Thread: Table saw & router dust control ideas

Sorry - can't make Any sense of the diagrams.
Some labels might help.

RE: Finally, I'm curious about the idea of a negative pressure cabinet underneath a vented table, for sanding, drilling etc and handheld router work, and I'd be curious to know if you have any thoughts.

Sawdust can be classified into coarse chips (C), and medium (M) and fine (F - invisible) dust
Downdraft tables are not much use at collecting C and M as these tend to get thrown well outside the relatively slow speed air streams generated by these tables. These are best collected at source using shop vac.
Down draft tables can be better at collecting F but as this dust is usually warmed by the generation process this dust often rises outside the sphere of capture of the tables air stream.
To improve collection a cabinet/curtain around and on top of such tables seems to help.

A 6x9 garage is borderline for a 3HP DC so I wouldn't recommend a cyclone for such a set up as this will reduce the flow. A 6x9 shed shed should really be serviced by a 14"+ impeller and a 4HP DC. If you made it a 15" impeller then you could add a cyclone.

If you stay with a 3HP DC I would add some additional force ventilation to deal with the escaped fine dust from power tools.

Originally Posted by BobL

Sorry - can't make Any sense of the diagrams.
Some labels might help.

RE: Finally, I'm curious about the idea of a negative pressure cabinet underneath a vented table, for sanding, drilling etc and handheld router work, and I'd be curious to know if you have any thoughts.

Sawdust can be classified into coarse chips (C), and medium (M) and fine (F - invisible) dust
Downdraft tables are not much use at collecting C and M as these tend to get thrown well outside the relatively slow speed air streams generated by these tables. These are best collected at source using shop vac.
Down draft tables can be better at collecting F but as this dust is usually warmed by the generation process this dust often rises outside the sphere of capture of the tables air stream.
To improve collection a cabinet/curtain around and on top of such tables seems to help.

just as a bit of an "out of the box thinking, would a blower or fan, located directly above the table (assuming it doesn't really get moved much) which might help direct the dust and airflow back down into the table?

Originally Posted by havabeer69

just as a bit of an "out of the box thinking, would a blower or fan, located directly above the table (assuming it doesn't really get moved much) which might help direct the dust and airflow back down into the table?

Moving dust (especially fine dust) by blowing is about as effective as hearding cats. Chances are the dust just gets further distributed around the shed.

Thanks BobL and sorry about the lack of labels - the diagrams are just a cross-sectional side view. The green bit is the table saw cabinet (the grey shapes inside it are the approximate motor positions). The second pic shows a router added to the right of the saw.

I confess I don't understand how the size of the workshop affects the suitability of a DC system? Do you mean the ~ 9m of 150mm duct is borderline too far for a 3hp system? Or are you suggesting the internal volume of the workshop needs to be taken into account, regardless of the machines being used? Also, the cyclone wouldn't be 'added' to the system, it would replace the bag setup - does that change your opinion of it?

Regarding Havaneer69's suggestion, I've been considering something along those lines as well. Not trying to 'blow' the dust, really, more creating a distinct airflow path by using both positive and negative pressure, instead of just negative pressure which would draw from all directions and therefore be significantly diminished the further you get from the intake. By directing a stream of air past the dust source and towards the intake, surely you'd see more effective collection of dust?

Originally Posted by Dusty Dave

Thanks BobL and sorry about the lack of labels - the diagrams are just a cross-sectional side view. The green bit is the table saw cabinet (the grey shapes inside it are the approximate motor positions). The second pic shows a router added to the right of the saw.

OK thanks

I confess I don't understand how the size of the workshop affects the suitability of a DC system? Do you mean the ~ 9m of 150mm duct is borderline too far for a 3hp system? Or are you suggesting the internal volume of the workshop needs to be taken into account, regardless of the machines being used? Also, the cyclone wouldn't be 'added' to the system, it would replace the bag setup - does that change your opinion of it?

Not really. A 3HP DC with 8" of WC pressure nominally loses about 20% of its pressure from the moment a cyclone is connected. A 3HP with clean twin filters loses about 6% of its pressure. Sure the pressure loses increase as the filters clog and they need to be cleaned. By clean the don't need to be pristine they just need the layer of dust shaken off the inside of the filter. Most people are lazy and don't want to do this and hook up a cyclone and eventually look in their collection bags after the cyclone and see very little fine dust so they think they are in front but all that has happened is that the loss of flow from the start means that the fine dust has simply not been collected. Collection bags do need to be emptied when they are about 1/2-2/3rd full and filtered bags need regular shaking but that way you are always in front in terms of fine dust collection. Cyclones do work when a 4HP motor and a 15" impeller are used - they can afford to lose pressure and still will collect fine dust.

We're not talking about much fine dust here. A 10th of a teaspoon of really fine dust suspended in air is all that is needed to contaminate the average DIY shed.

