CNC Processing China Service: What to Check Before You Send an RFQ

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  Key takeaways
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  What to check against what your part actually needs
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  Tolerance and surface finish: read the drawing before the price
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  Machine mix: what the shop can actually reach
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  Certifications and inspection: what an auditor will ask for
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  Lead time, MOQ and quoting: the commercial filters
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  Materials and finishing: where schedules quietly slip
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  Confidentiality and file handling
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  How to run a first order without surprises

Most RFQ arguments start with one number: ±0.005 mm. That figure is achievable, but it is a process window, not a shop-wide promise. On a 20 mm bore it is routine on a mill-turn center with a Ø400 mm rotary table. On a 2,000 mm frame, thermal growth and workholding deflection eat the budget before the cutter touches metal. Ask the supplier which features carry the tight callout and how they plan to hold them.

Surface finish follows the same logic. Ra 0.2–0.8 μm usually means a finishing pass with a sharp tool, light depth of cut and a stable setup. Ra 0.8–1.6 μm is a normal machined finish for mating faces. Ra 1.6–3.2 μm is fine for brackets and covers. If your drawing calls for Ra 0.4 μm across a 300 mm face, expect polishing and a slower cycle, and say so on the RFQ.

Dimensioning style matters too. A drawing full of ordinate dimensions from one datum is easier to inspect than a chain of stacked tolerances. If you stack five ±0.05 mm dimensions, the stack-up can exceed ±0.005 mm and no machine can fix that. Fix the drawing first.

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  Send real 3D and 2D filesSTEP plus a PDF with GD&T. A PDF alone forces the shop to guess the datum scheme.
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  Flag the critical fewMark the 3–8 features that matter. Everything else can run at general tolerance.
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  State the finish per faceA blanket Ra callout on a whole part adds cost where nobody needs it.

A CNC processing China service is not one machine. It is a mix, and the mix decides which of your parts fits. 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 across three plants covering 7,600 m². That spread matters when you need a turned shaft and a milled housing from the same supplier.

Travel limits are the first filter. Large work fits the 4,000 × 400 × 150 mm envelope. Mid-size parts run on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm machines. Compact parts go on 500 × 500 × 450 mm or 500 × 310 × 200 mm centers, which hold tolerance better because the structure is stiffer. Sending a 40 mm bracket to a large gantry is not wrong, but it costs more per part.

Five-axis work is where setup count drops. A part with features on five faces can be cut in one setup instead of four. Each eliminated setup removes a datum transfer and its error. For complex geometry, that is often the difference between holding ±0.005 mm and chasing it.

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  Ask for the travel envelopeGive your bounding box. A supplier who answers in one line is telling you they checked.
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  Match machine to batchPrototypes favor 5-axis and mill-turn. Simple volumes favor 3-axis with fixtures.
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  Check the turning sideMill-turn centers handle shafts with cross-features without a second op.

Certificates are not decoration. They tell you which industries the shop already serves and which paperwork exists before you ask. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The first is the baseline for general machining. The second is what automotive and EV programs require. The third covers medical devices. The fourth covers information security, which matters when your CAD files leave your network.

Inspection is the part buyers forget to specify. A 99.99% qualification rate only means something if you know how it is measured. Ask for the inspection plan: raw material check on incoming bar and plate, in-process monitoring at defined steps, and final inspection before shipment. Every part is inspected before it ships, and dimensional reports are available on request.

For medical and automotive work, ask for lot traceability back to the heat number. If the shop cannot show which bar became which part, an audit finding is coming. Settle this during quoting, not after the first shipment.

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  Ask for the certificate scopeA certificate with a narrow scope may not cover your process.
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  Request a sample reportOne page of a past CMM report tells you more than a brochure.
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  Confirm who inspectsIn-house inspection beats a subcontractor you never meet.

Three numbers decide whether a supplier fits your program: quote turnaround, production start and shipping window. At GreatLight, quotation and free DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. The historical late-delivery probability is below 2%. Treat those as the shop's stated capability and ask how they are tracked, because a date without a tracking method is a hope.

MOQ is the next filter. There is no minimum order quantity here, from one prototype to 10,000+ part runs. That matters if you are still validating a design. A supplier with a 500-piece MOQ forces you to buy inventory for a revision you may throw away. A supplier with no MOQ lets the prototype decide.

Quotingscope also varies. Some quotes list machining only and add finishing later. Ask whether the number covers material, machining, surface finishing, inspection and packing. A low quote that excludes anodizing is not a low quote.

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  Ask what the quote includesMaterial, machining, finishing, inspection, packing. Get it in writing.
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  Check the DFM responseFree DFM in 12 hours should include specific notes, not a generic reply.
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  Confirm the ship date basisCalendar days or working days? Ask before you promise your customer.

Material choice narrows the supplier list fast. Aluminum grades 6061, 7075 and 6082 cut cleanly and hold tolerance. Stainless 303 and 316L machine well; 17-4PH needs more care. Titanium TC4 (Ti-6Al-4V) and Inconel are slow, generate heat and wear tools, so cycle time and cost rise. If your part is titanium, ask whether the shop has run it recently, not whether it can.

Plastics behave differently. POM and PEEK hold dimension better than ABS or PP, which flex under clamping. Carbon fibre needs diamond tooling and dust control. A supplier who treats plastic like aluminum will scrap your first run.

Finishing is the most common schedule risk. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing all add days. Laser marking has a minimum character height of 1.5 mm, so check your label design early. Ask who runs the finishing line and who inspects the coated part.

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  Name the grade, not just the family6061-T6 and 6061-O machine differently. Specify the temper.
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  Plan masking before coatingThreads and mating faces often need masking. It affects the drawing.
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  Check marking sizeBelow 1.5 mm character height, laser marking gets unreliable.

Your CAD files carry your product. Before uploading, ask where they are stored, who can open them and how long they stay. GreatLight holds ISO 27001:2022 for information security, and uploads are handled as secure and confidential. An NDA is available on request if your legal team needs one on file before the first file transfer.

A practical test: ask the supplier to describe their file intake process. A shop that answers with a named system and a retention period is likely to pass your security review. A vague answer usually means files sit on a shared drive.

If your program runs under export control or a customer NDA, raise it during quoting. It changes what the shop can subcontract and how parts are labeled in the plant.

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  Sign the NDA firstIf your legal team requires it, do it before the first upload.
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  Ask about retentionHow long do files stay, and can they be deleted on request?
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  Flag controlled programsExport control and customer NDAs affect subcontracting.