper power settings, standoffs, focus distances, feed rates, and gas pressure for optimum performance along each cut segment. Based on settings from the library, for example, the controller ramps all laser parameters into cutting mode after piercing. As the beam approaches a corner, the controller will reduce speed and power and adjust focus.

Whitney's machines can adjust focal distances on the fly because each focus axis contains a servomotor that moves its optics cartridge up and down based on recommendations in the tool library. The machine can make the adjustments even while the assisting gas is exerting pressures that can run as high as 260 psi (1.8 MPa). Although the technology has existed for a while, many builders continue to use a fixed focus or have thumb-rolled focus for tweaking the position because of the development expense. Berry notes, however, the situation is beginning to change.

Linear motors, such as this one from Finn-Power, are driving more laser cutters to boost acceleration and velocity.

Macros make the performance of Laserdyne's precision lasers more or less independent of the postprocessor by optimizing motion for the particular laser cutter model.

On-the-fly adjustments are also part of 5-kW and other laser cutters from Trumpf Inc. (Farmington, CT). "Higher power lasers have been used for welding for years, but it takes a while to adapt them for cutting applications because beam quality and delivery are much more critical in a cutting machine than in a welding machine," says Dan Robinson, laser products group manager. One way the company accommodates higher power in different materials at various distances from the resonator is by deforming the mirrors in the delivery system to adjust focus point on the fly.

When the controller needs to adjust the copper mirrors' concavity, it changes the water pressure behind them. "By varying water pressure through the mirror, we can change its shape at will," says Robinson.

"Although movement is minuscule, only a few microns or so, it makes a big difference in the beam's focal characteristics." This method of computer control can move focal point up and down by 20 mm almost instantaneously. The controller retrieves the required amounts from the 11 pages of cutting data it has in its library for each material for automatic setup and on-the-fly adjustments in the cut.

Because operating at higher powers puts the lens at greater risk, Trumpf protects it in two ways. First, a sensor in the cutting head measures visible light above the lens. "If there is a problem with the lens, a stress fracture or dirt particle, the laser beam will cause it to burn and give off visible light as opposed to just the laser's invisible light," explains Robinson. If the sensor detects too much visible light, the controller stops the machine and tells the operator to clean or replace the lens.

Another way Trumpf protects its lens is by moving the cutting head away from the material while it is drilling or piercing, which is the messiest part of laser cutting. "The most dangerous time for the lens is when the laser first drills through the material because a large shot of molten material comes back at the cutting head," notes Robinson. He attributes the ability to move the head out of range during drilling and closer to the material during cutting to adaptive mirror responsiveness.

Four-kilowatt laser cutters from Bystronic Inc. (Hauppauge, NY) also adjust cutting parameters on the fly using feedback from optical sensors on the cutting head. "If it sees that it's losing cut quality, it will slow up or restart without any operator intervention," says Michael Zakrzewski, vice president, Metal Processing Systems Div. Because in-process monitoring prevents the laser from producing scrap, the machine can run untended during the day or in lights-out mode overnight.

SOPHISTICATED SOFTWARE

Another continuing trend in laser cutting is the development of increasingly sophisticated software. Machine builders have encoded decades of applications experience into their controllers. Consequently, today's controllers simplify setup and tune process parameters, such as laser power, cutting speed, gas pressure, and focal distance, as the laser is working. "Today's software guides the operator in a much more simplified way to adapt the machine to new materials and processes," says Lutz Ehrlich, a spokesman for Finn-Power International Inc. (Schaumburg, IL). Because of the growing use of CAD/CAM systems to program its three-dimensional laser cutters offline, Laserdyne has developed a series of macros, or canned subroutines, that make the performance of its precision lasers for the aerospace industry more or less independent of the postprocessor. "By definition, universal implies compromises," says Terry Vander Wert, vice president, Laserdyne Systems Div., Prima North America Inc. (Champlin, MN). "We can still use the universal post, but macros, subroutines, and other software features optimize laser system motion."

Use the Air - It's Free!

Since the air is 78% nitrogen, you might ask why are you spending all that money on nitrogen? You're not alone. The engineers at Amado America Inc. (Buena Park, CA) asked it, too. They went a step farther, though. They provided an answer: EZ Cut, a device that extracts nitrogen from the air and produces a gas even richer in nitrogen.

