FIRE HAZARD with CBN wheels

[John K Jordan]

New to me.

I keep some strong magnets under the front and behind my CBN wheels to try to contain some of the fine steel dust until I can hook the grinders up to the dust collector. Today some steel dust ignited and started burning and glowing red hot. It was "smouldering", the heat moving through the steel fluff. Unnoticed it might have caused a shop fire.

I have one powerful magnet just to the side of the front of a 600 grit wheel on a bench grinder that has collected a bunch of fine powder since the last time I cleaned it, probably 3/4" of steel fuzz.

This morning I was sharpening some scrapers to get them ready for a demo next week. As usual, I had dribbled some Trend diamond honing liquid on the wheel before sharpening. The sharpening was not aggressive and I don't remember seeing sparks while sharpening.

I suddenly noticed smoke from the magnet and some spots of orange-red glow deep inside the glob of powdered steel. My first thought was to grab it and carry it outside - a mistake since the increased air flow from walking made it burn faster and hotter. So I carried it back and dunked the magnet in some cooling water to put it out.

Now I'm sure every teenage science nerd knows how easy it is to ignite fine steel wool. In fact, sprinkle a bit of fine aluminum or silver powder into the steel wool first and you will have thermite, capable of welding railroad rails or at least dropping gobs of molten steel on the floor. But I never imagined the minimal sparks from a 600 grit CBN wheel setting the steel dust on fire!

I had been sharpening for 1/2 hour or so so I have no idea if the dust was just then ignited or if it had been smouldering for a while.

In my case I have all the grinders on a stainless steel table which may have minimized the damage if I hadn't noticed. I suspect the heat could burn into and possibly ignite a wooden table.

I think the problem at my grinder was actually the use of the magnet. The collected dust formed a big fuzz ball with longer tendrils of incredibly fine steel dust from the 600 grit wheel, most spread out so with lots of surface area in the air. This fuzz ball was in the perfect position to catch a spark. On a coarser wheel a some distance away I had no magnet so some steel dust collected on the table under the wheel - it was subject to lots of sparks but probably didn't ignite since the dust wasn't spread out in the air.

Action items for me:
- inform others (check)
- inspect the area carefully after each sharpening session (check)
- keep the steel dust from accumulating cleaning up every day (check, new policy)
- experiment with the Trend diamond honing fluid and steel dust/wool and see if it if flammable and if it might contribute to the problem
- experiment with igniting steel powder stuck to a magnet to see if it might smoulder unnoticed for an extended time
- devise an effective spark arrestor for the dust collector and use with the grinders

My plan is to position a dust collector pickup nozzle behind the wheel. I've been reading up on spark arrestors, commonly required in off-road motorcycle mufflers and in chainsaws in some areas. Most consist of a simple metal screen. I'm imagining the strong airflow from the cyclone may pull sparks through a simple screen so I want to experiment with multiple screens and perhaps a baffle and/or a change of direction of the airflow. I think I can set up a test stand with clear plastic tubes and watch the sparks in the dark and see what it takes to stop them. One promising thing is a glowing steel particle from a grinding wheel doesn't travel very far before it burns out. It may be that a vertical run with screens and baffles would work. I've also read about a centrifugal arrestor.

Researching industrial requirements and solutions is on my list. I'm not all that interested in burning the shop to the ground.

Comments?

JKJ

[Peter Blair]

Thanks for bring this to our attention John. I will be certain to keep my space clean. I seldom get any buildup but non the less worth keeping an eye on.

[Wayne Jolly]

I am way out of my element here, but you mention using honing oil. Is it possible that this could cause spontaneous combustion in all the filings stuck on your magnets to get things started??

Wayne

[Richard Dooling]

Thanks for the heads up John. I also use magnets to catch at l east some metal dust. The minimal sparking off the CBN wheel has made me less aware of this potential hazard.

Time for some better housekeeping.

[Alex Zeller]

With a CBN you don't need to use the metal guards like you would with a stone wheel, right? If so could you put a tray like a Tormek would have but doesn't touch the wheel with water in it with the magnet on the bottom? That way the metal dust would end up in the water. It'll rust but that shouldn't matter.

[John K Jordan]

I am way out of my element here, but you mention using honing oil. Is it possible that this could cause spontaneous combustion in all the filings stuck on your magnets to get things started??

Wayne

I don't think so. The fluid (not oil) was applied sparingly to the wheel with not enough to drip on the magnet. That's one of the things I'll carefully check when I get a free day (probably in November!)

JKJ

[John K Jordan]

With a CBN you don't need to use the metal guards like you would with a stone wheel, right? If so could you put a tray like a Tormek would have but doesn't touch the wheel with water in it with the magnet on the bottom? That way the metal dust would end up in the water. It'll rust but that shouldn't matter.

No guards needed, in fact the stock guards are too narrow to even fit over any of the CBN wheels I have.

