Heavy Welded Fire Pit, 287kg of Steel

I built this fire pit back in 2018. I wanted something that was going to last and could take a bit of punishment, so I came up with a design on paper and got it laser-cut at my local steel supplier, Surdex. I went with 10mm plate.

I then undertook my biggest welding project to date, with minimal skills and budget equipment.

The result is a 1.2m square pit (at the top) that weighs a fair bit and takes a serious load of wood. It's been through plenty of big burns since and shows no sign of giving up, aside from a bit of 'character' warping.

It sits on concrete blocks, with loose bricks plugging the ends underneath so I can open or close them to control airflow.

Dimensions: 10 mm (3/8") thick mild steel plate, 1200 mm (47") opening at the top, 942 mm (37") square base, 483 mm (19") high.he shape, I've since learned, is an inverted frustum — basically a pyramid with the top chopped off.

Weight: The delivery docket says 287 kg (633 lb) of steel, but I never weighed it.

Tools

1. MIG welder, this was done with a fairly basic gas-shielded unit from Bunnings (Bossweld MST195, 195A)
2. Angle grinder + discs: cutting/flap discs for cleaning up, a grinding disc for dressing welds
3. Welding mask, gloves, the usual PPE
4. Clamps, magnets or anything to hold plates square while you tack

Materials

1. 5× 10mm mild steel plates, laser-cut to the diagrams: 4 sides + 1 base. It's bog-standard hot-rolled plate. (In Australia ordered as GR250, Grade 250 structural to AS/NZS 3678.) Mine came in at 287 kg.
2. MIG wire to suit (0.9mm is what I use)
3. Shielding gas (CoreGas 512 from Bunnings)

The whole build hinges on accurate plates. I drew the design up on paper, then handed it to Surdex to laser-cut.

To assist the reader I have since redrawn and dimensioned the diagrams the diagrams using FreeCAD. Note that the side plate diagram is shown upside down compared to final build. The fabricator had no problems with my hand drawn diagrams, so the new ones should be fine.

A few things worth knowing before you send a job off:

You need 5 plates:

1. 4× sides — two pairs. The walls flare outward, so each plate is a trapezoid: 1200mm along the top, 932mm along the bottom, 500mm tall, with the sides raked at 75°.
2. 1× base — 942 × 942mm square, with a 4×4 grid of holes.

The holes are all ⌀50mm. The four side plates get a row of four near the top edge; the base gets sixteen in a grid. These feed air to the fire, and once it's roaring they glow and look great.

The 75° rake is what gives the pit its flared shape and means the four walls meet cleanly at the corners.

This is the fiddly bit. Five heavy plates that all want to lie flat or fall over, and you need them held at the right angles long enough to tack. And watch your toes!

First, put the base plate flat on a sturdy table. Then carefully stand up two of the walls and hold them in place however you can. I used long timber off cuts wedged down to the floor, plus a few spacers here and there, and strong magnets on the base plate to stop the sides slipping.

I used the edge of the base plate to line the walls up exactly. Note that this isn't their final position on the base, I'm just borrowing the base edge as a straight edge to get the walls sitting true while I tack them to each other.

So at the bottom, the two walls should sit flush with the base plate edges, and where they meet at the corner they should close up tight — the inside corner touching all the way from top to bottom. Once it's sitting right, tack the vertical seam top, middle and bottom.

Then do the other two walls the same way, this time making sure all the verticals line up and the corners close. Use the base-edge alignment again to keep it square.

At this point you've got four walls tacked into a box.

Now check the fit against the base. Sit the wall shell onto the base so the base sits about 5mm proud of the walls all the way around. That 5mm isn't slop, it's the joint. The walls are meant to land on the edge of the base, so you get a clean corner to weld. A few taps with a hammer sorts out small adjustments.

Once I was happy, I added a few more tacks to stiffen the four walls up. Then I flipped the whole shell over, open end down on the floor, base end facing up, rested the base plate on the four wall edges, and tacked it in place.

This was my biggest welding job to date, on a fairly basic machine. I used heaps of passes letting it cool a bit between. This was also necessary for the welder because at full bore it's only supposed to run at about 10% duty cycle.

I don't exactly recall how many, but I reckon at least 6, and I had everything on the machine turned up to full.

I ground the edges to expose bare metal and then warmed the joints with an oxy torch before welding. I'm not sure of the preheat was necessary, but I thought it might help.

There are two corner types, and they weld differently:

1. Side-to-side (the vertical corners): the two walls meet so their inside faces close up at the corner, which leaves a wide groove, almost 90°, running down the outside. That groove is your bevel, built in by the geometry. Fill it from the outside with passes.
2. Base-to-side (the bottom): the walls land on the edge of the base, and you run a fillet along the outside and inside where the wall meets the base plate. Same deal, multiple passes to build it up.

What I'd do differently:

On the vertical corners I also welded the inside, not just the outside groove. In hindsight I wouldn't bother. The outside welds already hold everything — the inside runs didn't add much, and with my skill level I couldn't make them look good anyway, so they just ended up messy in a spot people can see. If you're a tidy welder you might feel differently, but I'd put that time into making the outside welds clean instead.7

Warping: The finished welded product showed no signs of warping, but over the years with massive fires, 'character' warps have formed in the sidewalls.

This is the step that makes it look like one piece of steel instead of four plates welded together.

I took a grinding wheel to the outside corners and rounded them off, which blends the weld back into the shape of the plate and effectively hides it. Wherever I found a low spot — somewhere there wasn't enough weld to grind into a clean round, I shoved the MIG back in, filled it up, and ground it again. Fill, grind, check, repeat until the whole corner runs smooth and round with no dips.

Once the corners were rounded, I switched to flap discs to blend the weld into the surrounding steel.

The result: the outside corners look like one continuous piece of steel. You'd have to look hard to see they were ever separate plates. Eight years later, a quick wire brush, and they still look great - see second pic.

Real welders might wince at my technique, but I figured with 10mm plate with this many passes, there's plenty of weld there to shape without weakening anything.

The pit needs to sit up off the ground for two reasons: airflow into the base, and not burning whatever is underneath it. I had some old concrete blocks lying around and cut them down to make a stand. Nothing fancy, it's really just there to lift the pit and let air through.

The base has 16 holes in a 4×4 grid. I set the blocks as two parallel rows running under the outer edges of the base, so each row sits under a column of holes. That blocks off 8 of the holes, the outer ones, and leaves the middle 8 open, with a clear channel running underneath between the two rows of blocks.

Then I put a brick at each open end of that channel. The bricks are the control: air comes in from the ends, runs along the channel under the base, and feeds up through the 8 open holes into the fire. Slide a brick out and you open the end up — more air in, hotter burn. Push it back in and you choke it down. It's a simple damper, but it works, and you can tune the fire without touching it from above.

The holes in the sides may do the same job higher up, but I put them in mainly for show.

To finish the project off, just recently I made a 1:10 scale model (except wall thickness, I made this 2mm just for print ability, rather than 1mm) of the pit using FreeCAD, then 3D printed it. It's a good way to see the proportions before committing.

I've included the STEP file if you'd like to print one yourself. And there's a pic of the 3D print and the original, now 8 year old fire pit.