Thread: Bandsaw ..Improved Dust Extraction

Hi Paddy, well done with that effort I might just nick that design and adapt it to my old BS.
Cheers
Rumnut

Just checking you realise that
a) a vacuum cleaner simply cannot generate sufficient airflow (CFM) to capture invisible dust from a machine like a bandsaw
b) It takes 150 mm ducting or 2+ 100mm ducts to draw enough air to capture invisible dust from a machine like a bandsaw
b) the small amount of fine invisible dust the vacuum cleaner does capture goes straight through the vacuum cleaner and into the shed

If you are not worried by invisible dust then no problem.

Originally Posted by BobL

b) the small amount of fine invisible dust the vacuum cleaner does capture goes straight through the vacuum cleaner and into the shed

If you are not worried by invisible dust then no problem.

Most vacuum cleaners today have a HEPA filter which has a minimum efficacy (by law in the USA) of 99.97% to 0.3 micron

Originally Posted by elanjacobs

Most vacuum cleaners today have a HEPA filter which has a minimum efficacy (by law in the USA) of 99.97% to 0.3 micron

Not so, most (especially cheap) vacuum cleaners don't use HEPA filters and some of those that say they do - simply do not. Some that run HEPAs are poorly made and leak like sieves. If a decent HEPA based vacuum cleaner is used in a workshop and does not have something like a major dust trap between it and the machine it will clog up very quickly and be rendered useless.

Whether a HEPA filter is used or not is largely irrelevant. No vacuum cleaner sucks enough air to capture fine dust at source when used with machinery. Anyone that thinks they have solved the fine dust problem from WW machinery with a vacuum cleaner does not understand the problem.

Hey again Paddy,
I had a look at my old carbatec B/S today & it has a cast body and no room at all to fit in your dust catcher,
However it has stirred me into finding an alternative way of doing it.
Cheers
Rumnut

You know, I was wondering why you need those really high air volumes (the 6" pipe, 5HP industrial blower business) for just a little tablesaw or whatever, when it is pretty clear that a much lower spec system can suck the chips up. And if the heavy chips are getting sucked up, then surely the lightweight dust is also getting sucked up too?

Also, how come Festool and so on are just using essentially a vacuum cleaner (with the expensive filter bags) and everuone says they're so great, if you need a 6" pipe on everything?

So it is pretty easy to go with your common sense and say, well the 6" pipe, 5HP industrial blower crowd seem to be living in a parallel universe. If I'm sucking the chips, I'm sucking the dust. Stands to reason, right?

There was a thread earlier this year over on Sawmill Creek,
DC chip vs fine dust collection
which I think nails it. The first post asks the question, the third post gives a satisfying answer as to why you need the 6" pipes. Basically the heavy chips shoot off directionally with high force and you can suck them up. Damn, you can just hold a bucket down at the shute there at the back of the cabinet and collect them. But the finer the dust the more floaty it is and the more it tends not to shoot off, but just waft off into the workshop air. Since pretty well everyone agrees that fine dust really is a health concern (I've been looking into the industrial ventilation and industrial air quality textbooks, not just listening to Bill Pentz), the concern is to capture the really fine invisible floaty dust as close to the source as you can.

A couple of other points help to get the picture (I know, because I'm just getting the picture now). First is a couple of diagrams showing the air velocity at the end of a suction pipe, in particular a machine hood. The black and white textbook page is from a US Government EPA manual, you can read yourself, but the essential point is this "At approximately one-hood diameter away from the hood entrance the gas velocities [i.e. air velocities] are often less that 10% of the velocity at the hood entrance". The table at the bottom of the page has been reproduced from the ACGIH Industrial Ventilation manual, which is the industry standard text. It indicates the air velocities required for particle capture in various industrial processes under various conditions. Take 400 fpm as about the requirement for a woodwork machine.

