Updraft Sanding Box

[Poobah]

Just completed my down, err, updraft torsion box. THis was mostly to practice for the mobile saw base. Luckily I realized before it was too late I need to allow for airflow, so the ribs are staggered here and there (think Star Wars when Luke goes in to kill the Death Star).

I made one long air channel that snakes its way from the far corner over to where the hose connects.

I was worried the dust collector would be so strong it would make the box self-implode or something, but actually the holes are so big in the hardboard that I am going to have to plug a few. There is strong airflow for about a foot after where the hose connects.

Bottom is 1/4 ply, top is 3/8 hardboard and sides and ribs are 1/2 MDF 1 inch wide (so the box is almost 1-1/2 inches thick). Spacing is about 6.5 inches, but keep in mind not every rib touches both the top and bottom.

The box is amazingly rigid (about 40x22 inches - could even make a sweet extension table for the table saw).

The mobile base is next this weekend. Hope I can cut my 96 inch strips straight.

[LCHIEN]

I know its easier to criticize than to create, but you might take a page from the original Ryobi BT3000 downdraft/updraft table.

The air inlet was from the bottom or end rather than the top losing workspace.
The internal air path was somewhat serpentine, the table had some gates which could be used to close off the farthest portions of the table in sections. The strategy was this - place the workpiece over the corner closest to the air inlet/outlet, close off areas of the table outside say 6 inches to 12 inches past the workpiece. The workpiece would block off the holes directly underneath and only the holes int he active strip around the work piece would have suction, losing less suction thus hodling down the workpiece and having strong suction nearby for sawdust. The magic is in limiting the number of holes open rather than making the holes smaller.

Seems like if you wanted to you could take the top off and put some thin (1/4" plywood) gates over a few of the channels where it passes by the front of the table. You'd have to cut 1/4" slots in your front member, though.

[Poobah]

Never thought of being able to close off sections. That's a cool idea. My low tech solution so far has been to put a sheet of paper over parts of the top.

I did intend for the opening to be below for a traditional downdraft setup, but then I was thinking I would prefer to be able to set this on top of a workbench. The next thought was to put the connection on the side of the box, but somewhere my little brain picked up on the difference between the 4 inch diameter hole and the 1 inch high ribs ... so in the end I thought putting it on top would be OK.

The interior is serpentine as in the diagram below. I have a few more ribs inside, but for clarity this represents the build.



I did a little math last night while I was falling asleep, so it might be totally wrong ... but I think it makes sense now that I don't have nearly enough airflow.

The area of the 4 inch diameter hole is 8*pi sq in (2*pi*r^2). There are about 900 1/4 diameter holes in the top. I'm guessing at least 100 are covered by ribs. Each hole has an area of 1/32*pi sq in. That means 8 / (1/32) = 256 small holes have the same surface area as the 4 inch connection. There is no way I'm sucking enough air out of the box.

I think what I might do is put another sheet of 1/4 ply on top of the hardboard and drill a more appropriate number holes.

[LCHIEN]

...
The area of the 4 inch diameter hole is 8*pi sq in (2*pi*r^2). There are about 900 1/4 diameter holes in the top. I'm guessing at least 100 are covered by ribs. Each hole has an area of 1/32*pi sq in. That means 8 / (1/32) = 256 small holes have the same surface area as the 4 inch connection. There is no way I'm sucking enough air out of the box.

I think what I might do is put another sheet of 1/4 ply on top of the hardboard and drill a more appropriate number holes.

reducing number of holes to reduce area is not necessary if you block it off either with paper, carboard or gates. That will effectively reduce the holes and area. If you have fixed limits e.g. number of holes only then you will have too much restriction and won't flow any air to keep the DC happy when you have small or large opjects.

too bad you ran the serpent back and forth instead of up and down... I could see cutting a slot at every turn at the front and installing a plywood gate with a couple of large holes cut in it. Push it in or back it out to allow air to flow past that point when the holes align with the passageway.