I’m working on a simple design for a downdraft table for use with my dust collector. I’ve searched the archives here and read several threads, which have been very helpful, but I’m still uncertain as to what would be the best sizing and spacing of holes in the table top. Some have used 1/4″ peg board, in some cases enlarging and/or chamfering the holes. Others have drilled holes in plywood, of various sizes and at various spacings. Would anyone care to offer an opinion – or a scientifically based definitive answer – as to what would work best? It seems that it would be about the same amount of work to enlarge or chamfer the holes in a piece of pegboard as it would to drill new holes in a piece of plywood according to an ideal sizing and spacing. Pegboard would also have to be reinforced for stiffness.
The other question I’m still uncertain about is how to shape the box underneath – whether it makes much difference if I slope the sides into sort of a square funnel shape (and, if so, what slope) or just make a square-sided box.
I’m not trying to engineer the world’s most efficient downdraft table, nor do I expect to collect 100% of the sanding dust, but it would be good to get the most out of my 650 cfm DC.
Finally, would any of you air flow experts care to tell me if I’m out of my gourd to think I can make a downdraft table with a top surface of 36″ x 24″ that will work reasonably well with a 650 cfm DC? I could make it somewhat smaller, but not anything smaller than, say, 22″ x 30″ wouldn’t meet my needs. BTW, I roll my DC around the shop and connect it directly to each machine, so there will not be a lot of loss due to long ducts, etc. In the future, I’ll probably upgrade to a larger DC and run ductwork around the shop.
Thanks!
Replies
Most of what a down draft table collects is the dust that would have settled onto a solid benchtop anyway, the table just gets it out of the way and prevents the settled dust from being stirred up and kicked back up into the air.
A simple downdraft table doesn't do a good job of capturing dust that has gotten airborne. Once the airborne dust has gotten more than a few inches above the table surface the air flow isn't enough to reliably pull it back down and into the filter assembly.
Bottom line: make the grid as open as possible for the greatest airflow and use as big a fan or dust collector as you can get to power it, details about hole size and spacing and the shape of the box will make little difference, basically keep it simple and move as much air as possible.
John W.
Edited 12/1/2004 8:35 pm ET by JohnW
The main design factor is to keep a negative airflow around the piece you are sanding. You can always place some scrap materials over the open holes if you don't have enough down draft. I have a 1200 CFM unit on a 24x36 table and it sometimes doesn't have enough DD on smaller peices.
You'll have to experiment with what works for you. MDF is cheap and machines well.
I really don't use the table much anymore since upgrading to Festool sanders and vac. It isn't warranted with these sanders.
Todd I think you may be over thinking the design for such a small table.
The spacing of the holes is not critical as long as your dust collector is not starved for air. Your table will be small enough that you will see a significant difference with sanding dust not becoming airborn. I made my downdraft table with a squirrel cage fan and the actual sanding area is approximately 24x72 inches. You can see the sanding table and construction at http://www.superwoodworks.com/Projects/SandingBench.htm
One of the reasons that I used pegboard for my top
is because it is replaceable and it does not mark up the bottom of the piece that you are sanding because it is hard and smooth. You will need plywood or tortion box type construction to support the pegboard. The box below it does not need to be any deeper than your connection allows, just make sure that your collector can always draw air no matter how holes are blocked by the piece that you are sanding.
It is likely that a slope will be an advantage. If you look at the down draft table artical in the recent Fine Woodworking Tools and Shops issue on page 48 I think you will find a desgn that will suit your needs.
I disagree with the statement made by JohnW above that
>"
Most of what a down draft table collects is the dust that would have settled onto a solid benchtop anyway, the table just gets it out of the way and prevents the settled dust from being stirred up and kicked back up into the air.
A simple downdraft table doesn't do a good job of capturing dust that has gotten airborne. Once the airborne dust has gotten more than a few inches above the table surface the air flow isn't enough to reliably pull it back down and into the filter assembly."
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You will likely notice dust particals being pull down from the close by walls and shelves.
I use my down draft table to help clear the air in the shop after blowing off the walls and cieling to prepair for large finishing jobs. If I turn on the sanding table the fog of dust is usually not visable to the eye in less than 10 minutes. The shop size is 28 x 40. I know my design is differs greatly from yours but the point is you will notice a significant difference in the shop air.
Garry
Garry,
The argument that I was making is that while actively sanding, a certain amount of dust, usually the finest, will be thrown into the air, stirred up by the sander's cooling fan, the movements of the operator, and turning and moving the object being sanded and that this dust will stay airborne rather than being drawn back down into the box.
This is because even a foot or so above the grid the downward velocity of the air becomes quite slow and won't overcome the air currents stirring up the dust. Eventually this dust, as you pointed out, will drift back towards the table and be captured, though it might be some time later after the dust has drifted around the shop for a while.
My basic point was that there wan't much to be gained by trying to create a optimum design for the table, the basic physics is that much of the dust will go down and some will drift away pretty much irregardless of the downdraft table's design. The best you can do is move as much air as is possible through an effective filter.
I would use a top grid that is much more open than a piece of peg board to get the maximum airflow. Your design, while elegant and beautifully crafted, cuts the total airflow considerably by forcing the air to flow through many small openings and passages.
John W.
For years I used random orbit sanders hooked up to shop vacs. That system did not do a very good job. After expanding my shop, I built a 4' X 8' downdraft table. I used 1/4" pegboard for the top, a furnace blower & pleated furnace filters. I do not know what the CFM is on the blower. The blower exhausts out the bottom of the table cabinet. It does an excellent job of collecting dust. It does such a good job that I did away with the shop conection to the sanders. I have reinstalled the dust bags. No more tripping over hoses. I do not get dust floating in the air. I wish I had built one years ago.
I find the same benefits as you do Chris with the same basic design.
http://www.superwoodworks.com
Thanks John W, Craig, Garry, Chris, and Bill for sharing your experiences, observations, ideas, and suggestions. It was all very informative and helpful. Special thanks also to Bill for posting the photos and drawing. I'm building the table today. It's actually part of an overhaul of a large used workbench with a pine benchtop that I got for free from a friend who was reorganizing his shop. I cut a big hole in the top at one end of the bench and am in the process of building in the downdraft apparatus. So far, so good.
Downdraft table
Hello, I'm seeing this post was from 2004, but I am currently wondering the same thing that you were over 13 years ago. I chose to use a plywood for my sanding table top for more stability, and I am going to be using my cnc router to make the holes into the wood for the downdraft. I was curious what you decided on and what you have learned about it over the years. I bought a "big gulp" funnel attachment for inside the table that will go to the dust collection system. The area that I will be putting holes into will be about 15"x11" it's not a big area, but hopefully it will help. How big and how close together should the holes be?
Todd, attached are two pics of my dust table and one drawing. This was my own design out of the top of my head, no calculating cfm's. I hooked it up to my Oneida 3hp and it works quite well. The slots were cut with a dado blade on the table saw raising the blade into the wood but making sure the peice was properly held with hold downs etc. The plenum starts 5" tall and tapers to 2" tall over 8' as can be seen in the drawing. If you can enlarge the drawing you can see the details a little better. Let me know if you have more questions, good luck.
Bill
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