Down draft table use the suction power from you shop vac or dust extractor to pull air downwards below your work surface whilst you work, before the nasty bits of dust have chance to get airborne. This makes them great when sanding, routing or otherwise making a dusty mess.
Don't expect it to magically make all dust vanish, as it relies heavily on the amount of air your vac can pull through the table top.
Here we make an efficient downdraft table to help keep dust down whilst working safely and minimising clean up time!
Step 1: Make the Box.
You can make the box from what ever you have, solid lumber, plywood, MDF, even plastic/acrylic sheets.
Cut the parts to make the box to your own dimensions, but bare in mind that the larger you make it the more air the vac needs to pull through the top., I Made mine to fit into a rolling trolley that used to be a welding surface, but you can easily make a bench top version.
Step 2: Make the Hole to Attach the Vac Hose.
I'm using the same fitting from My table saw, but what ever method of attaching the hose you use, the hole needs to give a snug fit to the box.
Drill a suitable hole in one side of the box with a suitable sized drill, I'm using a Forstner bit, but a holesaw or Flat fit also works.
If like me you don't have the right size drill bit, you can also use a router to cut the hole with a circle jig (I'll have an intractable on that soon!)
I actually only needed to enlarge the hole I drilled so used special router bit to do this.
Step 3: Make the Main Body or Box.
With the right size hole in the side of your box, go ahead and fit the base. I cut slots in all 4 sides of the box to accept some thin Plywood on the table saw, I made sure that it was nice and close to the bottom of the hole to stop dust settling on the bottom the box It also means a smaller amount of air inside the box, and hence less air needed to be moved by the vac, making it as efficient as possible.
Once your happy with the fit, go ahead and fix it all together, I used glue and a brad nailer for speed.
Later on you can seal up all the joins with some silicone, but hold off for now.
Step 4: Make the Inside As Efficient As Possible
In order to make the inside as efficient as possible I made some slopping ramp inside to help direct the air and dust towards the hose. This also means that the volume inside the box is kept to a minimum and helps the airflow smooth and giving an easy directional flow towards the vac.
To do this first measure and cut a piece to slope towards the hose from the opposite edge. Test the fit to make it as snug as you can.
When you're happy use a hand plane or sandpaper to chamfer the back edge to give you a flat surface to glue to and glue in the ramp.
Step 5: Fit the Side Ramps.
Now at this point you could skip this step, but to make it as efficient as possible, it's well worth cutting the side ramps, to increase the tables efficiency still further.
The triangles look a bit complicated to work out, but really all you need is a large compass and a straight edge.
The triangle needs to fit between two corners of the box, and a point next to the hole for the hose attachment.
Use the compass to give you the distances between one corner and the hose, and transfer it to the piece that is to become the ramp.
Now do the same thing between the two box corners.
Now if all has worked out, you can check your measurements by checking the distance between the second corner and the point next to the hose and it should all line up perfectly.
Just use a straight edge and pencil to mark the hypotenuse, and cut out your triangle. Plane or sand the back and fit in the same way as the first ramp.
Now cut the second triangle.
Step 6: Seal Up the Inside.
I like to seal up the inside to keep it as airtight as possible, its also an idea to varnish/seal the surfaces to make them smooth as well if you like. before the top goes on.
Step 7: Cut, Mark and Drill the Top.
With the bulk of the work done we now just need a top.
I used 18mm Ply for this as i had some, and I want it to last a long time.
Cut the top to fit the box you've made, and mark out where you want the holes.
I drilled 20mm holes, at 100mm centres, but the pattern is up to you, the smaller the holes and less of them you have the stronger the pull of air, but if there aren't enough you can put undue stress on the vac motor. Too many holes, or really big holes let the air through, but can lessen the pull of the air.
This size and pattern I felt was a god compromise for my set up, and the vac I have (which is a heavy duty industrial/workshop type vac).
Step 8: Sand and Finish.
I also decided to chamfer the tops of all the holes, this not only helps a bit with airflow, but also helps stop things snagging on the edge of the holes when i use it as an outfit table for my table saw.
With it sanded smooth, i screwed it to the box and applied a varnish to smooth it out still further and help prolong the life.
Step 9: All Set and Ready for Use.
With everything in place, It's great to use as an assembly table, out feed table sanding station and router station.
Although it doesn't pull all the big ships through it's great at pulling all the fine dust out of the way before sweeping the big bets across the top and down through the holes. In this case a very messy router surfacing job is now half as messy and requires 30 seconds of clean up time!