Key takeaways
- A 2,500 mm steel panel at a 190 C cure wants to grow 5.7 mm; held at both ends it develops about 479 MPa, well past 250 MPa yield, and comes out permanently bowed.
- Every 10 C of cure temperature adds about 0.3 mm of free expansion on a 2,500 mm panel, so a 180 C and a 200 C cure are not the same process for flatness.
- Stitch weld stiffeners at 25 mm on 150 mm centres instead of running a continuous fillet, and keep the weld on the neutral axis where the drawing allows it.
- Never mix an aluminium stiffener with a steel skin on a coated panel: the two expand 5.2 mm apart over 2,500 mm at 190 C, and the panel bows to absorb it.
- Call flatness as 2 mm/m after coating, measured on a 1,000 mm straightedge with a feeler gauge, and state the measurement temperature and support points.
A 2,500 mm coach panel that leaves the press flat can leave the oven with 6 mm of bow, and the drawing will blame the laser. The oven is the more likely cause. At a 190 C cure, a steel panel of that length wants to grow 5.7 mm, and a fixture that holds both ends turns that growth into about 479 MPa of compression, which is nearly twice the 250 MPa yield of the sheet.
The panel does not crack. It buckles, yields locally, and cools into a shape that is stable but not flat. Every later operation inherits the shape: the fixing holes no longer line up with the coach datum, the panel gap opens at one end, and the trim covers a gap that was created eight metres earlier in the paint line.
Coach panel flatness is therefore a process tolerance, not a cosmetic one. It has to be specified after coating, measured on the supports the panel will see in service, and designed so that the panel can expand and contract without being restrained.
Why does a coach panel bow?
A long panel bows when one part of it grows or shrinks more than another and the panel is not free to move. Three sources dominate, and only one of them is the cut edge.
The first is thermal expansion against a constraint. Steel expands at about 12 x 10-6 per kelvin. On 2,500 mm and a 190 C rise, that is 5.7 mm. If the panel is clamped at both ends during cure, or if it is hung from hooks that grip, the expansion cannot happen and the sheet yields in compression.
The second is weld shrinkage. A continuous 4 mm fillet along a 2,500 mm stiffener deposits heat along one face and contracts as it cools. The panel resists, the stiffener side shortens, and the panel curves towards the weld.
The third is an asymmetric section. A skin with a stiffener on one face only has its neutral axis outside the skin, so any differential strain between skin and stiffener bends the panel rather than stretching it.
The cure oven is a thermal load case
Treat the oven as a load case, not a finishing step. The free expansion is easy to calculate: length times coefficient times temperature rise. On 2,500 mm of steel it is 5.4 mm at 180 C, 5.7 mm at 190 C and 6.0 mm at 200 C.
The stress appears only when the expansion is restrained. Fully restrained, the thermal stress is E times alpha times delta-T, which is 454 MPa at 180 C, 479 MPa at 190 C and 504 MPa at 200 C. The sheet yields long before that. With a 250 MPa yield and a modulus of 210,000 MPa, the elastic limit is reached at a rise of about 99 K. Any full restraint above roughly 100 C therefore guarantees a permanent set.
That matters because the seven-tank pretreatment line includes a dry-off stage before powder, and the powder cure follows at 180–200 C. A panel can yield twice in one pass: once on the way into the dry-off, and again in the cure oven.
The fix is not a slower oven. It is a support that lets the panel grow. Hang long panels vertically from three hooks, not two, so the panel is not asked to span 2,400 mm under its own weight. Set the hooks so the panel can move sideways, and keep the contact points off the visible face.
Coach panel flatness limits
The table is written for a 2,500 x 1,200 mm interior side panel in 2.0 mm CRCA, powder coated, with stiffeners at 600 mm centres. Values are after coating unless stated.
