Engineering changes are inevitable — uncontrolled changes are not
Roughly 25% of manufacturing quality issues at contract manufacturers trace back to revision-control failures: wrong-revision drawings on the shop floor, verbal-only change communication, or missing effectivity dates. The cost of a wrong-revision production lot is typically 5–20× the cost of the ECO process that would have prevented it. This guide gives you the framework to manage changes cleanly with any external manufacturing partner.
What Is an Engineering Change Order?
An engineering change order (ECO) is a formal, documented authorization to modify a released engineering design. It specifies what changed, why, and how the change flows to manufacturing, quality, and supply chain.
Why ECOs exist
Without a controlled process, changes happen through emails, phone calls, and markup PDFs with no audit trail. When something goes wrong — and it will — there is no way to determine which revision was built, who authorized the change, or when it took effect. ECOs create the paper trail that separates controlled engineering from chaos.
What triggers an ECO
Common triggers: design optimization (cost reduction, performance improvement), field failure correction, regulatory compliance change, material substitution due to supply-chain constraints, customer-requested modification, or manufacturing feedback (a feature that cannot be produced as drawn). Each trigger should be documented in the ECO rationale.
Cost of skipping the process
A verbal change ("Hey, can you make that hole 0.010″ bigger?") that is not documented results in: the shop floor running the wrong revision next time, the inspector checking against the wrong drawing, and the next engineer who touches the design not knowing the change was made. The cost of a wrong-revision lot (scrap + rework + schedule delay) typically runs $2K–$25K for CNC parts, depending on lot size and material.
ECOs apply to prototypes too — in a lightweight form
You do not need a full change-control board for prototypes. But you do need revision-controlled drawings (Rev A, B, C) and a clear record of what changed between revisions. The minimum: a revision table in the title block and an email transmittal to your manufacturer referencing the specific revision.
ECO Lifecycle: 5 Stages
Every engineering change, regardless of complexity, should flow through these five stages. The rigor at each stage scales with the production phase — lighter for prototyping, more formal for production.
| Stage | Owner | Key Action | Output |
|---|---|---|---|
| 1. Request | Design engineer | Document the proposed change, rationale, affected part numbers, and impacted documents | Engineering Change Request (ECR) form |
| 2. Review | Cross-functional team (design, manufacturing, quality, supply chain) | Assess impact on cost, schedule, tooling, WIP, and regulatory compliance | Impact assessment with go/no-go recommendation |
| 3. Approve | Change control authority (engineering manager or CCB) | Authorize implementation, assign new revision level, define effectivity | Approved ECO with signed-off drawing set |
| 4. Implement | Manufacturing (internal or contract manufacturer) | Update drawings on shop floor, modify tooling/programs, disposition WIP to old revision | Updated production package, WIP disposition record |
| 5. Verify | Quality / design engineer | Confirm change was implemented correctly via FAI or inspection of first production parts | FAI report or inspection record confirming new revision |
Pro Tip
For prototyping, stages 2 and 3 can be a 5-minute Slack thread with your lead engineer. For production, they should be a documented review with sign-off. Scale the process to the risk — but never skip stages 4 and 5 (implement and verify).
What to Include in an ECO Package
When you send a change to your contract manufacturer, include everything they need to implement it correctly on the first attempt. Incomplete packages cause back-and-forth that adds 1–2 weeks to the change cycle.
Updated drawing (complete, not just the changed sheet)
Always send the full, current-revision drawing — not just a marked-up page or a partial update. The shop floor needs a single, unambiguous document. Include the 3D model (STEP or native CAD) if the supplier uses CAM programming from the model.
Recommendation: Name files with the part number and revision: "MKS-1234_RevC.pdf" and "MKS-1234_RevC.stp". Never use "final", "updated", or dates in filenames — they create version confusion.
Redline showing what changed
In addition to the updated drawing, provide a redline (markup) of the previous revision highlighting exactly what changed. This allows the machinist and programmer to focus on the delta rather than re-reading the entire drawing. Use cloud markups or revision triangles per your drawing standard.
