China CNC Machining Service: How Engineers Vet a Supplier

A shop's price means little until you know which spindle your part will run on. A 300 mm aluminum bracket with two tight bores is a different job from a 3,800 mm frame with datums on four faces. Ask for the work envelope of the machine that will actually cut your part, then check that the part fits with room for fixturing. At GreatLight, the largest travel is 4,000 × 400 × 150 mm, with medium envelopes at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm.

The second question is how many setups. Every extra setup adds a re-datum and a chance for stack-up error. Parts with features on five faces, deep pockets seen from an angle, or tight true position between bores usually belong on a simultaneous 5-axis machine. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, 127 high-precision CNC machines in total across three wholly-owned plants (7,600 m², 150 technicians).

Turned parts with milled flats are a common sourcing trap. If a supplier quotes a shaft with cross-holes as two operations at two vendors, concentricity drifts. A mill-turn center cuts the OD, the bore and the cross-features in one chucking, which keeps runout where you drew it. Ask which machine type is in the quote. If the answer is vague, the process plan is vague too.

For long parts, check the rotary table too. A Ø400 mm rotary table sets the practical limit for parts that need indexing around a large diameter. Anything bigger may still run, but it will run as multiple setups with added cost and risk. A supplier who tells you this up front is easier to work with than one who discovers it after the order.

Tolerance is a system, not a number. A shop can hold ±0.005 mm on a 40 mm aluminum housing with stable fixturing and temperature control. The same callout on a thin-wall stainless part that moves after stress relief is a different conversation. When you send a drawing, say which dimensions are functional and which are reference. The functional ones carry the inspection cost.

Surface finish follows the same logic. As-machined aluminum lands around Ra 1.6–3.2 μm, a good machined finish for mating faces sits at Ra 0.8–1.6 μm, and fine finishes reach Ra 0.2–0.8 μm, usually after lapping, polishing or a coating step. If your drawing says Ra 0.4 μm on a pocket floor just because it looked good in CAD, you are paying for time you do not need.

Material choice drives both. Aluminum 6061-T6 and 7075 machine cleanly and hold thin walls better than 304 stainless, which work-hardens and pulls. Titanium TC4 (Ti-6Al-4V) and Inconel cut slowly and eat tooling, so tolerance and finish budgets should be set with more margin. Plastics like PEEK and POM move with temperature and moisture; a ±0.005 mm callout on a long POM part is often unwise unless the drawing controls temperature at use.

Post-processing changes dimensions too. Anodizing builds a few micrometres; hardcoat builds more. Electroless nickel and plating add thickness on every surface. If a bore is already at the top of tolerance, tell the supplier before the finish is applied. GreatLight handles anodizing (clear, colour, hardcoat, conductive), electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing, plus laser marking with a minimum character height of 1.5 mm.

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  Hold ±0.005 mm whenRigid part, stable fixturing, functional datums called out, aluminum or steel.
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  Loosen the callout whenThin walls, long plastic parts, titanium or Inconel, or a surface that gets coated.
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  Call out datumsA tight tolerance without a datum scheme is a guess, not a requirement.

Certificates matter differently by industry. ISO 9001:2015 tells you the quality system is audited. IATF 16949:2016 matters if the part ends up in a vehicle or an EV powertrain, because it brings PPAP-style discipline and traceability. ISO 13485:2016 applies to medical devices and their components. ISO 27001:2022 covers information security, which is relevant when you upload CAD files. GreatLight holds all four.

Ask how inspection is documented. A useful answer is specific: raw material check on incoming stock, in-process monitoring at defined steps, and a final inspection before shipment, with reports on request. GreatLight inspects 100% of parts before shipment and reports a 99.99% qualification rate. That number is only meaningful alongside the sampling plan, so ask for the inspection report format on your first order and keep it as the baseline.

First article inspection is the cheapest insurance in the project. One part measured against the drawing, with a ballooned report, catches a misread callout before 500 pieces are wrong. For repeat orders, ask what changes trigger a new FAI: tool change, fixture change, material lot change, or a program revision. A supplier with a written rule is usually one with a stable process.

Traceability closes the loop. Material certificates tie the stock to a heat or lot number. If a part fails in the field, that link is what lets you narrow the problem to one batch instead of suspecting the whole program. Ask for mill certs with the first shipment, then confirm they arrive with every lot.

A quote without manufacturability notes is only half a quote. The useful reply points at the features that will cost you: a corner radius smaller than the cutter can reach, a pocket depth more than four times its width, a thread too close to a wall, a tolerance that forces a second operation. GreatLight returns a quotation and free DFM analysis within 12 hours, and production can start within 24 hours once the drawing and material are confirmed.

Lead time is a schedule, not a slogan. Parts ship in 3–5 days on standard work, and GreatLight records a historical late-delivery probability below 2%. Treat that as a planning input, not a guarantee for your specific part. Complex 5-axis work, special material, or an outside finishing step will extend the window, and a supplier who says so early is doing you a favour.

No minimum order quantity changes how you phase a program. GreatLight runs from one prototype to 10,000+ part runs with no MOQ, so a design can be proven on a single part before tooling and fixtures are committed. Use the prototype to check fit, finish and the inspection report; only then scale.

Confidentiality should be settled before files move. Uploads are kept secure and confidential, and an NDA is available on request. If your drawings carry export-controlled or customer-restricted content, say so in the first message so the right agreement is in place before anything is uploaded.