China's CNC Processing Service: How to Vet a Supplier

The first filter is simple geometry. If a part has features on five faces, or needs a compound angle drilled in one setup, a three-axis shop will either turn it into three fixtures or quote it at a price you will not like. Ask how many simultaneous 5-axis machining centers the supplier runs. A shop with 16 of them and 12 four-axis mills can hold position across a complex part in fewer setups, and fewer setups means fewer datum shifts and less stacked error.

Work envelope matters just as much. A 4,000 mm maximum processing size covers long extrusions, rails and fixture plates that most job shops cannot touch. Mid-size travels like 750 × 1,150 × 550 mm and 600 × 600 × 600 mm handle the bulk of automotive and industrial housings. Compact travels of 500 × 500 × 450 mm and 500 × 310 × 200 mm are where small, tight-tolerance parts belong. A Ø400 mm rotary table is what makes 4-axis milling of round or indexed parts practical.

Do not treat a big machine list as a quality claim. It is a capability claim. What you are checking is whether the supplier has the right machine for your part, not the largest one. Send the drawing and ask which machine the job would run on. A vendor who names the machine and the number of setups has already thought about your part.

±0.005 mm (about ±0.0002 in) is a real number for a well-run shop, but it is not automatic across every feature. It holds on features the shop can reach in a single setup with stable fixturing and a controlled temperature. Deep bores, thin walls and long cantilevers are harder. When a supplier quotes a blanket tolerance, ask which features it applies to and how they verify it. A useful answer names the CMM, the fixture and the machining strategy.

Surface finish follows the same logic. Ra 0.2–0.8 μm is a fine finish, usually reached by grinding or a dedicated finishing pass. Ra 0.8–1.6 μm is a normal high-quality milled or turned finish. Ra 1.6–3.2 μm is as-machined and is often fine for internal features nobody sees. Specifying a fine finish on a non-functional face only adds cost and time.

For parts that need both tight tolerance and a decorative face, the sequence matters. Machining first, then finishing, then any re-machining of critical datums. If the shop anodizes before the final bore, the coating thickness will move your dimension. Ask for the order of operations in writing.

• 1
  Tight toleranceReserve ±0.005 mm for functional features, not the whole print.
• 2
  Fine finishRa 0.2–0.8 μm costs more; use it where sealing or wear matters.
• 3
  As-machinedRa 1.6–3.2 μm is usually enough for internal faces.

A shop that only cuts 6061 aluminium is not the shop for a Ti-6Al-4V bracket or an Inconel manifold. Look at the full list. Aluminium grades like 6061-T6, 7075 and 6082 are everyday work. Stainless 303, 304, 316L and 17-4PH show up in medical and food-contact parts. Steels such as 4130, 4140 and 4340 are common in motorsport and heavy machinery. Copper and brass grades like C110 and C36000 need different feeds and tooling than steel.

Titanium and nickel alloys separate the field. TC4 (Ti-6Al-4V) and Inconel generate heat fast, work-harden, and punish poor coolant strategy. Ask what tooling and coolant the shop uses on these. If the answer is vague, the risk on your part is real. Plastics behave differently again: PEEK and carbon fibre need sharp tools and controlled dust, while POM and ABS are far more forgiving.

GreatLight, for reference, machines aluminium, stainless, steel, copper and brass, titanium and special alloys, and engineering plastics from ABS through PEEK and carbon fibre. That range is not a claim of superiority. It is a signal that a supplier has fixtures, tooling and cutting data ready for your material instead of learning it on your order.

• 1
  Aluminium6061-T6, 7075, 6082, ADC12 for most frames and housings.
• 2
  Stainless303 and 316L for medical; 17-4PH for high-strength parts.
• 3
  Titanium and nickelTC4 and Inconel need heat control and rigid setups.
• 4
  PlasticsPEEK and carbon fibre need sharp tooling and dust control.

Certificates are a starting filter, not a finish line. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and matters if your parts go into a vehicle program. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which is what you want when you send CAD files and drawings to an overseas shop. A supplier holding all four has been audited on process and on data handling.

Then look at inspection practice. 100% inspection before shipment, with raw material check, in-process monitoring and final inspection, is the baseline for precision work. Ask what happens on a first article: who signs it off, what is measured, and whether the report comes with the shipment. Reports on request are fine, but you want to know that before the order, not after.

A 99.99% qualification rate is the kind of number a shop tracks internally. Treat it as context, not a guarantee for your part. The useful question is what they do when a dimension drifts. A documented nonconformance process with a rework or replace decision is worth more than a headline percentage.

A quotation with a free DFM analysis inside 12 hours is a reasonable target for a well-staffed shop. DFM feedback is where you catch a wall that is too thin to hold, a corner a tool cannot reach, or a tolerance that adds cost for no function. Read the DFM notes even if you disagree with them. They tell you how carefully the shop read your print.

Production can start within 24 hours once the design is frozen, and parts typically ship in 3–5 days. There is no minimum order quantity, so a single prototype and a 10,000+ part run both fit the same process. That matters when you want to validate a design before committing to volume. A historical late-delivery probability below 2% is a useful benchmark, but keep your own buffer on critical dates.

Confidentiality is a real concern when drawings cross borders. Uploads should be treated as secure and confidential, and an NDA should be available on request before you share anything sensitive. If a supplier hesitates on an NDA, that is your answer. For regulated programs, ask how files are stored and who inside the company can open them. ISO 27001:2022 is the evidence you want behind that answer.