In terms of ducting length I recommend 3HP DCs (with no cyclone) be restricted to 6m ducting runs of 6" ducting but of course sometimes longer runs cannot be avoided and you live with it. For years I had 3HP DC and used an 11m run to my TS BUT I also have a 1200 CFM ventilation fan directly above the saw that clears fine dust that was not collected by the DC.

Regarding Havaneer69's suggestion, I've been considering something along those lines as well. Not trying to 'blow' the dust, really, more creating a distinct airflow path by using both positive and negative pressure, instead of just negative pressure which would draw from all directions and therefore be significantly diminished the further you get from the intake. By directing a stream of air past the dust source and towards the intake, surely you'd see more effective collection of dust?

The "drawing from all directions" on the inlet of a fan works in reverse on the outlet ie the air/dust collides with a wall of air and "scatters" outwards in all directions. This is why sucking air/dust from a shed is nominally better than blowing dust from a shed. If you have spare fan capacity it would be better to use it to vent the shed. Blowing dust is like herding cats.

Blowing dust helps provided a much higher amount of air/power is needed. An example of this is at our 140m^2mens shed where a 3Phase 4kW Evap AC fan in the roof sucks air in from outside and blows it down into the shed.

Have you considered one of these? It seems to me to be the most sensible option for collection dust at a router table ....

DustRouter - Milescraft



Regards from Perth

Derek

(Woops, double post)

Thanks BobL, that's a great help understanding the relative merits of cyclone vs bags. I may also try just directly venting, there is plenty of space behind the workshop (I could even incorporate a composting area beneath the outlet).

I confess I'm still a little in disagreement with your views on positive pressure. Yes, theoretically the pressure must disperse in the same way the vacuum does, but used together either side of a cutting tool it's difficult to see how it would not help direct dust into the intake by creating a stonger, more directed flow. I'm certainly keen to try it, anyway.

@derekcohen - thanks, that is essentially what I would have by using the AEG's dust shroud in combination with above-table / fence collection. And I'm still curious what would happen if instead of sucking from that side port, I was blowing gently, and working with the router's internal airflow to push all the waste up and towards the intake...

image.jpeg

Dave, it just strikes me that the DustRouter fixture is a more direct and comprehensive method - following Bob's maxim of dealing with dust at the source. I have ordered one for my own router table (which is built into the outfeed of my table saw), and will provide feedback one of these days.

Regards from Perth

Derek

Dave if the area being collected is contained or shrouded with the suction on one side and the air coming in on the other it would assist grabbing the fine dust. Picture a sandblast cabinet. If you try to push the dust with a fan in an open area like above a downdraft table it will disperse more especially if the airflow of the fan blowing is greater and I suspect even close to what the dust collector can draw.

With your climate putting a large fan in the wall at bench height, with the bench against the wall would be the more effective way to draw off fine and large dust. I set up exactly that behind my fathers lathe. Two 16" fans from an old industrial heater grabbed all the sanding dust and most of the chips which landed on the ground 10+ feet away. Great in the summer but no fun turning in the winter. You had to have the doors open at the other end of the shop.

The best downdraft table I ever was around use a water bath that the dust laden air was drawn through. The table was enclosed on three sides and above. After the air went down through the water it hit filters that allowed the air though but not the water. Then through the 5hp 3,000cfm impeller and was ducted over the top of the table area where it was directed down through a slot across the front of the operator. That formed an air curtain that helped contain the dust in the sanding area. In addition there were plastic strip curtains that could be drawn beside the operator help contain the dust above the table. That table was about 3' x 6' so the flow was about 166CFM per square foot of the table. You can do the conversion to metric if you prefer. The dry downdraft tables we had were 4' x 8' and had 3,000 cfm impellers too but the larger surface made the flow 97CFM per square foot. More dust got away. To size yours you will need to know the actual flow at the table and size the top accordingly.

Pete

Originally Posted by Dusty Dave

I confess I'm still a little in disagreement with your views on positive pressure. Yes, theoretically the pressure must disperse in the same way the vacuum does, but used together either side of a cutting tool it's difficult to see how it would not help direct dust into the intake by creating a stonger, more directed flow. I'm certainly keen to try it, anyway.

Part of my day job over my 40 year working life involved working in, designing and building ultra clean (dust free) laboratories. Over that time we tried many things including trying to push dust through air and it generally proved to be a sure fire way to redistribute dust around a lab. The times we were successful in pushing dust was using high-flow low-speed air movement through a large area filter (typically 1200 x 600mm) into one side of a room to overpressure the room so that the only escape was though a large area vent on the opposite wall. Behind the vent was a slightly lower speed fan to draw the dust air away. This effectively converted the whole room into a duct. Using high air speeds (or narrow jets or streams of air) were nowhere near as effective.

