The short answer
To switch a CNC turning supplier without stopping supply, qualify the new one while the old one still delivers. Send the drawing with your inspection records and photos of the defects, agree how critical features are measured, approve first articles, run a trial batch through the full route, and hold enough stock to cover it all. Move the volume only when batches pass. A 2024 article on QualityInspection.org puts a typical production transfer at two to six months.
A note on who wrote this: we are a CNC turning factory in Dongguan, and part of our work is parts that buyers move to us from another supplier or from their own workshop. Each section gives the general method first, then how we handle it. Jump to the transfer plan
Switch, or add a second source
Repeated rejects, missed dates, a supplier that cannot grow with your volume, or a price you cannot work with are common reasons to move a part. If the current supplier is mostly reliable, adding a second source may be safer than replacing it: you keep a working supply and gain a fallback.
A second source has a cost. Splitting the volume means each supplier spreads its setup over fewer parts, and you qualify and audit two factories instead of one. How batch size changes the unit price How we answer: we take trial runs from 200 pcs and regular production from 2,000 pcs, so a first batch can be small while you decide.
Send what the drawing does not say
The old supplier has learned things that are not on the drawing: how sharp an edge may be, where the finish must not reach, how parts are packed, which feature fails most often. Your best record of that knowledge is your incoming inspection: the measured data, the reject reasons and photos of the bad parts. Send them with the current drawing revision, and ask the new supplier what it assumed wherever the drawing is silent.
How we answer: buyers rarely send us old parts. More often they send inspection records and photos of the defects, mark where the parts fail and ask whether we have seen the problem before. We reply with how we would make the part; usually it is a problem we have already solved on another order. We sign an NDA before you send drawings if you need one. A formal quotation follows within 1 working day of a complete drawing.
Settle gauges and fixtures early
Turned parts rarely need moulds, but many need a special gauge: a thread gauge for a non-standard thread, or a plug gauge made to fit a precision bore. If the new supplier measures a feature differently from the old one, the same part can pass at one factory and fail at the other. Agree the method for each critical feature before first articles, and agree who owns any gauge or fixture.
How we answer: if a special gauge cannot be bought, we ask you to provide it. If it can, we explain why it is needed and agree the cost with you according to the order value. On a small order where you already have the gauge, we use yours. Otherwise we propose a way to share the cost: for example, you pay for the gauge at sampling and it is credited against later production orders.
Qualify with first articles and a trial batch
First articles show the supplier can make the part; a trial batch shows it can make it every time. Ask for a report on every drawing dimension, measured the agreed way, and compare it with your records from the old supplier. Then run a small batch through the full route, including outside finishing, because a part that is right before anodizing or heat treatment can still fail after it.
How we answer: every order starts with a first-article check of every dimension on the drawing. A redacted first-article report from another order is available on request. Parts coming back from anodizing are checked for appearance, dimensions and coating thickness; heat-treated parts for hardness, distortion and surface cracks or scale. How we inspect every order
Keep supply running while you move
Hold enough stock from the old supplier to cover the qualification time plus one production lead time, so a failed trial does not stop your line. Plan the transfer for when you have the stock, not when the old supplier has already failed you.
Protect the part as well. QualityInspection.org warns that an old manufacturer may keep making and selling your product. Recover your gauges, fixtures and drawings, and confirm in writing that production of your part stops. How we answer: on the transfer below, sampling took about 2 months before regular supply began.
A real transfer: black anodized optical rings
An optics customer made a 6061-T6 optical ring of about Ø20 mm in its own workshop, and moved it to us because of unclean masking, output that could not keep up and unstable quality. The ring has an M18 fine-pitch thread and a precision side hole, and is black anodized. We met the same masking problem: masking the thread and hole before anodizing caused rejects of up to 50% during our own sampling, so we changed the route: anodize the whole ring, then re-machine the thread, side hole and functional faces on mill-turn machines.
Anodizing-related rejects are now about 4%. Every ring is inspected before shipment and all rejects are caught there; chipped black edges from re-machining are not yet eliminated. We have supplied 200,000 rings a month since December 2025. Read the full optical ring case Parts that do not meet the drawing are replaced or refunded free of charge, with freight both ways paid by us, if reported within 30 days of receiving the goods. Send a drawing and your defect photos
The transfer plan
Eight stages, in order. Build the stock before qualification starts, and move volume only after the trial batch passes.
| Stage | What you do | Done when |
|---|---|---|
| Decide the scope | Which part numbers move, and whether the new supplier replaces the old one or takes a share. | You know the parts, the share and the date the old supply must still cover. |
| Hold buffer stock | Stock from the old supplier to cover qualification, plus one production lead time. | A failed batch would not stop your line. |
| Send the full picture | Drawing and revision, material, finish specification, your inspection records and photos of the defects you see. | The new supplier has said what it assumed and how it would make the part. |
| Agree how parts are measured | The gauge or method for each critical feature, and who provides any special gauge. | Both sides would measure the same part and get the same answer. |
| Approve first articles | A first-article report on every drawing dimension, measured the agreed way. | Every dimension is in tolerance, and the finish matches your approved sample. |
| Run a trial batch | A small batch through the full route, including outside finishing and packing. | The batch passes your incoming inspection without sorting. |
| Move the volume | Release production quantities while you watch the first batches closely. | Several batches in a row pass incoming inspection. |
| Close out the old supplier | Recover your gauges, fixtures and drawings, and confirm in writing that production of your part stops. | Nothing of yours is left at the old supplier. |
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