How to Vet a Leader of China CNC Processing Service

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  Key takeaways
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  Five checks, and what each one actually filters out
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  Tolerance claims: read the number, then read the size
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  Capacity: what the machine list says about complex work
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  Certifications: each one points to a different buyer
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  Lead time: separate the quote clock from the production clock
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  MOQ, finishing, and the hidden handoffs
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  Five steps to qualify a supplier in one week

Almost every Chinese machine shop lists a tolerance on its website. The number alone tells you very little. What matters is the size of the part, the number of setups, and whether the feature is measured on the machine or on a CMM afterward. A shop can hold ±0.005 mm on a 50 mm boss and still struggle on a 900 mm housing with six faces.

Ask for the tolerance broken down by feature: bores, slot widths, flatness, true position. Then ask how the part is held. Thin walls move when the vise opens, and a number on a datasheet does not account for that. A supplier that has run your part family before will usually tell you which features worry them before you ask.

GreatLight holds ±0.005 mm (±0.0002 in) on parts up to 4,000 mm, with 100% inspection before shipment. That ceiling is not the same as a guarantee on every drawing. If your print asks for ±0.005 mm across a 1,200 mm weldment, expect a conversation about stress relief and datum strategy, not a yes.

Surface finish follows the same logic. Ra 0.2–0.8 μm takes a finishing pass and often a second setup. Ra 0.8–1.6 μm is a normal fine-machined target. Ra 1.6–3.2 μm is as-machined and costs the least. Specifying a finer finish than the function needs is one of the most common ways buyers add cost without adding value.

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  Tie the tolerance to a size±0.005 mm on a 50 mm feature and on a 1,200 mm feature are different problems.
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  Ask where it is measuredIn-process probing and final CMM inspection are not the same evidence.
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  Match finish to functionRa 0.8–1.6 μm covers most mating surfaces; go finer only when sealing or optics require it.

A shop with 20 three-axis mills can do a lot of good work. It cannot do a part with undercuts, deep angled pockets, and five faces of features without multiple setups. Each extra setup adds a datum shift, a re-clamp, and a chance for error. This is why axis count is the fastest way to sort suppliers.

GreatLight runs 127 high-precision CNC machines: 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. That mix matters more than the total. Five-axis handles the complex geometry, three-axis handles the simple volumes at lower cost, and mill-turn keeps turned parts from bouncing between two vendors.

Size range is the second filter. Travel of 4,000 × 400 × 150 mm covers long parts. 750 × 1,150 × 550 mm and 600 × 600 × 600 mm cover the middle. 500 × 500 × 450 mm and 500 × 310 × 200 mm handle compact work, and a Ø400 mm rotary table adds positionally machined features. If your part sits outside all of those, the shop will need a fixture plan, not just a quote.

Ask one more question: how many of those machines run lights-out or on a second shift? A large machine list with one shift behind it produces a very different lead time than the same list running three shifts.

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  Axis count firstSimultaneous 5-axis removes setups on angled and contoured features.
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  Then size envelopeCheck travel against your largest dimension before you check price.
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  Then shift patternMachine count without utilization data does not predict delivery.

Certificates are not a quality score. They are a signal about which industries a shop already serves and which paperwork it can produce without a scramble. Read them that way and they become useful.

ISO 9001:2015 is the baseline. It says the shop has a documented quality system and an audit trail. Without it, larger buyers usually stop reading. IATF 16949:2016 is automotive. It brings PPAP-style discipline, change control, and traceability expectations that most general machine shops do not maintain. ISO 13485:2016 is medical devices, where process validation and clean handling matter. ISO 27001:2022 covers information security, which is the one buyers forget until they are uploading proprietary CAD.

GreatLight holds all four. For an aerospace or automotive program, the IATF piece is often the deciding factor. For a medical device enclosure or instrument component, ISO 13485 answers the first question in the supplier audit. For a startup sending a novel design, ISO 27001 plus an NDA on request is the practical concern.

Check the certificate scope and expiry date. A certificate covering a different plant or a lapsed audit window does not transfer to your order.

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  ISO 9001 is the floorNo system documentation means no audit trail when a part is out of spec.
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  IATF 16949 for automotiveExpect PPAP, change control, and traceability language.
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  ISO 13485 for medicalProcess validation and contamination control come with it.
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  ISO 27001 for your filesIt speaks to how uploads and drawings are protected.

Turnaround promises get vague fast. Break the timeline into four separate clocks and ask for each one: quotation, DFM feedback, production start, and shipment. A shop can be fast at one and slow at another, and the slow one is what your program feels.

GreatLight quotes and returns a free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days. The historical late-delivery probability is below 2%. Those are the numbers to hold a supplier to. If a shop quotes 12 hours for a price but two weeks for DFM notes, you have not learned much about manufacturability.

Also ask what starts the clock. Does production start when you approve the quote, when the material arrives, or when first article is signed off? On a first-order complex part, material lead time for 17-4PH or Ti-6Al-4V can dominate the schedule, and no shop controls that fully.

Finally, ask about the finish. Anodizing, plating, powder coating, and laser marking add days if they are subcontracted. A shop that keeps finishing in-house, or manages it as a scheduled step, removes a handoff from your critical path.

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  Four clocks, not oneQuote, DFM, production start, and ship are separate commitments.
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  Ask what triggers the startQuote approval, material receipt, and FA sign-off are different events.
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  Watch exotic materialTitanium and Inconel stock can set the real date.

A low unit price means little if you are managing four vendors to get one finished part. Every handoff adds a shipping step, a re-inspection, and a place for tolerance stack-up to hide. Count the vendors in the chain before you count the savings.

GreatLight has no minimum order quantity, from a single prototype to runs of 10,000 or more. That range matters because the first article and the production run should come from the same process. Moving from a prototype shop to a production shop often resets the learning curve.

Finishing is where the handoffs usually appear. Anodizing in clear, color, hardcoat, or conductive; electroless nickel, zinc, silver, and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing, and polishing; laser marking and engraving with a minimum character height of 1.5 mm. If most of that list is out of house, build the extra days into your plan.

Materials deserve the same scan. Aluminum 6061, 7075, and ADC12; stainless 303, 304, 316L, 17-4PH; steel 1018, 4140, 4340 and tool steel; copper and brass grades; titanium TA2 and TC4; Inconel; magnesium AZ31B; and plastics from ABS to PEEK and carbon fiber. A shop that stocks what you need shortens the front end of the schedule.

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  Count the vendorsEach outside process is a handoff with its own queue.
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  No MOQ helps iterationOne prototype and a 10,000-part run use the same process route.
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  Check material stockCommon grades ship faster than specials.