Understand what pulls
Molten metal occupies more space than solid, so as a bead freezes it contracts and drags the parts towards it. A single continuous bead along one side of a joint pulls that side shorter — which is why a box frame welded corner by corner ends up out of square in a predictable direction.
Tack everything first
Tack all four corners, check square across the diagonals, adjust while the tacks are small enough to break, and only then weld. Magnets position the work for tacking; they cannot resist shrinkage, so clamp mechanically before running a bead.
Alternate and back-step
Weld a short length at one end, then the opposite end, then the middle — never continuously from one end to the other. On long joints, back-stepping (welding short runs backwards along the joint) cancels much of the pull. Both techniques are about spreading the heat rather than concentrating it.
Let it cool
Heat that has not left the assembly is heat still moving it. On anything precise, weld a pass, let it come back to hand temperature, then weld the next. Cooling with water is a temptation and it hardens the weld zone on some steels — better to wait.
The questions welders ask
Can I straighten a frame after welding?
Sometimes, with heat rather than force. Localized heating on the long side, allowed to cool, shrinks that side — the same physics that caused the problem. It takes practice and it is far easier to weld in the right order in the first place.
Does tacking more often help?
Yes, up to a point. More tacks mean the assembly is held in position by more places, so no single weld can pull it far. On a long joint, tacks every 100 to 150 mm (3.94 to 5.91 in) are a reasonable starting point — and each tack is itself a small shrinkage, so they should also be placed alternately.
Last reviewed 2026-08-14