Lack of preventive maintenance = lackluster laser performance
December 15, 2011
By keeping tabs on laser optics, gas flow, chiller performance, and machine cleanliness, a metal fabricator can ensure that a laser cutting machine is performing as it should and possibly lengthen the machine's working life as well.
For some, laser maintenance is a given. The meticulous metal fabricator knows how important it is to keep the laser cutting machine at its top efficiency and productivity levels. The equipment is probably under a service contract, and the fabricator religiously follows scheduled preventive maintenance recommendations.
For others, laser maintenance is out of sight and out of mind (see Figure 1). These carefree fabricators are more concerned with uptime than scheduling downtime for maintenance. It’s not until they experience a problem with speed or accuracy that they go into troubleshooting mode. But at this point, because the fabricator has not stayed on top of recommended maintenance activities, the equipment operator is at a loss where to begin the investigation. The problem can be due to any number of variables, and precious productive cutting time is lost while the search for the cause of the production disruption takes place.
That doesn’t have to be the case, however. No matter what the approach to laser maintenance may be, a metal fabricator can reduce future hassles significantly by staying mindful of just a few commonly overlooked maintenance areas.
After any type of maintenance is done to a machine’s optics, it is critical that they be returned to OEM specifications. Otherwise, out-of-spec optics can affect mode quality, beam quality, cutting performance, and optics life.
Beam analyzer and beam profilers are available to address this issue. These tools are large camera-based systems that measure the beam characteristics when the laser beam passes through. However, this equipment is typically very expensive to purchase and complicated to use in a shop environment. Implementation can be tricky when adapting the tool to the laser’s beam delivery system. Because the beam must pass through the large analyzer box, it needs to be placed on the machine table, underneath the Z axis, a tight spot.
Some manufacturers eliminate the need for that costly equipment by imprinting and attaching original mode burn profiles on a piece of acrylic attached to each machine before it leaves the factory. This allows fabricators to reference and check quickly for proper shape, size, power setting, and beam optic specifications. As the fabricator checks out the resonator and the beam delivery components, he or she is looking for cleanliness. If spots on an optic are ingrained onto the surface, meaning they cannot be wiped away, they will continue to grow and will need to be replaced. At the processing area, the operator checks primarily the processing lens, but also confirms that all optical components throughout the system are aligned.
Hot and cold optics checks can be performed both at the resonator and at the processing area. Hot burns most often affect the transmissive optics—or those that absorb heat—specifically the output coupler and lens. If any optics are causing thermal distortion, the evidence reveals itself immediately. Distortion won’t necessarily be obvious on the surface of the lens, but the kerf width will change throughout, affecting the cut. Kerf change is easily seen in mild steel, medium sheet cutting with oxygen. If this is the case, the operator is able to isolate and identify the culprit: output coupler or lens.
Of course, today’s advanced CNCs are programmed with step-by-step instructions for performing various maintenance checks, simplifying optics checks and maintenance efforts on the shop floor.
Most laser machines use an assist gas for cutting. Assist gas flows through the laser head and out the cutting nozzle with the laser beam to achieve a cleaner cut. Oxygen, nitrogen, and shop air are the most common assist gases. Oxygen requires the lowest flow, but leaves oxidized edges on the part after the cut. Nitrogen usage requires greater pressure to deliver it to the cutting head, but it also results in a clean, precise cut without oxidation. Shop air is the fastest-processing assist gas and is widely used for cutting stainless steel, mild steel, and aluminum sheet metals.
If the fabricator’s assist gas of choice is shop air, compressed air needs to be contamination- and moisture-free. Therefore, it’s essential that the lines are always clean and the system maintains proper air pressure and air filtration. Checking the lines and compressed-air source should be part of a shop’s general maintenance routine. Most compressor OEMs issue recommended maintenance guidelines, but a monthly check is standard. Contamination of the air source affects cutting performance, machine life, and productivity.
Fabricators also should have a dedicated compressor for the laser cutting machine to support the assist gas supply. This prevents downstream processes from using the same air source and decreasing air pressure to the machine.
