December 12, 2002
CO2 lasers were used predominantly for cutting flat sheet metal for many years. Advancements in laser beam quality, power, manipulation, and material handling features have propelled the CO2 laser into new areas of fabrication. Multidimensional cutting, increased cutting capacity, and the ability to cut a wider range of material types make the CO2 laser a popular thermal cutting process in today's metal fabrication industry.
As in all cutting processes, productivity largely is determined by cutting speed. Because of increased laser power, higher than ever processing speeds are being realized today. Laser cutting capabilities are limited only by the laser process and the machine components.
Advancements in linear-drive technology provide increased axis acceleration, 20 meters per second squared or more, while maintaining traverse speeds of more than 200 m per minute. Linear-drive systems are free of components such as elastic power transmissions or the conventional rack and pinion. Linear-drive systems provide contact-free power transmission, producing little wear on the system and resulting in less downtime.
With laser power of 5 kilowatts or more, today's CO2 lasers can cut 20 to 30 m per minute on 1-millimeter-thick materials. The ability to supply laser power to dual cutting heads also adds to the CO2 laser's productivity. Because only part of the laser power is needed to cut in the 5-mm range, it can be shared by multiple heads. Productivity increases of more than 50 percent can be realized with dual laser heads.
Cutting techniques also have changed, and the assist cutting gas chosen can affect cutting speed. For years oxygen typically was chosen as the assist gas to cut mild steel sheet metal, but this is not the case anymore. The use of compressed air can increase the cutting speed, lower assist gas cost, and increase productivity.
The use of compressed air is not without some limitations: Thickness is limited to 0.08 in. and the edge quality may suffer. An alternative choice is an inert gas, such as nitrogen, instead of oxygen.
Oxygen's limitation as an assist gas is the cutting process itself. In an exothermic thermal cutting process such as laser cutting, the cutting speed is determined by how fast the oxygen burns the material. Adding more oxygen will not increase the cutting speed; in fact, the process speed may decrease because a larger cutting kerf is produced.
Cutting with nitrogen is not a chemical process, it is a mechanical one. As the power of the laser has increased, the ability to drive the cut speed with laser power versus cutting process has also increased. When nitrogen is used as the assist cutting gas, the speed of the cut then is limited by the power of the laser, not the assist gas. Nitrogen can be supplied in higher pressures and flows to cut faster on mild steel and other materials. Although nitrogen costs more, the increase in productivity may offset the additional cost.
Another benefit of nitrogen cutting is a clean-cut edge that does not require a secondary process before the material moves to the next operation. This reduction in material handling lowers the overall production cost.
Limitations of nitrogen cutting are the higher pressures and flows required. In addition to the higher gas cost, a more complex assist gas delivery and storage system is needed.
In addition to cutting flat materials more quickly, the laser can be used to cut tube and pipe. Round, square, and triangular shapes 20 feet by 10 inches or more in diameter can now be handled efficiently by laser materials processing.
More power not only increases a laser's processing speeds, it also increases its material thickness capacity, but this has not always been the case. More laser power puts more demand on the optics to maintain a precise, focused beam. Without a coherent beam output, the beam mode quality would suffer, defeating the potential productivity of the laser cutting process.
Advancements in CO2 lasers have made possible
cutting of materials from 1 to 25 mm thick.
Improvements in the optics design plus the use of an adaptive optic system enable lasers in the 5-kW range to produce precise, controlled, focused laser beams. Mild steel 25 mm thick, stainless steel 20 mm thick, and aluminum 12 mm thick can be processed easily.
The ability to cut a variety of materials and thicknesses is attributable to improvements in the quality of the laser beam itself (see Figure 1). Higher-purity resonator gas improves the laser's efficiency and beam quality. Impurities such as moisture and hydrocarbons interfere with the lasing reaction. The photon light is defracted or absorbed as it comes in contact with the impurity, and the subsequent beam output is less than optimal with the lower power density. The cost to cool the resonator increases as a result of the defracted light and lower efficiency. The use of advanced optics to transmit the higher-powered laser beam is a necessity to prevent premature failure.
These purer gases may be more expensive. However, lower gas consumption rates help offset the higher unit cost. Maintaining the integrity of the laser gas from the gas supply to the laser is also important; the use of high-quality components and a filtration system is recommended.
A laser processing center capable of operating in the X, Y, and Z axes opens up a completely new world for laser materials processing. Not only can this type of machine perform cutting, but it also can be used for welding and surface modifications. Because of the ability now for the working area to be in three dimensions, new shapes can be cut effectively in a single pass. Processing areas of 12 cubic yards or larger allow bulkier items previously too large for laser cutting now to be processed.
Lasers can create narrow, deep weld seams with minimal thermal distortion. Multiple-axis capability increases the laser's flexibility relative to the geometry of the material, thus increasing process productivity. Out-of-position cutting is now commonplace, and movement of the laser cutting head along the contour of the material substrate produces a burr-free area without additional handling of the material.
Designed to meet the demands of precision metal fabricators, today's laser systems can supply high-speed, high-accuracy processing of a variety of materials and thicknesses.
For more information, contact Richard Green, product manager, CONCOA, 1501 Harpers Road, Virginia Beach, VA 23454, phone 800-225-0473, fax 757-422-3125, e-mail firstname.lastname@example.org, Web site www.concoa.com. CONCOA is a manufacturer of gas pressure and flow control equipment.
Photos courtesy of TRUMPF Inc., Farmington, Conn.
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