Industrial laser cutting goes mainstream

FIGURE 1. With 3 kW of cutting power, this laser center can process up to 3/8-in. stainless steel and 3/4-in. mild steel, as well as 3/8-in. aluminum plate. Piercing techniques help ensure accuracy in thicker work pieces.

FIGURE 1. With 3 kW of cutting power, this laser center can process up to 3/8-in. stainless steel and 3/4-in. mild steel, as well as 3/8-in. aluminum plate. Piercing techniques help ensure accuracy in thicker work pieces.

With the development of material-handling equipment that helps laser systems cut parts almost continuously, more manufacturers are applying lasers in mid- to high-volume production. Productivity gains can be dramatic-two- to fivefold for some flexible laser cells and systems with automated loading equipment. Applications range from processing of two-dimensional (2-D) sheet metal (the most common) to cutting of more-complex three-dimensional (3-D) parts.

At the same time, laser metal cutting faces tough competition from other processes such as abrasive water-jet cutting in 2-D processing applications. Some firms have developed laser cutting systems that use high-pressure water streams to guide laser beams to the target and/or cool the workpiece.

Why laser cutting?

Unlike conventional machine tools that cut metal with tools made of metal or other materials, lasers do not suffer from tool wear. They can, therefore, cut tough materials, such as the superalloys used in turbines and aircraft engine parts, cost-effectively without operators worrying about tool changes.

For 2-D sheet-metal cutting, equipment operators can program the laser to cut new shapes and a variety of diverse-gauge sheets fast (see Fig. 1). One benefit is the capability to schedule different workpieces across the laser system with no set-up time in-between.

And, unlike conventional cutting processes that can generate burrs, lasers can produce a fairly clean kerf or cut edge. This often eliminates secondary processing steps.

Discrete-parts manufacturers typically cut parts with continuous CO2 or pulsed Nd:YAG lasers. The process can use an assist gas, such as nitrogen for cooling or oxygen to facilitate the cutting process. The typical range for average output power is 1-5 kW, although some CO2 systems can produce significantly higher power. When cutting sheet metal, some systems can cut plates up to 1.5 cm thick at 1 cm/s. Thinner parts are cut faster, with one laser cutting 2-mm-thick sheets at 10-12 cm/s.

Hole production can be done with several different processing modes.1 Sheet-metal processing most often involves continuous cutting, with the laser beam moving over the surface, often in combination with worktable movement. To drill small holes (<0.2 mm) in thin plates, single laser pulses partially vaporize and partially melt the metal to expel it from the hole. In thicker sheets, holes with medium diameters to 0.6 mm can be produced by percussion drilling, in which repetitive pulses are directed onto the same spot. The largest hole diameters may require trepanning, with the laser cutting around the edge of the hole in a circular pattern.

Exploring automated systems

A flexible laser machining cell or system cutting 2-D sheet metal can comprise a number of distinct subsystems. In addition to the laser, pumping source, and controls, there is a drive system to accurately move and position the laser over the part. Some computer-controlled ac servo drives provide traversing speeds of about 1 m/s with accelerations of 5 m/s2 and positional accuracy of about 50 ?m. Linear drives can do considerably better with accelerations to 40m/s2, speeds exceeding 2 m/s, and higher accuracy.

Another key subsystem is the loading equipment that moves sheets onto the work area and removes finished parts (see Fig. 2). Some laser machines may still require the operator to manually direct a swivel loader to move the sheets into the machine. Others have fully automated palletized loading systems. Here, after a sheet is cut, the device exchanges the pallet it is on with another containing uncut sheets. Unloading the second pallet to a part conveyor belt and loading that pallet with a new sheet occurs off-line, so the laser can continue cutting.

In some automated loading systems, a robotic arm sorts finished parts, which can maximize laser productivity and allow untended third-shift processing. Such high automation can be more difficult with conventional machine tools, where operators must deal with issues such as tool breakage and changeover.

