Laser Safety: Fire Danger!

The K40 and all other high power lasers are burning tools. They work by burning. If the high temperatures and airflow manage to start a self-supporting flame inside the box, this can – and has – expanded to open flames engulfing the entire machine, with the real possibility of burning down the building. This has happened.

Basic Operational Safety

These rules may save your life.

  • Stay present at your laser at all times while operating it. Do not leave it, even for 30 seconds. If you have any urgent need to leave, stop/pause the job first.
  • Keep a CO2 fire extinguisher immediately at hand, and use it if flames start. You must install interlocks, and you will depend on them to avoid eye damage when you need to use the fire extinguisher.
  • Add air assist and use it. This may “blow out” smaller flames at the focus point before they build into self-sustaining flames
  • Keep the inside of the machine clean. Condensed plastic fumes on the bed or scraps of work under the bed can ignite.

Why are the fire risks so substantial?

Your new/prospective laser is not a complete solution that you can simply open the box and run. To get it to be that cheap, the manufacturers (and there are several or many) cut a lot of corners. One big range of corner cutting was in the original objectives. The K40 was originally designed strictly for engraving or cutting rubber stamps. Very few users have desires that limited, so the expectations you probably have are higher than the machine was designed for. You’re going to have to work at it.

I (@keen) personally would never plug my K40 into the wall power without having a five pound CO2 fire extinguisher ready in case there is a fire. But then I value my family, myself, and my house, in that order. The cheapest CO2 fire extinguishers cost US$80 and up at the time of this writing. $100 is a better estimate. Replacing your K40 will cost $400, replacing the upgrades another $100-$300. Hospital stays for smoke inhalation get rapidly more expensive.

I have seen some university web sites where they post instructions and cautions to students that use their CNC laser labs in which they advise/require the students to keep a plastic spray bottle of water beside the laser while lasing. They are instructed to spray water on any “minor” flames if they appear. I have a horror of spraying water into a working electrical machine, but then maybe this would work OK. I have not tried it. You have been informed. And warned.

Real Pictures of Real Fires

Here are some saved pictures and comments in the discussion that may help you decide whether to get a fire extinguisher and watch every burn job like a hawk. Thinking about this and being prepared might save your K40, your house, or in extrema, your life.

The posts and photos that follow are graphic illustrations of what can happen with your K40 in only a few minutes. They show why you should

  1. Never, ever leave the K40 or any CNC laser running without you watching it 100% of the time.

  2. Do not allow wood or acrylic residue to build up on the bed as this could catch fire. Also do not allow small off cuts to build up under the bed which could then be fuel for a fire.

  3. Some people have reported good results on small flames with a plastic spray bottle of water kept beside the machine. This requires you to be waiting beside the machine, not having “stepped away from the machine for a moment”.

  4. Have a fire extinguisher close to hand. The right fire extinguisher is a CO2 extinguisher, not a home-improvement-store “ABC” powder type. Both will extinguish a fire, but once the fire is out, the CO2 simply vanishes from the machine into the air.

    People who have used powder fire extinguishers on their lasers say that the powder gets into everything, sticks to surfaces, is nearly impossible to remove, and may well be corrosive. The fire may not kill your laser, but a powder fire extinguisher may well make it impossible to salvage the machine. On the other hand, you get to keep your house and life. ACK! Use CO2.

  5. Be prepared to handle the situation when (not if) a fire starts inside the machine. Be ready to kill all the electricity to the machine with some kind of cutoff switch, even if you just pull the cord of the outlet strip that runs the machine out of the wall. Be sure you can reach the electrical cord to yank it even if the machine has an internal fire. Place your fire extinguisher nearby, but not so close that a flaming laser would keep you from grabbing the extinguisher. Have an exit strategy. Don’t let a flaming laser keep you from exiting the room if it gets out of hand.

You really, really can burn down your house by getting too comfortable with the machine running all by itself.

What follows are posts and pictures (used by permission) from people who have had laser fires, in order of increasing severity. Sure, K40s don’t catch fire very often. But as these folks will tell you, it only takes once.

23 Feb 2020 Anybody want a slightly used K40? One careful owner.
I NEVER walk away while it’s cutting…Until today ?:frowning:

Nobody hurt thank God. Massive flames from such a little thing and my God those covers smell when they’re burning! Lucky I had the extinguisher to hand. Was only cutting 3mm birch ply. Last 2 of the order as well. Typical

Another discussion participant posted in response:

I quickly nipped to the house for a pee all gone in 8 minutes

All in all around 16k in loss on this ******* lol

[this prompted the question]

how does that happen in 8 minutes 0o

[the reply?]

a fire in a small enclosed wooden space. We called the fire brigade it took them 8 minutes to get to us by that time it was all over.

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Why a CO2 extinguisher vs an ABC extinguisher? Is it for ease of cleanup/damage to components? CO2 extinguishers aren’t the best option for wood/paper. Is it a voltage issue?

Primarily for easy of clean up.

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That I can understand. I was a firefighter for the first ten years of my career. Those things make a right mess!

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And not just a pain to clean up; they are corrosive…


If were going for fire extinguishment and ease of cleanup, let’s go for Halon!

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Just… Don’t breathe!

Well, you won’t have much a choice either way if it goes off.

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Just saw it in the OM group today.

Do not ever leave any laser unattended, always have a CO2 fire extinguisher at hand.

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