Key takeaways
- A yield strength change from 240 MPa to 340 MPa at the same R10 and 2 mm raises springback from 1.5° to 2.2° at 90°, which displaces the end of a 60 mm flange by about 0.7 mm.
- A weld heat input change from 0.72 kJ/mm to 0.54 kJ/mm moves angular distortion, because distortion follows the heat put into the joint, not the weld size on the drawing.
- Powder from a different resin batch at the same RAL number and the same 75 µm film can fail a cross-cut that the previous batch passed, so adhesion is requalified per batch, not per colour.
- Requalify any change that touches six things: substrate grade, thickness tolerance, forming tool, weld process, pretreatment chemistry or coating chemistry, even when no dimension changes.
- The requalification record must carry the change reference, the measured characteristics, the gauges used and the result; a passed sample with no record is a sample, not an approval.
The part number is the same. The revision is the same. The drawing, the material grade and the 2 mm thickness all read exactly as they did last quarter, and the parts coming off the line are 0.6° over-bent with the flange end 1.1 mm out of position.
That is what a supplier change without requalification looks like. A coil from a different mill, a powder batch from a different resin, a fixture re-clamped after maintenance or a weld moved from manual to robotic can each move the part while leaving every document intact.
This is not a paperwork problem. It is a measurement problem: the drawing describes the finished part, the process decides whether the part arrives at that description, and a change in the process is invisible until something is measured. What follows is which changes need a first article, what to measure, and the arithmetic that shows how far a part moves.
What does the drawing actually control?
A drawing controls geometry, material grade, finish and, at best, a tolerance band. It does not control yield strength within a grade, thickness within a coil tolerance, coating resin chemistry, weld heat input, edge condition or fixture clamp position. All six decide the part.
ISO 9001:2015 clause 8.5.6 requires changes to production to be reviewed and controlled, and the review has to consider the consequences on the product. In practice that means a change note that names the characteristic at risk, not a note that says the process was updated.
The distinction matters because most change notices are written by the person making the change, and the characteristic at risk is usually in a different department. A coil substitution is a purchase decision; the 0.6° of extra springback it causes is a forming consequence.
The four variables that move silently
Yield strength within a grade, thickness within a coil tolerance, weld heat input and coating chemistry. Each changes a property the drawing cannot see.
Yield strength moves springback. Thickness moves springback, flat pattern and stiffness. Heat input moves distortion, penetration and the heat-affected zone. Coating chemistry moves adhesion and, at the same film thickness, corrosion performance.
A fifth variable sits behind all four: the fixture. A re-clamped fixture or a rebuilt checking gauge moves the part without touching the material, and it is the change most often made without a note at all.
Supplier change without requalification: which changes need a first article?
| Change | Visible on the drawing? | What moves | Requalification needed | Ganesh limit or note |
|---|---|---|---|---|
| Coil from a different mill, same grade | No | Yield strength, and springback with it | First article plus mill certificate | Certificate checked at goods-in, heat number recorded |
| Grade change, 240 to 340 MPa | Only if the callout changes | Springback, flat pattern, tool load | First article, tool check, die try-out | Press tonnage re-checked on the 20–300 T range |
| Thickness within coil tolerance | No | Flat pattern, stiffness, weld fit-up | First article on critical characteristics | Measured at three points per blank |
| Press tool replaced or re-ground | No | Bend angle, flange length, burr side | First article plus fixture check | Tool room records the regrind |
| Laser replaces press for the same cut | No | Edge condition, hole taper, flatness | First article plus edge check | 12 kW laser, 40 ft × 16 ft bed |
| Manual MIG to robotic MIG | No | Heat input, distortion, weld profile | Weld coupon, then first article | Three CLOOS cells, fixtures built in-house |
| Fixture re-clamped or rebuilt | No | Hole pattern position, twist | First article on the fixture characteristics | Checking fixture measured against the drawing |
| Powder batch from a new resin | No | Adhesion, gloss, cure window | Cross-cut plus DFT on a coupon | Seven-tank line, 18 ft × 14 ft booth |
| Pretreatment chemistry changed | No | Conversion coating mass, adhesion | Coupon test, then first article | Bath chemistry logged per shift |
| Blast media changed | No | Anchor profile, edge radius | Coupon profile check | 16 ft × 16 ft chamber |
| Sub-tier process moved | No | Everything the process touches | Full PPAP level as agreed | PPAP and first article inspection available |
The pattern is that the drawing is a poor detector of process change. Every row above has a blank in the second column and a consequence in the third.
