Downdraft table ideas?

Well, we’re making progress settling in to the new place. One of the things that I’m looking forward to is doing more woodworking inside. For the last 5 years it’s all been in the driveway. But that means i need to figure out how to sand parts without creating clouds of dust.

I’ve got two box fans and a portable dust collector. I’m thinking of building a downdraft table with the two box fans, but I’m not great for design and features. I suspect the fans won’t be phenomenal at this, but they are advertised at something like 1000 cfm each, which maybe means 800-850. Probably cut in half again (or more) by taping filters to them. Maybe even more by restricting flow through the table top. But still…better than nothing.

About as far as i can get is a box, 22x35x40 give or take, holes in the sides for the fans, filters taped on for simplicity, and a hinged top of mdf or plywood with (lots of) holes cut in it.

Is anyone familiar with an actual design?

My other idea is to make a shallow top and install a fitting to plumb the dust collector. It handles the bigger stuff better, so I won’t have up worry much about cleaning out any trays or figuring out baffles to extend filter life, and the air flow might only be a little less than two fans, or maybe more after the filters and everything. Then I could just hang the fans out in the open. Somebody on YouTube measured a diy scrubber built from box fans, and it worked pretty impressively. Does that sound like a better idea?

My mental model on fans and vacs is that air volume is only half the story. Air pressure is just as important. They may be able to move 1000cfm when there is almost no resistance, but if you put a piece of cardboard on the intake, can they even hold it up?

I like to think of the electronics analog of a power supply with 100A capacity, but only 1V. You won’t power your Rambo with that, even if it can supply 100A.

My guess is that you will need to make a balloon holes in a downstate table to get any air flow. And it will not pull the dust very strong, but it can move a lot of air through.

That’s just my guess though. I would try first with something like a cardboard box before building a big table for it.

I have seen designs with box fans that filter the ambient air. Since they move a large volume, they should be able to clear the fine particles in a space quickly. You can just attach a furnace filter to the inlet.

The dust collector is going to be better at making a down draft table. That’s my guess.

Matthias Wendel has a bunch of interesting diy dust collection machines on his YouTube.

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Thanks for coming in, Jeff. As usual, you got me thinking. Buckle up, I’m rambling (and maybe learning) again. I’m sure we’ve got a member or three that studied something relevant to all this and can correct me, but here goes.

That’s basically what I’m getting at (re fans). I have a cnc or two to make all those holes for me. My mental model of fluid says the biggest hole I need is the size of the fan, and the smallest hole I need is only a little bit smaller unless I want to live with restrictions. If I’m only going to get half the airflow because of a filter, maybe I need something less than half the cross section for a hole, but I don’t know how much less (it may even be 0 less).

But I’m not trying to suck up dust. I’m just trying to create a draft table, vs a suction table. I really don’t think strong suction is necessary for this, and higher volume is better. (I think I found the rub near the end of this post).

I used a box fan to collect the powder coat overspray from a quick cabinet I made. You may or may not remember pictures of something I posted demonstrating why a guy like me needs a cnc, because the cabinet was pretty rough compared to a spray paint shelf i cnc’d, lol.

At any rate, it did a great job at that. The fan was pointed out the back, slightly above the work (space constraints) maybe 18 inches away from the coater with a wide open front, and had a large opening for the fan, covered by a pretty good filter. I’d used a cheap one at first, but it let too much through and pretty well wrecked the fan. Even stuck at low speed with a better filter, it did a good enough job that we still used it, so the fans DO have some pressure.

I figure for a downdraft table specifically for sanding, gravity is going to do a lot of the work. I only occasionally use an angle grinder. Most of it is a palm sander, followed by hand sanding, then dremel, then belt. Maybe we can classify dust into two categories : heavy enough to be significantly affected by gravity, and everything else. Each might get ejected at speed from or by a machine, or not so much, and basically all hand sanding would be slow dust.

Heavy slow dust will land on the table without much help, but the better the flow(down), the less it will travel.

