Guide to Chinese CNC Processing Suppliers: 7 Checks Before You Send a PO

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  Key takeaways
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  What to verify in Chinese CNC processing suppliers
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  1. Tolerance: read the number, not the slogan
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  2. Certification: check the scope and the site
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  3. Machine mix and part size: the geometry test
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  4. MOQ, lead time and how the quote is built
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  5. Materials and finishes: where cost quietly moves
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  Step by step: how to vet a supplier in one week

Every supplier claims tight tolerance. The useful question is which feature gets which tolerance. A block with a ±0.05 mm profile can still hold a ±0.005 mm bearing bore if the shop re-clamps or finishes the bore in a separate setup. If the quote lists one tolerance for the whole drawing, the shop is quoting a general machining capability, not your feature.

Ask how the tight features will be held. On a 5-axis center, bores and faces can be cut in one setup, which removes the stack-up from re-fixturing. On a 3-axis machine, the same part usually needs two or three setups, and each one adds error. Our own baseline for a normal job is ±0.005 mm on critical dimensions, with inspection on a CMM before shipment.

Surface finish follows the same logic. Ra 1.6–3.2 μm is standard as-machined. Ra 0.8–1.6 μm takes a finer step-over or a finishing pass. Ra 0.2–0.8 μm usually means a dedicated finishing operation or a secondary process, and it should be priced as such. If a quote promises Ra 0.2 μm at as-machined rates, ask which operation produces it.

One more thing: tolerance and part size interact. A ±0.005 mm callout on a 20 mm bracket is routine. The same callout on a 2,000 mm frame is a different problem, because thermal drift and fixture stiffness start to dominate. Send the whole assembly drawing, not just the tight detail.

ISO 9001:2015 tells you the shop runs a documented quality system. It does not tell you the shop can support an automotive PPAP or a medical device file. Those are different audits with different evidence requirements. If your program needs IATF 16949:2016 or ISO 13485:2016, ask for the certificate and read the scope statement, not the logo.

The site matters too. A certificate is issued to a legal entity at an address. If your parts are quoted from one plant and produced at another, the certificate on the wall may not cover the machines that cut your parts. Ask which plant will run the job and confirm the certificate names that site.

For defense, aerospace and some medical work, data handling is a separate risk. ISO 27001:2022 covers information security management, which is why it shows up alongside machining certificates on supplier lists. It is not a machining standard, but it answers the question of how your CAD files and drawings are stored and who can open them.

Where NDA coverage is required, get it signed before drawings move. Secure upload and an NDA on request are normal at this stage, and a supplier that resists either one is a poor fit for proprietary geometry.

Send the part, not just the drawing, to the quoting engineer. Geometry decides the machine, and the machine decides the price. A part with features on five faces needs either a 5-axis center or a series of 3-axis setups with custom fixturing. The first option is faster and more accurate; the second is cheaper for one or two pieces but adds setups and inspection points.

Ask for the machine list in numbers. We run 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, 127 machines in total across three plants covering 7,600 m². Mill-turn matters for shaft-type parts with turned diameters and milled flats, because it removes a handling step and the concentricity error that comes with it.

Size is the other gate. Maximum processing size here is 4,000 mm, with travels of 4,000 × 400 × 150 mm on the large machines, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on the medium ones, and 500 × 500 × 450 mm or 500 × 310 × 200 mm on the compact ones. A Ø400 mm rotary table covers round parts that need indexing.

If your part is outside these envelopes, the honest answer is no, or a split design. A supplier that says yes to everything without asking about travels is not reading your drawing.

Minimum order quantity is a strong signal about how a shop is organized. A supplier with no MOQ has to be good at changeovers and small batches. That same setup skill usually carries into production runs. We quote from one prototype to 10,000+ part runs, which means the first article and the production batch come off the same process plan.

Lead time should be broken into stages, not given as one number. A useful quote states when the DFM feedback arrives, when material is released, when the first article is inspected and when parts ship. Our own sequence is quotation and free DFM analysis within 12 hours, production start within 24 hours after approval, and parts shipping in 3–5 days for standard work.

Treat any single delivery date with suspicion if the material is not in stock. Titanium, Inconel and some engineering plastics have their own lead times, and those are outside the machine shop's control. Ask whether the material is on the floor or on order. The answer changes the schedule more than the machining time does.

Finally, ask what the price includes. Material certificate, heat treat, finish, laser marking and dimensional report are separate line items in most shops. Two quotes that differ by 20% are often quoting different scopes, not different hourly rates.

Aluminum is the default for prototypes and most enclosures. Grades 6061 and 6061-T6 machine cleanly and anodize well. 7075 gives higher strength but is less forgiving on thin walls. 2024, 5052, 5083, 6063, 6082 and ADC12 cover the rest of the common aluminum work, including die-cast parts.

Stainless and steel follow different rules. 303 and 304 are the general-purpose stainless grades, 316 and 316L for corrosion exposure, 17-4PH when you need strength plus corrosion resistance. On the steel side, 1018 and 1045 are common for fixtures and shafts, while 4130, 4140 and 4340 show up in stressed parts. Titanium TA2 and TC4 (Ti-6Al-4V) cut slowly and cost more per hour, so design thin ribs with care.

Finishes change both price and lead time. Anodizing in clear, color, hardcoat or conductive form is the usual choice for aluminum. Electroless nickel, zinc, silver and gold plating cover conductivity and wear requirements. Powder coating and black oxide handle larger steel frames. Bead blasting, tumbling, brushing and polishing set the cosmetic grade.

Laser marking deserves a check of its own. Minimum character height is 1.5 mm, so a 0.8 mm serial number is not going to be legible. Move the mark to a flat face with clearance for the marking head, or accept a larger font.