"When used for cutting stainless steel, the gas that EZ Cut produces provides a read-to-weld, cosmetically acceptable edge in most cases," says Amada's Jerry Rush. "When used in processing carbon steel, the gas mixture produces an edge that is absent of most, if not all, of the oxides that prevent paint from adhering firmly to the material's edge."

He adds that the device is economical. Not only is the unit less expensive than a conventional nitrogen generator, but also the nitrogen-rich gas it produces is much cheaper tha\n bulk liquid nitrogen. The gas from EZ Cut costs $0.29 per 100 ft^sup 3^ (2.8 m^sup 3^), whereas bulk liquid nitrogen costs between $0.60 and $1.70 for the same volume.

Amada is not the only builder tapping the air as an assisting gas for laser cutting. "Reducing operating costs is important, and one way of doing it is using shop air as the assist gas," says Richard Neff, manager, laser products, Cincinnati Incorporated (Cincinnati). "Compressed air turns out to be a very good gas to cut with as long as it's run through a filter and refrigerated drier to clean out oil and water."

Not only is air cheaper than other assist gases, but it also can cut faster. "In light-gage materials, you might find that you can cut twice as fast with air as you can with nitrogen or oxygen," says Neff. The reason is, when exposed to laser light, it creates a plasma near the material's surface with a quality that allows focusing to a very small, hot spot.

One resonator that can produce such a beam is the 3.3-kW, diffusion-- cooled resonator from Rofin-Sinar Inc. (Plymouth, MI). Because of the beam quality, cutters using it can cut light-- gage materials faster than machines based on 4- and 5-kW resonators. For instance, Neff offers the example of a piece of 18-gage (1 .2 mm) stainless steel cut with nitrogen. Cutting speeds were 205 ipm (5.2 m/min) on a 2-- kW laser, 335 ipm (8.5 m/min) on a 4-kW laser and 520 ipm (13.2 m/min) on a 3.3-kW diffusion-cooled laser. With air as the assisting gas, the diffusion-cooled laser was able to cut the same material at 750 ipm (19.1 m/min).

So users cutting thin materials can save both the costs of using expensive gases and operating a higherpower resonator. And that is a breath of fresh air!

The Material Parameter Library inside the controller on Whitney's 6-kW PlateLaser cutter supplies the proper power settings, standoffs, focus distances, feed rates, and gas pressure for optimum performance along each segment of the cut.

Consider a job that involves trepanning a part containing a number of precise holes. To maximize throughput, the software combines center locations from the universal postprocessor with subroutines that generate the cutout pattern and set operating parameters. The aerospace industry is using this approach to make various shaped holes for controlling airflow in jet engines.

The builder also has software that works with sensors to reduce setup. The sensor that tracks workpiece surface during cutting (known as automatic focus control) also can scan surface features to identify surface orientation or angle, and find reference points on workpieces or fixtures. "Because software can set up plane rotations and offsets, the operator need not spend lots of time making sure the part is true in the machine," says Vander Wert. "It can save minutes to tens of minutes per part, depending upon how complex it is."

The builder's engineers also are deploying the latest computer technology, both hardware and software, to improve motion control. Like their counterparts at builders of flatplate cutters, they are trying to improve lookahead algorithms and servo update times to allow the machines to exploit available cutting speeds without sacrificing accuracy. "Just by having a better understanding of the control system's role and optimizing it for a given part can reduce by 15 to 20% the time it would take to produce a part," says Vander Wert.

LINEAR MOTORs BOOST SPEED

The linear motors driving LEF's PlateLaser are not an aberration. Most of Whitney's competitors already have fit their high-power machines with linear motors to exploit the faster cutting speeds possible with these high-power resonators. "Linear motors have made a big impact because we can make even small contours quickly with high precision and edge quality," says Ehrlich at Finn-Power. Because they have no backlash and no physical springback, the dynamics have changed quite significantly. You get more precise corners if you make a square box, for example."

Another important advantage of linear motors is they can maintain high accuracy over long distances, a fact that makes them cost competitive over conventional ballscrew technology. For this reason, Finn-- Power drives the 20' (6 m) X-axis on its new Laser Brilliance laser cutter with these motors, and Whitney uses them on its largest machine, which has a 10 x 20' (3 x 6 m) working area. Ehrlich expects this drive technology to become more prevalent as it matures and cost falls.

