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Second-Source Qualification: Requalifying an Invar Part at a New Shop

By Invar Machining editorial team· Published September 21, 2026 · How we write these guides

Adding a second source for an Invar part, whether for supply chain resilience, cost competition or because your primary shop cannot meet a volume increase, is not the same exercise as qualifying a second source for a simpler material. The process a shop uses to get to the print dimensions matters as much as the dimensions themselves, and two shops can both produce a part that passes inspection while behaving differently over time.

Why this is different from qualifying a second source for other materials

For many materials, second-source qualification is mostly about confirming the new shop can hit the same tolerances with comparable quality, and the specific process used to get there matters less as long as the result is dimensionally correct at inspection. Invar breaks this assumption because its behavior after machining, not just at machining, is part of the specification. A part that passes first article inspection can still drift out of tolerance over the following days if the new shop’s stress relief cycle, covered in stress relief for Invar machined parts, differs meaningfully from the shop that originally qualified the part, even if both cycles are individually reasonable.

Process differences to check between shops

  • Stress relief cycle. Temperature, hold time and cooling rate can vary between shops while each individually produces an acceptable part. If your original qualification data was generated with a specific cycle, a new shop’s different cycle is a variable worth understanding, not necessarily a problem, but one to document.
  • Fixturing approach. How each shop holds the part during roughing, stress relief and finishing, covered in Invar fixturing and workholding, can differ even when both shops produce parts within print tolerance at inspection.
  • Measurement practice. Confirm the new shop measures critical features after a comparable soak time and at a comparable temperature to your original source, discussed in Invar machining tolerances, since a measurement practice difference alone can make two shops’ inspection data look different even on identical parts.
  • Material sourcing. Confirm the new shop’s material traceability and certification practice, covered in Invar material certification, matches what your program requires, particularly if your original qualification specified a particular mill or heat lot range.

A practical qualification approach

  1. Share your original part’s process history, not just the print, with the new shop: the stress relief cycle used, measurement practice and any known sensitivities specific to this part’s geometry.
  2. Run a first article inspection against the same critical features and datums used in your original qualification.
  3. For thermally critical parts, compare measurements over time, not just at first inspection, since the behavior that actually matters, dimensional stability over days and weeks, is exactly what a single inspection point cannot show.
  4. Document any process differences between the two sources, even ones that do not currently cause a measurable problem, so future troubleshooting has a clear record of what changed if an issue does appear later.

When the new shop’s process differs and that is fine

A second source does not need to replicate the first shop’s process exactly to be a valid qualification. Different, equally sound approaches to stress relief and fixturing can both produce a stable part. The point of checking these differences is not to force uniformity, it is to understand what changed, so that if a dimensional issue appears later, you have a documented starting point for troubleshooting instead of two unexplained black boxes.

Documentation worth keeping

  • The stress relief cycle and measurement practice for each qualified source
  • First article inspection data from each source, ideally measured against the same datums and at comparable conditions
  • Any drawing notes or RFQ details specific to this part’s known sensitivities, so a new source starts with the same information your original shop had

Getting a second source quoted properly

When requesting a quote for an existing part from a new shop, include the process history and known sensitivities described above, not just the drawing, so the new shop can qualify against the same expectations your original source met.

Send your drawing and process history through our RFQ form, and we match you with shops that can review that history and tell you upfront whether their process aligns with it or where it differs.

Frequently asked questions

Why would a part that already works at one shop need requalification at a second?

Because dimensional stability in Invar depends heavily on the specific stress relief cycle, fixturing approach and measurement practice a shop uses, not just on hitting the print dimensions. A second shop with a different but equally valid process can still produce a part that behaves differently in service if the process differences are not understood and accounted for.

Is a first article inspection enough to qualify a new Invar source?

It is a necessary part of qualification but not always sufficient on its own for thermally critical parts, since dimensional drift can show up over days or weeks, not immediately at inspection. For the most demanding applications, comparing measurements over time, not just at first inspection, gives a more complete picture.

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