Workshop safety #1: Workshop fire – averted . . . and a “whuf” question

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Blog entry by lightweightladylefty posted 08-22-2016 05:37 PM 1569 reads 0 times favorited 11 comments Add to Favorites Watch
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In the process of making a jig, I routed a dado for T-track in recycled plywood. Since my trim router was already set up with a ¾” straight bit, I used it – making four shallow passes 9” long and a total depth of 3/8”.

After each pass I stopped and vacuumed the sawdust. After the final pass when I started to vacuum, I saw burning embers and immediately stopped vacuuming. (You can see the burn remaining in the dado in the photo above.) I knocked the sawdust out of the dado and onto the concrete floor, removed the filter from the shop vac, and dumped the contents of the vac on the compost pile. Then out of an abundance of caution, I sprayed the pile with the garden hose even though I saw no embers.

This started me thinking and I couldn’t get to sleep last night. As a child, I liked to play a game with friends. I called it “whuf.” “What if this happened? . . . What if that happened?”

Well, my adult “whuf” question is: What if I had my shop vac/DC connected to the router/project? Could I have started a fire in the vac/DC?

What caused the fire? We’ve all had burn marks from uneven movement – pausing too long or moving too slowly – especially when routing hardwoods like maple and cherry. But in all the miles of routing I’ve done, never has the sawdust caught fire.

Could it be the glue in the plywood? Or simply the build-up of sawdust in the dado overheating from the bit? Each pass was smooth and quick (and only 9” long) and I took plenty of time vacuuming up between passes. Does anyone have any ideas? I certainly don’t want this to happen again.

Thanks for any insight you can give me.

-- Jesus is the ONLY reason for ANY season.

11 comments so far

View Donna Menke's profile

Donna Menke

617 posts in 4717 days

#1 posted 08-22-2016 05:42 PM

Glad it was not worse. It seems unlikely- but a random staple could cause a spark.

-- "So much wood. . .so little time!"

View nomercadies's profile


590 posts in 2790 days

#2 posted 08-22-2016 05:47 PM

I don’t know, but I sure am going to tag along to see what people say. Good topic.

-- Chance Four "Not Just a Second Chance"

View DocSavage45's profile


8850 posts in 3294 days

#3 posted 08-22-2016 06:03 PM

Watching the Revenant Sat night and I was reminded about fires as that was necessary to life in wilderness. The principle for igniting tinder may be at work here? Dry wood. Cutter(bit) may be a little dull? Moving too slow in doing your cut?

This one is new to me. Glad you saw it.

-- Cau Haus Designs, Thomas J. Tieffenbacher

View ChuckV's profile


3226 posts in 3978 days

#4 posted 08-22-2016 06:15 PM

I wonder if the bit could be dull and/or in need of cleaning.

This is a good reminder that we should all have a working fire extinguisher handy.

-- “Big man, pig man, ha ha, charade you are.” ― R. Waters

View lightweightladylefty's profile


3367 posts in 4164 days

#5 posted 08-22-2016 07:30 PM

Donna, the staple scenario you suggested seemed to me, also, very unlikely, but I decided that since I hadn’t yet vacuumed up the sawdust that I dumped on the floor from the dado I would sift through it to see if there were any metal fragments. I found nothing with my fingers so decided to take my magnet. This is what I discovered:

There are two very tiny metal fragments at the 4 o’clock position. I don’t think your idea was so far-fetched after all. That may very well have been the culprit. The reason for the surprise is that there was nothing on the first three passes. Whatever it was must have been embedded in the manufacturing process.

I do keep my bits sharp and clean and move at a good, steady pace. I generally only have burn issues when running long pieces over the router table and making a slight hesitation when shifting position so this surprised me with such a short, shallow cut.

And, yes, I have a workshop fire extinguisher at the ready!


-- Jesus is the ONLY reason for ANY season.

