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The force isn't with you during laser processing, but the impact can be significant. Permanently mark titanium pacemakers to a depth less than 0.001" (0.03 mm) without damaging internal electronics. Slice through abrasive composite materials without high tool wear or delamination. Cut, drill, and weld with one laser, one setup. Use simple fixturing since cutting forces are infinitesimal.

Three to five-axis laser machining centers routinely outpace processes such as CNC milling or electrical discharge machining on the "difficult" parts, those that chew up conventional cutting tools or have complex geometries requiring multiple setups and long cycle times. Key benefits of 3-D laser machining include the following:

* Flexible part processing. For example, the Bystar CNC three-axis laser system from Bystronic Inc. (Hauppauge, NY) has a 5 x 10' flying optics format-stationary workpiece, moving cutting head that accommodates flat sheets and round or square tubing and profiles. Changeover to 3-D cutting takes 5-15 min (depending on the part) because the CNC rotary axis is integrated into the machine frame. High-speed cuts are consistent irrespective of part size. Less floor space is necessary than with a moving table and a stationary laser head, and there are no clamp dead zones.

Improved laser control. The laser on the 3020HT five-axis CNC laser processing machine from MC Machinery Systems Inc. (Wood Dale, IL) provides rectangular-waveform pulsing up to 3-kW peak power. Better pulse control--including control of pulse on and off times-means faster cuts, capability to make sharp points, improved small-hole drilling in thick work materials, and reduced heat-affected zones (HAZ). The 3020HT's cross-axial-flow resonator design also targets "maintenance-free" operation and lower operating costs-consumable costs are as low as $0.02/hr. In addition, there is no warm-up time, and the laser can begin producing parts in 45 sec (see Managing Editor Jim Koelsch's article, "Be an Enlightened User" in this issue for more about the reliability and uptime of production lasers).

Programming flexibility. Programming options for Prima US Inc.'s (Farmington Hills, MI) laser systems range from on-machine teach pendant programming, the drawback being the machine isn't cutting, to an off-line simulator that can mate with a tracing machine and Prima's Forma CAD/CAM system, designed specifically for 3-D laser cutting machines. This allows building the 3-D cutter path trajectories directly from the CAD files.

High accuracy and repeatability. Consider the TLM Series of five-axis carbon-dioxide (CO sub 2 ) laser cutting systems from NTC Laser Machine Group, Marubeni America Corp. (Southfield, MI). Positioning accuracy for the TLM-404, 408, and 608 is 0.001 ipf (0.08 mm/m) in X, Y, and Z axes and 0.01deg in Alpha and Beta. Repeatability is 0.0005" (0.013 mm). Accuracies are slightly higher for the TLM-G10, which has a work envelope of 74.80 x 122.04'. Laser options include Rofin-Sinar (Plymouth, MI) switch-mode CO sub 2 lasers-the 1500W model RS-1200-SM with 5-kW peak power or the 1950-W model RS-1700-SM with 10-kW peak power.

THE SHAPE OF THINGS

Three to five-axis laser machining systems provide more access to part surfaces, even complex geometries, than conventional machine tools. On flying-optics (moving beam) three-axis systems, laser movement typically provides the major axes of movement: X, Y, and laser head rotation (include Z and tilt for a five-axis system). Tom Burdel at Bystronic reports gantry systems, which typically have the largest work envelopes, provide flexibility to gang several part setups on one table or to replace the table with a shuttle-type positioning system or custom workholding. This can allow processing a larger part than might seem possible based on axes dimensions. Rotary tables can add 3-D contouring capability.

The midsize machine is often a moving-beam cantilever-type system with the laser mounted at an angle to the part instead of above it. For example, Prima US's five-axis Optimo laser centers are flying-optics, gantry laser robots with working volumes up to 160 x 88 x 40" (X, Y, Z). The company's Rapido 5 cantilever machine provides 125 x 60 x 24" (X, Y, Z).

Lumonics Eden Prairie Div. (Eden Prairie, MN) provides three Laserdyne five-axis machining configurations: gantry, cantilever, and composite. The smallest machine has three moving-beam axes and two that are moving table or part. It has a smaller footprint, with one part loaded at a time?, and is aimed specifically at processing small, intricate 3-D parts requiring high accuracy.