Thanks BobL, so from your description it sounds like maybe the dust leaves the tool at too high a velocity to be contained by a narrow flow of air, and actually needs a 'diffused' intake to draw it back in once it's lost its initial velocity.

Sounds like a fun project for a high speed camera...

Ok, I've re-read the 2HP thread a couple of times now and my head (and wallet) hurts, but my system is slowly. starting to take shape. I really have to thank the members who have contributed to the huge body of knowledge here, it is referred to in forums all over the world when hobbyists start to talk about dust control, so it is no small thing. Thank you.

I'm using the 3HP motor and blower from a H&F DC7, mounted to some steel racking outside the back of the shed and venting freely into fresh air, with no cyclone or bags. The intake is connected directly to a straight overhead 6m trunk of 6" DWV pipe, which turns vertical via 2 x 45° elbows then horizontal again at ground level via a 90° straight into the base of the TS using hard pipe all the way. There are a couple of capped Y inlets along the way for future general purpose connections.

I have cut out the TS port to 6" (although it currently still has a hard transition at the connection) and I've added a large vent to the top of the opposite side to form a diagonal flow down across the blade towards the port. I plan to do some further 'sculpting' inside the TS cabinet with expanding foam to ease the transition and fill some voids. I also plan to add an overhead guard with DC port.

Outside the shed I will add a simple tin cowl/roof to protect the (horizontally oriented) motor from rain. For giggles, I have oriented the blower to also eject horizontally, then bolted one of the separator rings directly to the blower outlet, with a 'skirt' of old shade cloth to help carry the vortex downwards and onto a large compost pile. My thinking here is that as well as directing the output, the separator will provide some additional weather protection to the impeller, as well as perhaps a less turbulent transition than a naked outlet, hopefully without adding a particularly great amount of resistance or having to fabricate anything new.

I was curious about whether the fine dust would exit at the top or bottom of the separator, but a quick check reveals that it functions much like a venturi and sucks additional air down through the open top so presumably all the fine dust is carried down to the pile too. I'm considering mounting a fine spray nozzle inside the separator ring to help calm the dust (even though it is safely away from harm) and keep the compost moist. It would be pretty easy to connect it to an irrigation solenoid so the mister could run whenever the DC does.

I'm yet to add the router, but I'm planning to build a table to the right of the saw (between the saw and the vertical 6" riser). This will make it easy to have a short duct both above and below the table.

Of course, I have a bunch of questions:

1. I have no test equipment currently, but I could pick up an inline power meter and make a water tube to test static pressure. Would that provide enough data to estimate my CFM?

2. The impeller intake is only slightly smaller than 6". I have removed the intake guard and rolled edge. Would I gain much more from modifying it further with a 6" BMH, given the rest of my setup?

3. I am planning to use short 4" ducts above and below the router table, and possibly for the TS guard, although I could potentially use 6" for some or all of them instead. Given that my trunk is 6", would having 2 x 6" ports on a machine be better or worse than 1 x 6" and 1 x 4" (on the TS) and 2 x 4" (on the router)?

4. Would I be likely to gain anything/much by going up to 9" pipe for the main trunk?

5. Would I be likely to gain anything/much by losing the separator arrangement and just fabricating a (presumably flared) rectangular outlet port?

The weakness in your system is the 12" impeller of the DC7. The 3HP does relatively

Originally Posted by Dusty Dave

1. I have no test equipment currently, but I could pick up an inline power meter and make a water tube to test static pressure. Would that provide enough data to estimate my CFM?

Nope - all the power meter or water tube will allow you to determine is if setup "A" is more (or less ) than "B", but unless you get into hot wire anemometers or pitot tubes, and lots of phapphing about there's no other way of measuring a CFM.

2. The impeller intake is only slightly smaller than 6". I have removed the intake guard and rolled edge. Would I gain much more from modifying it further with a 6" BMH, given the rest of my setup?

A few % would be my guess but given all you have is a 12" impeller you are going to need every few % you can get.

3. I am planning to use short 4" ducts above and below the router table, and possibly for the TS guard, although I could potentially use 6" for some or all of them instead. Given that my trunk is 6", would having 2 x 6" ports on a machine be better or worse than 1 x 6" and 1 x 4" (on the TS) and 2 x 4" (on the router)?

Given the 12" impeller setup 2 x 6 would not give you anything extra over 1x6 + 1/4". 2x4 or the router will be OK.

4. Would I be likely to gain anything/much by going up to 9" pipe for the main trunk?

I would advise against this as the air speed will be marginal and cause sawdust to fall out of suspension during any stalls and then not be able to pick it back up again.

5. Would I be likely to gain anything/much by losing the separator arrangement and just fabricating a (presumably flared) rectangular outlet port?

Any separator on such a marginal system is going to reduce the flow. I'd only use a separator is you are not prepared to keep filters cleanish and empty bags when they are no more than 2/3rd full.