Thread: Extracting/minimising metal dust in the workshop for better health

Originally Posted by D.W.

There is much less dust in the air from the belt sander (it's far slower than the contact wheel grinder - 2200 feet per second). It's not a great comparison, and I don't have dust laying around right now that would be as I don't have a hardened set of chisels to grind right now, but I will be doing that in the near future and get better comparables.

At slower speed, the belt sander does seem to make less fines, though...

Thanks for the swarf images, DW. It does seem that the shape characteristics of the metal swarf persists into the finer particle sizes and that contributes to its aerodynamic behaviour with longer settling times.

There is also evidence that grinding speed has a direct relationship to the fraction of fine particle sizes created off a given grit size abrasive. This is for grinding ceramics, but expect the results would be similar for grinding metal.

Fig 4.1d particle size off #320 wheel.jpg
Source: https://repository.lib.ncsu.edu/bits...16/865/etd.pdf

There is also evidence from the same source that grinding with higher speeds creates finer particles sizes across a range of abrasive grit sizes...

Figure 4.4 particle size vs surface speed for various grit number wheels.jpg

My belt sander runs at 6m/s, the 8" bench grinder at 2,800rpm and 5" angle grinder at 10,000rpm. Even without converting all those to m/s grinding surface speed it is clear which ones are going to produce the most fine particles regardless of abrasive grit size. Not surprising that you are getting the least fine dust off your belt sander, DW.

Originally Posted by D.W.

Dust hood on a common wheel grinder (any type that has a hose hookup) is such a wonderful thing to catch all of those fines, too.

Yes, but unfortunately they don't come wide enough to accommodate the wider CBN wheels. The CBN wheel suppliers could do a nice little bit of extra business by supplying inserts to widen the standard width hoods.

Originally Posted by BobL

I found a few metal dust posts in the MWF that may be of interest
Metal dust measurements1
Metal Dust Issues
Metal dust catcher

I had previously come across your measurement thread and that is what prompted me to further investigate and try to do something about the metal dust in my workshop, thanks Bob.

Originally Posted by NeilS

My belt sander runs at 6m/s, the 8" bench grinder at 2,800rpm and 5" angle grinder at 10,000rpm. Even without converting all those to m/s grinding surface speed it is clear which ones are going to produce the most fine particles regardless of abrasive grit size. Not surprising that you are getting the least fine dust off your belt sander, DW.

6m/s (~1200 fpm) is quite slow for steel and wood on a belt sander.
It's probably at the low end for even something like acrylics.
Such a slow speed will result in a slow rate of material removal which then means users may end up pushing harder which results in abrading the belt.
My 150 mm belt sander run between 2500 and 3600 FPM while the 50mm belt sander runs at 3500 - 5000 FPM.
These ranges are good for both wood and hardened steel although HSS is better on the 50 mm sander.
8" wheels @ 2800 rpm are about 5600 FPM which is a good speed for HSS.

I haven't done many measurements on grinding wheels with covers/hoods but the few I have undertaken showed they were not very effective at fine dust control.
The 10" grinder with fully covered in wheels with 2" dust ports was the most effective probably because the 2" port meant more air could be drawn through it than the smaller ports on smaller grinders.

Originally Posted by NeilS

Yes, but unfortunately they don't come wide enough to accommodate the wider CBN wheels. The CBN wheel suppliers could do a nice little bit of extra business by supplying inserts to widen the standard width hoods.

I agree on the sentiment. I wish the wheel makers would just make a narrower wheel, too. A 3/4" CBN wheel would be fine for many things (though I get why turners like to freehand on a wider wheel that's got both a wideflat area and radiused edges).

