The drawing hasn’t changed. The die was copied from the old source. Yet the first parts from the new domestic shop come off with the 90° flange sitting at 91.5°, and the holes in that flange no longer line up at assembly.
This is one of the most common first-article failures when a formed part moves to a new supplier, and the cause is rarely the shop’s skill. More often, the steel is different in a way the material callout never captured.
A grade is a floor, not a target
Most structural sheet and strip specifications set a minimum yield strength. ASTM A1011 SS Grade 40, for example, requires at least 40 ksi (275 MPa). It does not say how far above that a coil may be. Two mills can both ship conforming Grade 40, one running close to the minimum and one well above it.
Springback grows with yield strength. A bend that was tuned for a softer coil will open up when stronger steel goes through the same tool, and the effect is larger when:
- the part is air-bent rather than bottomed or coined,
- the inside bend radius is large relative to thickness,
- thickness sits at the top of its tolerance, or the die clearance is wide,
- yield varies from coil to coil, so the angle moves with every coil change.
Check these, in this order
- The measurement. Measure the same part twice, and with a second method if you can. Is the part located on the drawing datums, and is the flange held the way the drawing or inspection note requires? A flexible flange measured unrestrained can read out of tolerance on a good part.
- The mill test reports. Put the yield strength and thickness from the new source’s MTR next to the old source’s. A clear difference is usually the answer.
- Angle by coil. Measure parts from more than one coil and plot the angle in production order, marking each coil change. A step at each coil change points to material variation. A slow drift points to wear or warm-up.
- The bend itself. Air bending or bottoming? Die clearance? Inside radius versus thickness?
What to change, and who approves it
The usual corrections fall into two groups. The first can normally be made by the shop and toolmaker under the agreed process, with the result re-verified:
- add overbend to the forming station,
- bottom or restrike the bend,
- reduce die clearance.
The second group changes the design or the material requirement, so it needs the buyer’s engineering approval before anything is built:
- tightening the yield range with the mill (a material specification change),
- adding stiffening darts or gussets across the bend (a design change).
In DSQP these are marked (E). Any correction that changes the drawing, material, functional geometry, approved tool surface or controlled process goes through change control, even when it works.
Keep it from coming back
A fix that holds for five first articles can still fail on the next coil. Before approval, make the control part of the process:
- review yield and thickness on the MTR for every incoming coil, against the range the die is set for,
- run a first-part angle check after every coil change, not only at setup,
- write the reaction into the requirements matrix: stop, adjust the bend, sort parts made since the last good check,
- if the material source changes later, treat it as a change that needs notice and re-approval.
Springback is predictable once you know which steel is going through the die. The mill test report usually tells you before the protractor does.
Atlas Insights are general technical guidance. Customer, safety and regulatory requirements, and your engineer’s decisions, always govern.