Down Draft Sanding Table (On the Cheap!)*

I had a commercial down-draft sanding table when I had my own shop. It was great at keeping the dust in the shop at tolerable levels. But that table is long gone now. These days, I volunteer at our local SLO MakerSpace as the Woodshop “Guru.” We had a few “extra parts” sitting around, so I thought I’d build a new table for the Space because I really liked mine.

What we had:

Dust Collector fan and motor- 1hp 110v
Multiple sheets of used 3/4” Melamine. (need two at most)
4mm x 4’x7’ sheet of Kortron (Looks like Masonite with one side painted shiny white.)  1/2 sheet of 1/4” or 1/8" Melamine or plywood will work ok.
Flooring from a Mercedes van. (It has a great looking grey bumpy finish, but is a little thin at 8mm.)  3/4” plywood would be better and not need any bracing. Need 30”x48”.
Assorted pieces of scrap ply, “one-by” and “two-by” lumber, screws, glue, and other odds and ends.
Biscuit Joiner & biscuits, or other means of joining casework.
CNC (handy but not necessary)
Woodworking tools.

What I bought:

Casters
Hinges: Piano,and door
Foam weather stripping: 1/2x3/8”x40’ and 3/4”x1/2”x20’ (these come in 10ft rolls,so you’ll have extra.)
Six furnace filters 16”x20”x1”; MERV-8. standard duty filters.
Electrical switch, duplex outlet,12g cord, switch cover, and box.
Misc odds and ends.

Total cost; about $200

I’d recommend you buy and build as you go rather than getting a bunch of materials at once. Each step covers a particular part of the build so you should be able to see what’s needed and buy for the job at hand. The only exception would be filters: Source and buy a set of six so you know the exact size of filter you’re getting and can cut the openings accordingly.

*Relatively speaking. The one I owned cost $1500 in 2000.

Step 1: Table Top

Materials:

30”x48” Mercedes flooring (or 3/4” ply)
3/4” x 4”plywood or solid wood aprons: 2@30”; 2@46.5”
1x4 Pine cross braces. (2)@28.5” I cut a slight arc on the bottom of these to reduce air drag.
1-1/4” and 2-1/2” screws for assembly. (Unless otherwise stated,all screws of 1-1/4" or longer are "general purpose" exterior flat head wood screws available from your hardware store.)

The table top was the best part of the build. I got to sit around and watch the CNC do all the work of cutting out my slots.

 I used a 1/2” two flute spiral up-cut bit to cut the slots from the back of the board, thus avoiding tearout on the face.
I did have to go back and round over the slots on the face using a hand held router, but that was relatively quick. (1/4” radius round over bit, not set to full depth.)
I did the same for the aprons (sides) of the top.
Because the top material I used is thin (8mm) I added cross bracing to stiffen the surface. If you use 3/4” material, you probably won’t need the bracing.
Everything is glued and screwed together, with the screws set well below surface.
I predrilled all the holes with a countersink bit so I wouldn’t split the wood.
I flush trimmed the outside edges and used the same round over bit as above to soften them.

Step 2: Cabinet

Materials:

(2) sides: 3/4”x 26” x 44”
(2) ends: 3/4”x 26”x 24-1/2”
Bottom: 3/4”x 25”x 43” (Presumes 1/4” dado in cabinet sides/ends to capture bottom.)
2-1/2" screws, 1/2" washer-head screws, biscuits, glue
1/8"x 1-1/4" strips of plywood ±40ft
Six furnace filters: 16'x20'x1"

Let me start out by saying I HATE MELAMINE! But, we had a bunch of it sitting around from some giant cabinets that had been torn out to make room for welding equipment and our laser cutter.
This stuff is really heavy and I had to get help to muscle it up onto the table saw.

