Controlling weld distortion in frames

Why a frame with sound welds still misses the drawing

Controlling weld distortion in frames is a sequencing decision, not a welding skill. Two frames with the same weld size, made by the same operator on the same shift, finish 2–4 mm apart on the diagonal if one is run from the centre outwards and the other from corner to corner. Both welds pass visual inspection. One assembly still fails at the mating panel.

The joint does not shrink as a free body. A metre of 6 mm fillet would shorten by roughly 18 mm if nothing restrained it: steel contracts about 12 × 10⁻⁶ per °C, and the weld cools through about 1,480 °C from solidification to room temperature. The surrounding plate resists that contraction, absorbs most of it as plastic strain in the heat-affected zone, and leaves 0.3–1.0 mm of net draw-in. Where that residual movement lands is set by the sequence and the fixture, not by the weld symbol.

For a buyer signing off a frame drawing, the consequence is direct. Distortion cannot be specified away with a flatness callout. What can be specified is the datum that must survive, the allowance the supplier is expected to hold, and the point in the route where the frame is measured.

What the diagonal tells you before the coating goes on

Measure diagonals rather than outside length. A 1,200 × 600 mm frame has a diagonal of 1,342 mm, so a 2 mm difference between the two diagonals is 0.15° of skew. On a 600 mm mounting foot, that skew moves the far hole by about 1.6 mm — enough to turn a slotted hole into a rework note.

Two rules make the measurement comparable. Take it at the same temperature on every frame, because a part moved from a 40 °C shop into an air-conditioned inspection room changes size before it changes shape. Take it before coating, because 60–100 µm of powder on one face hides the witness marks used to judge whether the frame moved during cure.

How you measure matters too. A tape hooked over two outside corners reads the corner radius as well as the diagonal, so two frames with different edge treatment can disagree by half a millimetre without either moving. Pin the frame on the datum holes, measure between the same two hole centres on each frame, and record the figure with the frame orientation. A 3-point check — both diagonals plus the distance between the two datum holes — separates a frame that has gone rhomboid from one that has simply shrunk in length, and the two need different corrections.

Heat input, travel speed and the number that changes both

Heat input per millimetre of weld is Q = 0.8 × 60 × V × I / (1,000 × v), with voltage in volts, current in amps, travel speed in mm/min and 0.8 as the arc efficiency of MIG. At 24 V and 200 A travelling at 300 mm/min, Q is 0.77 kJ/mm. Raise travel speed to 400 mm/min and the same joint falls to 0.58 kJ/mm — a 25% cut in heat per millimetre without touching the current.

That is the cheapest control available at a manual station: faster travel, shorter arc, less weave. In a robotic cell the same trade is made once, in the programme, then repeated on every part. What the robot cannot do is compensate for a gap that moves from 0.2 mm to 2.0 mm between parts, which is why fixture location and the quality of the cut parts matter more than any repeatability figure on the specification sheet.

Distortion controls and the limits we plan to

The values below are the planning figures used when a frame drawing arrives without a stated distortion allowance. They start the quotation; they do not replace the drawing.

Sequence rules that hold a diagonal

Four rules do most of the work. Weld from the centre of the frame outwards, so contraction runs toward the free ends instead of closing a corner. Alternate sides — never two adjacent sides in one pass. Backstep long runs in 200–300 mm increments, so each increment starts against material the previous one has already restrained. Leave the least critical joint until last, so the joint that sets the diagonal is completed while the frame can still move.

On a frame with more than one bay, treat each bay as its own sequence and let the shared member cool before the next bay starts. Two bays welded in parallel pull against each other, and the second one usually wins the argument.

Where the tacks go

Tacks are part of the sequence. Place them on the centreline of the joint, 15 mm long, at 150–250 mm pitch, and grind them back to sound metal before the root run. A tack that stands proud becomes a stress raiser and a cold lap in the finished joint, and a tack placed on the edge of a thin flange pulls the flange before the weld has started.

Where the frame is released

Releasing clamps is a step, not an event. Undo them in the reverse order they were applied, one at a time, and measure the diagonal after each stage on the first article. If the diagonal moves when one particular clamp comes off, that clamp was fighting the weld and the fixture needs changing — not the welder, and not the weld size.

Fixtures: locate the datum, not the outside profile

A fixture that clamps the outside of a frame grips the surface that will be coated and hides the surface that will be bolted. Locate on the assembly datum — usually two holes and a face — and clamp within 50 mm of the joint being welded, where restraint is useful and heat can be drawn away.

