Thread: Ducting update.

Bob
Pictures 1 & 2 in your last post seem to say it all. All the rest appears to be irrelevant regarding fine dust. The second photo with the extractor on shows all threads pulled into the hood even with the blade spinning. Does this not indicate that air is moving into the hood all round the lower edge of the hood? How can fine dust escape from the hood under such conditions? I notice that your hood in photo 2 is very close to the table. Ironically because the table saw hood is a semi enclosed environment it may not need large airflow to completely contain submicron dust. Larger particles may be a different matter

Your use of threads to show air flow is excellent - thanks for the idea

Could you please block the back of your hood to see if the wool threads are actually pulled in harder. If so blocking the back of the hood would not let fine dust escape and may help to also retain larger particles. To prevent larger particles from escaping it appears that we need more air entering at higher velocity at the front of the hood not the back.

So the remaining Q is what conditions produce an air speed between the front of the hood and the table that prevents the larger "sawdust" from also escaping.

Today I took the time to actually observe how sawdust (larger particles) were actually leaving the unguarded blade and where they ended up on the table. On my saw it was difficult to see all the dust as it left the blade, but it was clear that some if not most was actually coming from the back of the blade particularly when the wood had advanced such that the rear of the blade was also in the wood. Sawdust was distributed to the sides of the table on both sides of the blade ( even over the fence) as well as to the front of the table. Not all sawdust was thrown forward by the blade.

I have now decided to build a prototype where I can shift the duct entry between the front & rear of the hood. I will also try partially removing either the front or back to see which works the best. Doms use of fingers at the front may actually let in air at this end improving sawdust retention. Perhaps Dom might report whether his fingers at the front are sucked in or pushed out. He might also like to temporaily block the back of his guard to see if it improves sawdust retention. All food for thought
Ron

Originally Posted by ronboult

Bob
Pictures 1 & 2 in your last post seem to say it all. All the rest appears to be irrelevant regarding fine dust. The second photo with the extractor on shows all threads pulled into the hood even with the blade spinning. Does this not indicate that air is moving into the hood all round the lower edge of the hood? How can fine dust escape from the hood under such conditions? I notice that your hood in photo 2 is very close to the table. Ironically because the table saw hood is a semi enclosed environment it may not need large airflow to completely contain submicron dust. Larger particles may be a different matter

I found some interesting data I measured back 2012 that I never reported on what a spinning blade does in terms of air movement.
A 100 toothed 12" blade will move at net amount of ~30 CFM from under the table to above the table into the hood.
A 40 toothed saw will move around ~60 CFM
This effectively over pressures the hood so the air drawn from the hood must be at least that much even before centrifugal forces are taken into account.
This is why vacuum cleaners attached to saw guards may struggle because they in practice can only generate about 100 CFM and by the time filters clog and they leak they will be down to less than 50 cfm.

Something really important I forgot to mention last time is that it is difficult to maintain a very small gap between the bottom of the guard and a piece of wood at all times.
For example even a piece of wood wider than the guard has an exit point where it is cutting the last few cm - the wood is still under the guard lifting it above the table and then as that back edge of the wood passes under the front edge of the guard it leaves an ever widening gap and this is where I observe most of the chips escape and if there is insufficient flow fine dust will escape from here as well.
Having a longer guard means that gap is open for longer than if the guard is shorter.

If the wood is narrower than the front of the guard the same gap is there during the entire time the wood is being cut.
So you should never rely on being able to achieve a small gap, just like always DCs leak there will at some time be gaps under the hood at least equivalent to the thickness of the wood being cut.
A flexible front edge of the guard can help in reducing those gaps.

Could you please block the back of your hood to see if the wool threads are actually pulled in harder. If so blocking the back of the hood would not let fine dust escape and may help to also retain larger particles. To prevent larger particles from escaping it appears that we need more air entering at higher velocity at the front of the hood not the back.

The back of that hood in those photos is closed.
My previous hood is the hood in post 46 posted but with a bristle guard edge on three sides and the back open.
However this meant the port entrance is an even longer way from the blade edge and this is mainly why I wanted to upgrade the hood.
The highest velocity is needed between the blade and the port and this helped by making that path as short as possible.

The more I think about it the less I worry about air speed at the front edge and more about increasing the air speed between the chip source and the port as that is what ultimately controls the pick up of chips.
It may even be aided by closing the back and opening the front.

In the picture below the to maximise D the second option would seem better than the first as A would to some extent cancel B.
Some more measurements needed I think.

Guard1.jpg Guard2.jpg

So the remaining Q is what conditions produce an air speed between the front of the hood and the table that prevents the larger "sawdust" from also escaping.

I will try and measure some air speeds of my new hood in the next few days.

I have now decided to build a prototype where I can shift the duct entry between the front & rear of the hood. I will also try partially removing either the front or back to see which works the best. Doms use of fingers at the front may actually let in air at this end improving sawdust retention. Perhaps Dom might report whether his fingers at the front are sucked in or pushed out.