Start out by ripping the panels to height (26"). If you're using full 4'x8' sheets, then cross cut @ 26"x 48".
Then cut for each side at 44" for the long sides, 24-1/2" for the short sides.
The bottom is ripped @ 25" x 43".
Next rout openings for the filters. These openings are 15-7/8" wide by 19-7/8" high. My filters actual measurements are 1/4" less than the listed measurements. Source your filters before cutting the openings as some filters are up to 1/2" undersize. You want as little gap as possible around the filters to prevent dust leaking out.
I used our CNC to cut the openings, but you can just as easily cut them with a hand held router, flush trim bit, and a properly sized pattern. Or you can cut by hand with a jig saw. (BUT, if you have a full size & powerful enough CNC, you can just throw your panels on it's table, set up your code and let it do all your cutting. Ours isn't that beefy.)
I used our regular router table to cut 3/4" dados 1/2" up from the bottom of the sides. These are a fat 1/4" deep (±9/32").
Melamine is a full 3/4" thick or more, so it's necessary to readjust the table's fence after the first cut to make the dado a little wider.
Using a table saw & dado blade would be a better/faster way to cut these. We don't have the proper brake for our SawStop table saw to use dado blades.
A hand held router and fence would work well too.
I used a biscuit joiner & biscuits as my assembly method, so my next step was to mark and cut the biscuit slots. They're spaced about six inches apart. Allow for the dado so you don't cut into where the bottom fits in. Cut slots into the short sides' edges and slots into the long sides' inside faces: The long sides will overlay the short sides.
Cut up strips of 1/8" plywood and screw them to the back of the filter openings with the washer head screws. The strips should overlap the openings by 1/2". They act as a support to keep the filters in place. When the filter doors are in place, they will have a foam strip that presses hard on the filter edges to create a seal, so the filters need a good support behind them.
Before gluing up the case, do a "Dry Run" to make sure it all fits as it should.
Apply glue to the biscuit slots and on the space between slots; insert biscuits then short ends onto long sides. I used a clamp to hold the ends to the sides while I slid in the bottom. Remove the clamps and put on the last side. I used a rubber mallet to align my corners (the captured bottom does most of the work here). Then clamp it all up.
Measure the diagonals of the case to check for square.*
With the case still in clamps, predrill countersunk screw holes and screw the sides together; plus the sides to the bottom using 2-1/2" screws.
Once the screws are in, you can remove the clamps if you wish.

Whew! That's a lot of work. Let's take a break while the glue dries.

*If your case isn't square, you can make minor adjustments by moving the clamps in or out of alignment:
To shorten a diagonal, slightly loosen the clamps on one side.
For the corner of the diagonal you want shorter, move the clamp pad away from the clamped surface.
For the other end of that clamp, move the pad into the clamped surface.
Do this for all the clamps on that side then retighten the clamps. It won't take much to move a diagonal by 1/8" or more.
Remember that you only want to move a diagonal by 1/2 of the total distance you are out of square: so if your diagonals read 1/8" difference, you want to move the clamps to create a change of only 1/16" -- because as you change one diagonal, you are doing the opposite to the other diagonal.
You have to make these adjustments while the glue is wet and before you've drilled and put in any screws.

Step 3: Finishing Up the Cabinet Step

OK. Break's over!

Materials:

Four locking wheels (casters)
16 #12 x1-1/2" pan head sheet metal screws
Four 3/4"x4"x4" pieces of pine or plywood. (1/2" thickness would be fine also.)
Optional: 1-1/4" brads + nailer
±14 ft of 2x4 fir or pine. (optional: 1x4 hardwood with no dado cut into it.)
2-1/2" or 3" flat head "general purpose" screws

Place the cabinet bottom-up on the floor, glue and nail four 3/4" wood blocks in the corners (to clear the lip of the sides) then screw four locking casters through the blocks & into the melamine bottom with the #12 screws. (Mark and predrill holes first.)
Finally, the 2x4 is used to support the sanding top. If you have a planer, run the 2x4 thru the planer along it's face to get the surfaces smooth & even thickness.
Cut the frame so it is 1/4" larger overall than the sanding top which is 48"x 30". So cut your long sides at 48-1/4" and your short sides at 30-1/4". Cutting miters will give you better support at the corners.
Rout or cut a >3/4"wide by 1/2"deep dado that's 5/8" in from the inside edge of the frame pieces.
If you have a biscuit joiner, cut for biscuits in the miter faces. -- This will help align the corners and give strength to the joint.
Dry fit the frame to the cabinet top. If it's a too tight a fit (Rubber mallet doesn't persuade the panels to slip into the grooves) then widen the grooves a little. You could also chamfer the top edges of the cabinet.
BTW, if you don't have a biscuit joiner, you can screw the miters together after it's fitted & glued/screwed to the cabinet.
My miters weren't a very tight fit (bad math) so I sealed the gaps with silicon sealant.
Don't screw down the frame yet. It needs to come off for the next step.