| Parameter | Typical value | Ganesh limit or note |
|---|---|---|
| Panel size | 2,500 x 1,200 mm | longer panels need more support, not thicker sheet |
| Skin material | 2.0 mm CRCA, or 1.5 mm 304 | stainless moves less and holds flatness better |
| Free expansion at 190 C | 5.7 mm | 5.4 mm at 180 C, 6.0 mm at 200 C |
| Restrained stress at 190 C | 479 MPa | yield is reached at about a 99 K rise |
| Cure temperature | 180–200 C | confirm the actual peak, not the set point |
| Stiffener pitch | 600 mm maximum | 400 mm where the panel is visible from both sides |
| Stiffener attachment | 25 mm stitch on 150 mm centres | continuous fillet concentrates shrinkage |
| Stiffener material | same steel as the skin | aluminium on steel moves 5.2 mm apart at 190 C |
| Flatness before coating | 1.5 mm/m | check on a surface plate, not on the floor |
| Flatness after coating | 2 mm/m, 5 mm total | measured on the service supports |
| Straightedge length | 1,000 mm | shorter edges hide a long bow |
| Panel support in oven | three hooks, free to move | two hooks sag the panel between them |
| Coating thickness | 60–80 µm | seven-tank pretreatment, 18 ft x 14 ft booth |
| Fixing hole allowance | 1 mm clearance minimum | a clamped hole becomes a restraint |
Stiffeners: balance the section, not just the skin
A flat 2.0 mm skin over 1,200 mm is flexible out of plane. Its second moment about the weak axis is only about 800 mm4 across the full width, which is why an unstiffened panel needs handling frames just to stay flat on the bench.
Two 30 x 30 x 2 mm angles at 600 mm centres raise that to roughly 22,000 mm4, a 27-fold increase, and the panel stops oil-canning when a technician leans on it. The same stiffeners are the source of the bow if they are attached on one face with a continuous weld.
The rules that keep the stiffness without the bow:
- Balance the faces. A stiffener on the back and a return flange on the front edge put the neutral axis back inside the panel. If the panel is visible from both sides, use a top-hat section that is symmetric about the skin.
- Keep the stiffener shallow. A 30 mm angle at 600 mm pitch does the work. A 60 mm channel at 400 mm pitch adds stiffness the panel does not need and concentrates far more shrinkage at the weld.
- Stitch, do not run. A 25 mm weld on 150 mm centres welds about 17 percent of the length. The shrinkage falls roughly in proportion, and the panel keeps a series of short anchors rather than one long contraction.
- Weld from the centre out. Two welders working from the middle towards the ends balance the shrinkage. One welder running end to end pulls the panel into a single curve.
- Do not weld across a bend. The bend already has residual stress from forming. A weld toe at the bend line adds to it, and the panel kinks at that line after the oven.
If the panel needs no stiffener, leave it unstiffened and support it in the oven. A bare 2.0 mm skin is easier to keep flat than a skin with one-sided reinforcement, because it has no second member pulling against it.
Mixing materials and measuring the result
An aluminium stiffener on a steel skin is the fastest way to a bowed panel. Aluminium expands at about 23 x 10-6 per kelvin and steel at 12. Over 2,500 mm at 190 C the two want to grow 5.2 mm apart. The panel absorbs that difference as curvature, and no amount of stitch welding removes it.
The same applies to a stainless skin with a CRCA stiffener. Austenitic 304 expands at about 17 x 10-6 per kelvin, so the mismatch is smaller at roughly 2.5 mm over the same length, but it is still enough to show on a visible panel. The material rule is simple: skin and stiffener from the same family, and preferably the same grade.
There is a corrosion reason as well. A steel stiffener welded to a stainless skin creates a galvanic couple at every weld, and the powder coat does not reach inside a closed section. Condensation on a coach interior does the rest.
How to measure flatness so the number means something
Flatness is only meaningful when the support conditions and the temperature are stated. A panel that reads 2 mm on a bench reads 5 mm when it is held at four fixing points, because the bench is flattening it.
Measure the panel on three supports placed at the points it will use in the coach, with the panel free at the edges. Leave it at 20 ± 5 C for 24 hours after coating, because a panel measured warm is still moving. Lay a 1,000 mm straightedge across the face in three directions, measure the worst gap with a feeler gauge in 0.05 mm steps, and record both the mm/m figure and the total bow.
ISO 2768-2 covers general geometrical tolerances where the drawing does not call out a specific flatness value, and it applies to individual features rather than to a welded assembly. A coach panel is a welded assembly, so the general class is not a substitute for an explicit callout. State the flatness, the straightedge length, the support points and the measurement temperature on the drawing.
Fire performance is a separate specification. EN 45545-2 covers the fire behaviour of materials and components used on railway vehicles, and it is satisfied by the substrate and coating system, not by the panel's flatness. Do not let a flatness note imply a fire approval.
Related reading: the Cold store panel design: failure modes and the fix and the Metal furniture frame manufacturing: stamping, welding, coating take the same problem further. See the process and inspection scope on the capability pages, or send the drawing through the RFQ route.
