Weld distortion allowance: designing for the movement

A welded frame that leaves the fixture 3 mm short and 2 mm bowed has not been welded badly. It has done what the heat input told it to do. Every metre of fillet weld deposits molten metal that then contracts, and the contraction is restrained by the cold material around it, so the assembly moves in a direction and by an amount that can be estimated before the first part is cut.

Weld distortion allowance is the extra material and the design freedom you build in so that the movement lands inside tolerance instead of outside it. It is not a fudge factor. It has four components with different magnitudes and different fixes: transverse shrinkage, longitudinal shrinkage, angular distortion and bowing. Designing for the movement starts with knowing which one dominates the dimension you care about.

This is written for design and manufacturing engineers specifying welded assemblies in 1.5–6 mm CRCA, mild steel and stainless, where the frame carries an interface another part bolts to. It gives the allowance by joint type, works a 2,000 mm frame through the stack, and shows where a design change costs less than a longer weld sequence.

Where the movement comes from

The mechanism is thermal expansion followed by restrained contraction. Steel expands by 12 × 10⁻⁶ per °C, so a 1,000 mm rail heated 150°C above ambient near a weld wants to grow 1.8 mm. It cannot, because the rest of the frame is cold and holds it, so it yields in compression while hot. On cooling it contracts from a plastically shortened state, and the assembly ends up smaller than it started.

The restraint stress is easy to estimate and confirms why clamping does not fix it:

σ = E α ΔT = 210,000 MPa × 12 × 10⁻⁶ × 150 = 378 MPa

That is above the yield strength of common CRCA grades at 240–280 MPa, so the material yields on the first pass. Anything that has yielded stays moved when the clamps come off.

Heat input sets the size of the effect. For MIG welding, Q = kUI/v, where k is about 0.8, U is arc voltage, I is current and v is travel speed. At 26 V, 220 A and 6 mm/s:

Q = 0.8 × 26 × 220 / 6 = 763 J/mm, or 0.76 kJ/mm

Doubling travel speed halves that number and roughly halves the shrinkage on the weld, but it also changes penetration and bead shape, so it is a welding-procedure decision rather than a drawing note. The drawing can influence the number that matters more: how many welds there are and how much metal each one deposits.

The allowance by joint type

The values below are design allowances for 3 mm CRCA with a 4 mm leg fillet, welded in a fixture. They are starting points for the drawing, not guarantees: the first article measures the actual movement on the job and the cut lengths are set from that.

The transverse column is the one that decides most sheet metal parts. A single fillet pulls its two plates together by about half a millimetre to a millimetre, and that movement is perpendicular to the weld, so it acts on the dimension across the joint rather than along it. The longitudinal column is smaller per unit length but it acts on the longest dimension, which is why long rails come up short.

A worked example: a 2,000 mm frame with 12 welds

Take a frame built from 3 mm CRCA sections, 2,000 mm long rails and 1,000 mm cross members. Each of the four corners has two fillets, and four gussets add one weld each, for 12 welds at a 4 mm leg.

Rail length. Each rail has a fillet at each end, so it shortens by 2 × 0.5 to 2 × 1.0 mm, which is 1–2 mm. Cut the rail at 2,001.5 mm and the finished part lands near nominal. Cutting at 2,000 mm guarantees a frame that is short and a fitter who blames the saw.

Hole spacing. The cross member carries two mounting holes 800 mm apart, welded at both ends into the frame. Four welds act across that 800 mm dimension, each pulling 0.5–1.0 mm, so the holes close by 2–4 mm. At a ±0.5 mm callout this is not a tolerance question; the holes have to be drilled or punched after welding, or the mounting bracket needs slotted holes.

Angular lift. A 100 mm flange welded to the frame at 1.5° of angular distortion lifts at its free edge by:

100 × tan 1.5° = 100 × 0.0262 = 2.6 mm

A flatness callout of 1.0 mm over that flange cannot be held by welding. Either pre-set the flange 2.6 mm the other way in the fixture, or leave stock and machine the face after welding.

The stack. Add 1.5 mm of rail shortening, 2.0 mm of hole movement, 1.0 mm of fixture location error and 2.6 mm of angular lift, and the worst case is 7.1 mm against a ±1.0 mm interface. No weld sequence closes that gap. The design has to change: machine the interface, drill after welding, or move the datum onto the welded assembly rather than the pre-welded parts.

Design changes that remove the allowance

The allowance is the fallback. The cheaper route is to remove the movement from the design, and five changes do most of that work.

