CNC Fire!

[iclark]

This was posted by Chris Goris on the NC Woodworker forum. I am sharing it here with his permission. In discussing it with him, he said that he found out that it had also happened to someone else that he knew, but they never mentioned it to each other until now.

OK, now that I have everyones attention let me tell you what happened. I clamped a glued up table top to my CNC to flatten it. I routinely do this after glue up because its no handling of a large top to flatten after gluing. I use a 2" diameter shell type cutter (or a fly cutter) to accomplish this. I create a simple pocketing tool path slightly larger than the workpiece and take very shallow passes until Ive flattened the entire surface. typically, running the same program stepping it deeper into the material until Ive cleaned up the entire face. all the while, I have a dust boot over the cutter head with dust collection running. The top I was running was running was made of aspen. Aspen is considered a soft wood and tends to get stringy when machining it. The dust boot is just a sheetmetal box that encloses a cavity over the cutter that has brushes that crush to the mating material to create a vacuum seal. Tonight, on my second pass, unbeknownst to me, the stringy material was building up under the dust boot around the cutter starting getting tangled in the brushes. As I was watching it run, I suddenly noticed a spark fly up the DC hose (4" corrugated semi clear hose) , then before I could react, the hose was glowing orange for about 12" up inside!!!!. I slammed the E stop button the CNC and was scrambling for my DC remote control, which was in my pocket to shut it off. The dust boot has a hinged door you can open to inspect and I opened it before the DC had even begun to spin down and saw flames being fanned by the DC air so I pulled the hose off the boot and it went out, but now I was unsure as to how much burning material could have possibly made it to my DC in the next room. Scrambling over there, disconnecting the barrel and the saw dust bag from below the filter, I did have some very hot saw dust in the main bin, but nothing on fire or glowing thankfully. This all transpired within probably 30 seconds time. It happens fast and scares you after the fact when you can imagine the what ifs. I do know I need more than one fire extinguisher, it was way at the other end of the shop 70 plus feet away through a gauntlet of tools, lumber and projects to get to. Stay safe my friends!

Y'all be safe out there, ya hear?

[jteneyck]

This was posted by Chris Goris on the NC Woodworker forum. I am sharing it here with his permission. In discussing it with him, he said that he found out that it had also happened to someone else that he knew, but they never mentioned it to each other until now.




Y'all be safe out there, ya hear?

I've read other stories like that.  I got some smoke once from using a down cut bit to drill holes.  Won't do that again.  Leaving the machine running unattended definitely carries risks.  

John

[Mike L B]

I'm kind of curious what caused the spark.

[iublue]

Wow, that is scary!

I have never had a fire but not long after I purchased the home made CNC that I have I got a gig making parts out of MDF for a local company.  One day I had started it and went to run an errand.  As John stated, leaving it unattended does have it's risks and that applies to this time. 

The CNC had a cheap Performax router on it and evidently I did not get the bit tightened enough and it vibrated down some but not enough to make it quit spinning.  When I got back the bit was stuck in the MDF so bad it had stopped the router from moving so it was just sitting there spinning.

You can imagine how that built up the heat and the MDF was smoking and somewhat charred!  
  No fire yet but I had to wonder how much longer it would have been before ignition!

Scary indeed.

[paul2004]

Good story to share, I'm glad the guy did not get hurt or lose his house.
It's good to have a reminder like this, always watch the CNC.

I wonder if a bunch of the stringy wood shavings got tangled up in the bit and/or rubbed against the metal dust boot.

I have a spindle that is reversible. I once made the dumb mistake of running a bit in the wrong rotation, that burned up the wood pretty good, but I noticed it fast, no fire. Just another thing to check before we hit the "Start' button.

[paul2004]

Another thought, I have not used my CNC in awhile, so maybe I am not remembering this correctly.
I think if you use a Vetric software to make a pocket, the default is to do it in a spiral pattern , such as:
start at 0.0.. go in the positive X direction (size of board), go in the positive Y direction (size of board), go in the Negative X (back to about 0) , then go negative Y (back to about 0). Then it cuts a smaller "square" form the smaller perimeter.. this can cause piles of shavings to just get pushed around for the ride.

Maybe that's not the default, but it's an option.

It's better in his case to do a Raster X in or Raster Y only. Maybe the job takes a little longer, but the software will do something like this..
Start at 0,0, go in the positive X direction (move a 5-10 inches PAST the edge of the board, to allow sawdust to drop off) . Move a tiny bit (the stepover) in the Y direction, move the in the negative X direction.
This might take longer to cut, but it helps avoid crap building up under the dust boot.
It can be a little challenging to do this with clamping, you might have to divide the surfacing into a few smaller jobs.

[Admiral]

As my shop is about 18x40 with an "L" room 15x15, I have four equally spaced fire extinguishers down there. Thankfully I've never had to use them yet.

[EightFingers]

Another thought, I have not used my CNC in awhile, so maybe I am not remembering this correctly.
I think if you use a Vetric software to make a pocket, the default is to do it in a spiral pattern , such as:
start at 0.0.. go in the positive X direction (size of board), go in the positive Y direction (size of board), go in the Negative X (back to about 0) , then go negative Y  (back to about 0). Then it cuts a smaller "square" form the smaller perimeter.. this can cause piles of shavings to just get pushed around for  the ride.

Maybe that's not the default, but it's an option.

It's better in his case to do a Raster X in or Raster Y only. Maybe the job takes a little longer, but the software will do something like this..
Start at 0,0, go in the positive X direction (move a 5-10 inches PAST the edge of the board, to allow sawdust to drop off) . Move a tiny bit (the stepover) in the Y direction, move the in the negative X direction.
This might take longer to cut, but it helps avoid crap building up under  the dust boot.
It can be a little challenging to do this with clamping, you might have to divide  the surfacing into a few smaller jobs.

Yes, it does a spiral climb cut. At least it does on mine.

[Bøb]

I'm kind of curious what caused the spark.

FRICTION alone will do that ... look at all the times your cuts have been scorched ... a few degrees hotter, and fine shavings, you have fire.