Laser danger!

My primo is mounted on a piece of plywood, sitting on a wooden bench. It also has three layers of 3/4 and 1/2 inch MDF for the work surface/spoilboard. I have a 400x400 sculpfun honeycomb laser table that I set in the middle of the work area for laser projects. Last night, I set up a grayscale test on a piece of tile (Norton white tile method), and left the garage for a while. I have a camera, but hadn’t checked it in a while. Was on the computer in the house for half an hour or so.

When I came back out, I found the garage was full of smoke! After burning the grayscale test, the 10w laser had parked itself at its home point, over the wood spoilboard, at 100% power, and started burning its way through the wood, aided by the air assist. I’m very lucky that it did not actually catch fire, which it easily could have and probably would have eventually. As it is, it burned a hole an inch around and 1.5 inches deep.

So my question is: what are some things that could be done to help prevent this from happening again, or mitigate the risk if it does? Yes I know - don’t leave the laser unattended. But what else? Is there some sort of timeout code or something I could add? Something that could kill power when the program ends, or if the laser stays in one place for too long? Is there some material I could put at the origin point that could withstand the energy from the laser without becoming dangerous itself (like red hot)?

Thanks for any advice!


I have ceramic tiles that cover an area larger than the laser’s work area underneath the honeycomb table for just this purpose.

:rotating_light: :monkey: :rotating_light:
You already know… The other solution is to make absolutely sure that all gcode you send to your laser toting kill-bot ends with the appropriate command to turn off the laser. :smiley:

And I see that some sage advice is about to get dropped as well… :wink:



I read your post over on the LAHobbyGuy forum… and I think you said you’re running your laser with Marlin firmware on your MPCNC? I’m getting old and dull and sometimes have trouble explaining myself but here goes, off the top of my head…

I’m not sure what the state of Marlin laser functionality is currently but GRBL 1.1f and later features dynamic power adjustment with M4 gcodes… $32=1. What this does is to adjust laser power with the speed of laser head movement… to eliminate/minimize overburning at corners or line start/stop. Key with M4… no movement, no laser.

I take advantage of this often… pausing my job to go take a bathroom break, fix a sandwich, etc. Another feature of GRBL is that PAUSE takes effect almost immediately… and as soon as it stops the laser is turned off. RESUMING the job, it picks up immediately where it left off with no discernible effect on the burn. Also, at job end, when the laser head stops… so does the laser.

IIRC Marlin buffers commands differently and pausing a job usually results in a delay until the buffer empties? And I’m not sure whether it has gotten the dynamic power adjust feature yet.

I’m not trying to start a “holy war” here… but GRBL 1.1f and later have this “dynamic power” feature. If Marlin lacks it, I suspect you’ll always have the potential for future accidents.

I doubt I’ve explained this very well but please check this out… for you and your family’s safety.

– David


That is some awesome information, David! I did not know that about GRBL - one of my occasional frustrations with Marlin is that pause is just a suggestion, to be acted on when it feels like it - I know, technically when the buffer runs out, but depending on what it’s doing, that could cover a lot of time and distance.

So now I’m thinking I want to run with GRBL, at least for laser operations. Does anyone know if the SKR Pro 1.2 can run GRBL? If not, I can definitely make a completely separate control panel to use with GRBL - probably have most of what I need just laying around here…

My MPCNC is connected to the controller with DB25 and DB15 serial cables, so control is completely separate from kinematics.

David, do you - or anyone else - have recommendations for a GRBL setup? Board? Display? Can I build a pendant for it? I know lightburn works with it so I’m good there. Any tips would be appreciated!


Here’s a link to a recent conversation on this.


At the end of the day, I don’t trust any gcode (because I can’t really review it) and I don’t trust any firmware (this isn’t safety rated at all, and the boards they run on are also not safety rated). The parts can fail-closed (meaning they would leave the laser running). They can just freeze computation (which would also leave the laser stuck on). Or they gcode could just be wrong and it would be doing exactly what was in the file, and start a fire.

So I don’t trust my CNC as far as I can throw it. The fact that is usually works makes me trust it even less. If the one time it doesn’t, it destroys my garage, then I can’t leave it alone. The router is 600W+. The laser is obviously more dangerous looking, but that much energy can also turn your shop into a pile of ash.

Close call. I’m glad you’re safe.


When I first installed my laser, I had a similar problem. Once in a rare while, the laser failed to turn off. It only happened to me when the job was small, like a quick cut of a simple contour. I tried debugging it, and was never able to get a repeatable scenario. And it only happened rarely. My personal solution is sheet metal and an IOT relay. Some solutions:

  • Cover the working area of your machine with a piece of sheet metal or other non-combustible material. This has worked very well for me, and the metal will dissipate the laser, so even if it remains at high power in the same place, it will not burn the spoil board. I do a lot of cutting, and my sheet metal is black and covered in a tar-like substance. I’ve scraped it down twice. The spoil board underneath is unmarked.