The colour diagram is from a NASA occupational health and training powerpoint, showing an example where the velocity at the machine hood needs to be 400fpm to pull dust. But again, the fall-off in velocity is so rapid that at one diameter distance from the end of the hood, the velocity is only 10% of the velocity at the hood orifice. So to achieve 400 fpm velocity at one diameter away from the hood, you need 4000 fpm in the pipe itself. To get that kind of velocity, realistically you need 6" pipe. So that's the big deal about 6" pipe, and why a vacuum cleaner won't work for pulling the dust into a so-called "exterior hood" that sits out a little way from the cutting action.

Just to see what what means in a concrete situation, I found one of the diagrams from the very expensive ACGIH ventilation manual reproduced in a WHO publication. It shows the ACGIH recommendation for a tablesaw hood. Attached. Note the velocities are all just what was discussed above. It is internationally, pan-industrially, consistent advice. Its not just bl--dy Bill Pentz.

So what about the Festool type, with the vacuum cleaner? It turns out that this is called LVHV exhaust ventilation, and I'll just quote:
"The low-volume/high-velocity capture system ... uses an extractor hood designed as an integral part of the tool or positioned very close to the operating point of the cutting tool. The hood is empirically designed to provide high capture velocities, frequently greater than 10,000 fpm (50 m/s), at the contaminant release point. This velocity may frequently be achieved at airflows less than 50 cfm (0.02m3/s) by employing a small hood face area." (Burgess et al 2004 Ventilation for Control of the Work Environment, 2nd ed., p.192)

I had a diagram of this, on one of the zillion browser tabs I've got open, but I can't find it now. But to describe it, basically the intake is absolutely jammed right up against the cutting surface, built right into the guts of tool, with very small air ducts. You couldn't replicate it with a conventional vacuum cleaner and hose fittings.

P.S. in industrial ventilation and contaminant engineering, you talk about 'gas' not air (sounds pretty cool already, right?) and talk about 'exhaust' when a mere consumer would talk about an 'intake', because the hood is at the 'exhaust' of the industrial process (like a cutter head, or a welding tip, or whatever). Also 'dust collection' is gas-particulate separation. Sounds much more industrial. I just want you to know.

~Ian~

I think you will find that the air flow across the blade within the slot of the 40m diameter tube ,is far higher than would be achieved within the large diameter tube used with the bag collector. The lighter weight dust particles would certainly be collected by the air stream.The total lack of dust being deposited within the machine shows that overall collection is very efficient

Using the 4"extraction port provided with the bandsaw. The large volume lower velocity air flow of the bag extractor,is required to collect dust that has already been thrown off the blade into the lower section of the bandsaw and also has to strip the remaining wood particles from the teeth of the saw blade.

With this is modification the air flow strips the dust off the blade at source, hence air velocity is more important than volume moved.

I would agree that the extraction system is only as good as the final filtration on the system. This vaccuum cleaner has a very fine washable prefilter and then an additional filter around the motor and fan. This final filter has been replaced with a pleated P3 filter.

As I am extremely sensitive to dust particles, I always wear a powered mask with a p3 filter even when hand sawing or sanding. I do not work with mdf.

I think this has digressed a little from what Paddy had created with removal of dust from his bandsaw.

One item that does not need to be cleaned by hand as often well done Paddy.

Originally Posted by Paddy

I think you will find that the air flow across the blade within the slot of the 40m diameter tube ,is far higher than would be achieved within the large diameter tube used with the bag collector. The lighter weight dust particles would certainly be collected by the air stream.The total lack of dust being deposited within the machine shows that overall collection is very efficient

There are multiple misconceptions here but let me just deal with two

1) "Air flow" is not the same as "air speed".
Air flow is total volume removed per unit time, while air speed the linear speed of the air in length per unit time. For the same pressure a smaller tube has a higher air speed but the flow does not scale well for small pipes and they will usually have a significantly lower overall flow. A vacuum cleaner using narrow ducting might have double or even triple the air speed of a larger duct but it will simply not draw double or triple the air flow. I can drag out the ducting flow curves to show you this if you like.

