Octagonal Smokeless Fire Pit

I had wanted to make a smokeless fire pit for a long time. Most commercially made smokeless fire pits are round, but i don't have access to a bending machine and I also wanted to practice welding. So I decided on an octagonal design made from structural steel sheets. My project is inspired by this vendor.

Smokeless fire pits work on the principle of secondary combustion. Air enters through the holes at the bottom of the outer wall. Part of the air enters the holes in the grate, where the primary combustion takes place. Another portion of the air goes through the double wall, where it is preheated and then enters the combustion chamber at the top, where the secondary combustion occurs.

In reality, smokeless fire pits are more "smoke-less" than truly "smokeless" - there is usually some smoke, but definitely less than in regular fire pits (and almost none when using dry wood).

For this project, you will need:

1. sheet metal parts according to the project drawing
2. a welding machine (I used stick welding with 2,5 mm electrodes, MAG will be faster)
3. adjustable welding magnet and/or a 3D printed fixture (without magnet, I recommend at least two)
4. an angle grinder with a grinding wheel and an 80-grit flap disc
5. personal protective equipment for welding and angle-grinding

Get all the metal parts according to the attached drawing (PDF). CAD files with the geometry of all parts are included in the attached DWG or DXF file:

1. Option 1: (recommended): Cut the parts on CNC plasma cutter (or laser, what you have available). I had my parts cut this way by a local supplier. The total cost was less than 4000 CZK (164 EUR) for material and cutting.
2. Option 2: cut the parts with a hand-held plasma cutter. I don't have experience with this tool, but it is widely available. The accuracy will be probably much worse than in Option 1.
3. Option 3: cut the parts with an angle grinder. You will need to adjust the design a bit and improvise, as the complete geometry (especially the holes) cannot be cut only with an angle grinder. Drilling the holes would probably take a lot of time, so I'm not sure about this option. Also, the holes are not a standard diameter (because that's not needed for plasma cutting), so you would have to modify the design.

Most of the parts are made from 4mm sheet metal, except the outer wall, which is made from 3mm sheet. This seems to me like a good balance between sturdiness and mobility (approx. 44 kg). Commercial fire pits are typically made from thinner sheets and stainless steel (which is more heat-resistant than structural steel).

If you want to use just one sheet thickness, choose 3 mm for a more lightweight fire pite or 4 mm for a more durable one. But keep in mind that you well need to slightly reduce the width of a part no. 03 by about 0,8 mm (if using 4mm sheet). Or increase width of a part nr. 04 (if using 3mm sheet).

Weld parts. no. 01 and 04 (see drawing). I welded all parts at 80-95 amps.:

1. tack parts. no. 04 (wall II). Do this on a flat surface, ideally directly on part nr. 01. To hold the parts in place, use adjustable welding magnets or the 3d-printed fixture (here for download). I used one 3D-printed fixture, but recommend using two.
2. prepare the bevel for welds W2 and W3 (bottom). I prepared the bevel with an angle grinder, approximately 2-3mm at about 45°
3. tack parts no. 04 and 01
4. fully weld W1 and W2

(you should end without the part nr.2, that is in picture)

Weld parts from the previous step together with the outer wall and the cover (parts. no. 02 and 03):

1. tack parts no. 03 (W4). Use adjustable welding magnets or 3D-printed fixture. The fixture is deisgned for 4mm sheet; I adjusted the clearance with folded paper.
2. prepare the bevel for weld W5
3. tack and fully weld W3 (inner wall and cover)
4. tack outer wall and inner wall (W5)
5. fully weld W4 and W5

After weld W2, bottom (part no. 01) deformed slightly and become convex. When I wanted to place the ribs (parts no. 05 and 06), there was a gap of a few millimeters (about 3mm), which was too much for comfortable weld. I solved that by placing the fire pit upside down on a log and welding it in this position. The weight of the fire pit temporarily flattened the bottom, and I was able to weld the ribs with almost zero gap.

You may also need to adjust ribs with an angle grinder to make it fit the bottom space.

Grind the visible welds (W4 and W5):

1) Grind the welds coarsely with a grind disc (not a cutting disc!). In the picture, these areas are marked in red. Be careful not to remove too much material from the sheet.

2) Optional: Repair any welds, that were not done well and where holes appeared after coarse grinding. Than repeat the previous step.

3) Finish the surface and remove the sharp edges with a flap disc. In picture, these areas are marked in both red and green. I used 80-grid, and the surface become really smooth.

Now the fire pit is ready to use. Please note that hot coals will fall through the bottom, so do not place the pit on any heat-sensitive surface. I recommend placing the fire pit on a spot where you would make a fire even without this fire pit.

The fire pit will become smoke-less after a few minutes (the walls need to get hot to preheat the air). Using dry wood helps a lot, but once the pit is hot, it will "eat" wet wood as well without any problem. In any case, be prepared for higher wood consumption compared to a regular fire pit.

If you leave the surface untreated, it will start to rust soon. Some people like this as a "patina", others don't. See the next step.

I don't have much experience with surface treatment for high-temperature applications, but there are many grill paints available. I am planning to paint the outer surfaces (parts. no 02 and 03) with a heat-cured stove paint. I've already bought one, but I haven't had the time to apply it yet.

You can also make a grill attachment - it's entirely up to you. In the picture, you can see an example of mine, made from some leftover 4mm sheet metal and 25x2mm tube inside a 30x2mm tube