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Gaining an edge up in fiber laser cutting

How to keep the industry’s workhorse cutting

These days, speed is king. A high-powered fiber laser coupled with automation can feed an enormous number of downstream processes. It has made modern metal fabrication extraordinarily productive, even if some parts need deburring.

It’s common these days to see one high-powered fiber laser replace multiple CO2 machines. Fabricators now rely on fewer cutting machines to produce more than ever. That being said, if one of those extraordinarily productive fiber machines happens to produce bad parts that need to be recut, all those efficiency gains go out the window. If the machine crashes, the downtime can throw a serious wrench into the schedule and, at worst, starve the rest of the fab shop. Work flow grinds to a halt.

You can avoid this by following a few best practices, both at the machine and, most critically, during nesting and programming.

Narrow Window of Focus

Operators experienced in CO₂ laser cutting probably know the rule of thumb when it comes to focus. If you’re cutting with an exothermic process—that is, with oxygen assist gas—the oxygen is the real catalyst for the cut. The beam is mainly preheating the material. In this sense, the oxygen “catches” the heat from the laser and accelerates the cutting process through the thickness of the kerf.

For this reason, when oxygen cutting operators focus the CO₂ laser beam at the top or slightly above the material surface. This concentrates the beam’s focal point—the area with the most heat—at the top of the plate and, hence, helps spur the oxygen reaction that drives the cutting process.

When you cut with a CO₂ laser with nitrogen assist gas, you get an endothermic process. The heat from the beam melts the base material, and the nitrogen simply evacuates the molten material away from the kerf. The kerf is much smaller, and for good reason; the smaller the kerf, the less material the laser needs to remove, and the faster it can cut. To remove as little material as possible, the operator generally focuses the beam near the center of the material thickness. This also minimizes the top edge tapering in some situations.

For thicker stainless or aluminum, an operator might focus the beam near the bottom of the cut. He might not achieve optimal speed, but a low focus does tend to produce the best cut edge and the least amount of dross. All that concentrated heat near the bottom of the kerf evacuates the metal before it has a chance to solidify on the bottom edge.

In some ways a fiber laser changes the rules when it comes to positioning the beam focus. Outside of certain materials (say, 0.5-in.-thick or greater stainless or aluminum) a fiber laser beam cutting with nitrogen can focus almost anywhere in the material thickness—be it near the surface, in the middle, or near the bottom—and the beam won’t lose the cut. The best focus often tends to be near the middle of the material, sometimes near the bottom.

The fiber laser can be focused to a variety of positions throughout the material thickness—but that doesn’t mean it will cut well. In fact, with some materials, the fiber laser may have a very narrow optimal focus window, often ±0.040 in. or even ±0.020 in. Go outside this narrow window and edge quality will degrade quickly, causing dross to form.

Over the years machine manufacturers have mapped out focal points for various material grades. Some materials even have two optimal focal points, either very high or very low in the kerf, and both result in very good edge quality. Regardless, all these results have been entered into a machine’s factory settings. If these factory settings stay the way they are, the focus usually remains optimal. There’s rarely a need to adjust the focus unless a machine is cutting a material grade or thickness that’s not in the standard library. And even then, the focus adjustment is likely to be very small.

Figure 1
Heat and molten material can adversely affect a cutting nozzle. Here, the orifice has lost its concentricity, which will have detrimental effects on the beam and assist gas flow.

The same thinking goes for the nozzle gap, or the space between the nozzle orifice and surface of the sheet. Factory settings usually have this between 0.030 in. and 0.060 in., though it can be as high as 0.080 in. or as low as 0.010 in., depending on the application. If an operator changes the nozzle gap, he changes the amount of assist gas needed to make a clean cut, which changes the gas dynamics for the entire process. This, of course, isn’t good for cutting consistency.

The same goes for speed settings. For operators experienced with CO₂ systems, the speed settings can seem counterintuitive, especially for high-powered fiber systems (8 kW, for example). Yes, when cutting thicker material, optimal speeds will be slower. But cut 0.25 in. and thinner, and faster speeds can actually produce better cut edges. A 4-kW fiber laser might produce dross on thin and medium plate. But on an 8-kW system, with its combined high power and speed, dross issues can go away, and the cut edge gets cleaner and cleaner.

Modern fiber lasers are extraordinarily reliable machines, but they are also truly precision instruments. And in recent years, machine manufacturers have dialed in that precision through testing and establishing factory settings. Change the settings ever so slightly up or down, and edge quality may suffer.

Keep That Cover Glass Clean

A fabricator gets a new fiber machine and is getting ready to be wowed, not only by its amazing speed, but also by its simple maintenance. After all, the operator and maintenance tech no longer need to worry about mirrors and gas resonators. They need to keep the machine, chiller, and dust collector clean, but that’s about it.

A few days after installation and initial testing, the first-shift operator comes in and starts the next job only to find that the machine is producing terrible cuts. Why? He examines the cover glass window and finds a tiny spot. That’s the culprit. He cleans or simply replaces the cover glass window protecting the fiber laser’s sensitive optics, and he’s on his way.

Of course, the cost of constantly replacing cover glass windows can add up, not only for the cover glasses themselves, but also for the excessive downtime. Again, because throughput is so great, the cost of downtime is also great. It’s one reason that modern fiber lasers (and CO₂, for that matter) use lateral air jets during piercing. For thick plate in particular, the optimal pierce has beam parameters set so that the molten material bursts out sideways—and not straight up to the cover glass.

