Tech Topic: Avoid deburring when laser cutting sheet metal

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Precision metal fabrication has moved far beyond the inch-per-minute (IPM) metric in laser cuttng. Sure, talk is swirling about how some of the latest systems cut through thick plate so quickly it’s hard to believe. Some attendees at the last FABTECH peered through the green-tinted windows of 20-kW fiber laser machines, just to make sure what they observed on the TV screens adjacent to the machine was real. In truth, talk focused less on the speed and more on just how clean the edges were.

Deburring has remained the Achilles Heel of blanking and bending productivity. A fabricator might dive deep into automation, with automated parts stacking after cutting and automated bending via the press brake, folder, or panel bender. In between all this, someone manually sorts and feeds blanks that require deburring. Some fab shops rely on the laser operator to sort through which blanks need deburring and which don’t, depending on the cut edge quality and job requirements.

Robotically fed deburring machines are emerging on the market, so automated options are becoming available. That said, the best solution is to achieve a burr-free edge to begin with.

Today’s fiber laser beams offer various power density profiles as well as oscillating patterns to achieve better cut edges. New assist-gas mixtures are helping to improve edges as well. With all this new technology, though, it helps to understand exactly what makes a burr-free cut edge. Burrs, or dross, happen when molten metal from the kerf solidifies before it can be evacuated.

It boils down to knowing how the assist gas, the beam (including its focus), and the material interact. A focus spot too high in the material thickness leaves spiky dross; again, the metal melts and tries to evacuate, but then “freezes” near the bottom before the assist gas has a chance to flush it out the bottom. A focus spot too low within the material; thickness can lead to lower cutting speeds and bead-like dross. Buried low into the kerf, the focus melts a lot of material that, once again, the assist gas has a tough time evacuating in time before it “freezes” in place at the bottom of the cut.

Focus spot is just part of the equation; the other part is the assist gas. With the advent of in-the-shop nitrogen generation and ultrahigh laser powers, more shops than ever are relying on nitrogen assist gas for cutting, rather than dealing with the oxides left over from oxygen cutting. Some now use an assist gas mixture, such as nitrogen with a touch of oxygen, while still others use ultradry shop air (again, nitrogen with a touch of oxygen). Specific assist gases yield specific results, but the idea is to increase the temperature within the cut to allow time for molten metal to evacuate, resulting in a clean cut edge—or at least one that’s clean enough as to not require deburring. Some report such mixes eliminating the so-called fiber burr, even in dross-susceptible material like aluminum.

All this interacts with the cutting speed. For instance, a gas mixture might raise the temperature to a point, but slowing the cutting speed also raises the temperature—sometimes to an extreme degree. Slow the travel too much, and the laser starts to ablate, or vaporize the metal, which in turn disturbs the assist-gas flow dynamics, yet again leading to dross. In this case, increasing the cut speed slightly reduces the heat and the resulting ablation, allowing the assist gas to flow as intended through the kerf.

Nozzle designs play a role too, as do the consistency of gas flow throughout the system and, of course, general system maintenance. In these days of high laser power, consistent slat cleaning has become more important than ever. A high-powered fiber laser can cut extraordinarily quickly until a cut piece gets welded to gunky slats—a conundrum that becomes even more problematic in an automated setting.

Flat-part deburring machines will never go the way of the dodo, of course. Some parts need to have a certain grain finish. Some parts need microtabs to ensure cutting stability, especially in “sheet moving” blanking applications like punching and punch/laser combo machines. Some applications require rounded edges, which a laser simply can’t produce. And some part geometries are just challenging for any laser to cut with perfection.

Regardless, the more predictable laser cutting becomes, the better. Cutting extraordinarily quickly is great, but inches per minute remains just a piece of the efficiency puzzle. Cutting burr-free is another piece. Yet another is cutting on quality, laser-flat material that won’t bow and distort terribly after cutting, requiring part leveling, yet another secondary process.

Blanking in the metal fabrication department is all about tradeoffs. Sometimes deburring and part leveling is just unavoidable. That said, more operations continue to take a holistic look at cutting. They’re not worried about how many inches cut per minute. They’re worried about how many parts can be cut cleanly, stacked, and transported to bending or whatever the next downstream process happens to be. In this sense, blanking really isn’t “complete” until the next major process—be it bending, welding, coating, or anything else—can take those cut parts and run with them.

Looking for comprehensive specs on laser cutting machines?Check out The FABRICATOR's Laser Cutting Machine Buyers' Guide.

Source: Air Liquide