Step 4: Dust Funnel

Materials:

Half sheet of 1/8" or 1/4" Kortron or Melamine or finished plywood. (the smooth surface will help the sanding dust slide towards the bottom.)
±8' of 1x2 pine, or similar (scrap).
9"x9" square out of 2x10 fir.
3/4" #8 washerhead sheetmetal screws (or similar)
3/4" #6 flat head wood screws (Spax® work very well.)
Silicon sealant
masking tape
Seven or eight inch square of wire mesh with 1/4" openings.

The sanding table's "Funnel" caused me the most consternation of the whole build until I had an epiphany one morning at 3am.

I drew out my rectangular cone shape using an old CAD program that only ran on an old ibook that required my booting it up with an even older operating system. (I've tried newer different brand programs, but my shriveled brain just can't figure them out.) Anyway, I was able to create my extruded shape and get all the weird angles I thought I needed as well as the dimensions of the length (depth?) of each trapazoidal shape.
Then I changed my cabinet's dimensions. I wasn't sure I could go through all the gyrations of the CAD program again to get new measurements.

That's when I had my epiphany and remembered my tenth grade geometry: A squared plus B squared equals C squared! A right angle triangle! I had measurements for A and for B; what I needed was C, the hypotenuse. Finding the square root of a number used to be hard; now we have calculators!

Just to be sure things would fit, I cut out a full size mock-up in cardboard.
Determine your measurements: My cabinet's inside length and width are 42-1/2" x 24-1/2". I want the depth of the funnel to be 10". I want the bottom flat (where the vacuum connects) to be 8" square.
So, I want "C". "A" is 42.5 minus 8 divided by 2; equals 17-1/4". "B" equals the depth; 10".
Therefore "C" is 19-15/16" for two of the funnel's trapezoids: 8"x 19-15/16" x 24-1/2". Dimensions for the other two are: 8" x 13" x 42-1/2".
These pieces are shaped like isosceles trapezoids, with the perpendicular distance between the two parallel ends being "C".
Looking at my drawings, there are 7"x1/2" tabs at the narrow ends of each trapezoid. This 1/2" is in addition to the length "C". These tabs will fit into slots cut into the 9" square.

Cut out your funnel:

The CNC made light work of this for me, but laying out the patterns full size and cutting with a jig saw, etc, makes the work almost as easy.
Clean up your edges with sandpaper or a surform
I found I needed to chamfer the mating edges to keep within my dimensions. I used a 45° router bit on the router table to achieve this. 45° is too much of an angle but it worked ok.
Fold your sides into a funnel shape. Tape to hold in place.
Hot melt glue or construction cement would be a good choice for gluing the sides together, but I didn't think of that.
Lay out the 9"x9" square:
1. You want a 4" hole dead center.
2. From center, mark an eight inch square.
3. From your taped or glued-up funnel, figure out what angles you need to cut into the square.-- One angle for the longer pieces and one angle for the shorter pieces. (Here's where you're going to have to dig up your Jr High math again. Don't you wish you had paid more attention in class instead of passing love notes to Mary-Jane or Billy-Bob {as the case may be}?)
4. Cut these angles on the table saw; at the eight inch line, ±1/2" deep. They'll need to be a little "fat" so your funnel can slide in without too much trouble. (I'd give you these angles, but I forgot to take notes.)
With your square done, fit it onto the funnel's bottom. Screw a couple of washer head screws through the panels & into the square.
I used 1"x2" battens cut at (about) 16° to sandwich the funnel sides together. I temporarily fixed them in place with a brad nailer, predrilled screw holes, and then screwed the battens in place with the #6 Spax screws.

Take your funnel and drop it into your cabinet. If you're lucky -- as I was -- it will fit snugly, otherwise you'll need to make some adjustments to get it to fit.

Mark 1/2" from the top of the cabinet sides and screw the funnel to the sides. The washer head screws work best here too. They're less likely to punch through the thin panel walls.

At this point, add some caulking around the top edges of the funnel and at the bottom where the funnel sides fit into the bottom piece.

Screw the wire mesh over the dust collector hole. -- You don't want big pieces of debris getting sucked into the collector's fan blades.