How far does a change move the part?
Yield strength and springback
Springback for a bend is estimated as Δθ = 3σyRθ/(Et). Take a 90° bend at R10 in 2 mm steel with a 210 GPa modulus.
At 240 MPa the recovery is 3 × 240 × 10 × 90/(210,000 × 2), which is 648,000/420,000, or 1.54°. At 340 MPa the same expression gives 918,000/420,000, or 2.19°. The change is 0.64°.
On a 60 mm flange, 0.64° displaces the end by 60 × tan(0.64°), or 0.67 mm. Two such flanges in a channel put the hole pattern 1.34 mm apart, which is more than the 1 mm positional tolerance on most brackets.
Nothing on the drawing changed. The bend program overbends by a fixed angle, and that angle was set for 240 MPa.
Heat input and distortion
Weld heat input is Q = 60VI/(1000v) in kJ/mm, with V in volts, I in amps and v in mm/min. A manual MIG at 24 V, 180 A and 360 mm/min gives 60 × 24 × 180/(1,000 × 360), or 0.72 kJ/mm.
The same joint on a robotic cell at 480 mm/min gives 60 × 24 × 180/(1,000 × 480), or 0.54 kJ/mm. The weld is the same size on the drawing, the penetration is adequate in both, and the heat going into the plate has fallen by 25%.
Angular distortion in a fillet follows that heat. On a 2 mm panel, the shorter, faster weld pulls less, which is the good news; on a 4 mm frame it can cool fast enough to change the bead profile and the toe geometry that the fatigue life depends on.
Coating chemistry at the same film
Two powders can both be RAL 7035, both cure at 180–200 °C, both lay down 75 µm on the same panel, and behave differently under a cross-cut. The difference is the resin, the filler loading and the cure window, none of which the colour code carries.
ISO 2409 grades the cross-cut from G0, no detachment, to G5, more than 65% removed. A change of resin batch that moves a G0 to a G2 has not changed the colour, the film thickness or the drawing, and it has changed the warranty.
Thickness inside the coil tolerance
A coil is ordered to a nominal thickness with a tolerance, and the tolerance is not zero. On 2 mm CRCA, a ±0.10 mm band means a blank can be 1.90 mm or 2.10 mm and still be to specification. Both are legal. Neither forms like the other.
Bend allowance at R2 with K at 0.44 is 1.5708(R + Kt). At 1.90 mm it is 1.5708 × (2 + 0.836), or 4.46 mm. At 2.10 mm it is 1.5708 × (2 + 0.924), or 4.59 mm. The difference is 0.14 mm per bend, which is small until it appears on four bends and at both ends of a box.
The larger effect is stiffness. Stiffness scales with t³, so 1.90 mm against 2.10 mm is 6.86 against 9.26, a 26% difference in bending stiffness across a legal coil band. A bracket that was stiff enough on the first coil can flex on the second, and both parts are to the drawing.
That is the argument for measuring thickness at the blank rather than trusting the nominal. Three readings per blank take seconds and turn an invisible variable into a recorded one.
What does a first article have to measure after a change?
Every characteristic the change can touch, plus the characteristics the assembly depends on, even if the change should not affect them. The second part is the one that finds the surprises.
For a coil change: thickness at three points per blank, yield from the mill certificate, the formed angle on every bend, the flange length, the hole positions, and the flat pattern dimension before forming.
For a weld change: a sectioned coupon from the first part, macro etch for penetration and heat-affected zone, the distortion measurement on the assembly, and the weld profile at the toe.
For a coating change: film build with a DFT gauge at nine points, cross-cut to ISO 2409, and where the part is outdoor-rated, a salt spray coupon to ISO 9227 at the duration the drawing class calls for.
For a fixture change: the fixture itself against the drawing, then the first part in the fixture. A fixture that is 0.3 mm out produces a part that is 0.3 mm out and a first article that measures it as correct, because the part is measured in the wrong place.
The first article inspection guide covers the record and the sample size, and the PPAP guide covers the submission level when the change is customer-visible.