"We use CO sub 2 and Nd:YAG [Neodymium Yttrium Aluminum Garnet] lasers from a variety of manufacturers, including Lumonics," says Lumonics' Ron Sanders. "YAG systems are more common on the composite frame, followed by the cantilever. Rarely have we mounted them to large gantry systems." High-power carbon-dioxide (CO sub 2 ) lasers remain the production workhorses, but continuous-wave (CW) Nd:YAG lasers have reached 2-kW average power. Another plus in harsh manufacturing environments is their capability for fiber optic beam delivery. The laser source stays in a clean, protected environment, while the light traverses along fiber optics to the shop floor. For example, the CW Nd:YAG laser from Hobart Lasers & Advanced Systems (Troy, OH) can transmit up to 2.4 kW through a 150-m-long, 0.6-mm fiber optic cable, using computer-controlled motion systems or articulated-arm robots, for applications ranging from cutting and welding to surface treating.

To help defray high costs associated with Nd:YAG lasers, fiber optic beam delivery allows two workstations to share one Nd:YAG laser beam using a production-rate multiplexer for beam switching. A station processing a thick work material also can draw power from two lasers with little loss in working volume.

Engineers at Motoman Inc. (West Carrollton, OH), a Hobart-Yaskawa joint venture, report CW Nd:YAG transmissive efficiency (the percent of loss along the fiber optic) is now better than 90%. A typical C02 beam delivery system requires a minimum of one beam bender per articulated axis. Transmissive efficiency of clean benders is generally 98-99.5%, but cutting efficiency drops as they accumulate dust and other contaminants. CW Nd:YAG lasers don't have this efficiency loss because the beam delivery system is sealed.

COMPARISON SHOPPING

The capital equipment outlay for a high-power Nd:YAG laser is till, one estimate, almost 20% higher than for a mutikilowatt CO sub 2 unit. Operating Costs also are higher. One reason is flashlamps required to produce the beam must be replaced after so many thousand operating hours. Nevertheless, the decision to buy an Nd:YAG or CO sub 2 laser depends more on the application: work material, part thickness, processing rates, even accuracy.

Both lasers produce a monochromatic, coherent light beam in the infrared spectrum that bombards the part surface to create localized heating, melting and resolidification as in welding, or complete vaporization required for cutting. The Nd:YAG wavelength is 1.06 microns. The CO sub 2 wavelength is 10.6 microns. To comply with safety standards, Rofin-Sinar Vice President Richard Walker says both lasers require shielding. The only difference is you can shield the CO sub 2 with materials like plexiglass. YAG systems require more costly shields.

Walker notes typical production YAG lasers provide 300-500-kW average power, while it is more common to have multikilowatt CO sub 2 systems. YAGs can cut and drill thicker materials because they provide more peak power and energy per pulse than CO sub 2 systems with the same average power. For instance, a 500-W YAG may generate 10-kW peak power. A CO sub 2 laser able to pulse will probably reach two to four times average power. Unlike CO sub 2 systems, however, YAG's often don't have adequate average power for production welding. And CO sub 2 processing rates are usually higher.

A YAG laser also can look through the focusing lens in real time to find a specific detail on the part, so locating required features before cutting is faster than with a CO sub 2 system. Expect CO sub 2 referencing capability to begin improving, however. Lumonics, for example, recently introduced CO sub 2 systems with Teach Vision, where small television cameras aid part location.

Comparing a 2-kW YAG to a 2-W CO sub 2 beam is a little like comparing apples to oranges. The wavelength is shorter, yet the YAG laser's mode structure is such that it will not focus to as small a spot. A 400-W YAG laser might produce a spot of 0.008-0.10" compared to the 0.004-0.006" spot produced by a CO sub 2 laser. According to Lumonics's Sanders, this translates to a large difference in processing capabilities.

Nonmetals absorb beam energy better from a CO sub 2 laser than an Nd:YAG because of the high absorption characteristics at the 10.6-micron wavelength. Both lasers slice through common metals, but the YAG can also process certain reflective materials like gold, silver, copper, brass and a few other highly conductive, reflective materials. There also are less problems with HAZ. For instance, Steven Dolan, vice-president, HGG Laser Fare (Smithfield; RI), which specializes in laser processing, estimates a 0.010" (0.25 mm)-diam beam will create approximately 0.002" (0.05 mm) HAZ (20% of beam diameter) on either side of the cut.

DRILLING BITS

While high-power CO sub 2 lasers do most laser welding, Nd:YAG systems see a lot of use drilling high-precision, small holes. While percussion drilling expends a lot of energy, it can outpace electrochemical machining and electrical discharge machining in some applications. In material less than 0.100" thick, the laser uses three to four pulses to break through it and the remaining bursts for sizing the hole and reducing taper. Operators also can produce holes at an angle close to the horizontal and in areas inaccessible to other machining methods.