View Donna Menke's profile

Donna Menke

617 posts in 4717 days

#6 posted 08-22-2016 08:05 PM

Well, I am happy you found the culprit. It was the ‘reclaimed’ plywood that alerted me to the possibility of a metal inclusion. Glad it was not worse.

-- "So much wood. . .so little time!"

View lightweightladylefty's profile


3367 posts in 4164 days

#7 posted 08-22-2016 11:16 PM

I’m still wanting my “whuf” question answered. What if my vac/DC had been attached? Would I have set the whole system on fire?


-- Jesus is the ONLY reason for ANY season.

View Mean_Dean's profile


6976 posts in 3599 days

#8 posted 08-23-2016 12:26 AM

I m still wanting my “whuf” question answered. What if my vac/DC had been attached? Would I have set the whole system on fire?


- lightweightladylefty

We’re never going to know for sure, but my guess is that there would have been a 50-50 chance of setting your vacuum bag on fire.

Reasons For: a hot ember can smolder for a few minutes—long enough to get sucked into the vacuum bag, sit there smoldering away, and eventually ignite a fire. Also, the force of air moving through the vacuum could fan the ember, increasing it’s likelihood of sparking a fire.

Reasons Against: a hot ember may not have stayed flammable long enough while traveling along its route to the vacuum bag to start a fire. Also, the force of the air through the system may have simply blown the spark out. Lastly, the ember may not have had enough heat energy to spark a fire to begin with.

All-in-all, I’d say you did the right thing by throwing the vacuum bag out and hosing it down. In this case, an ounce of prevention is worth a ton of cure!

I once vacuumed the ash out of my fireplace with a 5 gallon shopvac. The ash had been sitting there cold for 3-4 days since the last fire in the fireplace. So, I vacuumed it out, and put the shopvac back out in the garage. Several hours later, I went back out to the garage, and found the shopvac smoking! Apparently the ash was deep enough in the fireplace, that some hot embers were buried (and thus well insulated) by the ash, and were still hot enough to start a fire.

The point is: don’t ever take a chance with burning your house down! It’s a heck of a lot cheaper to throw the bag out—hell, the whole vacuum—then burn the house down!

-- Dean -- "Don't give up the ship -- fight her 'till she sinks!" Capt James Lawrence USN

View Donna Menke's profile

Donna Menke

617 posts in 4717 days

#9 posted 08-23-2016 12:27 AM

Exactly what Mean_Dean said.

-- "So much wood. . .so little time!"

View Nate Meadows's profile

Nate Meadows

1132 posts in 2658 days

#10 posted 08-23-2016 01:10 AM

Sister Lefty,

The metal is most likely the culprit. Did you inspect the router bit? Is it intact? Carbide can sometimes fracture when it hits things like that. This also would cause more friction thus more heat. There is more story locked away within the plywood to be sure. As for the fire question, whole articles have been written on the subject. It is possible that had the spark been sucked into an active collection system a fire would start, but it is equally possible that the O2 levels would be so low the spark would simply have burned out. The metal pieces were small enough heat would have dissapaited rather quickly. Other factors to consider are the air humidity, dust humidity within the collector and the type of dust within the collector. As it sits, I think it’s definitely His saving grace that saved your shop!

-- "With a little bit of faith, and some imagination, you can build anything!" Nate

View lightweightladylefty's profile


3367 posts in 4164 days

#11 posted 08-24-2016 02:57 AM

Thanks, everyone, for analyzing my problem and giving me a little peace of mind. Dean and Nate, I understand much better now with your explanations about the dynamics of the fire potential. I’m thinking that had each of the sawdust fibers entered the vacuum/DC individually as they were routed, the fire potential would have been greatly reduced compared with the situation I had where the sawdust sat in the dado for a few seconds and could begin to ignite from the metal sparks.

Feeling pretty certain it was those tiny metal fragments that ignited the sawdust greatly reduces my concern (not that I will be complacent). I nearly always use attached dust collection when routing at either of my router tables, but I don’t have a set-up for attaching dust collection when hand routing.


-- Jesus is the ONLY reason for ANY season.

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