# Downdraft table design



## CharlesA (Jun 24, 2013)

I'm considering making a downdraft table. I would use a 4" dust grate at one end, 4×10". I'm thinking of putting in a piece of 1/4" ply sloping from one end toward the dust grate, and then two triangular pieces at an angle that would extend the entire length, the lower end of each at the mouth of the grate. Here is a pic. The triangular pieces are just lines in this sketchup model.

Any thoughts on this approach?


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## pmayer (Jan 3, 2010)

I think you're on the right track. I don't know if I'd pinch it up that tight on the front edge (farthest from dust port) as your air flow might get a bit restricted at that end. I would also suggest using a 6" port if your dust collector has enough pull to keep the air speed up with 6".

Also, you want to make sure that you have great separation on your DC when you use one of these. I previously used mine with a DC that had poor separation and it plugged the filter constantly. Now, with a CV cyclone the problem has gone away.


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## CharlesA (Jun 24, 2013)

Paul,

Thanks. I had thought of the pinching, but got lazy when I did the sketchup. Thanks for the reminder. I have a 4" ductwork system (Bill Pentz would be disappointed), so the 4" here will have to do.

I hadn't thought about the dust collector getting more fine dust here than on my stationary power tools, but that makes sense. I have a very basic drop box separator now.


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## timbertailor (Jul 2, 2014)

I would try to move the vacuum port as close to the center of the box as possible to provide a more even flow through the table.

I studied some designs recently and I found a good idea. Have two boards slope to the center of the box, with one just a little deeper than the other. Provides a steeper angle, using gravity better, and creates a low pressure area that aids flow.

Here is the down draft table design I added to my favorites.

Hope it helps.


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## CharlesA (Jun 24, 2013)

Brad, what do you mean by center of the box? I put the hole for the box resting of the bottom floor of the box, centered in the middle of the end. Do you mean something different?


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## pmayer (Jan 3, 2010)

You might be ok with your separator, but it will be good to monitor your filter and CFM for a bit after setting it up. My previous setup was a 1.5 HP DC with no separator and a cartridge filter. That setup would completely load the filter up within 10 - 15 minutes of heavy sanding. I had thought of installing a Thien separator, and I believe that it would have made a big difference. But I never got around to it and ended up going to a cyclone which has also solved the problem.


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## CharlesA (Jun 24, 2013)

Brad, Thanks for the link. I looked at that design. I don't think I understand airflow enough. I was trying to make a funnel, more or less, straight to the dust port. What would make his design more efficient?

Charles


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## pmayer (Jan 3, 2010)

Charles, mine is setup with a funnel going vertically rather than horizontal. I figured you were going horizontally for some practical reason, as many people do when they build these to be portable benchtop units. If you have the option of permanently installing this with the dust port at the bottom, you will have better collection.

Here is a pic of my downdraft. 6" port connecting directly in the middle at the bottom. I have a slanted plane going from each of the four sides toward the dust port, just like a big funnel.


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## CharlesA (Jun 24, 2013)

Okay, by centering it, you mean in the center of the bottom.

Yes, I was doing it from the end for a practical reason. I have a small shop, and I need this to go on my workbench when using it, and put away when I'm not.


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## CharlesA (Jun 24, 2013)

I was looking at something like the one on today's projects page and wondering if a more funneled design would be better.


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## timbertailor (Jul 2, 2014)

> Okay, by centering it, you mean in the center of the bottom.
> 
> Yes, I was doing it from the end for a practical reason. I have a small shop, and I need this to go on my workbench when using it, and put away when I m not.
> 
> - CharlesA


Since you can not mount the hose to the bottom, then, try to make your design more square, like the one you just posted. That looks like the ticket.

I have been toying with the idea of building one that sits on my router table. It already has a port for suction, just build a sanding table that sits on top and takes advantage of the porting already there.

Just an idea, since I know you are pressed for space.


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## CharlesA (Jun 24, 2013)

Brad,

Still trying to understand. The design you linked to is even more rectangular and has a port on one end (although focused in a smaller underneath, long rectangular holding box. Why do you recommend a more square size?

I thought the grate might be better than just a round pipe, as most other designs have, but I don't know if that is true.

Charles


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## timbertailor (Jul 2, 2014)

> Brad,
> 
> ........................................... Why do you recommend a more square size?
> 
> - CharlesA


Suction will be more even across the work surface if the vacuum source is fairly equidistant to all the edges of the box. That is why a funnel design is so efficient. A semi circle would theoretically be the best design if you have to provide suction from the side. A square would be next.

You could try varying the hole sizes so that the ones furthest from the vacuum source were larger than those closer, but that could turn out to be far more involved than either of us care to speculate on.

I would have to do the math but there is a correlation between the amount of surface area, the amount of flow provided by the DC, and the size of the holes used. Too little flow, and you are losing performance\efficiency. Not enough surface area, or too little, and once again, not very efficient.

Is it that important? Probably not to most but something to at least think about.


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## timbertailor (Jul 2, 2014)

Well, I found the table size calculations on Bill Pentz's site based on CFM and peg board with 1/4" holes.


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## MT_Stringer (Jul 21, 2009)

"I have been toying with the idea of building one that sits on my router table."

Me too, Brad. Especially since I can adjust the table height to a comfortable sanding height. 
I like the link you posted of the sanding station.


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## CharlesA (Jun 24, 2013)

The router table ideas is interesting.


