Automated laser cutting still requires expert operation
February 15, 2013
The modern laser cutting machine operator must wear several hats: one for in-process quality assurance, another for recordkeeping, one for preventive maintenance, and one more for troubleshooting.
Editor’s Note: This is an updated excerpt from Using a CO2 Laser in the Shop: A Handbook for Installation, Use, and Maintenance (Rockford, Ill.: Fabricators & Manufacturers Association, 2002). Although much of the content applies to any laser cutting operation, the original material focused on CO2 laser technology. For more information on FMA’s book offerings, visit www.fmanet.org/store.
Technology has changed the laser operator’s job dramatically. Laser cutting has become an automated process. Auto-focusing lenses help reduce downtime when preparing for a new job with different material types, such as switching from steel to aluminum. Manually refocusing the laser used to take 15 minutes to an hour or even longer, especially on older machines. But an auto-focusing lens can switch over in less than a minute. Software advancements also have changed job descriptions, both in programming and machine operation.
Still, this doesn’t mean operators have nothing to do. Yes, they may leave their stations to perform other work as the laser runs unattended, but not all jobs can run unattended. Complex cut geometries still can cause interference problems, and cut quality still can degrade if the lens focus or the gas pressure isn’t set properly.
The modern laser cutting operator should know which jobs require attention and periodically inspect cut edges to ensure good part quality. To that end, the operator must wear several hats: one for in-process quality assurance, another for recordkeeping, one for preventive maintenance, and one more for troubleshooting.
While the laser is cutting parts, the laser operator must pay close attention to the process. He must ensure that parts and scrap do not tip up and interfere with the nozzle. It is not unusual for a just-cut piece to pop up slightly above the material surface, even if microtabs are used in the material nest. In fact, if parts are tabbed into place, the operator can’t fully inspect part edge quality until the blanks are snapped out of the skeleton.
Problems arise when using a height sensor. The sensor rides up on the tipped-up, just-cut part, which can cause a loss of the focus point and cut-edge quality problems. If the part is tipped up far enough, the sensor will not be able to ride up and over it, causing the head to crash. Not only can this cause serious downtime—up to an hour on some machines, especially older ones—but the crash also might damage one or more nozzle parts beyond repair.
During laser cutting, the operator must ensure that the conditions are correct for the part being cut. He must ensure that corners are not burned, edges are acceptable, and that dross is not forming on the bottom of the part. And, of course, the operator should spend time observing the actual cutting, ensuring all conditions are optimal.
Ideally, the operator should know what proc-esses blanks will undergo downstream and take those into account when setting and monitoring cutting conditions. For instance, if the laser cuts a steel part that will be welded, the operator should know that the edge condition is critical. Excessive dust or dross can make life harder in the deburring and welding departments.
The operator must also monitor assist gas pressure. If the assist gas pressure changes, the cutting will change. The assist gas makes laser cutting much faster by providing an exothermic reaction (on carbon steel) and blowing the melted material out of the cut face area. Depending on the material, it may not cut very well, or not at all, without gas. If gas flow drops, the operator should ensure there are no problems with the supply. A drop in pressure can mean the cylinder or dewar is almost empty.
If the gas pressure cycles (increasing and then decreasing), the system may have less obvious problems. For instance, if gas vents from a dewar under a high flow condition, sometimes the liquid can’t change to a gas fast enough. This means liquid flows for a short period before the flow turns entirely to gas. This can cause the regulator and (if used) tubing to ice. This makes the tubing very inflexible, and if it is inadvertently bumped, it can break, spewing gas and liquid—a very dangerous situation. This usually isn’t a problem with bulk tanks that use metal piping. Still, the cycling of cold and warm flows can stress the pipe joints and cause leaks over time.
Sometimes the operator must make a series of test cuts in a scrap area to ensure the machine parameters are set properly for the material at hand. Regardless of how well the machine cutting conditions are set, some materials always will form a little dross. In some shops, the operator is responsible for removing any residual dross to otherwise ensure the laser portion of the part processing is complete.
The operator needs to inspect and count parts to make sure the proper quantity is cut. Modern lasers may have automatic material loading and unloading. Programs may be downloaded in an instant. Downtime between jobs may be just seconds or minutes. But all these productivity improvements don’t count for much if downstream assembly processes grind to a halt because of a missing part.
In these days of dynamic nesting and kit-based production, different parts from different jobs may be on a single nest, and such an arrangement may make counting a bit easier. Instead of counting hundreds of brackets to be welded onto 50 subassemblies downstream, the operator or material handler can group disparate parts together in kits, as specified in the job traveler, and those kits flow downstream as a unit. This way, if a kit is missing one part, the operator notices immediately and, within minutes, can cut additional pieces if necessary.