  • Add a beep to the end of your g-code scripts. I do this automatically in g-code end script. It doesn’t help turning off the laser, but it signals me the job is done if I happen to be working on something elsewhere in the shop area.

  • Use an IOT relay. I plug both my laser and my router into an IOT relay, and use M106/M107 g-code inserted by the start and end script to power up/down my tools.

  • Use the enable pin to control either the power or the signal to your laser. This is just an idea, but not something I’ve tried. As part of the Marlin laser interface, there is an enable pin that is high when the laser is on, and low when the laser is off. By using a cheap relay module, you can control either the 12V power to your laser, or control the control signal to your laser.

Other than the sheet metal, these other solutions depend on Marlin not freezing or otherwise failing to execute g-code. All I can say is that the IOT relay worked for me. I surmised the root of the issue was that the laser pin was left floating, but that was just a guess, and I didn’t understand how that could happen.


Wow glad you are safe, you could probably get a sheet of aluminum polished and sandwich it in between your wood layers as a protection. Laser would probably burn through then hit reflecting aluminum layer and stop no ?
Also since it would be in a while you wouldn’t have laser reflecting all over as well.
As for movement indication I wonder if you could setup a arduino motion AND circuit. Like a small wheel that’s attached to the trucks and gives a rotational feedback that has to be seen in order to close relay to power laser. Even a optical mouse and wheel etc. Like how you can make spinners for arcades etc.
Or get real fancy and make a angled mirror section for your home zone. So if it burns through it reflects the laser to a spot that has a photocell to pickup on the laser burn through and shit down relay for laser power.
Idk if photocell will pickup laser tho and would be quite complicated. Anyways just some brain seeding thanks for sharing the danger


Cannot believe no one said it (unless I did not see it above) Walking away is not encouraged while this stuff is running. We are all making this and Most of us are not pros. Equipment needs monitored at all times.

You mean besides me, in the original post?

Yes I know - don’t leave the laser unattended.

I’ve been running a laser on my MPCNC for over 7 years and have gotten pretty lax with some safety precautions. I absolutely was careless and am lucky to have not had a worse outcome.

Thanks to some of the suggestions here, I hope to mitigate the consequences of being forgetful/lazy/foolish when it happens again.


Already had a fire start with laser when air assist tube had disconnected by itself, i was sat at my desk in my workshop with my primo behind me.
I never leave the room when a job is running but it is sometime still not enough, I saw the fire maybe few second after it start thanks to my enclosure top camera that i stream on my second display screen.
I do have a protective sheet of metal under my honey comb but it’s the stock and my 3D printed air assist nozzle that had started to burn…

Also had laser at 100% at the end of a job few times and each time i abort a job, for sure GRBL seems far more secure with laser.


With all the GRBl buzz going on, I might flash the rambo with GRBL - especially if GRBL is better with laser! When I pause/stop lightburn, it’s very common that the laser is ON. Depending on the material/background, I have to emergency stop the whole thing, since shutting off the laser from within lightburn/console is too slow. If GRBL eliminates this issue, it’s reason enough!

Yup, I had seen that a couple of times when pausing too, and had to e-stop. It’s always been over the laser table though, which has sheet metal under it. Never expected the laser to head back to the origin over the wood and sit there burning. Lesson learned!

That’s a new one, and definately not worth waiting for!

I see Ortur have included some form of flame detector into their products and the MKS DLC32 has an input (the probe input interface) that can be used as a flame sensor input ($37). I guess the sensor will be a simple light or infra red sensor that will detect in the wavelength range of 760nm-1100 nm like one of these.
Gotta get tinkering!

Last night my machine did it again! Right near the end of the burn, it abruptly moved to the origin and sat there burning. Luckily, I had placed a piece of ceramic tile there, just in case, so there was no damage. I wasn’t out of the room either, just working at the workbench nearby, so it wasn’t there for very long before I noticed.

Because of this, I’ve removed the spoilboard, and covered the whole area that the laser can reach with a sheet of metal. I will probably also leave a piece of tile at the origin as well, to absorb energy, just in case.


Exactly the same also append to me 3 times and 3rd time was during my last laser use.
I was able to see in console that bad commands were sent:

echo:busy: processing
echo:Unknown command: "�{26"
echo:Unknown command: "�"
echo:Unknown command: "�Y33.556"
echo:Unknown command: "GG1 X138.99 Y30.098"

Didn’t took time to dig in that issue but i would suspect on of this:

  • Wrong baudrate(maybe need to slow it down)
  • Heavy load on my laptop
  • Weak usb cable (related to baudrate to high ?)

That looks like two senders were trying to send on the same serial line.

If it was baudrate, I wouldn’t expect any of the chars to get through. But it could also be noise. The double G makes me think there is a dongle and a computer using the same Rx lines though.

The USB connection has built in error checking and acks (or so I have read, I haven’t actually read the spec). Errors in USB are most likely to be slower transmit rates, pauses, or disconnects as the USB can’t get the data through and has to retry. Once it reaches the USB chip, it doesn’t have error checking on the serial lines.