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## CharlesA (Jun 24, 2013)

Thanks for the discussion. I have realized that the router table topper idea is my best option. I've raised up my workbench to help with my back issues, and if I do a 5-6" tall downdraft box on the router table, it will be about the same height. It will be 24×32". I cannot put the port in the center beneath the box with a 4" connection-there's not enough room. So, I will need to put the port on one end. So, I have three options for the design of the box.

1) my original idea, with a slope from the far end toward the dust port, using two triangular pieces to make a funnel. I can use either the 10" wide dust grate or a standard 4" port. Something like this:








2) I could use this nifty design that brad suggested, with two sloping boards, staggered so the dust drops from the one into a long rectangular box with a 4" port on one end.








3) Something like this more conventional design 









Which would you do and why?


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## timbertailor (Jul 2, 2014)

So, according to the chart, you would need over 700cfm of flow for a table that is 24"x32".

Not going to get there with a 4" port or most DC solutions under 3 or 4HP.

I do not think a lot of people appreciate just how much fine dust sanding produces.

A sanding station has to be really small or have a dedicated DC system, just to keep up.

Kind of discouraging, actually. I think my unit is around 425-450cfm with the Thien Cyclone, 4" ducting, and Wynn Filter.


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## CharlesA (Jun 24, 2013)

Right, but I already have violated Pentz's calcs by using 4" ducting throughout. I was fitting it to the size of the router table. I wear a 3M 7500 dust mask.

I appreciate Pentz's research, and can't argue with it. But he says that I won't have adequate dust collection without a 220v 5 hp dust collector with 6" ducting. That's not in the cards for me at the moment, so I do the best I can.


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## pmayer (Jan 3, 2010)

Brad makes a good point. Downdraft tables require a lot of air movement in order to work effectively. My previous DC delivered just a hair over 600 CFM at the inlet of my downdraft table, and in addition to the air/dust separation problems that I had, neither the CFM nor the air speed were adequate. When I upgraded to the Clear Vue cyclone, I measured nearly 1300 CFM at 7200 FPM, and that does an amazing job. I can barely pry the work piece off the top of the downdraft table due to the suction, and it grabs the lions share of fine dust from sanding operations.


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## CharlesA (Jun 24, 2013)

I just looked at the router range again, and I do think I'll at least drop it to 24×24, maybe even 18×18.

Any thoughts on the design (other than size)?


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## MT_Stringer (Jul 21, 2009)

I like the idea presented in picture #3 with a top similar to picture #2. 

My design will most likely have the dust port on the end. I am cramped for room and that would be the best spot for the hose to connect. The table would sit in a working area where I will be standing at the longer side. I am planning to make the box so it can be clamped to the table, which is 32×37 inches. Hmmm…maybe 20×24 or something like that.

I have a Grizzly air cleaner that hangs from the ceiling directly over head, and I can sit my box fan/filter thingie on the workbench adjacent to the sanding station. I also hook up my ROS to a shop vac with a Dust Deputy. Every little bit helps.


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## AlaskaGuy (Jan 29, 2012)

A Festool sander connected to a Festool vacuum eliminates the need for a downdraft table. Takes up less space and is more portable.


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## CharlesA (Jun 24, 2013)

yes, I guess instead of the $20 in parts and scrap plywood I could spend $700 on the Festool setup. But I would run short on my Porsche payment this month.


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## AlaskaGuy (Jan 29, 2012)

> yes, I guess instead of the $20 in parts and scrap plywood I could spend $700 on the Festool setup. But I would run short on my Porsche payment this month.
> 
> - CharlesA


I guess it's about priority's. I'd pass on the Porsche any day to get woodworking tools that work well.

I have tools that will out last any car.


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## CharlesA (Jun 24, 2013)

How would the Festool sander/vacuum combo help with the fine dust from hand sanding?


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## joeyinsouthaustin (Sep 22, 2012)

Not much time to comment, but think about designing the DC out of the loop. HVAC filters, and a HVAC blower motor. My design uses troughs to prefilter, and removable cleanable filters to catch fine dust. For downdraft you need movement much much more than suction. So think hvac.


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## TopamaxSurvivor (May 2, 2008)

No expert, but I think a sanding hood with an open front would probably provide more dust collection than just a down draft table. Air balancing with a low air flow on a table would seem to be more critical to accomplish collection than a hood. Just thinking out loud here. Maybe some air balancers are out there who could comment?


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## CharlesA (Jun 24, 2013)

Turns out Pentz recommends a downdraft table and a hood.


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## TopamaxSurvivor (May 2, 2008)

That is what I mean was a hood over the down draft table. Guess I did not get that our very well ;-) The air flow collection would be from one direction rather than having to collect particles from the surrounding atmosphere. I have worked on controls for hoods in labs, paint booths, ect. That experience was the catalyst for my comment.


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## CharlesA (Jun 24, 2013)

I built one this week. I used the conventional design, with angled pieces on both sides. I made it 24×16 because that was the size of scrap I had around. I but a round 4" port at one of the long ends. How does it work? I don't know how to evaluate it precisely, but I tested hand and using a ROS (without a vacuum attached) with it on and with it off. The thing I noticed was that with it on, I didn't smell the sawdust. But when it was off, I immediately began to smell the sawdust. That indicates to me that it must be doing some work with the fine dust.


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## TopamaxSurvivor (May 2, 2008)

The sniff test is a good indication things have improved.


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