What Ganesh runs for coach panels
Coach panel blanks are cut on the 12 kW fibre laser with the 40 ft x 16 ft bed, so a 2,500 mm panel and its window or fixing cut-outs are cut in one setup and the pattern does not shift between nested parts. Forming runs on the 200 T, 5-axis brake with the 4,450 mm bed, which folds a full-length return in one pass rather than in two overlapping hits. Stiffeners are welded on the three CLOOS robotic MIG cells or at the manual stations, with the stitch pattern set in the program rather than left to the operator.
Coating runs through seven-tank pretreatment into the 18 ft x 14 ft powder booth and the curing oven, with DFT and cross-hatch adhesion recorded per batch. Panels are hung vertically and checked for flatness after the oven, on supports, before packing. The same route produces coach side panels, seat frame parts and luggage rack components for railway builds, and Ganesh supplies Indian Railways among other customers.
The coach panel manufacturing guide covers the full route, the laser cutting guide for coach panels covers the cut edges and the bending guide for long panels covers the forming stage. The railway parts range lists the family, and the railway and infrastructure hub collects the related standards notes.
What to put on the drawing
- Panel size, material grade and thickness, with the rolling direction if the panel is visible.
- Stiffener section, material, pitch and attachment: stitch length and pitch, or adhesive.
- Cure temperature and time, taken from the coating system data sheet rather than assumed.
- Flatness after coating in mm/m, the straightedge length, and the total bow allowed.
- Support points used for measurement, and the panel temperature at measurement.
- Fixing hole size and clearance, so the panel is not clamped by its fasteners.
- Coating system, colour and thickness range, with any masked earthing points.
- Whether the panel is handed, and the sequence it is fitted in on the coach.
When a sheet metal panel is the wrong answer
A welded steel panel with stiffeners is the right route for most coach interior and exterior panels up to about 3,000 mm. It is the wrong route when the drawing asks for 1 mm/m flatness over 3,000 mm on a 1.5 mm skin, because the oven will not hold it and the panel will need a framed sub-assembly or a composite core instead.
Ganesh does not make aluminium honeycomb panels, does not anodise and does not carry out fire testing to EN 45545-2. Where a project needs a tested fire-rated composite, that is a different supply chain, and we will say so at enquiry rather than after the first article.
Send the panel drawing with the stiffener layout and the flatness callout, and the quotation will state the cure temperature, the support method and the flatness we will measure after coating.
Frequently asked questions
Why does a coach panel bow after powder coating?
The cure oven heats the panel to 180–200 C. A 2,500 mm steel panel grows 5.4–6.0 mm at that temperature, and if the conveyor or fixture holds the ends, the restrained expansion exceeds yield and the panel takes a permanent set. Weld shrinkage and asymmetric stiffeners add to it.
What flatness tolerance can a 2,500 mm coach panel hold?
Specify 2 mm/m after coating and expect 5 mm total over 2,500 mm on a 2.0 mm CRCA panel with stiffeners at 600 mm centres. A 1 mm/m callout is possible on a 1.5 mm panel with balanced stiffening, but it needs the panel checked on a surface plate after the oven, not before.
Should coach panels be stiffened on one side or both?
Both, or neither. A stiffener on one face only puts the neutral axis outside the skin, so the panel bows when the stiffener and the skin cool at different rates. Where a single-sided stiffener is unavoidable, keep it shallow and stitch weld it.
How is coach panel flatness measured?
Lay the panel on three supports at the points it will be fixed to the coach, put a 1,000 mm straightedge across the face and measure the gap with a feeler gauge. Report the worst gap in mm per metre and the total bow, and state the panel temperature, because a panel measured warm reads differently.
Does laser cutting cause the bow?
Rarely. Laser cutting leaves a narrow heat-affected zone at the edge, but a 2,500 mm panel cut on a fibre laser does not develop enough residual stress to bow on its own. The bow appears at the oven, or at the weld, and the cut edge only decides where the panel starts to move.
Sources and further reading
- ISO 2768-2 — General tolerances for geometrical tolerances on features without individual tolerance indications · reviewed 3 Nov 2026
- RDSO — Research Designs and Standards Organisation, Indian Railways, coach and wagon specifications · reviewed 3 Nov 2026
- BIS — IS 513 cold reduced carbon steel sheets and strips for cold forming · reviewed 3 Nov 2026
Have a coach panel drawing with a flatness callout?
Send the panel size, material, stiffener layout and coating system. We will confirm the forming and cure route, and quote the panel with flatness measured after coating.
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