When I run my larger grinder with dust hoods and do heavy grinding, especially with one side that uses a cutting wheel (the smell off of that is vile - like a combination of fire and electricity), I run through a dust deputy cyclone and it does a great job of getting the particles out of the air (I have two high intensity overhead LEDs where do a great job of showing any kind of significant particle or smoke presence - they're not a precise way of seeing things, but they do indicate when there's way too much in the air)....but the vac and cyclone don't get rid of all of the smell - just half of it, perhaps. I still give that whole setup the box fan and door treatment (I have no practical way in my garage to put in a hood or extraction system, but wish I did for heat treatment and quenching, paint/lacquer, and grinding...). Grinding is tolerable in the winter here, even though it's often below freezing - but the ability to spray lacquer is a no go as it crazes in cold temperatures. quench and temper, I put everything near the door and just behind the fan, or if doing something quick, literally use a battery leaf blower and turn it on and place it above the quench toward the door - it's incredible visually how precisely it can shoot the quench smoke out in a straight line and not have the wife notice any odor in the house at all - which is something that is loudly protested (quench oil seems like a thin paraffin oil and it's not that stinky, but it's stinky enough and it smokes like crazy).

Originally Posted by D.W.

I . . . . . . . not have the wife notice any odor in the house at all - which is something that is loudly protested (quench oil seems like a thin paraffin oil and it's not that stinky, but it's stinky enough and it smokes like crazy).

Had to laugh at that. SWMBO has the nasal sensitivity of a dog. If I fart anywhere inside the house she notices and often comments. She often comes down the back to the shed and sticks her head inside the door and says, "Something Burning?" or "Watcha doing now?"

I took some readings today. As I couldn't grind and snap the counter at the same time I set my little old camera to record as a video thinking I might manually transcribe the numbers as a time sequence (like every 5 or 10 secs) into a spreadsheet then graph.

Here is a sample of a screen grab from video... yeah, an awful out-of-focus image, but that was the best I could do with my little old camera vibrating away on its makeshift holder.

Screen shot from video.jpg

Bob, which numbers would you extract to be of most value?

I took videos of four test grinds, two of about 4mins after grind and two for 15mins after grind, plus one as a baseline without any workshop activity. Had to abandon outside baseline reading because of wood fire smoke from neighbours again.

At the 3min mark the test grind levels were mostly looking like this...

Screen shot from video - 3min mark.jpg

I'd use PM2.5 concentrations.

Why?
Those detectors don't measure PM10 correctly while they do fair job of PM2.5
Recommended PM2.5 values are more common.

Remember below about 10 µg/m^3 the numbers on those detectors are not very reliable,

If the dust is all metal (steel) which appears to be the case as they drop so quickly . then the concentrations need to be multiplied by 7.8 !

for PM2.5 37 x 7.8 = 289 µg/m^3 which is getting up there but its only for 3 minutes so not really much to worry about.

assuming the PM10 is correct for PM10 its ~480 µg/m^3

By way of comparison
Screen Shot 2021-08-29 at 4.12.22 pm.png


Most of the metal dust will be iron so you're about 1/10th of the way to the ACGIH TLV (American Conference of Governmental Industrial Hygienists Threshold Limit Values)

Originally Posted by BobL

I'd use PM2.5 concentrations.

Why?
Those detectors don't measure PM10 correctly while they do fair job of PM2.5
Recommended PM2.5 values are more common.

Remember below about 10 µg/m^3 the numbers on those detectors are not very reliable,

If the dust is all metal (steel) which appears to be the case as they drop so quickly . then the concentrations need to be multiplied by 7.8 !

for PM2.5 37 x 7.8 = 289 µg/m^3 which is getting up there but its only for 3 minutes so not really much to worry about.

assuming the PM10 is correct for PM10 its ~480 µg/m^3

Thanks Bob, that's very helpful... and reassuring!

I had been looking at the National Institute for Occupational Safety and Health (NIOSH) recommended exposure levels, but I'll also have a look at ACGIH TLV when I've processed my data (adjusted up by the factor in your above advice), and then got my head around what I am actually getting and exposing myself to in the workshop.

This discussion has motivated me to finally get a particle counter. Something I never did working wood as I just more or less eliminated power tools instead, and do very little sanding.