Into the mainstream

One company now using lasers in mainstream production is Roach Manufacturing Corp. (Truman, AR), which manufactures 100 models of conveyor belts in several sizes.2 The firm`s goal is to fill catalog orders in 24 hours. As sales increased, though, part production on seven turret presses fell behind, leaving no time to make parts for inventory. The plant was producing parts "just in time" by necessity, not by design.

To increase production, Roach installed three STX Hi-Pro 510 laser cutting machines from Mazak Nissho Iwai (Schaumburg, IL) with capabilities to process 1.5 x 3-m sheet metal. The metal is fed from a ten-shelf storage tower, with each shelf holding more than two tons of steel sheet. Controlled by a system PC, an elevator moves to the correct shelf and moves it to floor level for feeding sheet metal to an automatic loading system.

Adding a flexible manufacturing system to its shop floor immediately increased Roach`s capacity by 60%, even though the system initially produced only one-of-a-kind orders. The system now processes all 3000- to 5000-piece runs of parts up to 0.5 in. thick and, unlike the turret presses, requires no shearing of the sheet metal before cutting. The flexible manufacturing system requires only one operator per shift and a first-shift programmer for a total 120 man-hours per week. It replaces turret presses that required 360 man-hours per week-the result is a 200% increase in labor productivity.

But that was just a start. With its own conveyor-belt expertise, Roach designed and installed a conveyor system that organized a flow of material to and from the laser, which allowed untended overnight operation. The machine now runs almost continuously seven days a week. The company has doubled production without any increase in manpower and with an estimated 500% increase in productivity over the turret presses.

The competition

Despite the rapid strides made by laser metal cutting, the technology does not have the field to itself. There is some competition from plasma cutting systems and perhaps even more so from abrasive water-jet machining, in which the workpiece is cut by a slurry of water and abrasive projected against it in a high-pressure stream or jet (typically 400-700 MPa).3 Initially introduced to cut wood, plastics, and then glass, abrasive water-jet machining has since been applied to a range of metals including cast iron, stainless steel, aluminum, copper, titanium, and high-carbon steels.

As with lasers, the process can cut very hard materials, requires minimal set-up time, operates at high speed, and is amenable to automating. Unlike laser cutting, abrasive water-jet cutting is a cool process, which limits any heat damage to the part. For some applications, such as drilling of very deep holes, it has clear advantages over laser cutting.

However, there are also distinct drawbacks. The very high pressure required, greater than 5000 atm, mandates considerable initial equipment investment and creates safety hazards. In addition, the abrasive slurry has to be recycled or disposed of, making laser cutting a cleaner process. Abrasive water-jet processing also requires periodic nozzle changes due to wear. Lasers also tend to be faster than water jets and require less maintenance.

While both techniques have their strengths, researchers have long sought to combine them, using a water jet to guide a laser beam to a target, with the water simultaneously cooling the workpiece. Such water-jet guidance would also allow the laser to be positioned much further from the workpiece, instead of focused onto it, promoting more-intricate 3-D cutting, as well as enhanced micromachining applications.

The problem has been that the laser`s heat changes the refractive index of the water and defocuses the beam. This hurdle has been recently overcome in macroscale processing. Amada America (Buena Park, CA) offers some laser cutters for conventional sheet-metal processing with a water cooling jet.

Exploring the future

Despite the competition from abrasive water-jet machining, laser-based metal cutting appears to have a bright future. While laser metal cutting is moving into the basic high-production fields previously dominated by stamping and conventional metal cutting, it is also pressing ahead into newer applications, such as micromachining (see "Excimer lasers refine grooves in spherical bearings"). For example, Synova AG (Lacunas, Switzerland) has cut minute parts such as medical stents with a 15-kW Nd:YAG laser beam directed at the part through a water jet. 4

Even without water assist, laser technology continues its micromachining inroads. ATZ-EVUS (Vilseck, Germany), for instance, has cut microgears only 300 ?m in radius and 150 ?m thick in seconds using a copper-vapor laser with a pulse duration of 50 ns and a pulse repetition rate of 1.4 KHz. No secondary processing was required, and high-quality surfaces were obtained with one cutting step.

