Thread: Sawdust extraction devices

I'm sure BobL and many of the other more knowledgeable guys will be along to advise you of proper dust extraction, but from what I know I can tell you that a "shop vac" / wet dry vac will not be enough for machines that produce fine dust in significant quantities. Such as tablesaws, sanders and so on. A wet/dry vaccuum is a "high pressure low volume machine" and mostly suited to portable power tools such as sanders, biscuit joiners etc.

A better recommendation would be one of the larger units called dust extractors. These are the units with 2 bags and a motor and impeller seperately mounted on a base. See link

https://www.carbatec.com.au/dust-con...ld-code-fm-300

These types of units are "low pressure high volume" which means that they are much more suited to collecting the majority of dust created by machines.

I believe BobL recommends a 2hp model such as th carbatec one at the least, and venting or positioning the unit outside is highly recommended. If you are doing so a 3hp unit would be much better.

Hope that helps for the moment

Originally Posted by donnyb

I'm retired now and gradually acquiring some woodworking equipment for guitar building and repair. Its a hobby not a business, and Id be lucky to make 2 guitars a year ! But when I do make dust, there's a lot of it. I have sawdust makers of a belt sander, router table, table saw, radial arm and a bandsaw. Shavings from a drill press.

I'm concerned about the sawdust problem in the shed, and in my lungs. I wear a mask, but Id rather reduce the problem at the source.

Good move, masks are something that should be used AFTER everything else possible has been done to reduce dust, or where it's not possible to use dust extraction.
I'd say the first thing you need to do is go read stuff in the Dust forum.

Currently I don a mask and air blow the dust outside. Not ideal, but I live on acreage so no neighbours nearby.

While it's good to be in that position blowing dust just doesn't work - it's like herding cats - read on.

Can I use an "ebay" wet/dry workshop vac unit connected directly to the dust port of the saws ? Changing the hose to other saws as I use them.

The first thing you need to know is that there's two types of wood dust.
1) The stuff you can see, which doesn't do much except be a nuisance, and will be the stuff you can see being "blown out of your shed" because it can be directed by stream of fluid (air) in the same way as you can wash visible dirt off a garden park with a hose

2) The wood dust that causes health issues is extremely fine and can be considered "invisible" , until so much of it settles out of the air it forms a fine visible layer over surfaces - this is the stuff that needs to be controlled.
You won't be blowing much of this our of your shed. This stuff moves along with air currents like a fart. If you try and blow it anywhere almost as much of it moves back into your shed as you move out
Think of how much total air you move out of your shed by blowing, the answer is zero, because as much moves back in again to equalise the pressure.

Once the invisible dust leaves the source Vacuum cleaners are near useless at controlling this dust, in fact some make as much dust as they collect because their motor cooling loop continuously minces coarse dust into finer duct. Vacuum cleaners only collect small quantities of air (<100 cubic ft per minute - CFM) so are effectively limited to collecting fine dust at source for processes that generate a point source of dust and there are very few machines that do that. As Deverel l says, for Table, radial arm and band saws, and belt sanders you need a high air volume air flow (i.e. many 100s of CFM). The sorts of flow rates recommended by folks that have done the research is ~1000CFM. Router tables can use a vacuum cleaner above a table but don't collect enough air under a table so even for these machines you need high volume dust extraction.

I don't recommend the generic 2HP DC in its stock form. The only way to really move the amount of air needed to control fine dust in workshop is to use 6" diameter ducting na dot open up all the posts on machinery to take advantage of the ducting. The generic 2HP uses 4" intakes and 5" flexy between the impeller and filter housing so this will become a serious bottle neck in the system. A 2HP needs some serious modification to make it move sufficient air. Even then the ducting (including to outside the shed) must be kept as short as possible so this solution is very limited. A bigger impeller and motor is needed to really utilise 6" ducting and ensure longer ducting runs with multiple junctions can be handled.

Note, I can locate the vac unit outside of the workshop , about 3 metres of vac hose from the saws, as I am guessing the filtration systems on these cheaper vacuum cleaners is not fantastic ? Recycling dust particles within the workshop, I mean.

Aside from the filters, all dust collectors leak, so moving the dust collector outside makes a lot of sense.

Finally, I have a niggling concern about dust explosion, not by static electricity, but by vac motor brushes sparks in the unit.

The chances of this are very low. There must be 100's of thousands of woodworkers using these setups world wide and there have not been any reports of explosions. There have been reports of fire but these were because something hot was sucked up by the dust extractor. If you are concerned then moving the dust collector outside reduces the damage from the possible eventuality.

Finally don't forget general ventilation - you don't really want to be using your dust extractor to ventilate your shed - this is better done using something like the fan from an eva air con or better still a squirrel cage fan which use less power and make less noise.

Many thanks to you both, and for taking so much time to provide a detailed answer BobL. Scary stuff this dust. I will need to read your post a few times, and look at the Dust forum.

BobL, Could you point me to a new item brand etc , that would be OK (longevity-wise) for my limited home use , but would still shift a decent amount of amount of air and the nasty dust, even if it's volume capacity is at the bottom end of the acceptable range ?

