Reprinted from MAN-Modern Applications News December 1996
Copyright ©1996 Nelson Publishing
Cutting and welding spatter, as well as contaminated assist gases, can destroy a laser lens and turn precision work into scrap.
Today’s high-powered CO2 lasers all have a common feature that is essential to their operation. Whether the system optics is supplied with the laser, or purchased separately from one of several lens manufacturers, the beam delivery of laser energy is the important “working end” of the system.
The specific lens material, zinc selenide (ZnSe), is important in delivering the maximum energy in the part of the spectrum where the laser operates; specifically, a wavelength of 10.6 microns. Treating the lens with an anti-reflective (AR) coating boosts the transmitted energy to 90%, instead of the 70% usually transmitted by an uncoated lens (30% lost to reflection).
Whether the laser is being used for cutting or welding, the lens in the system is constantly exposed to contamination from spattering material or from particulates in the assist gas (usually nitrogen, but “dirty” shop air is often used). Being bombarded with these contaminants has a rapidly degrading effect on the optical characteristics of the system and increases the chance of damaging the lens. If the AR coating is damaged, the reflectivity of the lens increases and less energy is transmitted to do the cutting or welding.
As the contamination continues, the lens becomes more distorted and starts to absorb more energy. As the lens heats up, it distorts even more and can actually start to melt. Usually, the first indication that this is happening is that the system is not cutting properly, or the welds are not properly formed. When the performance has degraded noticeably, removing the lens and inspecting for spots or other contamination will verify that the lens should be reconditioned or replaced.
Trying to recover a contaminated lens is usually only a temporary solution in restoring a laser to full operation. A one-time recoating of the lens could cost as much as $300, where a lens replacement runs from $500 to $600. Although a lens may operate properly for months in relatively clean application, spattering and other high-particulate situations can damage a lens in a few days. Depending on the material and its thickness, cutting stainless steel and aluminum almost always causes the most mess. Add a contaminated assist gas, like shop air, and a laser system will use up lenses in a hurry.
Lens Protection Saves Big Bucks
Using a low-cost protective barrier between the lens and the work area takes the beating from the contaminants instead of the more expensive lens. In the case of the Lens Saver, this barrier costs less than 10% of the cost of a lens and keeps the lens isolated from any damage. In routine application the Lens Saver has a life of nearly half that of an unprotected coated lens. With the cost differences involved, this means an overall savings of nearly 70% of the cost of a new lens, with the original lens usually lasting for a long time.
The potassium chloride (KCl) Lens Saver also has a lower refractive index than the ZnSe lens, enabling the use of a lens that doesnít require an AR coating at the operating wavelength. Without the AR coating on the lens, and with reflection losses for the KCl protector of 6.8%, more power is available at the workpiece. The KCl protector material has a lower rupture modulus than the ZnSe lens material, so mounting the Lens Saver requires using a patent-pending zero-pressure design. Pressure equalization reduces stress in the standard and custom Lens Saver mounts provided for all available laser systems.
With the present cost of high-power laser systems running in the half-million dollar range, anything that can cut operating costs and protect valuable optics, and the laser itself, is an absolute must. As more higher-power lasers become available to do heavier cutting and welding jobs, lens and system protection is becoming more than just a cost-saving measure –itís the only effective way to do the job.
The Laser Cutting Company Story
The Laser Cutting Company in Clinton Township, MI has four Amada laser systems used primarily for cutting stainless steel and aluminum precision blanks. The high-pressure assist gases used include 80% to 90% oxygen, about 2% shop air and the balance nitrogen, determined by the flammability of material being cut. The company is primarily involved in prototype and short-run production. According to plant manager Rick Schultz, the operation is comparable to stamping, except that the CNC-programmed laser requires only 15 minutes to program a part, and no setup and tooling costs.
Before using lens protection, Schultz reported that the operation was consuming about 20 lenses per year at a cost of about $9,000. Using the Lens Saver cut lens consumption to about one-third with a Lens Saver cost of about $200 per month. The bottom line savings is about 50% overall. A personal touch from the International Crystal Labs (ICL) people involved designing a custom fixture for mounting the Lens Saver. The fixture was one of the pressure-equalized adapters, according to Schultz, and they have been using the protectors for about six months without any problems.
“All I know is that using a Lens Saver doesnít really require any additional power,” says Schultz. ìIn other words, when we are cutting thicker material… we run with Lens Savers 100% of the time.” As far as knowing when to change the Lens Savers, he says, ìWe never take them out unless they crack or need to be cleaned. (Since) the Lens Saver takes all the spatter, we just run the laser until (we) notice a decline in the quality of (the) parts. Then weíll pull the head off… and check the Lens Saver. If itís spattered, …we just take it off and throw it away.”
The Central Cal Metals Story
Joe Alandt is one of the owners of Central Cal Metals in Fresno, CA. This metal fabricating shop employs about 40 people in a 75,000 square foot facility that houses a variety of punch presses, press brakes, shears and machining centers. The operation also includes 12 Mazak lasers, nine of which are running from 1.5 kW to 2 kW. Alandt noted that these nine lasers were originally equipped with another brand of lens protector, but he switched to ICL Lens Savers after checking into pricing and specific features.
Originally, lenses were being replaced on the systems at the rate of one per month on each of the nine lasers. Based on a nominal lens cost of nearly $400, this added up to a total of about $43,000 annually. After outfitting the lasers with Lens Savers, lens replacement dropped to about one lens per month for all nine lasers, with one lens protector replacement each week on each laser. The comparable cost figures for laser operation after Lens Savers protection were about $19,000, for an annual savings of more than 50%.
The Mazak systems needed only standard mounts for the lens protectors, so no special modifications were necessary. Alandt stated, “We were able to incorporate them in the mounts we had –the torch heads that we had. We had enough room to accommodate the thickness of the (protector).”
At right, Lens Savers® Lens Mount for Mitsubishi HC 1.5″ & 2.0″- Work End View