What Ganesh actually runs
A change to a running part goes through the same route as a new part: the change is written against the part number and revision, the characteristics at risk are listed, and the first part off is measured against the drawing by the inspection team rather than by the operator who made it.
Material arrives with a mill certificate, and the heat number is recorded against the job so a coil substitution can be traced from the finished part backwards. Forming runs on presses from 20 T to 300 T, on the 200 T 5-axis brake with its 4,450 mm bed, or on the 12 kW fibre laser where the change is a cut process rather than a form. Welding runs in three CLOOS robotic MIG cells or at 30+ manual stations, and a change between them is treated as a new process, not a new setting.
Coating changes go through seven-tank pretreatment and the 18 ft × 14 ft powder booth, with DFT and cross-hatch adhesion recorded on the first part. Parts for Mahindra, Godrej & Boyce and TKE are qualified this way, and PPAP and first article inspection are available where the customer's system requires them.
The plant and quality hub indexes the rest of the system guides, and the stamping capability page sets out the presses and the tool room that support a change of this kind.
What goes into the change record?
- The change reference and the date it became effective, written against part number and revision.
- What changed, in process terms: coil, mill, grade, tool, weld process, fixture, chemical, media.
- The characteristics at risk, with nominal, tolerance and the reason it is at risk.
- What was measured, with the gauge or CMM identified and the calibration status.
- The material certificate or batch record for the changed input.
- The result: pass, conditional pass with a deviation, or reject.
- The approver, and the customer approval reference where the change is customer-visible.
- The disposition of parts made before the change, because a bin of mixed parts is the second failure.
When a first article is the wrong response
A first article is the wrong response when the change was never declared. If a coil is substituted mid-batch and the substitution is discovered at goods-in, the honest action is to quarantine the parts made from it and decide with the customer, not to measure one part and call the batch good.
It is also the wrong response when the change is a design change wearing a process label. If the new tool produces a different flange length because the tool was made to a different drawing, the drawing is what needs revising, and the first article will only confirm that the part no longer matches it.
And it is the wrong response when there is no customer requirement to meet. A first article measures against a specification. A change with no drawing tolerance, no coating class and no weld standard on the drawing cannot be requalified, only sampled, which is why the drawing should carry those requirements before the part is first made.
Send the change description with the drawing, the characteristics the assembly depends on and the quantity affected to request a quote. The scope of the first article is agreed before the change runs, which is cheaper than agreeing it after the batch is in the bin.
The revision control guide covers the design side of the same problem, and transfer tooling from an existing supplier covers the change that brings a tool and its history into a new plant.
Frequently asked questions
Does a material change need a new first article if the grade is the same?
Yes. Two coils of the same grade from different mills can differ in yield strength by 40–60 MPa, and springback at R10 on 2 mm moves from 1.5° at 240 MPa to 2.2° at 340 MPa. The drawing cannot see the difference; the formed part can.
How long does requalification take?
Plan on one production run: the change is made, the first part is measured on every critical characteristic, and the coating and weld checks follow at their normal intervals. The measurement itself is hours; the plant time to make a representative part is the real constraint, so a change should be scheduled, not slipped in.
Is a process change on the drawing revision enough?
No. A drawing revision describes the part, and a process change describes how it is made. If a part moves from a press tool to the laser, the drawing is unchanged and the edge condition, flatness and hole quality all move. That needs a process change note and a first article, not a revision letter.
What should the requalification record contain?
The change reference, the date, the material certificate or batch number, the characteristics measured with their nominal and tolerance, the gauge or CMM used, the result and the approver. Link it to the part number and revision so the next auditor can find it from either direction.
Does a coating colour change need a first article?
Only if the chemistry, gloss or resin changes with it. A colour change within the same product family and resin is a batch check: film build with a DFT gauge and a cross-cut to ISO 2409. A different resin, a different gloss level or a different supplier of the same RAL is a requalification.
Sources and further reading
- ISO 9001:2015 — Quality management systems, clause 8.5.6 control of changes · reviewed 3 Nov 2026
- AIAG Production Part Approval Process (PPAP), 4th edition · reviewed 3 Nov 2026
- ISO 10007:2017 — Quality management: guidelines for configuration management · reviewed 3 Nov 2026
Planning a material or process change on a running part?
Tell us what is changing and which characteristics the assembly depends on. We will scope the first article, the coating or weld checks and the record before the change goes into the schedule.
Request a quote