His photos show that they are slightly sucked in.

Bob in your post above with the front of the hood open. Would a hi speed jet of air from a compressor at point A aid in pushing the dust and chips into the hose inlet at point C?

Pete

Originally Posted by QC Inspector

Bob in your post above with the front of the hood open. Would a hi speed jet of air from a compressor at point A aid in pushing the dust and chips into the hose inlet at point C?

Pete

I would worry about a high speed air jet disrupting any sort of smooth flow into the port.
OTOH it might already be so turbulent that it might not matter.
Rather than a jet I would think an air curtain or air knife would be better but they use quite high volumes of air.

To maximise air speed you either increase flow by adding more air or reduce gap area but any increase in air speed by forming a narrow gap is very localised.
I might have measured 10 m/s at the gap but that is only for a few mm close to the gap and immediately either side of the gap the air speed will drop quickly.
A chip with sufficient momentum opposite to any air flow at the gap appears to easily penetrate a local small volume of high speed air found in such a gap.
It would depend on the aerodynamic braking of the chip.

gap.jpg

Thanks Bob. I was thinking it could be tested by directing the blast from an air nozzle under the lip of the guard while making a cut to see what the effect was. Naturally you would need a second person to work the nozzle for a test cut. Air pressure could be played with to see how much air would be effective and what pressures and airflows might create turbulence in the hood if at all.

Pete

Originally Posted by QC Inspector

Thanks Bob. I was thinking it could be tested by directing the blast from an air nozzle under the lip of the guard while making a cut to see what the effect was. Naturally you would need a second person to work the nozzle for a test cut. Air pressure could be played with to see how much air would be effective and what pressures and airflows might create turbulence in the hood if at all.

Pete

It's worth a try. My concern is that unless a back pressure is right across the full width of the hood gap I think it might scatter some dust out of the hood,

Moving dust by blowing is akin to herding cats. I watched a bloke yesterday using compressed air to sweep the shed. He sure did get rid of all the stuff on the low down surfaces and the floor and you could see the big bits going out the door. However after doing this a few times unfortunately with the DC turned off there is now a pall of fine dust on all the higher surfaces he cannot reach.

Originally Posted by BobL

It's worth a try. My concern is that unless a back pressure is right across the full width of the hood gap I think it might scatter some dust out of the hood,

Moving dust by blowing is akin to herding cats. I watched a bloke yesterday using compressed air to sweep the shed. He sure did get rid of all the stuff on the low down surfaces and the floor and you could see the big bits going out the door. However after doing this a few times unfortunately with the DC turned off there is now a pall of fine dust on all the higher surfaces he cannot reach.

We used a trick back in the old factories; that is a bucked of sawdust and add water. Throw wet sawdust on ground then sweep it up. Keeps the fine dust under control. Appears to work surprisingly well. Knowing what I know now about fine dust, I just wonder how well it did work.

Originally Posted by Pearo

We used a trick back in the old factories; that is a bucked of sawdust and add water. Throw wet sawdust on ground then sweep it up. Keeps the fine dust under control. Appears to work surprisingly well. Knowing what I know now about fine dust, I just wonder how well it did work.

Not that well - you 'd have to get the wet sawdust over evenly over every surface in the factory i.e. even machinery.
Sweeping a shed that has poor dust extraction raises the fine dust levels in a shed to about the same extent as RO sanding for about 15 minute which send the air well above any recommended OHS level,

Originally Posted by BobL

Moving dust by blowing is akin to herding cats. I watched a bloke yesterday using compressed air to sweep the shed. He sure did get rid of all the stuff on the low down surfaces and the floor and you could see the big bits going out the door. However after doing this a few times unfortunately with the DC turned off there is now a pall of fine dust on all the higher surfaces he cannot reach.

That's why I don't like cats. Not herd animals.

I had to clean out my father's shop after he passed away a number of years ago. A leaf blower was more effective than compressed air. It also helped that the shop had low ceilings and a garage door across the end. I see it as being akin to pulling air with a DC compared to a vacuum cleaner. The larger volume of air takes the fines away better than a concentrated stream at higher velocity.

Sorry for the little hijack.
Pete

Originally Posted by QC Inspector

I had to clean out my father's shop after he passed away a number of years ago. A leaf blower was more effective than compressed air. It also helped that the shop had low ceilings and a garage door across the end. I see it as being akin to pulling air with a DC compared to a vacuum cleaner. The larger volume of air takes the fines away better than a concentrated stream at higher velocity.

A leaf blower will be no better at moving fine dust than a compressor. What folks forget is that they both move dust by generating a high pressure stream of air (the leaf blower just moves more air) but in doing so the blower has to create a low pressure volume in the vicinity of its air intake. The air stream propels visible dust because they are heavy or have a large cross sectional area. Eventually the air stream loses speed and the visible particles fall out of suspension but the invisible particles will ride the air currents and guess where they end up going? Well right back to the low pressure volume generated by the blower. In essence blowers rapidly recycle fine dust over and over again and generate every increasing concentrations of invisible dust.