Setting the pierce for a CO₂ can be a complicated affair, with various beam parameters that need to be tweaked and optimized. For instance, on thick mild steel you might have a “drill pierce” where the focus changes from high to low throughout the cycle. All this is to avoid a “mound” of molten material that can wreak havoc when the system converts from piercing to cutting. The mound can throw off the head’s height sensing and move it upward, changing the focus; or even worse, the mound can cause the head to crash. Improper pierce settings also can cause excessive heat buildup, particularly when cutting mild steel with oxygen. The beam can cause the metal to overburn and lose the pierce hole; when this happens, molten metal blows back into the nozzle (see Figure 1).

With the fiber laser, the story changes. The fiber laser can achieve the throughput it does in part because of its cutting speed, in part because of its piercing speed. Advances in a fiber laser’s piercing ability has been a huge boost to productivity, particularly in thicker materials. The fiber laser’s pierce has always been fast; but the piercing cycle has been susceptible to blowback, which is why manipulating the beam (often in proprietary ways) and the side air jet has been so critical during the pierce cycle.

Material Considerations

A fiber laser can’t magically cut cleanly through poor-quality material. If an operator pulls a part out of the nest and sees heavy striations, the problem often can be traced back to the material.

When a beam hits a plate surface with mill scale, craters, or other imperfections, it can create a hot spot that penetrates the material and creates a vein down the side of the cut. Pitting and cratering also can change the assist gas flow dynamics and, in effect, change the way the laser cuts. The smoother the plate, the more consistent laser cutting will be.

Figure 2
Cut quality can plummet when parts are nested too close together without proper tabbing, as the narrow web section moves and distorts during cutting.

Programming and Nesting

Many edge and overall quality problems these days come not from a bad material surface or improperly tweaked pierce or cutting parameters, but instead from improper programming, nesting, and tabbing.

In many applications microtabs are a fact of life. They’re needed to ensure parts stay where they are after they’re cut—especially critical when considering the airflow dynamics around a fast-traversing cutting head. The last thing a second-shift manager wants is for a part tip-up to cause a crash during unattended operation in the middle of the night.

Where to place a tab depends on the part geometry. Proper tabbing must accomplish two things: Keep the part in place to ensure the laser cutting process is reliably consistent and take downstream processing into account. A small bump protruding into a hole for hardware can wreak havoc at the hardware insertion press. A small pit (rather than a bump) left by the microtab would likely be acceptable. In this case, the laser should cut the tab so it’s ever so slightly into the part edge.

Smart tabbing also plays a role when cutting certain pieces subject to bowing. Of course, it’s ideal to purchase the best, laser-flat material one can. But when a laser cuts a long, narrow piece, the act of cutting releases internal stresses and the piece is likely to bow, making a perfect target for a head crash. In these cases, tabs should be placed strategically where the most heat deflection will occur, so the part is held in place throughout the cutting cycle.

In a quest to optimize material, programmers nest extremely tightly and, in so doing, sometimes cause problems during the cutting cycle, especially without proper tabbing (see Figure 2). A nest can have a hole or slot very close to a sheet edge, and during processing the web between the slot and edge—sometimes only 1/16 in. wide—can wobble and cause interference (see Figure 3). Strategic tabs, holding the piece in multiple places between the slot and narrow web section, can prevent that web near the sheet edge from wobbling and, hence, prevent a crash. The slot’s slug itself could tip up from air pressure, causing a lost or bad cut or some welding.

One final issue involves challenging part geometries, often in 0.25-in.-thick plate and thicker, such as a sharp sawtooth pattern. A machine’s cutting conditions are written for myriad part shapes—but not all. A machine may have a cutting condition for, say, a corner that’s 80 degrees, but it doesn’t have a cutting condition for a 20-degree corner. So the head doesn’t slow down sufficiently to make the turn, and the beam melts the tip of the sawtooth. Cutting conditions can be customized, of course, but before going through the trouble, shops should consider if the part geometry is necessary.

For instance, looping at the end of each sawtooth is an option, but before building an entirely new cut path, a shop can turn to design for manufacturability. Does this part need a 20-degree sharp corner, or could a radius be put on the end?

The Small Things Really Matter

If something’s amiss with a fiber laser, it’s often not the focus, gas flow settings, nozzle offset, or other elements of the cutting-conditions recipe. It’s often the small things: A beam that’s not entirely centered, which means the operator needs to check it, be it with a tape shot, acrylic puck, or spark shot on sacrificial plate (the spark and etch pattern, if concentric around the nozzle, shows the operator if the beam is centered). If it’s not the nozzle, it may be a tiny spot on a cover glass, a tab in the wrong place, or a narrow web section that makes parts unstable.

Take care of the small things, and the fiber laser will continue to be the fab shop’s extraordinarily productive workhorse, today and years into the future.

Hank White is national product manager for Mitsubishi Laser at MC Machinery Systems Inc., 85 Northwest Point Blvd., Elk Grove Village, IL 60007, 630-616-5920, www.mcmachinery.com.

Figure 3
Parts nested too close to the edge of a sheet can cause problems, as the narrow web section wobbles and causes interference during the cut. Strategic tabbing can prevent such problems.