Trepanning to make a hole uses a tightly focused beam and the motion system or rotary tables to start a hole in the middle of the circle (sometimes by percussion drilling), and then cut the circle perimeter. Orbiting, an outgrowth of trepanning, rotates the focusing mirror to make the hole.

The drilling process required depends on the hole specifications, work material, and volume to be removed, notes Dolan. For instance, drilling a composite with conventional machining techniques can be difficult due to the work material's inhomogeneous composition and its hardness and abrasive nature. Other potential problems are excess tool wear and delamination. Laser processing melts and seals the material not vaporized, leaving an unfrayed hole or edge.

In one application, HGG Laser Fare had to trepan 8800 0.125" (32-mm)-diam holes in a slightly bowed Kevlar noise-suppression panel. At first, engineers used a 1200-W CO sub 2 laser in pulse mode, with 300 W overall power output. The laser beam was stationary while the part moved under it. This method had several drawbacks. First, the 6 hr/panel cycle time was too high. Tooling configurations limited cutting speeds, and inertia problems caused occasional over-travel and slightly elliptical holes.

To solve the problem, engineers attached an orbital cutting nozzle to the lens focusing assembly. This allowed rotating the lens to focus on a stationary part. Cutting speed increased because the nozzle assembly can move faster than the tooling table. Engineers also designed an autofocus device for the orbital lens assembly. Since conventional capacitance-measuring techniques only work on conductive materials, they used a HE-NE laser beam to triangulate positions and automatically adjust focal distance from the Kevlar part.

Manufacturing engineers can now make holes in 0.25 sec. Setups are less complex, so what once took six hours is now done in two. Higher cutting speeds also reduce beam exposure time to help eliminate part damage such as charring.

Another application involved percussion-drilling thousands of 0.001-0.002" (0.025-0.05 mm)-diam holes in a titanium-based composite. Here, Dolan's company used a YAG laser fitted with special optics. To be cost effective, processing had to be done on-the-fly (by synchronizing pulses with work material movement, and having each pulse vaporize a hole). To avoid repeatability problems related to synchronizing positioning equipment with the laser controller, the engineers developed software to coordinate beam pulse speed (pulses/sec) with travel parameters, such as speed and distance, while accounting for laser firing.

DOING MORE WITH LESS

F&B Manufacturing Co. (Phoenix, AZ), which makes hydroformed parts and assemblies for aerospace, computer, and medical applications, also received substantial productivity gains with laser machining. On a 625 Inconel detail part that used to require CNC milling of the flange perimeter, Engineering Manager John Stewart reports boosting feed rate by 60:1.

"The transition to laser machining went smoothly," says Stewart. "We were making parts the first week and in full-scale production within a couple months. We justified the system by identifying the parts that would derive the most benefit from it. As a result of this evaluation, we now target the following parts for laser machining: hard-to-machine high-temperature alloys such as Inconel 718; complex trim geometry; parts requiring pierced holes smaller than one stock thickness; parts requiring cutting, drilling, marking, or welding; and parts requiring expensive dies or next-day turnaround."

One part benefiting from laser processing is an aerospace combustion liner. Before, it required turning, milling, and drilling. Pierce and slot dies were necessary; setups were numerous and time-consuming. The process flow involved trimming, refixturing, hole drilling, often at shallow angles to the surface, and then cutting complex shapes. Another operation serialized the part. According to Stewart, the company now machines and marks this part and others on a five-axis Lumonics Laserdyne 780 BeamDirector(TM) with a T-slot table, rotary index table, a 1.5-kW CO sub 2 laser, and standard clamps.

Programs control all axis motions and laser process parameters. Feed rates average 30-60 ipm (760-1520 mm/m). Most applications involve making 0.015 to 0.020" (0.38-0.51-mm)-diam holes in 0.020-0.032" (O.81-mm)-thick stock, although operators have cut 0.125" (3.2 mm)-thick aluminum, 0.5" (13 mm)-thick cold-rolled. steel, and 0.375" (9.5-mm)-thick stainless.

"Most aerospace work has rigid specifications controlling edge conditions, including allowable taper and recast layer," says Stewart. "F&B checks and proves these conditions by sampling macro cross sections of the laser cut. Edge conditions and cutting parameters are recorded and maintained in the laser cutting schedule. During first-article inspection, operators can keep the tooling set up on the work table, while we machine another part. This boosts machine uptime dramatically."

Copyright Society of Manufacturing Engineers Jan 1994
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