Finally, put the 2x4 frame back on the cabinet and screw it in place.

Step 5: Filter Doors

Materials:

About 40 feet of 3/4"x 2" solid wood or plywood. Solid is better, but I had a lot of scrap plywood laying around. Note: 1x2" furring strips are only 1-1/2" wide, so won't be wide enough.
18gauge x 5/8" brads + nailer. Or headless brads and a hammer.
Wood glue.

Verify that your wood thickness is close to 3/4". Real lumber yards have hardwoods that are normally "random width and length. Thickness is either 7/8"-1" (rough sawn) or 13/16" S2S (surfaced two sides). Softwoods are dimensioned with a thickness of 3/4". Much plywood is ±18mm thick rather than 3/4".
I don't recommend using MDF or particle board as they don't have much structural strength.

Determine how many feet of 2" stock you need. My filter openings are 19-7/8" x 15-7/8" (16"x20"). I want the doors to overlay the openings by one inch all around. So my doors should be 22" tall by18" wide: equals 80" per door; times 6 doors, equals 480", equals 40 feet.

Rip your stock to two inches wide. Rip extra to allow for mistakes etc.
Crosscut twelve+ pieces at 22".
Crosscut twelve+ pieces at 18".
I'm using a bridle joint to construct the doors. If you use a different joinery method, you may need to adjust these measurements.

I used a mortising jig with the table saw to cut my joints, but you can do without it if your careful. Other joinery options are dowels, loose tenons (Festool Domino), biscuits or a half lap joint.Of these joints, the bridle joint is probably the strongest.

Cut shoulders on the 22" stiles using the table saw. As the door stock is two inches wide, the shoulder should be a "fat" 2 inches from the end.
The wood is 3/4" thick so set the blade height at a fat 1/4" high: you want to divide the joint into thirds.
Next, cut the cheeks of the tenons using a mortising attachment or home made jig. (here's a nice Instructable by notsosharp.)
1. Set the saw blade at just under 2" high.
2. Adjust the jig so you're cutting less than 1/4" off the outside.
3. Flip the stile and cut the other cheek.
4. Measure the resultant tenon and move the mortising jig as needed to achieve a tenon that's about 1/4" thick -- ≤17/64" would be a good thickness. Remember, with each adjustment of the jig you are doubling the amount of wood you're cutting off the tenon
5. Cut all the stiles.
Now cut mortises in the 18" rails.
1. Raise the sawblade to just over 2" high.
2. Center the blade on the rail and make your cut
3. Flip and cut again.
4. Adjust your jig until the mortise slips over your tenon with only slight resistance. Remember that with each adjustment you are doubling the amount you're removing from the inside of the mortise.
5. Cut all the rails.

Fit up all the doors then begin glue-up.

Because these doors aren't fancy furniture doors, I didn't use a lot of glue; plus I shot a couple of 5/8" brads into each corner joint to secure it. I didn't feel the need to clamp anything.

Step 6: Fitting Doors to Cabinet

Materials:

40' x 3/8"x 1/2"wide closed cell foam window seal (4 ten foot rolls)
Nine feet of piano hinge, or six pair hinges of your choice.
3/4" solid wood scraps. (To be ripped and Xcut for latches.)
(4) 3/4"x 1"x 20" strips of solid or plywood
(8) 1/4-20 x 3" hex bolts
(8) 1/4-20 nylon insert Locknuts
(16) 1/4-20 flat steel washers
(8) >1/8" thick x 1" diameter washers (made from scrap).

This step is a little weak because I forgot to take pictures of some of the processes.

After the glue has dried on the doors, you need to rout a 1/2" wide by 1/8" deep groove around the inside of each. This is best accomplished with a router table & fence using a 1/2" bit.
Later, you'll fit the window seal into these grooves to improve the seal between the doors and the filters.
1. Set the fence so the bit will cut a groove 1-1/16" in from the outside edge of the door.
2. Mark the fence where the cut should start and where it should end: 1-1/6" from the outer edge of the router bit and the end of the door.
3. Hover over the spinning bit with the edge of the door against the fence and the leading end of of the door in line with your start mark.
4. Gently drop the door down onto the bit and feed the door into the bit until the trailing edge of the door reaches your end mark.
5. Lift the door off and away from the bit.
6. Repeat for all four sides and all six doors.
Install your hinges on the doors. I used lengths of piano hinge cut at 18".
Set the hinge so it will hold the door about 1/8" above the cabinet side. (You don't want to completely crush the foam seal against the filter when the door is closed.) It's always best to predrill your holes. I use a self-centering drill bit to help center the hole.
Fit the window seal into the groove of the doors. I found it easier to cut the seal at each corner rather than try to wrap around the corner.
Lay your door over the filter opening on the cabinet (filter removed). You want the foam to fit within the opening. When you feel it's centered in the opening, drill and screw a couple of screws to fix the hinge in place. Check that the door opens and closes freely, then attach the rest of the screws.
Put a filter in the filter opening. The doors won't close without effort. So you'll need to make or buy latches that will keep them closed. (A design feature, not a flaw.) I made latches:
1. Cut four strips of 3/4" thick wood or ply @ ~1-1/8"x 20".
2. Drill 1/4" holes two inches from each end and centered side to side.
3. Secure these strips between the double doors and 1/8" away from the end doors using 1-1/4" brads.
4. Drill through the 1/4" holes and through the cabinet sides with a 1/4" bit.
5. Cut eight strips of 3/4" thick solid wood @ 1-1/4"x 5"
  1. Drill a 1/4" hole centered in the wood
  2. Cut a taper on the strips as shown in the pictures: Angle is about 16°. Leave a flat of about 1-1/4" square in the center where the hole is.
  3. Sand the tapered edges to allow the latches to ease over the doors as they squish the doors closed.
6. Cut eight "washers" out of some 1/8" or 1/4" ply. I used a 1-1/8" hole saw to make my washers.
7. Attach the latches to the cabinet with the 1/4-20 bolts & nuts: Bolt>steel washer>latch>thick washer>through cabinet>steel washer>1/4-20 nut. Tighten down snug, then back off so the latch turns easily.
8. Close the doors with filters in place and latch -- to be sure everything works smoothly.

Step 7: Dust Collector Installation

Materials:

Dust Collector motor & fan
4" flex hose or metal elbow
(2) 4" or bigger hose clamps
4" dust port.
4 gang electrical box, 20amp outlet, 20amp switch; 110volt.
Matching cover plate.
10 feet 12gauge electrical cord.
15amp or 20amp male plug.
1/4", & 1/2"or 3/4" plywood (scraps).
1" &/or 2" thick wood (scraps).
screws, nuts, & bolts as needed.

The dust collector motor and fan installation is dependent on the unit you have available.

Our SLO Makerspace's unit was an old used 1hp Grizzly. When we got it the bags were full of holes; it would have cost >$50 dollars to replace them, and the unit wasn't worth it. It would have been nice to have had a more powerful collector -- say 1-1/2hp or 2hp. But, you use what's at hand.

When I got to this point, I discovered I didn't have as much room under the funnel as I thought I would have. Also, the fan's exhaust port was too close to a filter door so I had to move it off center, then use a flex hose instead of a adjustable metal Elbow to connect to the funnel.

Attach your 4" dust port to the bottom of the funnel.
Set your collector on blocks, as needed, so it sits level/square to the cabinet & funnel base. (I used a couple of 2x6's.)
Using either flex hose or an adjustable elbow, connect the collector fan to the dust port.
An Elbow is better, if you can get it to fit, because there is less air resistance; thus a better flow of air/greater vacuum.
Adjust your collector's position so that you're not too close to any one of the filter doors.
Bolt the collector to the cabinet base. DON'T USE SCREWS. The vibration from the collector will rip the screws from the cabinet.
I made a deflector for the fan's exhaust port so that all the dust wouldn't be directed at just one filter.

Wiring:

If you're not comfortable with electrical wiring, PLEASE have someone knowledgeable (like an electrician) do this for you. AN ELECTRICAL SHOCK CAN KILL YOU.

