CNC Processing China Factory Guide: 7 Checks Before You Place a PO

• 1
  Key takeaways
• 2
  What to check against what your part actually needs
• 3
  Tolerance: read it per feature, not per drawing
• 4
  Machine fit: why the wrong machine costs more than it saves
• 5
  Lead time: separate the quote clock from the production clock
• 6
  MOQ, materials and finishing: the cost drivers nobody quotes
• 7
  Certification, inspection and your data
• 8
  Step by step: qualifying a cnc processing china factory

A cnc processing china factory will usually quote a general tolerance block, something like ±0.1 mm on all dimensions unless noted. That number is fine for clearance holes and brackets. It tells you nothing about the two features that decide whether the assembly works.

The number that matters is what the shop can hold on the specific geometry you drew. A Ø20 mm bore in 6061-T6 on a rigid 5-axis center can hold ±0.005 mm. The same bore at the end of a 300 mm thin-wall tube will move as the material relaxes. Same shop, same machine, very different result.

So write the drawing the way you want it quoted. Put tight tolerances only where the function needs them: bearing seats, mating faces, dowel holes, seal grooves. Leave the rest at general tolerance. Every tight dimension you add raises cost and inspection time, and it raises the risk that a good shop declines the job or a careless one accepts it.

When you evaluate a supplier, ask two questions. Which features do you probe in-process, and what do you do when a trend shows the tool is drifting? A shop that answers with probing intervals and tool-wear offsets is running a process. A shop that answers 'we check at the end' is sorting parts, not controlling them.

Every quote is a machine decision. A part with features on five faces can run on a 3-axis mill in three or four setups, or on a 5-axis center in one. The 3-axis route has a lower hourly rate and a higher total cost once you count fixtures, re-clamping error and the scrap from setup three.

The rule we use: if the part needs more than two setups, or if any feature is not reachable from a single spindle direction, price it on 5-axis or mill-turn. Mill-turn centers are the right answer for parts that are mostly turned but carry milled flats, cross-holes or slots. Turning then milling on separate machines adds a second datum and a second queue.

Size decides the rest. A 4,000 mm maximum processing size covers long rails and beams. Rotary tables up to Ø400 mm handle round work with cross features. Compact travels in the 500 × 500 × 450 mm range suit small, high-mix parts where setup time dominates.

Ask the shop which machine they plan to run and why. If the answer is only a price, you are not talking to the person who will make the part.

Lead time claims are easy to make and hard to verify. Break the number apart. How fast does a quote and DFM review come back? How fast can chips actually fly after you approve? How long is the run itself, and how long is inspection and packing?

In practice, a quotation and free DFM analysis can come back within 12 hours, and production can start within 24 hours of approval. Parts can ship in 3–5 days for straightforward work. Those numbers assume the drawing is manufacturable and the material is in stock. If your alloy is 17-4PH or a titanium grade, add sourcing time and say so on the RFQ.

The useful question is not 'what is your lead time' but 'what is your lead time for this part, this quantity, this material, and what would delay it'. A supplier who names the risk is easier to plan around than one who promises a date with no conditions.

Historical late-delivery probability below 2% is a shop-level figure. It does not mean your job cannot slip. It means the shop tracks it. Ask to see how they track it for your order.

No minimum order quantity sounds like a gift. Treat it as a process question instead. Can the same shop run one prototype and a 10,000+ part run without changing the process, the fixturing philosophy or the inspection plan? If the prototype is made on a different route, the prototype proves nothing about the production part.

Material choice drives both price and risk. Common aluminium grades like 6061-T6, 7075 and 6082 are stocked and machined daily. Stainless 303 and 316L are routine. Titanium TC4 (Ti-6Al-4V), Inconel and beryllium copper machine slowly, wear tooling and need conservative feeds. Plastics such as POM and PEEK behave differently again: they move with temperature and clamp pressure, so tolerance and finish need a different plan.

Finishing is where quotes quietly diverge. Anodizing, electroless nickel, zinc plating, powder coating, black oxide, bead blasting and laser marking are all standard, but each one changes dimensions slightly and each one needs a masking decision. Laser marking needs a minimum character height of 1.5 mm to stay legible.

Ask for the finish callout as a number, not a word. Ra 0.8–1.6 μm is a machined finish. Ra 0.2–0.8 μm is a fine finish and costs more time. 'Smooth' is not a specification.

Certificates are only useful when you read the scope. ISO 9001:2015 covers a quality management system. IATF 16949:2016 is the automotive standard and matters if your part goes into a vehicle program. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security, which is the one that protects your CAD files.

Inspection is the daily version of the same question. 100% inspection before shipment sounds strong, but ask what is inspected. Raw material check, in-process monitoring and final inspection are three different activities. A shop that can send dimensional reports, material certificates and first-article data on request is a shop you can audit later without surprises.

A 99.99% qualification rate is a shop-level outcome, not a promise about your part. Use it as a floor for the conversation, then ask what happens to the 0.01%.

On confidentiality: uploads should be secure and an NDA should be available on request, before files move. If a supplier wants the drawing first and the NDA later, that is the wrong order.