FIRE HAZARD with CBN wheels
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