FIRE RISK, CNC, Dust, Shop Vacs and Air Filters - a cautionary tale

It seems like this comes up every few weeks here and on every CNC forum, usually in relation to people saying they are comfortable walking away from a running CNC machine for a while - with or without cameras monitoring it. Don’t, it’s unsafe and the fire risk is real - I know.

I recently joined a local Makerspace that has a 4x8 Shopbot professional CNC. It has high rapid and cutting speeds and lots of power.

I was using it last weekend to cut a large project (3’x5’) and the controller or PC connection failed. We now believe this was due to a power brownout caused by someone running a shopvac nearby on the same circuit. This caused the CNC to crash and lose it’s home coordinates.

The crash had happened once before during the same session and it hadn’t lost home, so I (due to enthusiasm and lack of experience) incorrectly assumed the second time it was ok (N.B. ALWAYS check zero positions after controller failure before re-launching a job). I restarted the job. I was using a fairly beefy bit, a 3/4" cutter for making dog holes. You can probably guess what happened next…

The machine rapidly moved to the target position and before it registered with me that it was about 4" further than it should be on the Y axis, it plunged deeply in to full thickness 1" baltic birch at high speed in the wrong place. The area should have been substantially pre-cleared for the depth and speed the cutter was set to, but since it was in the wrong location it went badly.

As quickly as I could hit the emergency stop I still didn’t get it to stop before it made essentially the whole plunge and it burned the wood, nice and brown and threw a lot of sawdust. It all happened in a split second.

After emergency stopping it I withdrew the bit and inspected the wood. It was browned and now I had a hole where I didn’t want one, but otherwise seemed ok. I WAS WRONG.

I reset zero and continued with my job. About 40 minutes later I was vaccuming off the table. The dust shoe is not on the CNC machine so you have to manually clear the sawdust after the job - which I’m actually thankful for given what happened.

As I used the shopvac to remove the two inch thick layer of dust near where that incorrect plunge took place, the force of the vaccum uncovered some embers that were sitting on the table top. My heart rate spiked when a few black embers suddenly burnt bright red from the fresh oxygen being drawn over them as they shot up the vaccum hose and in to the portable shopvac I was using.

Time slowed down for a moment while my brain said “Did I just really see what I think I saw? Uh oh…” I immediately lifted the top off the shopvac and sure enough a portion of the top layer of the substantial sawdust was now black and lightly smoking, but more importantly the filter had a 4" hole burning through it. No flames but it had burned back a 4" circle with red smoldering edges on the filter and a lot of smoke was now present.

I took both pieces out the loading bay door that was thankfully open and about 20 feet away and got it all away from the building. I ran back to grab a fire extinguisher. I put them out fully and avoided a major incident but the building did have some smoke build up and there were about 60 people in at the time (there was an event on elsewhere in the building). We aired it out with fans and all was well, which we are all thankful for, as it is a giant tinderbox of potential fuel. Makers like combustible things it seems.

I also got two give gallon buckets of water and suspended all the remaning sawdust that was directly involved in water in a heavy garbage bag and we left it all outside to ensure it burnt out completely. Resist the urge to take anything back inside until you are 100% sure it is out.

A few learnings I wanted to share:

  1. DO NOT leave a cutting router unattended and even while attending be mindful of any sign of burning even if it seems mild or isolated - stop and investigate thoroughly. One tiny ember is enough.

I was wearing a respirator that is N95 rated so I actually couldn’t smell the smoke or burning smell. It makes visual inspection all the more important. And with so much sawdust generated from routing it is tricky. You also can’t see inside your shopvac but it’s worth checking it as a matter of course if you have any concerns. Better safe than sorry.

  1. Make sure you have a fire extinguisher at hand when running jobs. There was a nice unit nearby but it was stored under some mezzanine steps and someone had leaned a few sheets of 3/4" plywood up against the steps (presumably for a future job) and that hindered my access by probably at least 30 seconds while I moved things. That’s a long time in fire time. Make sure you keep your extinguisher accessible all the time - I recommend at eye height and unobstructed. And know how to use it, if you’ve never used one buy a cheap one and practice.

  2. If you’re in a commercial or shared space, make sure you have a fire alarm pull station near the nearest exit to the machine. I had an open bay door to exit through, thankfully, but the pull was not nearby. In hindsight I should have pulled the alarm first and dealt with everything else after but I was new to the space as not properly oriented. Alarm first then everything else.

If you’re in a residential environment, I highly recommend adding a smoke detector to your work area. And do not block your fire exit when working. It’s so easy to lean stock up “just for a minute” but that minute could be critical in an emergency.

  1. Shopvac filters burn fast. High speed air and paper and spark = very fast burn. The shopvac center plastic frame below the motor was quickly deformed and melted by the heat from the smoldering filter, within seconds, even though we hadn’t reach actual flame level. Thankfully we got it unplugged and outside before any damage was done to the eletrical motors and wires, which could have complicated things.

  2. The adjacent woodshop happened to have about 4-5 different people sanding during all this. Literally everyone in the area was sanding so there was A LOT of fine dust in the air. We were fortunate it was contained quickly as fine dust + spark = explosion potential. This is how mills explode and burn:

Dust is also very bad for your lungs. As others have pointed out MDF is full of nasty things and plywood isn’t great either. If you don’t have good dust extraction and an air cleaner in your work area, please change that.

Ridgid shop vacs are affordable and readily available at HomeDepot, as are many others. For affordable routine air cleaning of fine dust, check out Izzy Swan’s video here about using a simple 20"x20" cheap box fan and furnace filters. I’ve done this at home and it works amazingly well for the fine dust:

Overall, this was probably one of the scariest experiences I’ve had working on things, but I think it went fairly well all things considered. It has certainly upped my safety awareness which I already thought was pretty good. Even so, I’ll be improving things related to how to respond to fire risk at home and at the Makerspace. I hope this info helps keep you and your family safe.



Good post dust explosion is nothing to mess with.

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