What buyers really want from Chinese CNC suppliers

Every supplier advertises tight tolerance. The useful question is which tolerance holds across a batch, on the features that matter. On a 5-axis part with a reamed bore, a face datum and a bolt pattern, the bore and the pattern stack up. If the shop quotes ±0.005 mm (±0.0002 in) on all three, ask how they hold the pattern position when the part is re-fixtured for the second operation.

Process capability is what you are buying. A 16-station 5-axis cell can cut five faces in one setup, which removes most of the re-fixture error. A 3-axis job with three setups adds a fixturing deviation at each move. Neither is wrong. The 3-axis route is cheaper on a simple bracket. It is the wrong choice for a housing with true position callouts on two opposed faces.

Material choice changes the answer too. Aluminium 6061 and 7075 cut clean and hold size. Stainless 316L and 17-4PH move more after roughing, so the shop has to leave stock and semi-finish before the final pass. Titanium TC4 (Ti-6Al-4V) and Inconel need slower feeds and more tool changes. If a quote does not mention stock allowance or stress relief on those alloys, the tolerance claim is not backed by a plan.

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  Ask for the setup countOne setup per critical face is a good sign; five is a risk.
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  Match alloy to tolerance17-4PH and Inconel need semi-finishing before the final pass.
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  Check the datum planThe drawing datum and the machining datum must be the same feature.

A certificate of conformance tells you the paperwork exists. A dimensional report tells you what the part measured. Buyers who have been burned once ask for the second, and they want it tied to the drawing balloons rather than a generic template. Raw material check, in-process monitoring and final inspection are three different gates. Ask which one produced the numbers you are reading.

First article inspection is the cheapest insurance on a new part number. It costs the supplier an hour of CMM time and saves a rework cycle. If the first article passes, the production run follows the same setup and the same tool offsets. If it fails, you find out before 200 parts are cut. A supplier who resists a first article on a 200-piece run is telling you something about how they plan to control the process.

On high-mix work, inspection reports also function as a handover document. When the next order comes in a year later, the report shows which features were tight and which were open. That history is how a shop keeps a part stable across repeat orders without re-learning it each time.

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  Ballooned reportNumbers mapped to drawing item numbers, not a blank form.
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  First article on new partsRun it before the batch, not after.
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  Keep the reportIt sets the baseline for the next repeat order.

The most useful thing a supplier sends is not the price. It is the note that says the 3 mm wall will chatter, or the corner radius is smaller than the tool that has to reach it, or the thread callout is not standard. Buyers who source from Chinese machine shops regularly rank this above a few percent on the unit price, because a DFM note costs nothing and a redesign after tooling costs weeks.

Good DFM feedback is specific. It names the feature, the problem and a fix. "Reduce the pocket depth from 40 mm to 32 mm and we can use a 12 mm cutter instead of a 6 mm one" is a usable note. "Please optimize the design" is not. When you get the second kind, you are talking to a sales desk rather than a process engineer.

DFM also has to arrive early enough to matter. A quote that comes back in 12 hours with two DFM comments is more useful than a price that arrives in four days with none, because the comments change the drawing and the drawing changes the cost. If your supplier only reviews the model after the PO is issued, every change becomes a change order.

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  Specific beats generalName the feature, the problem and the proposed fix.
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  Before the PODFM notes after order release become change orders.
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  Small shops can do thisA process engineer reads the model, not an algorithm.

If your part is a product rather than a bracket, the model is the asset. Buyers want to know who sees the file, where it is stored and whether the shop will sign an NDA before the first drawing is uploaded. A mutual NDA is standard practice and should not be a negotiation. If a supplier hesitates on this, that is a decision point, not a detail to sort out later.

File handling matters as much as the agreement. Restricted access on the shop floor, no photography of customer parts, and a clear rule that models are not reused for other customers. Some buyers also want the CAM program and the fixture drawings returned or destroyed at the end of the project. Ask, and put the answer in writing.

For regulated industries the paperwork goes further. Medical device work under ISO 13485 and automotive work under IATF 16949 both require traceability from the raw material lot to the finished part. That traceability is also what protects your design, because it limits who touches the job and when.

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  NDA before the fileSign it before the model is uploaded, not after.
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  Named access listKnow which programmers and machinists see the data.
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  TraceabilityLot-to-part records are required in medical and automotive work.

A lead time is only useful if it comes with the assumptions behind it. "Three to five days" means nothing without knowing whether the material is in stock, whether the finish is outsourced and whether the inspection report is included. Ask for the split: material lead time, machining days, finishing days, inspection days. Then you can see which step is the risk.

Repeat orders are faster than first orders, and they should be quoted that way. The first run carries programming, fixturing and first article inspection. The second run reuses all three. If a supplier quotes the same lead time for both, they are quoting a generic number rather than your job.

Shipping is the part buyers underestimate. Air freight from Dongguan to a US or EU door adds days that the machining quote does not show. Plan the finish and the inspection so that the parts are ready to ship when the freight booking is made, not two days after. A shop that flags this in the quote is saving you a slipped launch.

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  Ask for the splitMaterial, machining, finishing, inspection, freight.
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  Repeat ordersShould be faster than the first run. Ask why if they are not.
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  Book freight earlyFinishing and inspection must finish before pickup.