Nitrogen generators as a source of assist gas are growing in popularity. This process feeds compressed air into a membrane that purifies the air into nitrogen, which is then used as the assist gas.
Even after checking on optics and compressed air, a fabricator can’t relax. The chiller needs to be looked at.
Chiller performance is all about temperature, so it’s important to keep an eye on the chiller’s temperature gauge. Variations in operating temperature could affect the machine’s performance. Refrigerant leaks or similar problems trip internal equipment alarms and power down the machine until it’s serviced by an HVAC expert. In the most extreme cases, a temperature change can lead to resonator damage that requires costly and time-consuming repair.
Today all chillers have either a temperature sensor or a temperature display to prevent such problems. New units are usually so well-integrated with the laser cutting machine that they require little maintenance beyond cleaning condenser coils and filters.
New control technology in laser cutting equipment has eliminated the need for a laser operator to be cognizant of ambient temperature and its effect on the chiller. Chiller units can be set to hold a constant temperature in any climate or region.
Also, if a laser cutting machine is idle for a long weekend, the water in the chiller unit tends to build up conductivity—or, more simply stated, its ability to conduct an electrical current. This occurs because water is naturally conductive. When conductivity in the chiller water is high, the machine’s self-diagnostic features prevent the resonator from starting until the level is more suitable for efficient operation. To lower the chiller’s conductivity level, the operator should run the chiller unit for a few minutes before laser cutting.
Many chillers require additives and chemicals, which complicate maintenance efforts. Newer chiller technology, however, requires only that the operator watch the water level and change the resin when necessary. Resin deionizes the water and is used to lower conductivity.
One of the most obvious yet overlooked areas of laser maintenance is keeping the equipment clean. Processing areas, support slats, scrap drawers, slugs, and pallet ways bear the brunt of negligent maintenance. It doesn’t take a fancy tool or gauge to tell the fabricator if these areas are not being maintained properly. The buildup of debris, dust, grease, and grime on these surfaces (see Figure 2) not only make the equipment look neglected, but it also affects the machine’s performance.
Buildup on the machine slats affect slat support and can cause the material to shift. This creates material handling difficulties and may affect cut consistency, accuracy, and overall productivity. Excessive buildup also causes scrap drawers and slugs to become jammed shut.
More noteworthy, perhaps, is that uncleaned processing areas, scrap drawers, dust collectors, and ducting create a thermite hazard. Thermite is a mixture of aluminum and an oxide of another metal, typically mild steel, in fabricating environments. The potential hazard associated with thermite buildup is not good: When ignited, extremely high temperatures are produced that can cause significant damage to the equipment.
Gas delivery to the resonator requires basically no maintenance, unless a hose pops off or there’s a leak. Sometimes leaks can go unnoticed, but a machine operator will see a difference in the machine’s output power or gas usage.
Here’s a good guideline for fabricators to follow: Every time the resonator gas bottles (see Figure 3) are changed, apply soap to them. This tells the fabricator right away if a leak is present and prevents any major loss in productivity.
Most laser machines use up to three separate bottles to deliver the appropriate gas combinaton for a cutting job and, as a result, require a mixer. Advanced resonator designs on the market, however, are far simpler, requiring only one bottle of premixed gas. These designs also call for less gas consumption during cutting.
Laser manufacturers today are making it easier for fabricators to practice healthy maintenance. For example, a controller on the latest laser cutting equipment has a monitor built into it with a full list of maintenance options. Users can schedule and set a maintenance regime that makes sense for their production schedule. The controller also store notes, dates, and historical data for quick reference if future maintenance issues arise.
Those metal fabricators that can’t qualify their preventive maintenance effort as a best practice have another option: Most laser cutting equipment manufacturers offer preventive maintenance contracts. These eliminate a great deal of service down the road and reduce overall maintenance costs and unpredicted machine downtime. With prescheduled maintenance appointments, a service team from the equipment manufacturer can run a comprehensive inspection on the laser to keep it operating at peak performance.