Modern controls can store information digitally, but it may be a good idea to record a paper backup. After all, like any other computer, the laser also can lose its data, including years’ worth of cutting conditions. That’s why, over time, a separate job logbook becomes the bible for anyone operating the laser cutting machine.
In one section of the log, the operator maintains a record of the number of hours the laser was on versus actual cutting time. This ratio can indicate the real efficiency of a shop operation. Some machines have internal counters that provide the power-on hours, cutting hours, and similar data.
A second section of the log can have daily, weekly, and monthly records of all preventive and corrective maintenance. A calendar cut to fit the logbook or binder can be helpful here.
A third section can be used to record the time required for all aspects of each job. This includes the clock time at the job start and end, which can be fed back to estimating personnel so that future jobs can be estimated more accurately. It can also include the amount of time the operator spends during a shift changing cylinders or dewars (if the shop doesn’t use a bulk tank), changing the focusing lens, performing minor maintenance, as well as dead time waiting for job data.
This log may include information that can reveal some obscure problems. Say at 7 a.m. every morning the laser stops or malfunctions briefly. This may indicate that the AC powerline is being switched by the electrical company. Most homes and businesses probably don’t give these brief power outages a second thought, but they can wreak havoc on a laser cutting machine.
The quality assurance department may need to inspect the first part, and then reinspect parts at certain intervals during the run, as specified by the customer or company policy. This loss of run-time often isn’t included in the part run-time calculations—but it can be, if there’s a record of it. Recording this time can help reveal how much time a job really takes and, most important, how much it really costs.
If any changes are made to the expected cutting conditions, they also should be recorded in the log. An excessive number of changes may indicate a bad lens or mirror. If a laser is cutting a new material, the operational log should note this, and the cutting conditions finally used for the material should be transferred to the master cutting conditions log verbatim.
This cutting conditions logbook should have a page or pages for each material that the shop cuts; this can include carbon steel, stainless steel, aluminum, other metals, and nonmetals. Each page should be headed with a specific material name, such as 10-gauge hot-rolled steel or 10-gauge cold-rolled steel. Every time the conditions are changed for cutting that material, the log should reflect the date; the person making the change; the reason for the change; the conditions used at the beginning; the modified conditions at the end; and any other data, such as edge quality information.
The quality of the material is important as well. Depending on where the buyer obtains the metal, the metal composition can vary. So while the laser conditions may have been established for normally used, high-quality material, “garbage” sheet can still be difficult to cut.
Controls on older lasers may have limited storage capacity for cutting conditions, so the operator keeps only the most used in digital form. When a rarely used material has to be cut, the operator can enter conditions from the logbook and be assured he is starting out just about correct.
On machines in which cutting conditions are embedded as part of G and M codes, the cutting conditions log will make the programmer’s job very easy. Instead of developing a program based on new or modified cutting conditions, he can refer to the log.
Yes, preventive maintenance can be mundane, but it’s also extremely important. Reactive maintenance on any machine can reduce shop throughput significantly, and the effects are even more dramatic at a shop’s primary cutting process.
Although more linear-drive systems are making their way into the market, many lasers today still have cutting tables driven by ball screws. A ball screw is a long, ground rod with a nut filled with a ball bearing rather than a thread profile. The nut is surrounded on each end with wipers that are supposed to wipe off the ball screw before the nut goes forward. As dust and dirt build up, however, the wipers can become embedded with the dirt and lose their ability to wipe properly. This is why the ball screw must be lubricated according to the manufacturer’s instructions.
The laser generates a lot of swarf (basically, laser sawdust). The dust consists of very fine metallic particles that can go everywhere, especially into the laser’s electronic equipment, so the filters must be cleaned periodically. In addition, the electrical cabinets should be blown out every few weeks using either cleaned and dry air or dry nitrogen assist gas. Nearby equipment, particularly personal computers, should be opened and cleaned periodically as well. Care should be taken with how the dust is blown out, however, because it can overwhelm adjacent machinery.
Operators also should monitor the condition of the cutting lens and nozzle and record how often they need to be changed. This will be dictated to some extent by the material being cut, because some materials splatter more than others; aluminum and stainless steel are the worst offenders in this area. In time, however, a pattern should establish itself that will indicate the required replacement intervals.
With CO2 lasers, the beam bending and cavity mirrors generally require cleaning every month or so, depending upon use. They typically begin to show signs of wearing out after about three or four months.