Reduce the weld volume. A fillet's cross-section is leg² / 2, so a 4 mm leg deposits 8 mm² and a 6 mm leg deposits 18 mm². That 56% difference in deposited metal is a 56% difference in shrinkage force. Most over-welded parts carry a 6 mm leg because it looks stronger, not because the calculation asked for it. Specify the leg the joint needs and no more.

Weld near the neutral axis. Bowing is caused by shrinkage acting at a distance from the neutral axis, so a weld on one face of a long panel bends it while the same weld on the centre line shortens it without bending. Where a panel needs stiffness, a bead on each side of the neutral axis cancels the bow, at the cost of twice the welding.

Balance the sequence. Weld from the centre outwards, alternate sides, and use back-step on long seams. The total shrinkage does not disappear, but it stops accumulating in one direction. Balanced welding typically halves the net bow, and it is the cheapest change available because it costs only a work instruction.

Preset the fixture. If the first article shows a 2 mm pull, build the fixture 2 mm the other way. Preset is standard for long rails and for flanges that lift at the free edge. It requires the allowance to be measured rather than assumed, which is why the first article matters more on welded parts than on cut parts.

Design a joint that absorbs movement. Slotted holes, tab-and-slot location, shims at the interface and a machining allowance on the mounting face all give the assembly somewhere to move. A slot of 12 × 9 mm for an M8 bolt gives 3 mm of adjustment without weakening the joint, and it costs one punch feature.

The one change that does not work is tightening the tolerance on the drawing. A ±0.5 mm callout on a welded interface does not make the weld shrink less; it moves the argument to the inspection bench.

What to put on the drawing

• Weld symbols with leg size and length, on both sides where the joint is balanced.
• A note that straightening after welding is not permitted, because a straightened frame carries residual stress that the drawing did not intend.
• The assembly tolerance to ISO 13920 with the class stated, plus individual callouts on any interface a customer measures.
• Machining allowance on a mounting face where the tolerance is tighter than the weld can hold, with the stock size stated.
• "Drill after welding" on hole patterns whose spacing the shrinkage will change, or slotted holes where adjustment is acceptable.
• Flatness or angular tolerance on flanges that lift, expressed over the flange length rather than as a general note.
• Material grade and thickness, because shrinkage scales with heat input and section.
• A datum scheme taken from the welded assembly, not from the pre-weld parts.

ISO 13920 covers general tolerances for welded constructions: dimensions for lengths and angles, plus shape and position, in classes A to D with A the tightest. It is the right default for the frame outline and the wrong tool for a machined interface, which needs its own callout. ISO 2768-1 covers general tolerances for linear and angular dimensions where no individual tolerance is given, and it does not address weld movement at all.

What Ganesh actually runs

Three CLOOS robotic MIG cells and more than 30 manual MIG/MAG, spot and SPM stations weld the assemblies, and the fixtures are designed and built in the in-house tool room. Fixtures locate on the drawing datums rather than on the outer profile, so shrinkage pulls against a fixed reference and the movement appears where it can be measured and corrected rather than at the interface.

Cut parts come off a 12 kW fibre laser with a 40 ft × 16 ft bed or off presses from 20 T to 300 T, and bends up to 10 mm run on the 200 T, 4,450 mm brake. The first article records as-welded dimensions before anyone straightens or adjusts anything, which is the only way to know the real allowance for that job. PPAP and first article inspection are available across five units in Mumbai (Vasai) and Nagpur, under ISO 9001:2015 and ISO 14001:2015. The welded assemblies capability page covers the cells and the fixture route.

When the allowance is the wrong tool

Below about ±0.5 mm on a welded interface, no allowance and no sequence will hold the dimension across a production run. The part needs stock left on the face and a machining operation after welding, or a design change that moves the interface onto a component that is not welded. A drawing that specifies ±0.3 mm on a welded face without a machining note is asking for a concession at the first inspection.

Stress relief, where the drawing calls for it, is a separate operation with its own specification and its own distortion. It is not a substitute for a balanced sequence, and it does not restore a dimension that welding already moved.

Aluminium assemblies are a separate case again: the thermal conductivity and the oxide layer change both the procedure and the movement, and a joint developed for steel does not transfer. Those parts are reviewed individually rather than quoted from a standard allowance.

Weld movement is predictable enough to design around, and the numbers are cheapest to apply before the drawing is released. Send the assembly with the joint details, leg sizes, material and the interfaces that must hold to request a quote, and the review will say which dimensions need an allowance, which need a sequence and which need machining. The bending and welding hub holds the related rules, the weld distortion guide covers the shop-floor correction methods, and the welded assembly tolerance article sets out the inspection side.