China CNC Processing: A Practical Guide for Engineers

The useful question is not whether a supplier owns CNC machines, but what geometry and tolerance it can repeat once a part goes into production. A shop with 127 high-precision machines, 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers can cover most prismatic and turned work without sending it out. That matters because every subcontract step adds a setup, a new fixture, and a new chance for the datum to shift.

Tolerance is where the conversation gets specific. We quote ±0.005 mm (±0.0002 in) on machined features when the part geometry, material and fixturing allow it. A thin-walled aluminium housing will not hold that number, and no honest shop should say it will. Surface finish follows the same logic: Ra 0.2–0.8 μm is achievable on a dedicated finishing pass, Ra 0.8–1.6 μm is the normal target for functional sealing faces, and Ra 1.6–3.2 μm is what you get on as-machined surfaces without extra work.

Size sets the machine. Parts up to 4,000 mm long fit the large travel of 4,000 × 400 × 150 mm. Mid-size work runs on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm travels. Compact parts go on 500 × 500 × 450 mm or 500 × 310 × 200 mm machines, often with a Ø400 mm rotary table for fourth-axis positioning. Matching the part to the right envelope is cheaper than forcing a small part onto a large machine, and it usually gives a better surface.

Five-axis work earns its cost on parts with compound angles, deep pockets, or features that would need three or four separate setups on a three-axis machine. One setup means one datum. On a complex bracket or an impeller, that alone can remove more error than tightening a tolerance band.

• 1
  Complex geometryCompound angles, undercuts and contoured pockets are cut in one setup on a 5-axis center.
• 2
  Simple prismatic partsThree-axis mills are faster and cheaper when all features are reachable from one direction.
• 3
  Turned parts with milled featuresMill-turn centers avoid a second operation and keep concentricity between the bore and the milled flats.
• 4
  Large frames and platesThe 4,000 mm travel handles long parts that would otherwise need splicing or welding.

A drawing that follows a few rules will come back with fewer questions and a tighter quote. Put every critical dimension on a common datum and say which datum it is. Mixed datums force the programmer to guess, and guessing shows up as a rework note later.

Call out tolerances only on features that need them. A drawing where every dimension carries ±0.01 mm reads as a request for a slow, expensive process, even when most of the part is cosmetic. General tolerances plus a handful of tight ones let the shop run the right speeds on the right features.

Note the material condition. 6061-T6 machines cleanly; 7075 moves more after roughing and may need a stress-relief pass. 17-4PH in the H900 condition is harder to cut than the annealed state, and that changes both tool life and cycle time. Post-processing matters too: hardcoat anodizing builds up on edges and can close a tight bore, so the finishing note belongs on the drawing, not in a separate email.

Threads, surface finish symbols and edge-break requirements are the three items most often left vague. A 0.2 mm edge break is a specification; “deburr all edges” is not, and it will be interpreted differently by two different operators.

Every part ships after 100% inspection. That means a raw material check when the stock arrives, in-process monitoring during the run, and a final dimensional inspection before packing. Reports are available on request, and for regulated work they are usually mandatory rather than optional.

Certification tells you which quality system the shop runs. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive and EV programs and adds traceability and change-control requirements. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security, which matters when you send CAD files and process data across borders.

For engineers working under an NDA, the practical question is who inside the shop sees the files. Uploads are treated as secure and confidential, and a signed non-disclosure agreement is available on request before any drawing changes hands.

Lead time is part of the specification. Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours of a released order, and parts typically ship in 3–5 days. Historic late-delivery probability sits below 2%. Those numbers describe the shop's normal rhythm, not a promise for every part; a first-article build with an exotic alloy will take longer.