How to weld square tube: corners and thin walls
Hollow section is thin metal in a shape that traps heat and concentrates stress at the corners. Two problems, and both are solved before the arc is struck rather than during welding.
Cutting and fit-up
Square tube is unforgiving about cuts. A mitre a degree out leaves a gap on one side and a hard contact on the other, and the weld then pulls the frame towards the tight side as it cools. Cut on a stop, check with a square, and file the burr off both the outside and the inside of the cut.
For a corner joint you have a choice: mitre at 45° for appearance, or butt one tube into the side of the other for strength and simplicity. The butted joint is stronger in most light frames and is far easier to cut accurately.
Tacking is the whole job
Tack two opposite corners, check for square across both diagonals, then tack the remaining two. Measure the diagonals rather than trusting a square — on a frame of any size the diagonals tell you the truth and a square does not.
Any adjustment happens now, while there are only tacks to break. Once the frame is welded, correcting it means cutting.
The welding sequence
Weld one face, then the opposite face, then the two remaining, again opposite each other. Each weld shrinks as it cools and pulls in its own direction; welding opposite pairs makes those pulls cancel. Running around the perimeter face by face lets them add up, and the frame finishes as a parallelogram.
On thin wall, stitch rather than run — the same logic as thin sheet. Let each stitch lose its colour before starting the next.
The corner itself
The corner is where the load concentrates and where welders most often go thin. The temptation is to swing the torch round the corner in one movement, which speeds up the travel exactly where more metal is needed.
Stop at the corner. Reposition your hands. Restart on the cold weld 5 mm behind the stop, so the new run fuses into the old one, and continue. Two deliberate runs with a proper overlap beat one continuous sweep every time.
Settings
| Wall | Wire | Current | Voltage |
|---|---|---|---|
| 1.5 mm | 0.8 mm | 70–90 A | 16–17 V |
| 2.0 mm | 0.8 mm | 90–110 A | 17–18 V |
| 3.0 mm | 0.8–1.0 mm | 120–140 A | 18–20 V |
| 4.0 mm | 1.0 mm | 140–170 A | 19–21 V |
A closed section holds heat: the second and third faces will need slightly less current than the first, because the frame is already warm. If the last face burns through on settings that worked on the first, that is why.
Frequently asked questions
What settings for 2 mm wall?
MIG short arc, 0.8 mm wire, about 90–110 A at 17–18 V. Test on an offcut of the same section: 2 mm wall behaves differently from 2 mm flat sheet, because the closed section has nowhere to lose the heat.
Do I need to weld all the way round?
Only if the drawing says so. A fully sealed section cannot admit moisture, which matters outdoors; but a continuous weld all round puts far more heat in and pulls the frame more. On light frames a stitched weld with sealed ends is often the better engineering answer.
The frame comes out out of square every time.
Sequence, not skill. Tack diagonally, weld opposite faces in turn, and use a jig or a stop block. A frame that is welded face by face round the perimeter will always pull towards the last weld.
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