Three details decide whether the fixture helps or hurts. Copper or aluminium backing bars under a seam sink heat and reduce draw-in on thin sheet. Over-clamping a stiff frame stores residual stress that releases when the clamps come off, so the part springs after it leaves the fixture. A fixture that blocks torch access forces a longer arc, which raises heat input and cancels the restraint it was built for.

Robotic MIG on repeat frames

Robotic MIG gives a frame the same sequence every cycle, and that repetition is the property that matters. Heat input matches a manual station running the same parameters; the difference is that it does not drift through a shift. Three CLOOS cells cover repeat frames, with fixtures designed and built in-house so the location scheme follows the drawing datums rather than the shop's habit.

Robotic welding does not rescue variable fit-up. A robot welds the gap it is given. If cut parts vary by 0.8 mm and the fixture does not close that variation, the programme lays a consistent arc into an inconsistent joint, and distortion starts tracking the gap instead of the sequence. That is why the cut quality upstream and the fixture design belong in the same conversation as the welding parameters.

Inspection points between welding and coating

A frame should be measured twice: once straight out of the fixture, before any dressing, and once after dressing and before pretreatment. The first measurement tells you whether the sequence worked; the second tells you whether grinding a weld or hammering a flange moved the datum. If the frame only passes when measured at dispatch, the route has a hidden step that will not survive a change of shift.

The first article sets the record: datum holes, both diagonals, flatness on the mounting face, and the weld sizes on three sample joints. Later releases check the same fields on a sampling basis, and every frame is checked for the diagonal before it enters the seven-tank line, because a frame that has to be reworked after coating loses the finish as well as the time.

What we run on frame work

Frame sheet is cut on a 12 kW fibre laser whose 40 ft by 16 ft bed takes a full side panel without a splice, then formed on a 5-axis 200 T brake with a 4,450 mm bed under Delem control, to 10 mm. Welding runs on three CLOOS robotic MIG cells or at more than 30 manual stations covering MIG/MAG, spot and SPM work, after which the frame is shot blasted, pretreated and powder coated. The welded assemblies route and the bending capability are the two stages that decide the frame, and both fixtures and checking fixtures are designed and built in-house so the sequence can be frozen at first article and repeated on the next release.

Programmes run under ISO 9001:2015 for quality management and ISO 14001:2015 for environmental management, with first article inspection and PPAP documentation where the customer requires it. Frame work for TKE, Eros Elevators and Indian Railways follows the same order: drawing review, fixture, first article, release. The bend geometry behind the cut file is worth checking in the bend allowance calculator before the drawing is issued, because a wrong flat length becomes a wrong gap at the weld.

What to put on the frame drawing

• The assembly datum set, marked A, B and C, with the two holes used to locate the frame.
• A weld symbol with leg size on every joint, plus a note that unspecified fillets are 3 mm rather than "as required".
• The distortion allowance: diagonal tolerance, flatness on the mounting face, and whether the frame is measured before or after coating.
• Straightening permission: whether press straightening is allowed and which surfaces it may touch.
• Masking and earthing points, with thread sizes that must stay paint-free.
• The revision of the mating panel, because the frame is usually the part that has to absorb the panel's tolerance.

Related reading that changes the same numbers: welded assembly tolerances covers how those allowances stack through a finished assembly, and fixture-controlled welding for dimensional accuracy sets out the locating scheme in more detail. Both sit inside the bending, welding and assemblies hub, alongside the bending and robotic welding pages.

When welding is the wrong way to fix this

Some frames should not be welded at all. On 0.8 mm cosmetic panels, spot welding or riveted construction moves less and leaves no heat-affected zone to finish. Where a mounting face has to be flat to 0.2 mm, the honest route is weld, stress relieve, then machine the face, and that machining step has to be planned and quoted separately. If the design needs furnace stress relief after welding, that operation sits outside the fabrication route described here and must be arranged and inspected as its own step.

The same applies to cosmetic expectations. A frame welded and straightened will show a slight waviness along the flange even when every dimension is inside tolerance, and powder coating does not fill it. If the frame is visible in the finished product, say so on the drawing: the A-surface requirement changes the sequence, the fixture and the amount of dressing time, which is a price difference rather than a defect.

Controlling weld distortion in frames is therefore a drawing conversation before it is a shop-floor one. Send the frame drawing with the datums, the weld sizes and the movement you can live with, and the sequence, fixture and allowance can be quoted together instead of argued about after first article.