Recommendation: Mark changed dimensions with a revision triangle (△C for Rev C changes). Include a revision table in the title block listing every change by revision.
Revision block and effectivity
The drawing's title block must show the new revision level, date, change description, and approval signature. Separately, specify the effectivity: at which serial number, lot number, or PO does the new revision take effect? Without explicit effectivity, the supplier does not know whether to apply the change to current WIP.
Recommendation: Use one of three effectivity types: "next order" (most common), "immediate" (stop and change now), or "use and exhaust" (consume old stock first, then switch).
BOM delta (if applicable)
If the change affects the bill of materials — new hardware, different raw material size, added or removed components — include a BOM comparison showing what was added, removed, or changed. This prevents the supplier from ordering wrong material or missing a new purchased component.
Recommendation: Format the BOM delta as a simple table: Item | Old Revision | New Revision | Change Description.
Transmittal with explicit supersession
Every ECO package should include a transmittal (email or cover sheet) that explicitly states: "This package contains [Part Number] Rev [X], which supersedes Rev [Y]. All copies of Rev [Y] should be destroyed or marked obsolete." This eliminates ambiguity about which drawing is current.
Recommendation: Request a written acknowledgment from the supplier confirming receipt and understanding of the new revision before they begin manufacturing.
Communicating Changes to Your Manufacturer
How you transmit the change is as important as what the change contains. Poor communication is the root cause of most wrong-revision manufacturing events.
Never rely on verbal communication alone
A phone call or Slack message can initiate a change discussion, but the formal change must be transmitted as a documented package (drawing + transmittal). Verbal changes get forgotten, misheard, or applied to the wrong feature. Require all changes to be in writing — no exceptions.
Use a single source of truth for drawings
If you email drawings, use a shared drive, and also upload to a supplier portal, you will eventually have version conflicts. Choose one channel as the official source. If you use email, the transmittal email IS the official notification. If you use a portal, make sure the supplier has notifications enabled and you can confirm they downloaded the latest revision.
Require revision acknowledgment before production starts
Add a line to your PO or standing operating procedure: "Supplier shall confirm receipt of the current drawing revision in writing before beginning production. Production against a superseded revision will be rejected." This creates accountability and prevents the "I didn't see the update" defense.
Maintain a revision log per part number
Keep a simple table tracking every revision of every part number you have sent to each supplier: Part Number | Revision | Date Sent | Supplier Acknowledgment Date. This log is your audit trail when investigating a wrong-revision event and is required for regulatory traceability in medical and aerospace programs.
Pro Tip
When sending a revised drawing, always attach the complete drawing — never just the changed page. A machinist should be able to manufacture the part from the single document you sent, without needing to piece together old and new pages.
Managing Cost & Schedule Impact
Every engineering change has a cost and schedule impact. Some are trivial; others trigger re-quoting, tooling rework, and WIP disposition.
Re-Quote Triggers vs. Non-Triggers
| Change Type | Re-Quote Required? | Typical Impact |
|---|---|---|
| Tighter tolerance on any feature | Yes | Slower feed rates, added inspection — typically +10–40% on affected features |
| Material change (e.g., 6061-T6 → 7075-T6) | Yes | Different stock cost, different tooling wear, possible coolant change |
| Geometry change affecting fixturing | Yes | New fixture design ($500–$3K), reprogramming (2–8 hr) |
| Added surface finish requirement | Yes | Additional operations (polishing, bead blasting) — +$5–$30/part |
| Added inspection requirements (e.g., CMM report) | Yes | CMM time: $50–$200/part depending on complexity |
| Quantity increase or decrease | Yes | Volume pricing adjustment, possible tooling impact |
| Title block correction (no dimension change) | No | Drawing update only — no manufacturing impact |
| Reference dimension change | No | Non-manufactured feature — drawing update only |
| Note formatting or drawing cleanup | No | Administrative change — no manufacturing impact |
Tooling impact assessment
If the change affects a feature that is produced by a dedicated fixture or custom tooling (e.g., a go/no-go gauge, a custom workholding plate), the tooling may need modification or replacement. Ask your supplier to assess tooling impact before approving the ECO — tool rework can take 1–3 weeks and cost $500–$5K.