Some people use a water bath with CBN wheels on the Tormek which turns very slowly, with the bottom of the wheel immersed in water. But the Tormek doesn't make sparks.

A magnet in water under the front of the wheel may work. In the long run, however, the steel dust really needs to be captured, just like fine sanding dust.

The strong magnet I'm using with the 600 grit wheel captures only a small amount of the total. The fine steel dust really goes everywhere so I suspect a water tray below wouldn't catch much but it might prevent the potential fire problem at that particular magnet. In the long run, however, the steel dust needs to be picked up since it goes everywhere - all over the table and on the floor. It even floats around the air like fine talcum powder - I've found it covering some magnets on the back side of my bandsaw at least 10' from the grinders. Reed Grey said he found black spots on the sheet rock after a demo - it was at every sheet rock screw. The fine stuff in the air probably isn't good for the lungs.

JKJ

[Glenn C Roberts]

Yep. Had it happen three times - that I've noticed. Not sure if it is because of the cbn or not. I have the 1hp Rikon with 1 and 1 1/2" cbn wheels, both with the guards installed. Yes, I had to modify one guard to accept the 1 1/2' wheel. My intent is to vent it outside using a separate fireproof venting system than the wood dust. No magnets and no honing oil. Still burned. There must have been a few shavings in the guard because I could smell smoke, even though I always where a face mask with the best filters I can find. Since then I always check the grinder before leaving the shop.

[Bill Boehme]

..... My plan is to position a dust collector pickup nozzle behind the wheel. I've been reading up on spark arrestors, commonly required in off-road motorcycle mufflers and in chainsaws in some areas. Most consist of a simple metal screen. I'm imagining the strong airflow from the cyclone may pull sparks through a simple screen so I want to experiment with multiple screens and perhaps a baffle and/or a change of direction of the airflow. I think I can set up a test stand with clear plastic tubes and watch the sparks in the dark and see what it takes to stop them. One promising thing is a glowing steel particle from a grinding wheel doesn't travel very far before it burns out. It may be that a vertical run with screens and baffles would work. I've also read about a centrifugal arrestor.

Researching industrial requirements and solutions is on my list. I'm not all that interested in burning the shop to the ground.

Comments?

JKJ

As an aerospace engineer, my initial thought was to install a metal plenum chamber in the vacuum line close to the pick up nozzle. The purpose is to create a section where the air velocity is very low and flow goes from laminar to turbulent. If you could install a screen baffle in the plenum that would be even better. The plenum wouldn't necessarily capture most of the dust, but it would cause it to cool down to where it isn't creating a fire hazard.

One possibility for a plenum is a small galvanized trash can.

[Van Huskey]

The good thing is the issue is not remotely unique and has plenty of commercial answers such as: https://www.baileigh.com/index.php/m...SABEgKx9vD_BwE

The even better thing is metal work is at least as popular as woodworking and so there are hundreds of DIY solutions that have been built for collecting the metal dust/sparks from grinding metal. Just pop over to youtube and sort through the huge number of builds for metal dust collection. There is zero need to try to reinvent the wheel.

[John K Jordan]

Yep. Had it happen three times - that I've noticed. Not sure if it is because of the cbn or not.

I think it's the metal dust only, nothing to do with the type of wheel unless CBN tends to make finer dust than conventional wheels. In my case it was due to the dust accumulating on the magnet. I wanted to post it here because I've read where others were also using magnets to help collect the dust. And woodturners tend to sharpen a lot more than some home shop woodworkers.

[Dave Fritz]

When i vacuum ashes from our wood stove I use a vacuum with a metal canister even though I'm sure the ashes are cold. I wonder about vacuuming steel shavings into a shop vac or other plastic container. Good things to think about, thanks for sharing.

[Michael Schneider]

I don't think so. The fluid (not oil) was applied sparingly to the wheel with not enough to drip on the magnet. That's one of the things I'll carefully check when I get a free day (probably in November!)

JKJ

John,

Could you put the magnet in a small trough of water? It would get rusty but would dissipate the heat.

Michael

[John K Jordan]

John,

Could you put the magnet in a small trough of water? It would get rusty but would dissipate the heat.

Michael

Alex Zeller suggested that too. Might be a good idea. I think it would be better to suck up the dust as it's being generated since the magnets only caught a little anyway.

[John K Jordan]

The good thing is the issue is not remotely unique and has plenty of commercial answers such as: https://www.baileigh.com/index.php/m...SABEgKx9vD_BwE

The even better thing is metal work is at least as popular as woodworking and so there are hundreds of DIY solutions that have been built for collecting the metal dust/sparks from grinding metal. Just pop over to youtube and sort through the huge number of builds for metal dust collection. There is zero need to try to reinvent the wheel.

The king of google research, beat me to it! Thanks.