Heavy fast dust needs high flow the most, but will settle to the floor (and other things) quickly where it will be swept, wiped, or vacuumed up.

Light fast dust will probably slow down quickly because momentum vs air resistance, and be more likely to catch a draft into the table.

Light slow dust will be most likely captured, as long as there is any draft at all close to it.

Learning starts here:
I double checked the listing for my actual fans and they claim 2300 cfm. But suppose I’m wrong about losing half the airflow. Maybe it’s actually closer to 75%, even, with a good filter, and they start out at 2k in the real world. Then for a pair of 20x20 fans at 500cfm each, if I can give 200 sq inches of holes (2x20x20x(1-0.75)) in a 1400 sq inch table, then the table top wouldnt be an EXTRA restriction. If I’m wrong and the filters are only only a reduction by half, maybe I need 400 sq in of holes. Either way, i think if there aren’t enough holes, the fans bog down way before air velocity goes up (and I don’t think it ever actually goes up, honestly), reducing effectiveness. If I have too many, the air moves somewhat slower than it might, reducing effectiveness.

I think that’s where the dust collector wins. Maybe not because of the actual extra power it has, but because the extra power means i don’t have to be so precise on the calculation. Does that make sense?

Added bonus, turning the fans into scrubbers means they don’t have to be in the same spot and can work in addition to the draft table.

The last company I worked for had a central dust collect system. If we were going to do some sanding we would open the gate and plug the ~4" flex hose to something like this,Downdraft Table Panels, set of 2 | Rockler Woodworking and Hardware, in the table in the middle of the room 24"x24". There’s was a little more commercial, but same idea. It worked well enough. The best thing they had was a dust pan hooked up to the vac as well. So you could clean up in a pinch. Showing clients a busy…yet extremely clean workshop was a priority.


A 1400 sq inch table, with 1000 cfm translates to 20.5 inches per second of average downward velocity. I don’t feel like I have good intuition about air flow, but I can calculate. And 20.5 inches per second is about 1.1 mph if that’s more meaningful as a wind speed.

Those speeds are at the table, and if it draws air from all directions, the speed might be much lower as you get higher above the table. If you could shield the table on three sides, it could have a significant effect on the air speed above the table, and effectively shield against minor drafts blowing from the side. It could also block the heavier particles from escaping laterally.

Similar to what Jeff said, I think of air flow as analogous to electrical impedances, so for example a single blower passing through two filters in parallel will have twice the throughput if the filters behave like resistors and if the fan behaves like a constant voltage (constant pressure) source. But I’m pretty sure a real fan wouldn’t behave like a constant pressure source, and I have no idea what the IV curve looks like. Centrifugal blowers and axial blowers are probably different.

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I hadn’t thought of calculating the mph, but it seems intuitively useful. I think 3 or 4 mph is average walking speed for a human. That seems like the lower bound for what I’d expect to be effective.

I’m pretty good at calculating, too, once I have an idea what I’m looking for, so I did this:

Assume my DC (advertised at 650cfm) is close to 550 after a short hose and the bag. That means I’d have to shrink my table to something like 12x20 or 15x15 and make enough holes to not slow it down any more. Since the inlet hose is already about 4in diameter, anything over 13 sq inches might be enough holes. That could actually be a useful size.

Getting up to that speed with two box fans would require about a 20x20 table, which is the same size as a single fan, so not large enough to not slow the fans down even more. If i could do with half that speed, the box fans become workable, but then the dust collector could support a table only slightly smaller with no extra filters or cleanup, and it’s ok if my assumptions are off a little.

I think I’ll go look for videos where anyone might have actually measured real cfm under any of these restrictions for the portable dust collectors and box fans.

I had a downdraft table for a few years (20 years ago) connected to my dust collection system. Note the past tense. The bottom line is that in a home workshop environment they are a great idea but In my case at least never seemed to work well enough to justify the bother.

