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Laser Technology

Crash prevention

By Nick Wright

Above: When the cutting head impacts material that sticks up on the table, laser head crashing can occur.

Programming power can prevent lasers from running into material, damaging the head

March 2015 - Mitigating malfunctions isn’t something that machine operators should have to worry about. This is especially true for laser cutting tables—usually the workhorse and moneymaker of any sheet metal fabrication shop. 

Maybe it hasn’t happened in your laser cutting cabinet, but those who’ve experienced the frustration of a laser head crashing know that it is a costly glitch. Laser head crashing occurs when the laser cutting head—whether it’s CO2, YAG or fiber—impacts material on the cutting table. The laser head can incur damage from cut parts or floating island scrap. When the cutting program brings the head over an already-cut section, loose material that might have shifted during cutting can damage the head.

If you’ve grappled with laser head crashing, there are solutions. Programming the cutting sequence to avoid areas through which the laser has already sliced is an effective fix. Cincinnati-based SigmaTek’s SigmaNest software can instruct the laser to automatically move around areas in which there’s a risk the laser will run into material. By reducing the likelihood cutting head collision, fabricators can reduce downtime and costly hardware replacement.

Scale of the problem

How many times have cutting operations been halted due to laser head crashing? Kevin Keane, SigmaTek’s machine technology manager, stops short of saying that it’s so widespread as to cause alarm. That said, the costs associated with worst-case scenarios should be known to operators. Damage to the laser, not so much the material, is the chief concern.

“Most machines are affected by it, including moving gantry-type machines with fixed tables, but also on tables where the sheet moves,” Keane says. “The lasers themselves have different methods of holding the heads, like plastic bolts, magnets or spring mounts. So, there are different methods to mitigate damage by tip-ups. But there’s still just the fact there are stoppages and sometimes damage.”

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Regardless of the type of laser, the fact that a machine is moving over material presents an inherent risk. The faster the laser moves, the greater the potential impact. New fiber machines are at a higher risk because they move quickly, Keane explains. Plus, the faster a machine is moving, the less time there is for an operator to intervene. Some of the lasers nowadays, which can traverse a 5 ft. by 10 ft. cutting table upwards of 13,000 in. per minute with 5G of acceleration, are so fast that even when an operator is watching with a hand on the control, he can’t necessarily stop the machine in time. In some instances, slug material in the slats can be tipped up from high gas pressure when the machine is moving along.

In a worst case scenario where the laser head is severely damaged, the cost to replace a laser head can fall in the $10,000 to $30,000 range, according to SigmaTek. Losses also accrue in the way of machine downtime and man hours devoted to replacing and recalibrating cutting heads, as well as ceramic spacer rings if they also become damaged.

“Every minute that the machine is not cutting correctly or has stopped costs you more money,” Keane says.

Breaking it down further, a fabricator has a limited number of hours each day to make a profit. “This imaginary line when a company begins making money does not come at the beginning of the day. In an eight-hour shift the transition won’t likely occur until the final couple of hours. So naturally any downtime shortens the work day, shrinking the window on profitability,” he says.

Built-in solutions

Programming the laser’s software to prevent crashing is an option to ensure the head doesn’t come near previously cut material, whether there are holes or floating pieces of scrap.

Keane explains the two basic programming methods. The first is by managing the cut sequence to cut in a nest pattern that protects the machine and maintains part quality and time efficiency. This is known as intelligent sequencing. Second is to program the machine to avoid crossing over previously processed areas that could present a tip-up, floating scrap or other threat. “The software will need to have the smarts to recognize areas of danger,” he says. A third option is to make the laser cut up or destroy threatening material so it falls away from the material surface.

“We’re looking to reduce and eliminate stoppages and crashing just as a function of the nesting software,” Keane says.

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Aside from programming the cutting sequence to avoid areas of the table where collisions could happen, it’s worth pointing out measures some manufacturers have taken to address the problem. In the past, solutions involved keeping an operator at the machine to manually intervene on the programming side.

Some solutions are engineered into the cutting head. Coherent Inc., Santa Clara, California, includes a crash sensor in its cutting heads. The laser nozzle is affixed to a magnetic mount with three electrical contacts. If anything bumps the nozzle, it will disrupt the electrical contact and stop the machine. As a failsafe, the entire nozzle will simply break free from its magnetic mount if there’s significant force (the nozzle assembly easily goes back into place).

Similarly, since the advent of high-speed lasers more than 10 years ago, Bystronic Inc., Elgin, Illinois, has virtually eliminated head crashes through slug-destruct, automatic collision avoidance positioning and automatic micro-tabs. In its ByAutonom line, cutting will stop if the head bumps something, after which the head returns to the centering station and realigns the beam. The control then skips over the area where the impact happened.

All this programming sounds like it might negatively affect overall cutting speed. However, in calculating the risk of a stoppage, it is often faster to have the laser circumvent a part than cross over it, says Keane. On one hand, less risky larger parts tend to lay flat after cutting. On the other hand, parts that aren’t as wide as at least three table slats are most problematic.

“These parts are probably less than 12 in. in either X or Y for most machines,” says Keane, who cited an example in a recent blog post. Take a 12 in. by 12 in. part, which would have a 16.97 in. diagonal. 

“To cross this part fully might take up to 0.339 seconds. On the other hand, to go around this part and avoid it entirely will be 24 in. of travel or 0.48 seconds. So in the literal blink of an eye, 0.15 seconds of travel completely eliminates the risk of a crash. So while it might be slower—it will probably be faster,” he wrote.

As any machine operator knows, productivity requires more than speed. Keane adds that programs like SigmaNest are effective at preventing crashes by smartly nesting sheet. When laser head repairs aren’t a worry, fabricators can leverage their machines for full profitability. FFJ

Sources

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