Thread: Dust extraction strategies for underneath the table saw?

The photos don't help much.

My Dad helped me ship this saw directly from the US so it had a 60Hz motor. In case some of you are wondering, the motor pictured is a 4HP S6 DOL Hammer motor. Considering that the saw is only a 10" machine, I would consider that a glorious motor upgrade!

But its the dust I'm hoping to conquer! Thanks. Sorry the photos are not the right way up.

photo.jpgphoto copy.jpg

I don't think it's value for effort to do anything inside the cabinet. I would just cut a 6" hole into the cabinet in the same place as the current 4" and ensure that there is enough air flow into the cabinet as directly opposite to the outlet as possible, that means up high on the other side of the cabinet. That might mean cutting an opening in that side and putting a louvred vent. You may still end up with a few dead spaces with big dust chips in them but they are not really that much of a problem.

BTW cabinet saws with their motors inside the cabinet are problematic fine dust generators. The impeller used to cool the big electric motor will be spinning at 2850 RPM and be churning of dust into finer particles. The fact that the motor is semi contained inside a cabinet does very little for the fine dust which will leak out of the cabinet like a gas under a diffusion gradient unless high air flow rates are used.

Ahah, I'm making it way too hard for myself. I think I was fixated on the piles of dust not remembering that said "piles" will not gather (or mostly will not gather) once I have good flow. A grate on the opposite side is definitely doable and a lot easier than some dodgy internals...

I think I understand the principle of maintaining the same cross-section to allow good airflows at 6". Are there any pointers though about how to quantify various cross-flow openings which have an impedance? For example, if I measure my throat plate opening around the blade on the tablesaw, there's not much flow potential. If I make a zero tolerance plate there would be even less. Is this why I've read that some drill holes in their throat plate? How does the saw blade rotation affect flow? And is there a generally known reduction in flow for a typical venting grate? Finally, should I bother with all the leaks in the cabinet, eg around the adjusting wheels, up underneath where the casting meets the sheet metal?

Apologies if I've missed this information in other threads. Feel free to direct me to them if that's easier. My missus is used to me reading these forums for hours on end of an evening. Almost getting in the way of actually making things in the shop!

Thanks.

Yes, the question of what size entry for extra air is an interesting one. I modified my Laguna saw to 6" and am now trying to decide what size opening to put in the door on the opposite side of the cabinet. The door already has a small area with three or four small louvres and I find that when I hold the door open, the further open I hold it the less air is drawn through the plate which seem logical. What is less logical is it seems that when the door is fully closed the suction through the plate is at its highest. This seems to be at odds with what most people say.

Originally Posted by safari

Yes, the question of what size entry for extra air is an interesting one. I modified my Laguna saw to 6" and am now trying to decide what size opening to put in the door on the opposite side of the cabinet. The door already has a small area with three or four small louvres and I find that when I hold the door open, the further open I hold it the less air is drawn through the plate which seem logical. What is less logical is it seems that when the door is fully closed the suction through the plate is at its highest. This seems to be at odds with what most people say.

Thanks for your reply Safari. I'm not certain what you mean when you say "suction through the plate is at its highest". Do you mean by plate as in "throat plate" up at the blade? If so, I wonder whether your louvres are doing the job and you have more or less 6" of cross-sectional air-flow? Perhaps opening the door provides too much cross-sectional area which reduces speed of the air flow and therefore would reduce the CFM around the throat plate. I don't know!

I know I have to commit myself to attacking my machines -- and just want to make sure I do it more or less correctly!

Originally Posted by LuckyDuck

I think I understand the principle of maintaining the same cross-section to allow good airflows at 6".

Nope - if you maintain the same cross section you will still add resistance to flow - to effectively wipe out this resistance you need at least double the cross sectional area - this assumes no grids or mesh.

Are there any pointers though about how to quantify various cross-flow openings which have an impedance? For example, if I measure my throat plate opening around the blade on the tablesaw, there's not much flow potential. If I make a zero tolerance plate there would be even less. Is this why I've read that some drill holes in their throat plate? How does the saw blade rotation affect flow? And is there a generally known reduction in flow for a typical venting grate?

Gaps narrower than a 2 mm can be ignored in any cross sectional contribution calculations and for all others subtract 2 mm from that dimension.Drilling holes in a throat plate is better than nothing but they must be bigger than 2 mm to add to the cross section and even then when you do the sums, holes will end up contributing very little to the overall flow.Blade rotation pushes air into the cabinet at the front and pulls it out at the back so there is no net contribution.While gaps of ~2mm or less can be ignored gratings or grids have to be much smaller than 2mm before thy can be ignored. There is no way to calculate it easily because it depend on a heap of stuff like the angle of the air entry and the shape and smoothness of the actual edges of the grate/grid. Fine meshes block too easily - remember all you have to do is keep fingers out so 5mm wide slots or holes will be fine. If there is sufficient distance inside the cabinet so that even if a long finger is poked into the grid that it won't contact anything then 10 mm holes with a 1 mm mesh will be effective. If a finger looks like it make contact then using a standoff box mounted to the side of the cabinet is easily enough done.

Finally, should I bother with all the leaks in the cabinet, eg around the adjusting wheels, up underneath where the casting meets the sheet metal?

Gaps up near the blade I would leave alone those down near the floor I would seal

Sorry Lucky Duck. I didn't make it clear. I did mean the throat plate and when I put my hand over, the suction felt is definitely more with the door closed. The three louvres measure 120mmx5mm so not double the area of a 6" pipe and there are no other holes apart from the throat plate.
Another of life's mysteries!

My experience is that the recommendations already given are pretty good. I built a cabinet for my old contractor saw with nothing more than a sloped ramp leading to the 6" outlet. A small amount of dust collects in the corners either side of the outlet, but I ignore it studiously. If it is not floating around in the air in the shop and not in my lungs, and if it is not on the shop floor, I ignore it.