2) "The total lack of dust being deposited within the machine shows that overall collection is very efficient" Assuming because no dust can be seen does not mean there is nothing there. The moving blade of a machine like a bandsaw is like a car moving along a road - it pulls an envelope of surrounding air along with it and slams it down into the workpiece and table. There is no chance for this moving air to all go through the gap in the small throat plate usual provided with a BS. As a result the air fluffs up and carries large numbers of mainly lightweight invisible dust particles that emanate like a halo around the cut. The only way to capture as much of this invisible dust is to have a large volume of air being sucked at above the machine while it cuts. Bill Pentz suggests something like a minimum of 1000 cfm at 4000 fpm is needed to do this. A vacuum cleaner might generate 8000 fpm of air speed (which is real good) but @ only 120 cfm, so it does not have a chance to capture the volume of air generated that contains the invisible fine dust.

If your setup was checked with a particle counter it would show large numbers of invisible particles being squirted all over the place.

A shroud that covers the work piece and cut, and reduces the ability of the air moving with the blade from creating the floaty stuff would be needed to grab this with a vacuum cleaner. Vacuum cleaners work better with power tools where these shrouds can be integrated right at the source of (not after) the cut into the vacuum cleaner setup, like the festool systems do.

I don't want to be disparaging about people trying different things to improve dust filtration but OTOH I just don't want newbies to think this is a solution to the fine dust problem when the well established science says otherwise.

Sh-t, and all I wanted was a way of getting rid of the dust covering my table and inside my bandsaw. Would do you think, could it work, if we put a 4" or 6" pipe in the same place but connected to a dusty and not a vacuum cleaner?

SB

Originally Posted by Superbunny

Sh-t, and all I wanted was a way of getting rid of the dust covering my table and inside my bandsaw. . . . . . .

That's fine if that's all you want then by all means go for it and let me not forget to congratulate Paddy for doing this with his setup. However, the problem with posts like these is there are many newbies (and still quite a few oldies) who come to this forum looking for a total dust solution and think they can solve it with a cheap vacuum cleaner and a bit of PVC. If only . . . :roll eyes:

Yeah, look, you've got a line of argument there, Paddy. This place isn't a university so no need to dig into it too much.

The textbook stuff is really just about air quality in places of employment. If you're on your own, you can make your own choices about that.

As long as you reckon you've come up with a solution, you're satisfied it's a safe example for others to follow and nobody gets hurt, that's all that matters.

BobL,
Hi,

Have you ever stepped out of they shed withe the 5hp dusty and the air filters and entered the real world on a windy day? You know? One of those days when you can SEE visibile dirt particles in the air?

Jeez, someone makes a post about catching all the dust that they would otherwise have to clean up off the floor and you bring up air purity standards that dont even exist outside the shed half the time.

From a workplace health and safety point of view an employer has to meet a minimum standard for his employees because they are there 40 hrs a week, but bloody hell what amateur is running a dust extractor for even an hour a week on average? I only wish!

In a good week I might spend 20 hours in the shed and be cutting or turning for 3 hours, but yes BobL you are right: the invisible dust may be present for the whole 20 hours.

But I am outside the shed for 148 hours of the week and how much of that time am I in an environment where the air quality is worse than in the shed? Every hour that we are outside we are breathing unfiltered air laden with microscopic particles, pollen and dust particles.

I take my health seriously, therefore I believe in efficient dust collection. I collest what I can with the equipment I have and my budget but are we trying to make the air in our workshops cleaner than what we are breathing when we step outside?

Doug

Hey guys,

Just wanted to keep my saw and workshop clear of visible dust. Never suggested that ALL airborn particles would be eliminated!

Would doubt if many home workshops would have space or funds to install an extraction system that would achieve this lofty ambition.

Thats why I always wear a powered face mask with P3 filter, even when hand sanding.