Skip to content

Engineering Change Requests on an Invar Part Already in Production

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

An engineering change request on an established Invar part looks routine on paper, a dimension moves, a tolerance tightens, a feature is added, but the alloy’s process-dependent behavior means a change that would be trivial on a simpler material can have effects beyond the specific line item that changed.

Why ECRs need extra scrutiny on Invar parts

Most of a machined part’s cost and process is set up around its specific geometry: fixturing, toolpaths, the stress relief cycle and how it interacts with the part’s mass and shape, and inspection datums. A change that seems small on the drawing, a tolerance tightened by half, a wall made thinner, a hole added near a previously unconstrained edge, can ripple into the fixturing approach covered in Invar fixturing and workholding, the thin-wall considerations in thin-wall Invar machining, or whether the existing stress relief cycle, covered in stress relief for Invar machined parts, still delivers the stability the new requirement needs.

Categories of changes and their typical impact

Loosening a non-critical tolerance. Usually low impact. Rarely requires requalification, and the shop can typically confirm this quickly.

Tightening a tolerance, even a small one. Worth scrutiny. A tolerance tightened from ±0.05 to ±0.02 mm on a feature that was previously easy to hold might now require a process change, additional inspection, or in some cases push the feature into territory where the existing stress relief and finishing sequence is no longer sufficient, a distinction covered in Invar machining tolerances.

Adding a feature near an existing one. Can change fixturing, since the new feature may need clamping clearance that conflicts with the existing setup, or it may introduce a new thin section or asymmetry that affects distortion risk.

Changing wall thickness. Directly affects the thin-wall and fixturing considerations covered above, and even a modest reduction can shift a feature from routine to one that needs the specific handling discussed in thin-wall Invar machining.

Changing the datum structure. Affects how the part is measured and can invalidate historical inspection data for comparison purposes, even if the physical part itself is unchanged.

Scoping an ECR properly

  1. State the underlying reason for the change, not just the new dimension. A shop that understands the change is driven by a fit issue with a mating part, for example, can sometimes suggest an alternative that achieves the same functional outcome with less process disruption than the specific change originally proposed.
  2. Ask the shop to assess process impact before implementing, not after. A tolerance change that looks like a one-line drawing revision can require re-evaluating fixturing, stress relief sequencing or inspection approach, and finding this out after parts have already been made against the new print costs more than finding out beforehand.
  3. Decide whether requalification is needed. For a significant change to a critical feature, a first article inspection against the new requirement, similar to the process covered in second-source qualification for a new supplier, confirms the change actually achieves what was intended before it rolls into full production.
  4. Update your documentation. Any change to a stress relief cycle, fixturing approach or inspection practice resulting from the ECR should be captured in the same process history discussed in second-source qualification, so future troubleshooting or a future second-source effort has an accurate record.

Cost and lead time implications

An ECR that changes the process, not just the drawing, can affect cost and lead time on both the specific change and, in some cases, existing inventory or work in progress made against the old print. Flagging an ECR early, before a shop has committed to a production run under the old requirement, avoids the more expensive scenario of parts made to the old print becoming unusable once the change takes effect.

Getting an ECR evaluated

Describe both the proposed change and its underlying reason when you send it to your shop, and ask specifically whether it affects fixturing, stress relief or inspection, not just whether the new dimension is achievable in isolation.

If you are evaluating whether an ECR might be better served by a different Invar shop entirely, or need a second opinion on process impact, send your drawing and the proposed change through our RFQ form, and we match you with shops that can assess it properly.

Frequently asked questions

Does a tolerance change always require requalification of a part already in production?

Not always. A loosened tolerance on a non-critical feature usually does not. A tightened tolerance, or a change to a critical feature or its datum structure, often does, since it can affect whether the existing process is still capable of meeting the new requirement consistently.

Can I make an engineering change without telling the shop why?

You can, but sharing the reason usually gets you better input. A shop that understands why a change is happening, a fit issue, a new requirement, a cost reduction effort, can often suggest a more effective way to achieve the same goal than the specific change you proposed.

Need a quote for this part?

Send the drawing. We match you with up to 3 shops that machine Invar and reply within 24 business hours.

Get matched with an Invar shop

Keep reading

All process guides