Sure you could remove dust, as long as you were only dealing with a small object and kept it above the table. If I was a model maker with a lot of hand sanding of small objects I’d consider it, but not for anything else.

In my shed a proper shopvac attachment to the sander removes a significant amount of the problem. A good bellmouth over the bench connected to a 2.5hp collector takes a lot of airborne dust, extractor fans keeps air turning over.
Then an air filter takes care of the rest!


That is straight out of Dr Seuss


It makes me smile every time I’m using it. :smiley:


There are plenty of them - pretty much all that is known is summarised somewhere on Bill Pentz’s pages though.


Found a video where a guy measured his fan (similar to mine) with an anemometer. Good news is that 2 box fans are absolutely capable of drafting the way I want. Bad news is that it’s gonna take A LOT of filter.

A single merv 13 (20x20 like the fan) brings the fan from 2000 cfm to about 250. 4 more filters to make a cube with the fan as a side boosts it back up to about 1000 cfm. So, i see a few ways to make it work.

Less capable filters (<merv 13). Not sure how much this will improve flow, but if the point is to capture dust well, it seems like the wrong direction. Better to make the cubes and hang them from the ceiling.

Stuff all the filters into the table. This would require some very clever baffling to divert the heaviest dust away from the filters without choking the airflow. Definitely beyond my design skills given my available time right now. Maybe i could figure it out with enough hours.

Thicker filters with more material in them. Probably the easiest way to make up the difference, but I can’t remember the last time i actually saw a decent assortment. I guess the internet could be my friend here, so i won’t rule it out.

Really small table with the dust collector. After reading Penz’s articles, i wonder whether it’s worth the hassle. He had a much larger DC, but remote, and I think it was pulling a little more than my portable would right next to it. I also think my standards are much lower though, since I don’t have allergies and i live in the desert anyway. This would still probably be the most effective, but I do need to do something about the dc bag. It may be time to just get a bigger dc.

Actually i have a couple more ideas, but I need to sketch them because they seem a little stupid in my head. Like a hood with a fan and filter array on top and another on the back so the heavy stuff falls away from the filters anyway. Just one idea.

Thanks for all the input and the links. I learned some more.


I had this in my maybe oneday folder from James Hamilton aka StumpNubs:


Current state. I can feel plenty of air blowing out of the fans, but, I can’t promise it’s all moving through the filters. When I stick my face in it, it cools off a little, which says at least SOME air is going the right direction.

The joints were pretty tight (fixed with liquid nails, too), but I didn’t spend any caulk make sure. If it works pretty well, it’ll be worth sealing up. If it barely works at all, caulk isn’t going to solve the problem. I also came across a different fan this morning that advertises 7000 CFM at $150. I’d bet it has a much higher static pressure, as well, so I could block off one side, replace the other fan with that one, and turn the two box fans into the proven cube-scrubbers.

The gap between the supports and the inside wall is about 2 inches, maintained all the way around. 4 inches from the top to the upper filter.

Got some powder coating to do soon, so that should give a pretty OK indication. Also have to make some cleanout shelves for those pockets on the bottom to help with the heavy stuff. Also planning LED strips inside at the corners if I keep it, and a few switches to control lights, fans, and power strip.


It’s absolutely everything i hoped it would be. This is the best I could do in terms of video. I tried 3 times, lol.

You can see my sander kick the dust out behind me and get drawn back in.

It won’t replace vacuum-on-tool. Even if it sucked up all of the little stuff, the heavies would still fall out of the cabinet onto the floor. And the filters would load up way faster and stop working.

But for what was in my head, this is a great result.

The second video is the missus spray painting a cup. I’d hoped she was ready for a color paint, but even with the white you can see that it’s not blowing back out.


Looks pretty good to me!

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Very different from the first idea I had. Thanks to the team here for the discussion, which motivated me to keep thinking.


I just vent my fines outside, but I cannot spray paint like that with mine. Also a note of caution on flammable paint fumes and static electricity… be careful. :wink: