This table starts out as the Cutting Table Kit from You-weld-it.com. The kit cost $225 which very reasonable for a kit of this quality. Plasma cutting generates a very fine brown dust which gets into everything. It can also generate fumes which are not good. Converting the table to a downdraft table does a lot to remove the dust and fumes and keep the shop clean.
You will need a supply of sheet metal, a length of 6" galvanized duct and cap, some piano hinge, and an exhaust blower.
Step 1: Build the Kit As Instructed With One Exception
There are several You-tube videos that show how to build the kit. so I won't bother with that here. I used a small 70 Amp MIG welder, and it did just fine. The main difference is the location of the bottom tray. I placed it down about 18 inches below the table top to make room for the exhaust duct. I wanted the duct far enough away from the top so there would be no danger of it getting cut.
Step 2: Paint the Frame and Cut the Slats
Since I planned to use galvanized sheet metal, I primed and painted the table after the weld-up. The table does not come with slats, since shipping costs for the slat would cost more than the material. Buy some 1-1/2" x 1/4" hot-rolled strap from your local metal supplier and cut it to fit your table. Slat length varies a bit from table to table depending how you did your weld joints. Do not paint the slats.
Step 3: The Exhaust Duct
The purpose of the exhaust duct is to make sure air is exhausted evenly across the entire top of the table. The exhaust duct is a length of normal HVAC galvanized ducting. When you buy it, the seam will be open, this is a good thing. Using a hand punch to punch a series of 1/2" holes on both sides of the seam. If you don't have a large hand punch, a step drill works very well with thin sheet metal. It would be a good idea to wear gloves during this process. Be sure to only put holes in the part of the duct that will be inside the cabinet.
After the duct was finished I placed a partial roll of aluminum widow screen inside the duct and let it unroll to fill the duct. This will help prevent sparks and small debris from getting into the duct.
Step 4: Cut the Back and Sides, Then Install the Duct
Cut a piece of sheet metal for the back. Secure it to the frame with self drilling screws. Both sides are symmetrical so they can be cut to the same size. Cut a 6" hole for the duct in one end piece. Then mount a 6" duct cap to the inside of the other end to hold the end of the duct. Make sure the duct is located a few inches above the bottom of the cabinet. I first made my exhaust with the open end on the left, but changed my mind a swapped it to exhaust to the right. Self drilling screws work well for securing the sheet metal together.
Step 5: Make the Front Panel and Mount the Exhaust Blower
You can make a solid front panel, but to clean the cabinet you would have to remove the slats. I decided to cut doors in the front panel to make clean-out easier. The doors are simple, held with piano hinges and sliding bolt latches.
The blower was left over from a Chinese laser cutter. It worked well for this purpose. Blowers like this can be found on eBay for very reasonable prices.
Step 6: Install the Table and Build an Optional Top Skirt
The table should exhaust to the outside through metal ducting. Make the run as short as possible. Make sure the duct exhausts towards non-flammable material (like concrete). It is possible a spark could make it through the blower and go outside. Putting the exhaust near grass or other flammable material would be a bad idea.
After installing the unit, I decided to make a removable skirt for the top. It consists of three pieces of sheet metal with piano hinges at the corners. It sits in z-brackets screwed to the sides of the top. The skirt can be easily removed and set aside if you have to cut large pieces of stock.
The last photo is our welding lab at The MakerBarn. Covering the walls with galvalume "R" panels makes to lab safer and cleaner.