Why does my weld pull out of square?

Weld metal shrinks as it cools and pulls the joint toward the side you welded. A full bead on one side of a corner shortens that side and swings the leg. Welding in a planned sequence balances those pulls.

Don't tack welds stop distortion?

Not on their own. Tacks hold the parts in position, but the full welds that follow are much bigger and shrink much more. In Fireball Tool's test, two identically tacked corners came out different, and only the one welded in sequence stayed square.

When should you check a weld for square?

After it has cooled completely. The weld keeps shrinking as it cools, so a corner that reads square while hot can be out once it's cold.

JoineryMetal & Tube JoineryWeld sequence for a square corner

Why does a tacked corner still pull out of square, and what fixes it?

"Tack it first, dummy." That's the comment Fireball Tool kept getting, so he tested it. Two identical mitered square-tube corners, tacked in the same spots, a dial indicator on each. The one welded with no weld sequence pulled out of square. The sequenced one didn't. Tacks hold the parts together. Preventing distortion takes a weld sequence.

Welding: a helmet at the right shade, fully covered skin (arc light burns like sun), leather gloves, and real ventilation or fume extraction. Never weld galvanized or painted tube without grinding the coating off first; zinc fumes cause metal fume fever. Keep the area clear of anything that burns and watch for smouldering sparks after you stop. A fresh weld stays hot enough to burn long after it stops glowing, so wear gloves to handle the corner and set the square down on it only once it's cool.

⚡ Rule of thumb

Tacks alone won't keep a mitered tube corner square; weld sequence does. After tacking, weld the outside corner first, work the remaining seams from the inside out, and run the fillet last. Let the corner cool completely, then check it with a square before you trust it.

Starting point

Tack → outside corner → inside-out passes → fillet last

Measured

Square = 90° between the legs against a combination square, checked cold

Confidence

Creator technique

▶ Source video — Tack it first, dummy — Fireball Toolwatch on Facebook ↗

Applies to

Process: MIG or TIG welding of frames
Materials: Mitered square or rectangular steel tube corners
Setup: Welding table, clamps or stops, combination square; dial indicator optional
Orientation: One leg clamped flat to the table
Not for: Code or structural work with a written weld procedure

Why it matters

Weld metal shrinks as it cools, and it pulls the joint toward whichever side it's on. Put a full bead on one side of a corner and that side gets shorter. The leg swings.

Tacks don't stop that. They're small welds too, and the beads that come after are far bigger.

Fireball's test makes it visible. One leg is clamped, an indicator rides on the other, and you can watch joint A walk as it cools. Joint B, welded in sequence, holds. Same tube, same tacks, same welder. Different order.

Order is the free part. It costs nothing, and it saves a straightening job.

What good looks like

The corner reads square against a combination square once it's cool
No daylight under the square's blade on either leg
Both legs lie flat on the table with no lift or twist

How to apply it

Fit and clamp the mitered corner square on the table, then tack it.
Weld the outside corner first.
Weld the remaining seams, working from the inside out.
Run the fillet last.
Let it cool completely, then check it with a square and on a flat table.

Test it on your setup

Run the creator's test. Tack two identical corners in the same spots. Weld one with no plan, fillet first and corner last (joint A), and the other in sequence (joint B).
Clamp one leg of each to the table. Put a square, or a dial indicator if you have one, against the free leg and watch it as the weld cools.
Compare both against a square once they're cold, then lay each flat on the table to check for twist.

Exceptions & tradeoffs

Heavier wall, longer welds and different processes shrink by different amounts. Sequence cuts distortion; it doesn't replace clamping a critical part to the table.

Mind you, a full frame is four of these fighting each other. Alternate corners and sides so the pulls balance, instead of finishing one corner before you start the next.

Structural or code work follows its written weld procedure, not a shop rule of thumb.

Common mistakes

Trusting "four good tacks on the corners" to hold square through full weldsSequence the welds, not just the tacks
Running the fillet first and the corner last (joint A)Outside corner first, inside-out, fillet last
Checking square while it's still hotCheck cold; the weld keeps pulling as it cools

Frequently asked questions

Why does my weld pull out of square?Weld metal shrinks as it cools and pulls the joint toward the side you welded. A full bead on one side of a corner shortens that side and swings the leg. Welding in a planned sequence balances those pulls.
Don't tack welds stop distortion?Not on their own. Tacks hold the parts in position, but the full welds that follow are much bigger and shrink much more. In Fireball Tool's test, two identically tacked corners came out different, and only the one welded in sequence stayed square.
What order should you weld a square tube corner in?Per the creator's test: tack it, weld the outside corner first, work the remaining seams from the inside out, and run the fillet last. Check it with a square once it's cool.
When should you check a weld for square?After it has cooled completely. The weld keeps shrinking as it cools, so a corner that reads square while hot can be out once it's cold.

Fireball ToolSource & visual reference

Side-by-side test of two identically tacked tube corners, one welded with no sequence (joint A) and one in sequence (joint B), checked with dial indicators and a square — "Tack it first, dummy." ↗ at 2:50–3:45.

Summarized and independently edited by Start Here from Fireball Tool's Facebook video (5:14): sampled frames and the caption, no audio. The A-versus-B setup, his on-screen labels ("tacks + no weld sequence" and "tacks + weld sequence"), the dial indicators and the square checks from about 2:50 are on screen. The weld order comes from his caption. What "inside out" means seam by seam is in his audio, which I didn't have, so the steps keep his wording instead of guessing. The indicator readings aren't legible in the frames, so this page doesn't say how far joint A moved. The flat-table twist check, the full-frame note and the mistakes are mine. Fireball Tool sells the fixturing in the video; the sequence works without it.