Originally Posted by donnyb

Many thanks to you both, and for taking so much time to provide a detailed answer BobL. Scary stuff this dust. I will need to read your post a few times, and look at the Dust forum.

BobL, Could you point me to a new item brand etc , that would be OK (longevity-wise) for my limited home use , but would still shift a decent amount of amount of air and the nasty dust, even if it's volume capacity is at the bottom end of the acceptable range ?

I won't point to a specific item for a variety of reasons but most readily available 3HP units will suffice for ducting runs up to about 6m including to the outside of the shed.
If you want to choose one or tow I can tell you the +/- of each one.
You will need to get rid of the usual 4" inlets these units come with and replace these with a 6" inlet.

Ducting via PVC stormwater pipe and fittings is the usual go.
Don't buy it from bunnings as it much more expensive there.
A specialised pumping supplies or PVC pipe and fittings store will be much cheaper.
If you buy a cheap DC be prepared to spend as much on the ducting.

If you can specify a more specific location than Australia then a more local member may be able to help with a local supplier.

Thank you again BobL.

What do you think of this one ? Only 2HP, Rexon circa 1997 that "works well" ?Dist collecotr- Rexon.jpg

I'd say you'll be much happier, healthier and cleaner if you upgrade your unit to use 150mm ducting everywhere as explained in the "Dust Extraction" sub forum.

Should be OK as a starter provided you are prepared to;
- modify the entrance to take a 6" duct
- use 6" ducting
- modify the Machine ports to take 6" ducting and
- and put the DC outside?

Thank you for your advice. I will make an offer on it .

Does anyone know a reasonably cheap way of measuring the airflow (and hence the CFM) in a dust extraction system. I bought a Jaycar Anenometer but didn't check the upper limit of the airflow range (it's around 6000 ft/min) and it goers off the scale when placed near the ducting.

Cheers
Dave

I've wondered the same thing. Just did a few quick calcs and if it was, say 6000ft/min, then it would equate to 1178 cfm in 6", 815 cfm in 5" and 513 cfm in 4" pipe. What size duct where you measuring? You would probably want to be able to measure near on twice that. They, unfortunately are not an accurate method of measuring air speed in ducting. I'm looking at rigging up a Hot wire airmass meter off a vehicle and doing some tests. I'm not after accurate figures, I'm only looking for variations with system mods.

Just had a look on the WEB. Most vane types, even some of the dearer units seem to stop at 5900ft/m. Just had a look at a $800 hot wire type and it peaks at 4950 ft/m

My shop Vac spec is 180 cfm and doing the calcs on a 2" hose at 6000 f/m, I get 130.8 cfm so 180 cfm would certainly max out your unit - 8256 f/m.

Duct is 6" and the anemometer fluctuated between 125 - 5900ft/min and was clearly beyond its limit and so none of the readings are reliable.
At this point the duct is less than 2m from the dust extractor with only one large radius 90 deg bend so I would expect it to be much closer to the 1700 cfm claimed by the manufacturer (that should give a speed of around 8700ft/min). Like you I am only chasing comparative readings so I can fine tune the duct work

Originally Posted by Dave Reed

Duct is 6" and the anemometer fluctuated between 125 - 5900ft/min and was clearly beyond its limit and so none of the readings are reliable.
At this point the duct is less than 2m from the dust extractor with only one large radius 90 deg bend so I would expect it to be much closer to the 1700 cfm claimed by the manufacturer (that should give a speed of around 8700ft/min). Like you I am only chasing comparative readings so I can fine tune the duct work

Dave - it sounds like you need to do a lot more reading in the dust forum.

1) Manufacturers DC flow data is highly dubious. Manufacturers claims are only for a naked impeller, i.e. no bags, entrance ports etc and done using a faulty measurement protocol. You should basically halve the manufacturers claim and you might be close.
2) A 6" duct simply cannot move more than about 1250 cfm using a normal DC - the limit is the pipe itself at the usual pressures of a DC.
3) before wasting further funds on instrumentation please read the third post in this Sticky at the top of the Dust forum.
Vane type anemometers are not suitable for small ducting and are not even accurate for comparative measurements.
That post also shows how to over come the speed problems but you cannot do it with a vane type anemometer.

It's all explained in that post.

Thanks Bob
After reading through your post on air speed measurement it all looks more than a bit daunting and far more complex than I naively assumed. Realistically, for most of us, it doesn't appear to be practicable to try and measure the airflow rather it would be a better option to just apply the principles of good duct work design (6" pipe, large radius curves, blast gates on unused branches etc etc) and assess the effectiveness of the system by the results at the machine
PS anyone want to buy a near new anemometer!!!!

Originally Posted by Dave Reed

Thanks Bob
After reading through your post on air speed measurement it all looks more than a bit daunting and far more complex than I naively assumed. Realistically, for most of us, it doesn't appear to be practicable to try and measure the airflow rather it would be a better option to just apply the principles of good duct work design (6" pipe, large radius curves, blast gates on unused branches etc etc) and assess the effectiveness of the system by the results at the machine
PS anyone want to buy a near new anemometer!!!!

Good call Dave.

On numerous occasions in these forums and elsewhere I've seen decisions made about changes to dust extraction based on dubious measurements.
I've even made a few myself and it has taken some time to understand the vagaries of what is involved.