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?

L/W

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.

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.

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!

L/W

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.

I’ve had some pretty hot stuff in my woodworking days, but never something that embered. Glad all was good.

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.