WIP disposition
When a change takes effect, what happens to parts already in production? Three options: (1) Scrap WIP — write off the material and labor cost; (2) Rework WIP to new revision — possible if the change is additive (tighter tolerance, added feature) but not if it is subtractive; (3) Ship WIP to old revision — acceptable only if the customer approves receiving the old revision. Document the disposition decision in the ECO.
Pro Tip
Batch non-urgent changes into a single revision whenever possible. Releasing three minor changes as Rev B, Rev C, and Rev D in the same week creates revision fatigue and triples the re-quoting effort. Instead, accumulate non-critical changes and release them together as one revision.
Common ECO Mistakes
These six errors account for the majority of wrong-revision manufacturing events, WIP scrap, and supplier-relationship friction.
Verbal-only changes
A phone call or chat message is not an ECO. The change never makes it into the drawing, the next production run reverts to the old design, and nobody remembers the conversation. Every change — no matter how small — must be reflected in the drawing revision.
No effectivity date
An ECO without an effectivity date leaves the supplier guessing: do they apply it to the current WIP? The next order? Immediately? This ambiguity is the root cause of mixed-revision shipments — where some parts in a lot are old revision and some are new.
Skipping FAI on changed features
A design change affects the manufacturing process. If you do not verify the changed features with a partial FAI, you are assuming the supplier implemented the change correctly — and assumptions in manufacturing create escapes. At minimum, inspect the changed dimensions on the first article after any ECO.
Sending a redline without an updated drawing
A redline (markup) shows what changed. An updated drawing is the manufacturing document. The redline is useful context; the updated drawing is the contractual document. Never send a redline alone — always include the complete, updated, revision-controlled drawing.
Revision confusion from inconsistent naming
Using "Rev 2", "v2", "updated", "final", "final_v2" in filenames guarantees confusion. Establish a convention (Rev A, Rev B, Rev C — or Rev 01, Rev 02, Rev 03) and enforce it. The revision level on the filename must match the revision level in the title block.
Not notifying all affected parties
An ECO affects more than manufacturing. Quality needs to update inspection plans. Purchasing may need to source different material. If you have multiple suppliers for the same part, all of them need the update simultaneously. Maintain a distribution list per part number and notify everyone in one transmittal.
Pro Tip
If you catch yourself saying “just change it, it’s a small thing” — that is exactly when the process matters most. Small, undocumented changes accumulate into large discrepancies between your design intent and what the supplier is actually building.
Conclusion
Engineering changes are a normal part of product development. What separates functional hardware teams from dysfunctional ones is not the frequency of changes — it is the discipline with which those changes are documented, communicated, and verified. A 15-minute investment in a proper ECO package prevents a $5K–$25K wrong-revision scrap event.
Three non-negotiables: (1) every change goes through a revision-controlled drawing, (2) every drawing transmittal includes explicit supersession, (3) every changed feature gets verified at first article. Follow these three rules and wrong-revision events effectively disappear.
Frequently Asked Questions
When does a design change require a new first article inspection (FAI)?
How should I revision-control drawings sent to my manufacturer?
What is the difference between an ECO and an ECN?
How do I handle a change to a part that is already in production?
What changes trigger a re-quote from my supplier?
How do I prevent wrong-revision parts from being manufactured?
Should I use a formal ECO process for prototype parts?
Related Resources
Need a Manufacturer Who Handles Changes Cleanly?
MakerStage tracks every revision, confirms receipt before production, and includes full inspection reports on every order. Upload your latest revision and get a quote in hours.
Get Free Quote Fast