To accommodate linear-drive technology, builders will have to design their high-speed laser cutters differently. They need more rigidity than conventional machines to support positioning speeds as high as 300 m/min and acceleration as high as 2g. "The challenge to maintaining accuracy here is not as much in the CNC electronics as it is in the mechanical design," says Ehrlich. "You need rigidity when moving masses around that quickly."

Although more machine builders are turning to linear motors to increase speed and acceleration along cutting axes, some are hesitant to jump into the technology just yet. The reason is the machine rarely reaches its full speed, especially when the job involves mostly short distances. "A linear motor has a tremendous amount of mass, meaning a typical linear motor machine takes time to reach full speed," explains Zakrzewski at Bystronic. "Most parts don't require you to move even 10" (254 mm) between holes, so you're never taking advantage of full speed."

Consequently, Bystronic has chosen to concentrate its development efforts on generating very high acceleration rates, instead of generating high speeds. Its strategy is to accelerate to top speeds quickly so the machine can exploit whatever speed it can reach for as long as possible. "With a high acceleration, you can take advantage of the top speed at very small move sizes," says Zakrzewski.

To execute this strategy, the builder's engineers reduced the mass of the machine's flying optics to keep the inertia of moving elements low and specified high-torque motors to overcome the remaining inertia. They also developed a proprietary helical drive in which a motorized nut runs up and down a stationary screw. By not rotating the entire screw, the design reduces twisting, wear, and the amount of inertia the motor must overcome.

Zakrzewski reports that improvements in cutting time can be between 30 and 50%, depending on part geometry and material thickness. "You will see the most advantage on jobs with a lot of contours, holes, and short point-to-point moves," he says. "The design also has a speed advantage in thinner materials because positioning time is a larger percentage of overall production time. In thicker materials, cutting the material takes longer than moving between holes."

Some experts believe laser power has hit a temporary plateau in cutting applications. "The practical limits of output power is around 4 to 5 kW," says Bill Shnowske at Mitsubishi Laser Div., MC Machinery Systems Inc. (Wood Dale, IL). "Getting a good beam for cutting is difficult at high power."

For this reason, Mitsubishi's development engineers are trying to do more with existing cutting power by focusing it into a smaller spot. Consequently, much of their effort is devoted to resonators that produce better quality beams. "The resonator is the heart of laser cutting," says Shnowske. "If you start with something bad there, it can only get worse, not better." Other efforts focus on experimenting with gas and nozzle technology.

STAY Busy WITH AUTOMATION

Although LEF has not invested in automation to load its fast, 6- kW laser cutter, a growing number of other laser shops have. "We don't make anything without a shuttle table anymore, and more than half of our machines go out with some kind of material handling equipment on them," says Zakrzewski at Bystronic. "More people are trying to get as much as they can from their equipment. Even if they don't have full storage and retrieval systems, they are still using some kind of loading device so they can turn the light off at 5:00 o'clock in the evening and let the machine run until it runs out of material." Some take the next step by installing storage and retrieval systems that can manage raw materials and finished parts in an untended operation over an entire weekend.

To make storage and retrieval systems appealing to a larger number of laser shops, Bystronic and others have designed material storage towers and handling devices to be modular so users can specify them for their own manufacturing environments.

For example, Bystronic's design has an additional off-loading station that is separate from the tower, but still connects with the automation. This feature gives users the choice of transferring parts to the next operation or storing them in the tower for later use. Users also can specify the combination of shelves used for storing raw material and finished parts.

Robinson at Trumpf adds that users can start with a stand-alone machine and add loading equipment and then perhaps a storage unit later as necessary. Like other builders, Trumpf fits its laser cutting machines with a pallet changer. "Because most lasers now are flying optics, meaning the cutting head has to move over the material, you can't unload single parts while you are cutting," he says. "So you need an offline pallet with all the cut parts on it, and you remove them from there, away from the cutting process."

To automate loading these pallet changers, Trumpf uses electric lifting devices that can swing forks into position under entire plates and piece parts. One device manages raw plates and lifts the web off the machine, and the other, a pick-andplace device called the Sort Master, stacks individual parts on a pallet. Because loading devices keep the laser fed, beam on-time can be as high as 80%, reports Robinson. And keeping the beam busy is a big bright spot for productivity.