Thinkhaus had a small fire back in 2011: http://www.thinkhaus.org/2011/04/03/lazzoring-is-serious-business/ and their post about it has become part of i3Detroit's standard laser cutter training. Thanks, Thinkhaus! 

At i3, we've had a few small fires in the chamber, though not by the apparently-common tray fires described by others in this thread. For us it's the material being cut, remaining aflame after the head moves away. I think corrugated cardboard is the usual culprit, since it has channels that can sustain "peaceful" combustion, even as the air-nozzle blasts down from above. (There's a project idea floating around, to use the oxygen-poor exhaust of a medical O2 concentrator, or just a plain old tank of argon, to supply the air assist blower with inert gas during the entire cut. I'd be super curious if anyone else has tried this and how it went! It should reduce edge char too, no?) 

Most folks just pause the job and Big Bad Wolf the flames, but the extinguisher has come into play at least once. 

I don't know how others' cutters are constructed, but on our big machines, the platform (tray?) is several inches below the cutting plane (there's the honeycomb, and then the slats, and then a gap caused by the slat bracing), and while the beam is still focused enough down there to melt through adhesive tape, I'm not aware of anything ever having caught fire in the tray. That's a good point worth making, though; we should give it a look after shutting down the exhaust, in case anything down there is smoldering. 

In addition to linking to the above Thinkhaus page from our tool-info page, I make a point to discuss the incident during training. (I don't know if other trainers do, but operators are supposed to be trained twice by separate trainers, so I think most have gotten The Talk.) We have a strict "do not walk away" policy, and have recently mounted a phone/intercom near the operator's position to help with this. Sometimes when I walk past, I make a point to offer to grab a soda for anyone babysitting an active job, even though I know they could easily pause the job and get their own, since I want to emphasize gratitude for their vigilance.

We have a checklist for operation, which includes "lift the fire extinguisher from its spot and set it back down" as the last step before pushing Start, with the intent of both confirming that it's always there, and building the muscle memory of how to unhook it. That's a 5-pound nitrogen unit, which is what the service place offered as a replacement when our beloved halon unit went out of date. (That's a tradeoff. Nitrogen's environmentally harmless, but pound-for-pound, inert gas isn't as effective as halocarbons at actually extinguishing fires.) 

In the future if we ever get a tool-auth system going, in addition to badging into the laser to prove that you're on the operators list, I'd like to rig a switch so it confirms that the extinguisher-mount changes state before enabling the machine. For the time being it's all manual.

Also sitting nearby is a 20-pound CO2, which every laser operator is encouraged to practice with before getting certified. We blast each other with it and generally treat it as nonthreatening, in hopes of building familiarity and reducing hesitation in the event of an actual fire. It's cheap to refill, and since it's not the "official" extinguisher for the area (that would be the nitrogen by your knee), I'm not worried about running it empty playing with it. That's happened once already, and I think the practice and familiarization was well worth the trivial refill cost. I plan to swap it for a 10-pound next time it's due; the 20 is clunky to handle. 

I've had a LOT of folks say it was the first time they ever actually pulled the pin on a fire extinguisher, much less discharged one. I think this is important -- we practice CPR on dummies, we practice fire drills by walking outside, even the AED has practice pads. Why isn't it more common to rehearse with actual extinguishers? They're cheap and fun!

There's another 5-lb clean-agent (halon or nitrogen, I forget) in the electronics lab, and the rest of the space has big dry-chemical extinguishers everywhere. Most are mounted right near the supply-stations (paper towels, tape, pens, markers) in each zone, to capitalize on the habituation of turning toward those during other instant needs, such as spills.

-Nate B-

