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Downdraft table design

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Forum topic by CharlesA posted 10-20-2014 06:32 PM 1781 views 0 times favorited 31 replies Add to Favorites Watch
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CharlesA

3025 posts in 1264 days


10-20-2014 06:32 PM

Topic tags/keywords: question sketchup downdraft sanding

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?

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson


31 replies so far

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pmayer

864 posts in 2531 days


#1 posted 10-20-2014 07:23 PM

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.

-- PaulMayer, http://www.vernswoodgoods.com

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CharlesA

3025 posts in 1264 days


#2 posted 10-20-2014 07:31 PM

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.

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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timbertailor

1592 posts in 890 days


#3 posted 10-20-2014 07:47 PM

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.

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

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CharlesA

3025 posts in 1264 days


#4 posted 10-20-2014 07:49 PM

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?

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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pmayer

864 posts in 2531 days


#5 posted 10-20-2014 07:50 PM

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.

-- PaulMayer, http://www.vernswoodgoods.com

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CharlesA

3025 posts in 1264 days


#6 posted 10-20-2014 07:54 PM

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

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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pmayer

864 posts in 2531 days


#7 posted 10-20-2014 08:27 PM

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.

-- PaulMayer, http://www.vernswoodgoods.com

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CharlesA

3025 posts in 1264 days


#8 posted 10-20-2014 08:47 PM

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.

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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CharlesA

3025 posts in 1264 days


#9 posted 10-20-2014 09:30 PM

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

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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timbertailor

1592 posts in 890 days


#10 posted 10-20-2014 11:38 PM


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.

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

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CharlesA

3025 posts in 1264 days


#11 posted 10-20-2014 11:45 PM

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

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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timbertailor

1592 posts in 890 days


#12 posted 10-20-2014 11:58 PM


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.

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

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timbertailor

1592 posts in 890 days


#13 posted 10-21-2014 01:38 AM

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

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

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MT_Stringer

2854 posts in 2697 days


#14 posted 10-21-2014 03:11 AM

“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.

-- Handcrafted by Mike Henderson - Channelview, Texas

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CharlesA

3025 posts in 1264 days


#15 posted 10-21-2014 03:16 AM

The router table ideas is interesting.

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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