The CO2 laser’s beam mode and alignment also should be checked at least once a month. The beam mode pattern should be labeled and kept so there is a visual record of the beam quality over time. The beam pattern is a short burn of the laser beam into a piece of 0.5-in. or so plastic. If working properly, the beam forms a cone in the plastic with an even cross section. An improper burn is larger in one direction than in the other. This can cause problems, depending on which direction the beam is cutting. The shop technician can clean and adjust partially reflective and totally reflective mirrors. All other internals typically can be serviced by the laser machine vendor.
Laser operators can run into any number of problems on any given day. The most typical problems occur in dewars, regulators, and the laser beam itself.
Many fabricators now use gas stored in bulk tanks, which the gas supplier refills periodically. The larger gas supply ensures that adequate gas is available at a nominal pressure. Gone are the problems of regulators, bad hoses, and empty cylinders or dewars. Fabricators may also use so-called “six packs”—six large gas cylinders piped together on a roll-out cart. They usually contain nitrogen rather than liquid oxygen.
Nevertheless, many fabricators do still use individual cylinders or dewars for certain laser machines. These arrangements can be efficient as long as operators inspect and maintain the gas delivery components regularly.
Also, operators shouldn’t forget about the gas supply hoses and piping, which also can be a source for problems. For instance, a bad hose may flake off particles that can clog the filter.
If a machine draws gas from dewars, the operator needs to make sure its valves aren’t leaking. A small amount of liquid can become a fair amount of gas. Frost forms on the valve, and vapor can be seen escaping from it. Tightening the valve usually solves the problem. The operator should always use gloves when working with frozen parts. Protective face gear and clothing also should be worn, and all company safety policies followed.
Older dewars can have other problems. Packing glands on the valve stems often leak vapor when the valves are turned from open to closed, or vice versa. Tightening the stem nut sometimes solves the problem. Older dewars also can have bad insulation that causes them to develop hot spots, which in turn cause the liquid to turn into a gas and vent. Valve insulation also can develop problems, like pinholes, causing the dewar to bleed off significant amounts of gas. Defective bleed valves can exacerbate the problem.
The operator should be sure to report any such problems to the supplier. In fact, full dewars with these problems may bleed off as much as half their contents. Dipsticks on dewars also can be inaccurate. In fact, it’s not uncommon for a dewar to be delivered somewhat less than full. The bleed valve also may have the incorrect pressure.
Gas regulators and hoses are another area for troubleshooting. High gas pressures correspond to high usage volumes that produce stress and wear on all parts of the delivery system, both inside and outside of the laser proper. A hose with a too-small diameter or the incorrect material can restrict the gas delivery by the time it gets to the cutting nozzle. Inside the machine, flow screens made from sintered material can clog and restrict gas flow as well. Frost buildup on the gas lines or valves can indicate a slow leak.
Regulators have delicate internal parts and can easily be damaged by improper valve operation—for instance, if they are opened too quickly. In fact, depending on company policy, an operator may not be qualified to work on a regulator, which can be dangerous. Because regulators are removed and installed every time a cylinder or dewar is changed, dust and dirt can easily enter and cause flow problems. It is very important that the assist gas reach full pressure as soon as possible and, conversely, drop in pressure as soon as commanded.
Considering all this, qualified personnel should check these valves and hoses to ensure they are in working order. Otherwise, a laser operation could have less assist gas than expected and result in unexpected downtime.
The most common troubleshooting can occur because of the laser beam itself, which is why establishing an effective preventive maintenance program is so important. The focusing lens can be splattered by the material being cut and so should be replaced as needed. On a CO2 laser, imperfections in the circular polarizer (sometimes called “bend mirror 1”) can cause the beam to become noncircular and thus cut differently in different directions. If the operator suspects problems, he should be sure to do a mode burn, which can indicate a possible solution.
New technologies—from linear-drive machines to solid-state fiber systems that don’t require laser gas—have changed maintenance regimens. And automation certainly has changed the laser operator’s job description. The person may no longer need to work lift cranes to load and unload material. He or she may not need to spend time focusing the lens, or managing the laser program and reviewing G and M codes.
But with lead-times shorter and throughput demands higher, any problem in the laser cutting area can send ripples of inefficiency throughout the rest of the shop. The bottom line: The operator must ensure parts of the correct quality and quantity emerge from the laser cutting system at the right time and are sent to the right places on the shop floor.
In the relay race for optimal part flow, the laser operator runs the initial leg. The operator may run incredibly fast (that is, the laser may produce parts extraordinarily quickly), but if the baton handoff (part flow) isn’t smooth, the race still may be lost.