I removed the existing electrical cord and switch from the collector and rewired the cord to the appropriate terminals on the motor.
I mounted a 4 gang electrical box to the cabinet and drilled a hole through a box knock-out and through the cabinet.
I ran the collector cord through the hole. I connected the black (hot) to an on/off switch and the neutral (white) to the neutral side of the female outlet. The green ground wire will be connected later.
I ran my 10 ft. 12gauge electrical cord through a fitting into the box. I connected the white (neutral) wire to the second neutral terminal on the outlet. I connected the black (hot) wire to the brass (hot) terminal of the outlet.
I ran a separate black wire from the outlet's second hot terminal to the second hot terminal on the switch.
I connected the two cords' ground wires together and then connected them to the electrical box.
I screwed the outlet and switch to the box and attached the cover.
Finally, I attached a male plug to the other end of the 10ft cord; I plugged it into a wall outlet and tested my wiring with a meter. when it read "OK", I turned on the collector -- it worked!

Step 8: Hinging the Top

Materials:

One pair Gate hinges (5" long?)
3/4" x ±27" dowel or stick
(2) screw eyes: equivalent to a #8 screw or there-about
Length of nylon twine (venetian blind cord works well)
(1) 1-1/4" wood screw, (1) 1-1/2" woodscrew, two small steel washers.
(1) 3" Bail handle (not shown)
13 ft of 3/4"wide x 1/2" thick closed cell weather stripping (requires the purchase of two ten foot rolls).
Two or four 3/8" hi spacers (I used 10mm nylon spacers used for mounting drawer slides.)

We're almost done. This is the last construction step.

Place the top on the cabinet and align it.
Mark for your hinges. (I was going to use standard 3" door hinges but the holes weren't going to line up with the wood once the weather stripping was in place.)
Screw the bottom of the hinges into the 2x4 framing with the hinge knuckles centered on the line between the top and the 2x4 frame. (See picture) I replaced the included wood screws with some long deck screws.
(Note that you may have to grind down the top of the hinge so it is not near the sanding surface.)
Place two 3/8" spacers between the top and the 2x4 frame; behind the hinges. (I used the weather stripping, but it might compress too much.)
Mark and drill the hinge holes and screw into place. Here, I used #8 machine screws, drilled through the plywood apron and bolted the hinges in place.
Take your two 3/8" spacers and move them to the front - unhinged side - of the top. Align with the hinges and screw or nail them into the edge of the top's apron. These spacers will allow the weather stripping to seal but won't allow it to be crushed from the weight of your sanding.
If you have something to rest it on (table or saw horse) open the sanding top 180° and apply the weather stripping to the edge of the top's apron.
Drill a 3/16" hole through the dowel; 1/2" from the end.
Position the dowel on the inside of one of the short aprons. Check that it can swing freely, then screw it into the apron with the 1-1/2" screw and two steel washers on either side of the dowel. Loosen the screw enough so the dowel can pivot easily.
Screw in the 1-1/4" screw into the apron near the free end of the dowel. This will act as a catch to hold the dowel in place when the top is closed.
Open the top. Bring the dowel out and position it so the top is open most of the way but not so much that it is about to fall backwards and rip the hinges off.
Mark the dowel's position on the 2x4.
Drill a shallow hole in the 2x4 with a 1" Forstner or Spade bit. This will provide a stop for your dowel-as-top-support.
On the other side of the top, I attached a cord to the two screw eyes screwed into the apron and the 2x4 frame. This was to allow me to open the top beyond the tipping point without fear that it would fall backwards and rip out the hinges.
Centered on the front apron of the top, attach your bail handle.

DONE!

Step 9: Further Thoughts

I'm hoping this instructable isn't too complicated. I wrote it for someone who has basic woodworking knowledge. An accomplished woodworker should be able to skim over my steps and use the info as a reference. I have captioned most images to help identify what they show.
Sanding Top: I love the look of the top, but it is proving too "slippery". The piece being sanded tends to slide around on the surface. We have had to add a really ugly drawer liner on the top. I hope to use either a spray-on truck "bed liner" or Dipit® spray (used on small tool handles) to improve the grip of the surface.
I don't have plans for the sanding table; with the exception of the top holes and the funnel. I had plans, but I made a number of changes as I built, including all cabinet dimensions, So what plans I have don't reflect what I built.
Filter doors: The routed grove for weather stripping should have been cut deeper -- perhaps as much 1/4" deep. I have found that the pressure applied to the filters, causes them to stick to the stripping and I have to peel the filter off the stripping -- which damages the foam.

I have not personally used the sanding table yet, but others have and I've gotten a lot of positive feedback about how well it works. ;-)