Downdraft Sanding Table

[KWaugh]

I have acquired a nice double squirrel cage fan which Grainger states is 2150 cfm https://www.grainger.com/product/DAYTON-Fan-4C756 and I think it is the perfect fan for a downdraft sanding table. I have the basic thoughts of a cabinet with a sloped deck under the top aiming to one side but falling short. A box that slides in from the side which will house the filters as a filter cartridge that can be pulled out for cleaning, which will divide the fan from the area where the dust falls. A sealed drawer that will help in cleaning the dust out. And of course long routed slots in the top to move as much air as possible.  Am I missing anything, does anyone have pictures of their downdraft tables to help in ideas. 
Ken

[Dara]

I built one a few years ago.
I would suggest buying the inserts from busy bee or harbour freight. Formed steel with holes and rubber grommets...they seem expensive but worth it in the long run.
I made the top with sides and end, inner top angled down to an opening with prefilter . fan and motor below that and cut a slot in the side for furnace filters...good ones and double up. through fan then a fine filter to finish it off. I built it right in to a big work table

[BigD]

Here are some pictures of one that I made using a squirrel cage fan. If you have any questions, just send me a private message.

   

   

   

   

   

   

   

   

   

   

[Steve N]

D how does it work? I have more motors than a motor repair shop. I've considered a smaller unit, or something smaller attached to a bigger roll around.

Is the black ?plastic? grill in front of the air filter just a grill? Or is it something else?

What are the specs on the motor you are using? I always wondered being around all that dust if a TEFC motor would be necessary? Didn't know they put those in squirrel cage applications.

How did you grid out, and hole out your top? Plus what hole size is that? Did you try a few sizes, or just pick one and went with it? I use a stupid simple thing I called the "Swiss Cheeser" to make aligned holes.

   

   

Made with whatever sized hole you want, get a pin, bolt, or whatever the inside diameter. Once you have drilled the field, you lose a line of holes, but your pin indexes you to the left or right, drill them out with a hand drill, and a good quality boring bit. The 1 1/2" depth is the perfect height to start you drilling 90*.

Quick work, accurate grid to whatever hole size you want. Use it to make a top like this, or for a FMT table type application. Plenty of hand tool helpers, or Festool accessories you can work with the correct sized hole.

[KWaugh]

Big D
That is just what I was looking for.  Do you just vacuum out the area before the filters? I was thinking to make some sort of drawer to remove the bulk of the dust. I think my design is going to be more like Dara's with the slope going almost across the whole table. Do you think you get better suction by sloping both sides to a common center point? Kind of looks like it might.

[BigD]

It works well. To remove the dust that is trapped by the filters, I just remove the side door (shown in the last picture) and vacuum it out. Note that there are two filters stacked. The first (what Steve N refers to as a black plastic grill) is what I call a "rough" filter. It is a heavy duty unit to catch the larger particles but is not a "fine" particle filter. The second is the "fine" filter.

If you just use the "rough" filter, it will not load up very quick, but will let the small particles through and back into the room. If you just use the "fine" filter, it will catch the small particles, but will become plugged up quickly. Using both works the best. When I check the filters, I can take the first (rough) filter outside and bang it on the sidewalk and that gets rid of most of what was caught. It can then be reused. I check the second (fine) filter and if it is becoming filled, I toss it and replace it with a new one. Also note that in my layout, the air passes through both of the filters BEFORE it gets to the fan and motor. This eliminates (or at least greatly reduces} any requirement for a TEFC motor on the fan.

As far as the layout, I like the double slope layout because it reduces the distance the particles have to travel before they get to the filters. If you only have on slope, it could possibly have some "dead or reduced" areas that could have some buildup instead of everything getting to the filters. I say possibly because there are a lot of other factors that enter into this such as overall size, volume of air being moved per minute etc. While I am a retired engineer, this type of design was not my specialty, but it is my best guess.

As far as the blower, it is one that I picked up from a local heating/cooling contractor. It has a 1/3 HP motor and while I do not have a rating for the air movement specs., it seems to do the job.

The hole layout was not a calculated layout based on any specifications. I just made a grid layout on three inch centers with every other row offset 1 1/2". This seems to provide enough cross-section area for good air flow. The holes are 1 inch diameter and then I used a router to put a 1/4" radius at the top of each hole (and yes, it took a while). I just felt that adding this radius would help to improve the air flow.

And last, note on the last picture, it shows the switch box with the conduit. This is not just an on-off switch, it is a timer switch that can run up to two hours before it shuts off. I find that this is helpful because in addition to running it when sanding etc., when I am ready to shut down for the day, if it seems like there is still some dust in the air, I crank it over and it runs (and shuts off based how it is set) after I am back in the house.

[Steve N]

D, thanks for the feedback.

I think your rounding over the holes will also allow that plywood top to last several times longer than not doing it. Left flush from the drill you get a lot of snags, and soon enough they start tearing back, the more that happens the quicker it goes. Do you remember where you got the black plastic grill (filter for rough stuff)?

[KWaugh]

I like the timer idea a second air filtration system in the shop.  I will use your idea for the 2 sloped sides since it is working good for you, plus I can make drawers on the dead side for sandpaper and such. Thank you for all your comments 'D' you have answered a lot of questions I had in mind.  This will be the next project on my list so I will post pictures of the completed set-up when done.

[BigD]

D, thanks for the feedback.

I think your rounding over the holes will also allow that plywood top to last several times longer than not doing it. Left flush from the drill you get a lot of snags, and soon enough they start tearing back, the more that happens the quicker it goes. Do you remember where you got the black plastic grill (filter for rough stuff)?

I can't say for sure, but I think it was Ace Hardware. It has been around 10 years since I made it.

[Steve N]

Thanks D, I'll check the local. Probably there are several products which could do that work. Would you say the open areas are close to "hardware cloth?"