This is why it is better to suck up all dust and transport it via ducting to be captured/filtered, or vented across a physical barrier like a shed wall.

Originally Posted by BobL

The more I think about it the less I worry about air speed at the front edge and more about increasing the air speed between the chip source and the port as that is what ultimately controls the pick up of chips.
It may even be aided by closing the back and opening the front.
Guard1.jpg Guard2.jpg


I will try and measure some air speeds of my new hood in the next few days.

All very interesting. It makes me want to try a few experiments myself, but the motivation just isn't there, as despite any shortcomings in the guard's design the results are at the point where I don't want, need or expect any noticeable improvement.

I'm doubtful that opening the front and closing the back would be successful in controlling the ejection of chips; they just have too much energy and need a hard-stop to contain them (unless you direct most of the airflow through the top guard rather than the cabinet perhaps).

And yes, the front "fingers" of my guard are pulled slightly in (why wouldn't they be?). If I close the 6" port that goes to the table saw cabinet the vacuum is too great and basically collapses the overhead guard down onto the table if it is down, or pulls the front rubber fingers right around the front edge if the guard is raised up. It also makes a hell of a noise.

As an aside, I played with the idea of fitting a compressed air jet directed at the blade underneath the table to clear dust off the blade before it was carried back around and fired out the top, however the noise it made was ridiculously, painfully loud. I imagine directing compressed air at the blade from the top would have a similar effect.

Originally Posted by DomAU

All very interesting. It makes me want to try a few experiments myself, but the motivation just isn't there, as despite any shortcomings in the guard's design the results are at the point where I don't want, need or expect any noticeable improvement.

I agree, the law of diminishing returns eventually kicks in.

I'm doubtful that opening the front and closing the back would be successful in controlling the ejection of chips; they just have too much energy and need a hard-stop to contain them (unless you direct most of the airflow through the top guard rather than the cabinet perhaps).

The chips vary in size but have the same initial speed, so as you say different KE. Depending on their shape they will have different (non-linear) aerodynamic resistance , the larger they are the pre resistance. More rounded, heavier chips will be more likely to make it through the moving air wall at the front of the hood. It could be tested by trying blades with different numbers of teeth.

And yes, the front "fingers" of my guard are pulled slightly in (why wouldn't they be?). If I close the 6" port that goes to the table saw cabinet the vacuum is too great and basically collapses the overhead guard down onto the table if it is down, or pulls the front rubber fingers right around the front edge if the guard is raised up. It also makes a hell of a noise.

This can also make it harder to move the wood

As an aside, I played with the idea of fitting a compressed air jet directed at the blade underneath the table to clear dust off the blade before it was carried back around and fired out the top, however the noise it made was ridiculously, painfully loud. I imagine directing compressed air at the blade from the top would have a similar effect.

A possible alternative to an "air jet" under a table is a half-length zero-gap plate under the table that the saw runs though.
This might assist in "wiping" some dust off the blade before it carries it out of the cabinet.
I put one of these on my big belt sander and it makes a big differences to the dust the belt can carry away from the collection hood.

This effect happens already to some extent for saws with a zero throat plate so would probably be of more benefit to saws which did not have a throat plate.
Any zero gap plate placed inside a cabinet would need to be attached to the trunion so it could tilt with the saw.
Although it will add to the noise the more I think about it, the more I wonder why this is not done.

Originally Posted by BobL

This effect happens already to some extent for saws with a zero throat plate so would probably be of more benefit to saws which did not have a throat plate.
Any zero gap plate placed inside a cabinet would need to be attached to the trunion so it could tilt with the saw.
Although it will add to the noise the more I think about it, the more I wonder why this is not done.

It's funny you mention zero clearance inserts. I understand what you mean by placing an additional zero-clearance insert under the table to scrub chips off the blade and it makes sense to me that this would work, however my experience with actual zero-clearance inserts at the table-surface in my table saw is that the amount of dust fired out the top is greater with a zero clearance insert than without. That is, the dust collection is better without a zero-clearance insert. I guess because there more air pulled into the cabinet through the insert for a normal insert.

Originally Posted by DomAU

It's funny you mention zero clearance inserts. I understand what you mean by placing an additional zero-clearance insert under the table to scrub chips off the blade and it makes sense to me that this would work, however my experience with actual zero-clearance inserts at the table-surface in my table saw is that the amount of dust fired out the top is greater with a zero clearance insert than without. That is, the dust collection is better without a zero-clearance insert. I guess because there more air pulled into the cabinet through the insert for a normal insert.

Correct and to a lesser extent less dusty air is extracted from the cabinet by the back of the saw because the throat plate gap is wider and bit more air can flow into the cabinet through the slot.