I'm not so much worried about absolute levels (they'll be what they'll be), but rather how effective low-tech methods may be at getting the counter to read multiples lower for one method vs. another.

Following are the PM2.5 readings (upscaled by a factor of 7.8 following advice from Bob) from grinding the same gouge four times, plus a buff on a chrome dressed rag wheel.

The gouge was a Thompson V10 bowl gouge being re-ground on a #360 CBN 8” wheel at 2,800rpm. The duration of each grind was about 30secs. The PM counter was located at head height behind the grinder.

Readings outside the workshop showed highly variable air quality from intermittent woodfire smoke that was likely to skew the readings inside the workshop, so I ran the cyclone inside the workshop before each test run to ‘scrub’ the air of any infiltrated smoke particles. A dummy run without any grinding gave me low and stable particle counts for the intended duration of the test grinds, which gave me confidence that the readings would only be coming from the metal dust. I had also given the workshop a thorough clean before all of this to avoid any disturbed dust becoming airborne again and skewing the results.

Here is a graph of the four grinds.

PM2.5 readings following four gouge grinds.png

The four test run grinds produced very similar results with the PM2.5μm/m³ dropping quickly away to about zero at about the 3 minute mark.

See Bob’s evaluation of the risks in his post above. Add Cobalt to that Recommended Exposure Limit (REL) list, which has a REL of 0.05mg/m³ (that is an average of 50μm/m³ over an 8hr shift).

I understand (well, I think I understand) that burning Chrome, as in the hot sparks that come off the grinder, converts some of the Chrome into Chrome (VI) in the same way that this happens when welding SS (https://www.jespear.com/articles/weldingchrom-1.pdf). If so, the recommended exposure level for that is much lower at 0.0002 mg/m³ (8hr-TWA) or 0.2μm/m³, but is also likely to be the smallest fraction of the swarf/fine particles coming off when grinding the HSS tool. It’s a known carcinogen, so one to watch out for. Wearing a respirator/mask is an extra bit of insurance there with that one. Otherwise, the levels of metal dust I’m getting in the workshop from frequently grinding HSS don’t look like they are an issue for me.

There was no difference with the PM2.5μm/m³ count for grinding with or without my magnetic metal dust collector, but the magnetic tray collector captured a lot more of the larger PM10μm/m³... so at least useful for keeping the visible metal swarf from spreading everywhere.

I also did a test run with a 8” chrome buffing wheel. Who knows what the composition of the particles coming off that might be: the chrome compound (Cr₂O₃) that I put on the buffing wheel is about ⅔ chrome and ⅓ wax, and the cloth wheel, although well run in, would still be letting go of some fibres. The readings went nowhere near as high as the metal swarf off the grinder, but remained airborne for at least 15mins and possibly could have remained airborne for much longer than I had time to measure.

PM2.5 readings following four gouge grinds & Chrome buff.png

Anyway, the Cr₂O₃ fraction of what comes off the buffer is the component of greatest risk and that has a recommended exposure level of 0.5mg/m³ (ie 500μm/m³ averaged over an 8hr shift), so I don’t expect to get anywhere near that going by the above reading and, unlike my very regular grinding of HSS woodturning tools, I don’t do that so often.

Originally Posted by NeilS

Here is a graph of the four grinds.

PM2.5 readings following four gouge grinds.png

The four test run grinds produced very similar results with the PM2.5μm/m³ dropping quickly away to about zero at about the 3 minute mark.

This looks very similar to what I've seen so based on particles size alone I'd say you have nothing to worry about.

I understand (well, I think I understand) that burning Chrome, as in the hot sparks that come off the grinder, converts some of the Chrome into Chrome (VI) in the same way that this happens when welding SS (https://www.jespear.com/articles/weldingchrom-1.pdf). If so, the recommended exposure level for that is much lower at 0.0002 mg/m³ (8hr-TWA) or 0.2μm/m³, but is also likely to be the smallest fraction of the swarf/fine particles coming off when grinding the HSS tool. It’s a known carcinogen, so one to watch out for. Wearing a respirator/mask is an extra bit of insurance there with that one. Otherwise, the levels of metal dust I’m getting in the workshop from frequently grinding HSS don’t look like they are an issue for me.