I did what Bob recommended. The bottom area of the cabinet is sealed. The top area is not, and dust collection is great.

Because mine is a contractor saw, there is a largish opening above the motor where air gets in. I blanked off some of the open area, but that which remains is larger than a 6" pipe, so I am confident I don't suffer restrictions that prevent enough air getting in to the cabinet.

Cheerio!

John

Originally Posted by safari

Yes, the question of what size entry for extra air is an interesting one. I modified my Laguna saw to 6" and am now trying to decide what size opening to put in the door on the opposite side of the cabinet. The door already has a small area with three or four small louvres and I find that when I hold the door open, the further open I hold it the less air is drawn through the plate which seem logical. What is less logical is it seems that when the door is fully closed the suction through the plate is at its highest. This seems to be at odds with what most people say.

Safari,

Maybe I am missing something here, but getting higher velocity through the throat plate with the door closed is what I would have expected. It seems that the air being drawn into and through the machine will follow the line of least resistance. If the door is open, most of the air will be drawn through the open door. If the door is closed, the cyclone will pull air from wherever it can, including the throat plate.

Could it be that when the door is closed you don't have enough air getting into the cabinet? High velocity of air through the throat plate might tell us naught about the volume of air through the machine, which is more important.

Cheerio!

John

What goes on at the throat plate with the blade turned off is quite different compared to when the blade is running and wood is being cut.

Whatever the air speed is at the throat plate (when the blade is static) is swamped by the spinning blade - the back of the blade easily overcomes any air drawn down through the throat plate and pulls air (and fine dust) out of the cabinet and sprays this in a circular swathe up into the shed above the blade/cut. At the front of the blade, some of the now expanded spray of dusty air is thrown down onto wood or any exposed part of table in front of the blade and onto the operator.
On top of this (at least some of) the throat plate gap is usually covered by the work being cut , so very little of the initially ejected air and dust passes down through the throat plate gap. By the time the work piece is removed exposing the throat plate gap the fine dust is too far from the gap to be captured by any air passing down through the gap. The air speed in the gap is actually not as relevant as the air speed out where the dust is hanging in the air and being swirled around by the blade.

The problem is made worse by a zero gap throat around a blade.

All this is why a throat plate gap should probably just be ignored in any dust capture possibilities and why high volume extraction hood above the blade is critical for controlling fine dust emitted by a TS.
The ideal place is as far forward on the guard as possible but this gets awkward so above the front is a reasonable compromise.

Some further info about using a blade hood is provided in this thread.
https://www.woodworkforums.com/showth...=157526&page=7 (post #98)

Hi LD,
PJT here, Based on the pics it looks like you have a similiar need for the mod I did to Roy's saw, I think I directed you to that thread? I am at John's atm so if tomorrow suits I could come over for a look, John may come as well.
Send contact details via a PM to John's PM and we can go from there.


Pete

Originally Posted by John Samuel

My experience is that the recommendations already given are pretty good. I built a cabinet for my old contractor saw with nothing more than a sloped ramp leading to the 6" outlet. A small amount of dust collects in the corners either side of the outlet, but I ignore it studiously. If it is not floating around in the air in the shop and not in my lungs, and if it is not on the shop floor, I ignore it.

I did what Bob recommended. The bottom area of the cabinet is sealed. The top area is not, and dust collection is great.

John

Hi John:

I plan to do as you have done. I can't believe how much I've read on these forums, and yet I still have a lot to read. Have you read the Cincinnati document linked from Pentz' website? Very enlightening reading -- if a bit heavy in parts.

Thanks for your input.

Originally Posted by BobL

Nope - if you maintain the same cross section you will still add resistance to flow - to effectively wipe out this resistance you need at least double the cross sectional area - this assumes no grids or mesh.

Gaps narrower than a 2 mm can be ignored in any cross sectional contribution calculations and for all others subtract 2 mm from that dimension.Drilling holes in a throat plate is better than nothing but they must be bigger than 2 mm to add to the cross section and even then when you do the sums, holes will end up contributing very little to the overall flow.Blade rotation pushes air into the cabinet at the front and pulls it out at the back so there is no net contribution.While gaps of ~2mm or less can be ignored gratings or grids have to be much smaller than 2mm before thy can be ignored. There is no way to calculate it easily because it depend on a heap of stuff like the angle of the air entry and the shape and smoothness of the actual edges of the grate/grid. Fine meshes block too easily - remember all you have to do is keep fingers out so 5mm wide slots or holes will be fine. If there is sufficient distance inside the cabinet so that even if a long finger is poked into the grid that it won't contact anything then 10 mm holes with a 1 mm mesh will be effective. If a finger looks like it make contact then using a standoff box mounted to the side of the cabinet is easily enough done.

Gaps up near the blade I would leave alone those down near the floor I would seal

Hi Bob:

Thank you very much for this level of detail. I think for the first time I am getting a better handle of what you mean by "resistance" when it comes to machine mods. Previously, I just thought you and others were referring to the resistance from small ports (e.g. 4" instead of 6"), whereas this is only part of the picture. I've finally (I hope) got the point into my thick head that we not only need the large port, but also need large openings opposite to maintain that flow. Thanks for persevering -- I don't find a lot of this stuff very intuitive. Cheers.

Originally Posted by BobL

Some further info about using a blade hood is provided in this thread.
https://www.woodworkforums.com/showth...=157526&page=7 (post #98)

Thanks for this. I've read that thread; I intend to attack my table saw above table as well. I wish all my dust mods could be done all in one day, but it is going to take me time as I still have to make some furniture. I hate the dust even more now that I know just how insidious it is.