Workshop safety #1: Workshop fire -- averted . . . and a "whuf" question

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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.

-- “Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.” Benjamin Franklin

  • Part 1: Workshop fire -- averted . . . and a "whuf" question

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I say dull bits. I’ve had embers before. You did the right thing by removing the dust from the shop and spraying it with water.

-- Losing fingers since 1969

Very similar thing happened to twice:
- first time when I was doing pattern routing with a dull bit. I was not connected o the dust collector.
The second time I was connected to the dust collector. most of the burning stuff were in my router box. I was concerned that some made it to the dust collector barrel. I took it out and emptied it on the drive way just to make sure.
I am sure I was rushing the cuts and talking too deep one.

-- Abbas, Castro Valley, CA

Jim, 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. (I had made four passes to route the 3/8" depth.)

Paul, It was a regular 3/4" straight bit. The only time I’ve used downcut bits was when cutting through-mortises so the sawdust can exit. What a horrible nightmare that must have been for your friend with the CNC! It’s a good thing he didn’t set the CNC up to route and then go to bed, thinking it would be all done in the morning. It would have been ALL DONE in the morning!


-- “Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.” Benjamin Franklin

You weren’t using a downdraft bit were you? They can pack and heat dust very dramatically on occasion. A friend almost set his really big CNC machine on fire recently that way. ….. Well let’s say he burned through the MDF vacuum base and started to melt the plastic sub-base. Scared the he** out of him though.

-- The early bird gets the worm but its the second mouse that gets the cheese.

I understand your concern but burns are fairly common and caused by a number of reasons: cutting too fast or slow,dull router bits,to heavy of a cut,type of wood being routed,or clogging of a groove or hitting metal,I’d say the biggest issue is vacuuming a newly router groove ,without that a fire is not likely unless you have flammable materials in close proximity.

-- woodworking classes, custom furniture maker

UPDATE: It was suggested that I could have hit metal. While I didn’t think that was very likely, I still had the sawdust on the floor and decided to filter through it. I found nothing with my fingers, but using my magnet, this is what I discovered:

There are two very tiny metal fragments at the 4 o’clock position. 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.

But I would still like to know what would happen if DC/vac were attached when such an event occurred?


-- “Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.” Benjamin Franklin

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