Engineering change control: how a drawing revision becomes scrap, or a silent non-conformance
- Engineering change control is the process that ensures a design change reaches every part that has not yet shipped, and that every part already made is traced and dispositioned.
- The two failure modes are opposite: mixed-revision lots, where old and new parts accumulate in the same bin, and silent non-conformance, where a part is quietly reworked to meet a new drawing and never recorded as a change.
- Most change cost is decided by one question: how far through the manufacturing route has the old revision already travelled?
- Control is cheap. A change notice that names the revision, the affected work orders and the disposition of existing parts costs nothing and removes almost all of the risk.
Why changes go wrong
A change goes wrong when the new drawing reaches some parts of the process and not others. The drawing itself is rarely the problem — the propagation is. In a contract machining environment there are four places a revision can be lost:
- The shop-floor copy. A machine operator works from a printed drawing or a setup sheet. If the revision is not distributed to the machine, the shop keeps making the old part to the old drawing, correctly and to specification.
- Work in progress. Parts at different stages of the route — some rough-machined, some finished, some in anodising — cannot all be changed the same way. A change that is free on unmachined stock may be impossible on a part already finish-ground.
- Outsourced operations. A plating or heat-treatment batch sits at another supplier. A revision issued while that batch is away is the most common source of a mixed-revision lot.
- Finished stock and assemblies. Old-revision parts already in your own stores may now be non-conforming against the new drawing — or may still be acceptable if interchangeability was considered.
Two failure modes, and why the second is worse
| Failure mode | What happens | How it is discovered | Cost |
|---|---|---|---|
| Mixed-revision lot | Old-revision and new-revision parts are made and packed into the same shipment or bin | At assembly, when parts do not interchange | Sorting, rework or replacement; customer trust |
| Silent non-conformance | A part already made is hand-reworked to meet the new requirement, and the deviation is never recorded | Months later, when a failure is traced back and the history shows nothing | Highest: the inspection record is now unreliable, which puts the whole quality document trail in question |
The second is worse for a reason that is easy to underestimate. A mixed-revision lot is a logistics problem that can be sorted. A silent non-conformance means the records no longer describe the parts, so no future investigation can rely on them. That is why a reworked part must be re-inspected and reported as reworked, not quietly returned to the batch.
The change control workflow
- Change requested in writing. An email or a drawing revision issued with a written instruction. A change mentioned in a phone call or implied by a new model file is not a change notice.
- Revision and scope confirmed. The supplier confirms back the revision number being applied, the date, and which purchase orders and work orders it affects. This confirmation is the single most valuable step in the whole process, because it catches the misunderstanding while it is still free.
- Impact assessment. Manufacturing identifies what is affected: stock not yet cut, parts already machined, parts at an outsourced finishing process, parts finished and packed, and any tooling or fixture that must be modified.
- Inventory position reported. Actual quantities at each stage are reported to the customer: how many finished to the old revision, how many in progress, how much raw material cut.
- Disposition agreed in writing. For each group of parts: rework, use as-is, scrap, or hold. Nothing moves until it is agreed, and the decision is recorded.
- Implementation. Drawing, setup sheet, programme, inspection plan and workstation copies are all updated, and superseded revisions are physically withdrawn rather than left on the machine.
- Verification and record. The first part made after the change is inspected as a first article, and the change is closed out against the revision number.
What a change notice must contain
| Item | Why it matters |
|---|---|
| Part number and new revision number | Unambiguous identification; "the latest version" is not a revision number |
| Effective date and effective scope | Whether the change applies to parts not yet started, or to every part from a stated date |
| A description of what changed | A marked-up drawing or a written list. Naming the change prevents a re-issued drawing being missed among unchanged ones |
| Interchangeability statement | Whether old-revision parts can be used with new-revision parts. This single line decides whether finished stock is scrap or acceptable |
| Affected purchase orders and work orders | Prevents the change being applied to one order while another continues on the old revision |
| Required disposition of existing parts | Rework, use as-is, scrap, or hold pending decision |
| Revised documentation requirements | Whether the inspection report or certificate needs to reference the new revision |
Cost impact by type of change
Change cost is not uniform. It is driven almost entirely by how far through the route the old revision has already travelled when the change arrives.
| Change | Cost impact | Notes |
|---|---|---|
| Tolerance tightened on a feature not yet machined | Low | Programme and inspection plan updated; may change the process route |
| Cosmetic or finish change before finishing has started | Low | Instruction to the finishing supplier only |
| Feature added to raw material not yet cut | Low | Additional operation; no scrap if material stock is uncommitted |
| Dimension change on parts already machined | Medium to high | Rework if material allows, otherwise scrap. Reworked parts must be re-inspected and reported as reworked |
| Change arriving while parts are at an outside finishing process | High | Mixed-revision risk at its worst; the batch may be unrecoverable |
| Material or process change | High | New material procurement, new cutting data, possibly a new route and a new first article |
| Change after tooling has been cut (die, mould, stamp) | Highest | A tool modification or a new tool, plus a sampling round |
Five practices that prevent most change failures
- Never rely on "latest revision" as an identifier. Quote the revision number in every instruction. A supplier cannot confirm a revision they were never given.
- Require a written back-confirmation. The supplier states the revision they are applying and the work orders affected. This catches the mismatch before metal is cut.
- Freeze a revision during production. If a change is expected, say so at the start and agree a hold point. If it is not expected, do not issue one mid-run — complete the batch and change at the next order.
- Decide interchangeability explicitly. Write down whether old and new revisions can coexist in one assembly. Without that statement, finished stock has no defined disposition.
- Treat rework as a deviation, not a repair. A reworked part is re-inspected to the new requirement and reported as reworked. This keeps the inspection record truthful, which is the only thing that makes it worth having.
Questions about change control
What is the biggest risk when I change a drawing mid-production?
Mixed-revision stock — old and new parts ending up in the same shipment. The second risk is a silent non-conformance, where a part is reworked to meet the new drawing without the deviation being recorded. The first is a sorting problem; the second invalidates the inspection history and is far more damaging.
Should I make the change now or finish the batch?
Compare the cost of the change against the cost of the old revision. If the old revision is still functionally acceptable and interchangeable, completing the current batch and changing at the next order is almost always cheaper. If it is not acceptable, stop and change immediately — every part made in the interim is scrap.
Who owns the cost of a change?
Normally, whoever caused it: a customer-initiated change is normally chargeable for material, work done and any tooling modification, while a supplier error is not. What matters more than the principle is that the disposition and the cost are agreed in writing before the work is done, so there is no dispute afterwards.
What if parts have already shipped to the old revision?
Report it and stop using them until the interchangeability question is answered. Parts already at the customer are the one group that cannot be recalled cheaply, which is why the interchangeability statement on the change notice matters more than any other line on it.
Cite this page
Boyang Hardware. "Engineering Change Control in CNC Machining: How a Drawing Revision Becomes Scrap." www.bycncmachining.com, published 17 September 2026, updated 17 September 2026.
Dated and versioned so it can be quoted with a reliable source date.
Related pages
Planning a change?
Send the new revision and the affected order numbers. What can be reworked, what has to be remade and what can still ship is confirmed in writing before any work proceeds.