Welding is a much higher temperature process than grinding so the chances of making Cr(iV) while grinding are very low especially when combined with the fact that Cr is nominally only about 3.5% of HSS . This is quite different to say welding or plasma cutting SS which contains ~12 - 30% Cr and a plasma (gas) is formed during the process. Grinding produces dust and the sparks coming off are not burned (combined with Oxygen) metal, they're just hot, If they were burnt the iron dust would be red or black (iron oxides) - they will eventually turn red/black by absorbing oxygen and water from the the air.

I also did a test run with a 8” chrome buffing wheel. Who knows what the composition of the particles coming off that might be: the chrome compound (Cr₂O₃) that I put on the buffing wheel is about ⅔ chrome and ⅓ wax, and the cloth wheel, although well run in, would still be letting go of some fibres. The readings went nowhere near as high as the metal swarf off the grinder,

but remained airborne for at least 15mins and possibly could have remained airborne for much longer than I had time to measure.

I'd say that's almost all fluff from the buffing wheel.

Anyway, the Cr₂O₃ fraction of what comes off the buffer is the component of greatest risk and that has a recommended exposure level of 0.5mg/m³ (ie 500μm/m³ averaged over an 8hr shift), so I don’t expect to get anywhere near that going by the above reading and, unlike my very regular grinding of HSS woodturning tools, I don’t do that so often.

CR203 contains the Cr+3 ion which is a very different kettle of fish from the carcinogenic Cr+6. The dust of Cr203 is classified as a nuisance dust, is poorly absorbed by the human body and can cause some allergies and gastro effects. If the Buffing compound contained Cr+6 you would not be able to buy it.

Originally Posted by BobL

Welding is a much higher temperature process than grinding so the chances of making Cr(iV) while grinding are very low especially when combined with the fact that Cr is nominally only about 3.5% of HSS . This is quite different to say welding or plasma cutting SS which contains ~12 - 30% Cr and a plasma (gas) is formed during the process.

Good oh, Bob, thanks for that.

I'll take your reassurance on the small amounts of Cr(VI) that we are likely to generate from grinding our HSS tools in our backyard sheds/workshops.

I had picked up on Cr(VI) being an issue from the industry compliance guidelines in the US...

This industry includes companies that produce cast metal products from steel and other metals and metal alloys. The primary source of Cr(VI) exposures... is the Cr(VI) fumes generated when chromium is melted as part of the pour or when grinding or welding the finished part.

...

Grinder operators are exposed to Cr(VI) containing fumes and dust that is generated in the grinding process that escapes capture by the ventilation system.

Pg.43, Small Entity Compliance Guide for Hexavalent Chromium Standards
https://www.osha.gov/sites/default/f...ntity_comp.pdf


But, hadn't processed that all the way through, as you have done, to get that in perspective for us non-occupational tool grinders in our backyard sheds.

Thanks, again.

The sort of grinding they're talking about is heavy duty industrial type grinding - something like cleaning up castings, or surface grinding blocks like flat grinding HSS stock which is often done together with coolant. I note in the article you referenced that they are specifically referring to grinding at Iron and steel foundries.

I was visiting my mates steel fabrication business a couple of weeks back and I noticed his large 10 x 3m bed plasmacutter cuts directly into a 250mm deep water bath under the bed.

The other perhaps more significant Cr exposure to be concerned about for DIY is likely to be from grinding/welding/cutting steel coated with a Cr(IV) coatings like a dichromate.

I'm not saying to take any of this lightly, its good to be aware of these things and employe any mitigation processes even if its just to wear a mask while grinding and for a short period thereafter.