BIFOCALS REVISITED

Across North-America, laser job shops are coping with inflationary costs of energy and raw materials and low-availability of credit. Dual-focus technology addresses this issue by providing a double digit increase of cutting speeds in production and thus reducing variable costs and generating open capacity that can be used to reduce JIT overtime requirements or process more orders.

All of these benefits are achieved without any need for retrofit of the laser machine or for capital investment. Until recently, dual-focus effect was offered by replacing a standard focusing lens with a bifocal focusing lens. After years of development, a new dual-focus technology has been successfully tested and is now available for high-power CO2 laser cutting.

This newest application uses a bifocal mirror which, in combination with a standard lens, yields a dual-focus effect. This combination has led to similar increases in production cutting speeds particularly, when laser-cutting stainless steel, galvanized steel or aluminum under high-pressure nitrogen-assist gas. Bifocal mirrors can be water-cooled much more efficiently than lenses, and due to its protected location in the laser machine, a bifocal mirror is effectively protected from sputters projected from the point of cut. This ensures a long mean time between cleaning and a long life for the bifocal mirror, even with very high power 6 kW systems.

For end-users, bifocal mirrors are consequently more economical and less prone to damage by operators than bifocal lenses. Some end-users have measured a net 15 percent instantaneous increase in throughput performance.

Bifocal Mirror Technology

The dual-focus technology, which was originally invented in Denmark, belongs to Air Liquide. The company carried out extensive research that led to the development of bifocal mirror products that have been used in demonstrations and installed on various laser cutting systems. With every end-user utilizing these bifocal mirrors, enhanced cutting speeds result in higher profitability and lower variable costs in such areas as labor, utilities, repair and maintenance. These advantages translate into significant annual net savings for companies.

Bifocal mirror technology delivers stability to the operation by employing a dual-focus mirror with two focusing points along the beam pointing direction. Upon incidence on a standard focusing lens, the outer part of the laser beam reflected from such bifocal mirror will propagate through the focusing lens of focal length f and forms the first focus point, while the inner part of the same laser beam will propagate through the same focusing lens and form the second focus point at a distance df below the first one.

During cutting, the top focusing point is positioned near the top surface of the material. The second focusing point is positioned towards the underside of the material. The focus point located near the underside of the workpiece works to avoid dross formation, while the other focus point, located near the upper surface, produces a high enough power density near the top surface to boost cutting speeds.

Technology in Practice

A number of laser job shops are already using the technology, including American Access (Gainesville, FL) a leading metal fabricating and welding shop. This fabricator uses dual-focus mirror technology in production on a Trumpf 5 kW laser machine. The company works with aluminum, stainless and steel. The majority of the material cut is 063 5052 aluminum. American Access was looking for a technology that would help reduce costs and time without compromising on the precision of the metal cut.

The company has seen significant improvements since using dual-focus technology for the past year and can now cut at a speed of 1100 ipm, an increase of almost 60 percent. The precision of the cut was not compromised. In fact, it increased. As a result, employees work less overtime and the company is able to provide its customers with a shorter lead time on products.

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Air Liquide Industrial U.S. L.P., 2700 Post Oak Boulevard, Suite 1800, Houston, TX 77056-8229, 713-896-2332, 713-896-2332, www.us.airliquide.com.