REFERENCES -

  1. J. V. Owen, Manufacturing Engineering, 119, 34 (July 1997).
  2. R. Kovacevic et al., Transactions of the ASME, 199, 776 (Nov. 1997).
  3. J. Hecht, New Scientist, 156, 16 (Nov 29, 1997).
  4. H. W. Bergmann et al., SPIE Proc. 2789, 33 (Aug. 1996).

    FIGURE 2. Elevator system to left of flexible manufacturing system allows rapid access to sheet metal of diverse gauges. Excimer lasers refine grooves in spherical bearings Under a microscope, bearing surfaces are actually a series of hills and valleys (top). Without adequate lubricant delivery to surfaces, such as provided by microgrooves (bottom), these surfaces would quickly fail from adhesive wear.

    Specialized spherical plain bearings play a critical role in the defense and aerospace industries because of demands for lower-maintenance, higher-performance, longer-life components. The problem is that, while bearing surface finishes of 3 ?in. are typical, under a microscope the surface appears to have hills and valleys (see figure). When these surfaces rub together under high pressure, the surface irregularities are thought to weld together and then break off. This event, called adhesive wear, ultimately results in a failed bearing.

    One way to inhibit adhesive wear is to develop a thin protective film between inner and outer ring surfaces. This film will be created dynamically in the rotating bearing with the right lubricating fluid-it must offer increased viscosity under increased pressure. With such a film, there is no rubbing or adhesive wear because high viscosity generated under high pressure separates inner and outer rings by a distance greater than the typical surface-finish roughness.

    To ensure that a bearing can develop the elastohydrodynamic film, designers purposely develop micro grooves of varying geometry. These create a self-priming and pumping action to allow a continuous flow of lubricant across bearing surfaces. If engineers have the flexibility to tailor these microgrooves to match specific performance requirements, they can extend bearing operation over a larger dynamic range. Until recently, this has not been easy.

    Neuman MicroTechnologies (Concord, NH) has solved this problem with a patent-pending process called N?Sphere. Using a seven-axis computer-numerical-control laser-etching technique, the equipment illuminates a mask that defines the groove geometry with a directed beam from a high-energy ultraviolet laser from Lambda Physik (Ft. Lauderdale, FL). Similar to a stencil, the mask is several sizes larger than the actual groove.

    The equipment then scans the mask in two axes, while the bearing is simultaneously scanned in four axes. The beam is defined and shaped by the mask and is delivered to the bearing spherical surface by a multielement imaging lens. The lens compresses the mask image to the proper size and shape. The reduction or compression ratio of the mask to target size determines the optimal energy density needed to etch the bearing material.

    Using a multiaxis controller, the system can simultaneously synchronize all axes of motion as well as the laser firing to precisely etch the grooves. It can produce grooves 2 mm wide and 0.25 mm deep within ?0.002 mm in bearing materials such as hardened steels, ceramics, crystalline structures, and polymers (with minor system adjustments). Due to the unique way that the ultraviolet excimer laser etches, the bearing material does not suffer from adverse heating effects, including the heat-affected zones common with lasers such as CO2 and Nd:YAG.

    The N?Sphere process can produce a variety of complex microgrooves on bearing surfaces ranging from tough spherical surfaces to simpler surface geometries required by spindle, radial, angular-contact, and thrust-bearing applications. By modifying current spindle and bearing assemblies, added performance and lifetime can be gained. Initially developed for high-end bearing assemblies, the process is suitable for a wide array of industries including the automotive, aerospace, medical, microelectronics, semiconductor, and commercial manufacturing.

    Microgrooves can be applied to both slow- and fast-moving bearing assemblies. The goal should be to deliver lubricants efficiently and/or to ensure that the lubricants flow continuously over bearing surfaces. Design tolerances will impact bearing requirements. If tolerances are very tight, specialized channels will be required to force the lubricants onto the bearing surfaces. These channels may take the form of microgrooves. For example, in turbine applications lubricants can take the form of aerosolized oils in the pneumatic lines feeding the turbine.

    TODD E. LIZOTTE is the director of R&D/new process development at Neuman MicroTechnologies, 26 South Main St., Concord, NH 03301; e-mail: tlneusales@aol.com.

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