A production order is running when engineering sends a drawing change: a hole moves 3 mm, and a tolerance tightens on a mating face. The change looks small in CAD, but the tool is already cutting, the fixture locates on the old hole position, and the inspection plan measures the old tolerance. Stop the line or finish the batch? The answer is not automatic — it depends on the change’s effect on the parts already made, the tooling, and the order’s acceptance criteria. Engineering changes during production fail when they are treated as drawing updates instead of order changes, because a drawing revision without a plan stops work, wastes parts, and starts arguments about who pays.
When to stop work: change impact on tooling and parts
A change request arrives, and the first decision is whether to stop the current work. Stopping protects the order from producing parts against the old revision, but it costs downtime and schedule. The rule is to stop when the change affects parts already in process — when the machining, the finish, or the inspection will produce parts that the new revision rejects. If the change only affects future lots and the current batch can be accepted under the old revision with the buyer’s agreement, finishing the batch can be the right call. The decision belongs to the buyer and the supplier together, documented in writing, because “we assumed you stopped” is the most expensive assumption in production.
The impact assessment should name what is in process at each stage: material on order, stock cut, parts machined, parts finished, parts inspected. A change that affects only the drawing but not the physical parts may be a documentation update; one that affects geometry, material, or finish is a real order change.
Assessing cost and lead-time impact before approval
Every change has a cost and a schedule effect, and both should be estimated before the change is approved. The cost includes scrapped or reworked parts, tooling or fixture changes, new material, and new inspection; the lead-time effect includes the time to implement the change and to rebuild any lost inventory. Ask the supplier for the impact in writing: what is affected, what it costs, and when the first parts under the new revision can ship. Approving a change without the impact estimate turns a controlled process into an open checkbook.
| Impact area | Questions to ask | Decision input |
|---|---|---|
| Parts in process | Which stages hold parts against the old revision? | Scrap, rework, or accept-old-batch decision |
| Tooling and fixtures | Does the change move datums or features? | Fixture modification cost and time |
| Material | Does the change alter grade or size? | Material cost and procurement time |
| Inspection | Do the acceptance criteria change? | New gauges, CMM programs, or reports |
The table is the impact checklist; the supplier’s answers become the change order’s cost and schedule lines.
Documenting the change: revisions and traceability
A production change is not official until it is documented: a new revision, a change description, the reason, the approval, and the effective point in the order. The effective point matters most — which lot or serial range is built to which revision — because traceability is what lets a field failure be traced to the change. The supplier should record the revision on the parts, the packing list, and the certificate, and the buyer should confirm which revision is being received. A change that is implemented on the floor without the documentation is a defect, even when the geometry is right, because no one can prove what was built.
The change record also feeds the quality file: the new revision’s first article or inspection results belong with the change, and the old revision’s results stay with the old parts. Traceability is not paperwork for its own sake; it is the mechanism that makes the next change review faster and the next failure investigation possible.
Tooling changes vs part-only changes
Distinguish changes that affect tooling from changes that affect only the part. A hole position change on a machined part may require a fixture or program change; a dimensional change on a molded part may require a mold modification, which carries its own cost and lead time. Tooling changes also have a validation step: after the tool is modified, the first parts must be inspected against the new revision before production resumes. Part-only changes — such as a finish spec or a documentation requirement — skip the tooling work but still need the effective-point control. The cost and schedule estimate should separate the tooling line from the part line, because the tooling change often dominates both.
If the tool is modified, ask whether the modification is reversible and whether it affects the tool’s life or maintenance. A mold that is welded and recut for a change may have a different service life than one modified in another way, and that difference belongs in the change decision.
Preventing change-driven defects with clear gates
The defects that follow engineering changes are usually process defects: parts built to the old revision, fixtures not updated, inspection checking the wrong criteria. Clear gates prevent them. Gate one is the written impact assessment before approval. Gate two is the documented effective point and the stop/continue decision. Gate three is the tooling or program update and its validation. Gate four is the first-article or first-lot inspection under the new revision before production resumes. Each gate has an owner and a record, and a change that has not passed a gate does not reach the floor. The gates are not slow; they are what makes the change fast to approve and safe to ship.
The communication guide on this site covers working with a manufacturing partner across time zones; this page is the change-control procedure that runs inside an active order. When the change is complete, review it once more: were the old parts dispositioned, the new parts verified, and the records filed? If yes, the change is closed; if not, it is still open and still a risk.
A cross-site example shows why the effective point matters. A program runs the same machined part at two suppliers while a change moves a hole 3 mm. Supplier A stops production immediately, documents the change, modifies its fixture, and inspects the first part under the new revision before resuming. Supplier B finishes the current batch under the old revision with the buyer’s written agreement, then implements the change at the next lot boundary. Both actions are correct because both are documented: the buyer knows that the first lot from Supplier A and the last lot from Supplier B are built to the old revision, and the packing lists and certificates say so. If either supplier had implemented the change silently mid-batch, the buyer would receive mixed revisions with no way to tell them apart — and a field failure would be untraceable. The example shows the real purpose of the gates: not to slow production, but to keep every part identifiable to a revision. The impact assessment, the effective point, and the first-article check are what let two suppliers, two revisions, and one part number coexist without confusion. When the change is closed, the records show which lots carry which revision, and the next change review starts from that clarity instead of from a guess.
Frequently asked questions
Who pays for a change that the buyer requested mid-order?
Typically the buyer pays for the change’s direct impact — scrapped parts, tooling modification, new inspection — unless the supplier caused the need for the change. The impact estimate should be approved before the work starts, so the cost is agreed rather than discovered on the invoice. If the change is required to correct a supplier error, the supplier absorbs it; the cause decides the cost, and the record proves the cause.
Can a small drawing change be implemented without stopping production?
Sometimes, if the change does not affect parts already in process and the supplier can implement it at a natural lot boundary. The condition is that no parts ship against the wrong revision and the effective point is documented. “Small” changes still need the impact check; the geometry may be small, but the fixture and inspection effects are not.
What should the change request include to get a fast, accurate quote?
The new drawing or model, a marked change description, the reason, the quantity affected, and the required effective date. The supplier can then assess the impact without chasing context. A change request without the reason or the effective point produces a slow, conservative estimate, because the supplier has to protect against the unknown.
The controlled change in one paragraph
An engineering change during production is an order change, not just a drawing update: assess the impact, decide the stop point, document the revision and its effective point, validate the tooling, and inspect the first parts under the new revision. The gates are the cost of control, and they are cheaper than the scrap, rework, and arguments that uncontrolled changes produce. The production line that handles changes well is not the one that changes fastest — it is the one that knows exactly what it built and when.
If you are managing a change on an active machining or molding order, the 6CProto team can provide the impact assessment, the revision documentation, and the first-article verification before the new revision ships.