How to Vet a CNC Processing Supplier in China

• 1
  Key takeaways
• 2
  Seven checks and what each answer tells you
• 3
  Tolerance claims and machine mix
• 4
  Order size, materials and finishing under one roof
• 5
  Certifications, inspection and what the quote includes
• 6
  Lead time, delivery risk and the cost of a low price
• 7
  Step by step: vetting a supplier in one week

Every supplier page says the same thing about accuracy. The useful question is narrower: which features on your part need ±0.005 mm, and how will the shop verify them? A bore position, a flatness callout and a shaft diameter are three different problems. A shop that answers with a feature list and an inspection method has thought about your part. A shop that answers "all dimensions" has not.

Tolerance also depends on the machine doing the work. A three-axis machine reaches tight geometry by repositioning the part across setups, and each setup adds stack-up. A simultaneous five-axis center holds the tool at an angle and finishes the feature in one setup, which removes that error source. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 16 mill-turn centers and 27 three-axis machines across 127 high-precision CNC machines.

The mix matters more than the total. A shop with 50 three-axis machines and no mill-turn center cannot turn and mill a shaft in one setup, so it will either move the part between machines or send it out. Ask which machine family your part will run on, and ask who owns the program.

Size is the other half of the machine question. Our largest travel is 4,000 × 400 × 150 mm, with medium envelopes at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact envelopes at 500 × 500 × 450 mm and 500 × 310 × 200 mm. If your part sits near the edge of an envelope, ask how the shop will hold it.

• 1
  Feature, not blanketMatch each tolerance callout to a process and an inspection method.
• 2
  One setup winsFive-axis or mill-turn removes repositioning error on complex parts.
• 3
  Envelope checkConfirm travel before you release a drawing with a long part.

Minimum order quantity is where prototype projects die. Some shops quote a good unit price and then add an MOQ of 500 pieces, which turns a design check into a purchase order. We run no minimum order quantity, from one prototype to 10,000+ part runs. That is a statement about setup, not generosity: a single-part job is priced around the setup and programming hours, and you should expect to see that in the quote breakdown.

Material availability is the next constraint. Common grades are rarely a problem. The list we keep in stock or source quickly covers aluminium 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630); steel 1018, 1045, 4130, 4140, 4340, A36 and tool steel; copper and brass C101, C103, C110, beryllium copper, C27400, C28000 and C36000; titanium TA1, TA2 and TC4 (Ti-6Al-4V); Inconel; magnesium AZ31B and AZ91D; plus plastics including ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre.

Exotic grades are where lead time moves. Inconel and Ti-6Al-4V cut slowly, wear tools faster and often need a stress-relief step between roughing and finishing. If a supplier quotes the same lead time for 7075 aluminium and Inconel, the quote was probably built from the aluminium job.

Finishing is the last item on this check. Anodizing in clear, colour, hardcoat and conductive variants, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing are all standard. Laser marking holds a minimum character height of 1.5 mm. Keeping finishing in-house or in a controlled partner network protects the surface you paid for.

• 1
  No MOQOne prototype through 10,000+ part runs, priced on setup plus cycle time.
• 2
  Exotic grades cost timeTitanium and Inconel need slower speeds and often a stress-relief step.
• 3
  Finish spec earlyAnodize type and masking notes change the price more than the colour does.

Certification is a filter, not a score. ISO 9001:2015 tells you the shop runs a documented quality system. IATF 16949:2016 adds automotive process discipline, and ISO 13485:2016 adds medical device traceability. ISO 27001:2022 covers information security, which matters when you send CAD files and drawings to an overseas supplier. GreatLight holds all four. Ask for the certificate number and the scope, because a certificate for a trading office does not cover a machine shop.

Inspection is the part of the system that touches your parts. We inspect 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection, and we provide reports on request. For a first article, ask what the report contains: dimensions only, or dimensions plus material certificate and finish thickness. That difference decides whether you can close your own quality file without a second round of measurements.

The quote itself is a document you can read. A usable quote names the material grade, the machine family, the finish, the inspection level and the assumptions behind the price. If a feature is quoted as machined but your drawing calls Ra 0.8–1.6 μm, the gap shows up at first article. Our quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours of approval.

Confidentiality belongs in this check too. Uploads are secure and confidential, and we sign an NDA on request. If your drawings are controlled, settle that before you send files, not after.

• 1
  Four certificates, one scope checkISO 9001, IATF 16949, ISO 13485, ISO 27001.
• 2
  Ask what the report holdsDimensions, material cert and finish thickness are three different documents.
• 3
  Read the assumptionsFinish, inspection level and setup count are where quotes diverge.

Lead time is quoted in stages, and each stage can slip. Our numbers are: quotation and free DFM analysis within 12 hours, production start within 24 hours, and parts shipping in 3–5 days. Historical late-delivery probability is below 2%. Those figures describe our shop, not the industry, so treat them as a benchmark and ask every supplier for the same three numbers.

The reason to ask is that a low unit price usually hides a step. The shop may skip a stress-relief pass, machine a tight bore with a worn tool, or outsource the anodizing to the cheapest line in the area. None of that appears on the invoice until the parts fail incoming inspection or a customer finds a coating defect six months later.

A better comparison is total cost per accepted part. Add the setup, the inspection reports, the finishing, the freight and the probability of a rework loop. On a 50-piece run of a complex aluminium housing, a shop that machines it in two setups instead of five often wins even at a higher hourly rate, because the second setup is where the scrap lives.

One more point on scale. A supplier that handles one prototype is not automatically a supplier that handles a 10,000-piece production run, and the reverse is also true. Ask how the shop plans the transition from first article to repeat orders, and who signs the first article report.

• 1
  Three numbersQuote time, production start, ship time. Ask all three.
• 2
  Total cost per accepted partInclude setup, reports, finishing, freight and rework risk.
• 3
  Plan the rampFirst article approval and repeat-order control should be one process.