China's CNC processing industry: what engineers should know before sourcing

The supplier base is no longer defined by a few large contract manufacturers. A typical Dongguan or Suzhou shop now runs a mixed floor: 3-axis verticals for plate work, 4-axis mills for multi-face parts, and a smaller number of simultaneous 5-axis centers for impellers, medical housings, and thin-wall structural parts. When a supplier quotes a complex part, ask which machine it will run on. A 5-axis quote that comes back at the price of 3-axis work usually means the part was re-fixtured four times instead of cut in one setup.

Setup count drives cost more than hourly rate. A bracket held in a vise and machined on three faces costs less than a housing that needs custom soft jaws, a fixture plate, and a probe cycle between operations. Send the 3D model with the drawing so the shop can quote the fixturing, not just the cut time.

GreatLight runs 127 high-precision CNC machines across 3 wholly-owned plants covering 7,600 m², with 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. Maximum processing size reaches 4,000 mm, and the largest travel is 4,000 × 400 × 150 mm. Those numbers matter when your part is long, thin, or needs one-setup access to five faces.

Published capability and repeatable capability are different things. A shop can hit ±0.005 mm on a 30 mm aluminum feature with a temperature-controlled room, sharp tooling, and a probe check. The same shop will not hold that on a 900 mm steel weldment that sat on a truck overnight. Tolerance is a function of size, material, and thermal history, not a single number on a website.

For most production parts, ±0.05 mm on metal is routine and inexpensive. Pushing to ±0.01 mm adds inspection time and slow finishing passes. Below ±0.005 mm you are paying for metrology as much as for cutting, and the design should justify it. Ask yourself what the mating part really needs before you tighten the block tolerance.

Surface finish follows the same logic. As-machined aluminum lands around Ra 1.6–3.2 μm, a good finish pass reaches Ra 0.8–1.6 μm, and fine finishing with a small stepover gets to Ra 0.2–0.8 μm. Specify finish only on the faces that seal, slide, or take a coating. Blanket Ra 0.4 μm across a part triples cycle time for no functional gain.

Inspection is where tolerance is proved. A supplier should tell you what is measured, on what machine, and how often. Raw material certificates, in-process checks, and a final report on request are the baseline. If a shop cannot describe its inspection flow in one sentence, the tolerance claim on the quote is a guess.

Aluminum dominates prototyping and enclosure work: 6061-T6 machines cleanly, 7075 gives higher strength for stressed brackets, and 2024 is common in aerospace where fatigue matters. Anodizing is the usual finish, and it adds 5–25 μm per surface depending on type. That growth is enough to close a tight bore, so mask or pre-size critical diameters.

Stainless and steel change the economics. 303 and 304 are straightforward on a lathe, 316L is standard for medical and food-contact parts, and 17-4PH covers higher-strength applications after heat treatment. Heat-treated 4140 or 4340 will need coated tooling and slower feeds, and hardened tool steel above 45 HRC usually means EDM or grinding after machining.

Titanium and nickel alloys are where many suppliers drop out. Ti-6Al-4V (TC4) and Inconel cut slowly, generate heat at the edge, and wear tooling fast. They are machinable, but cycle time may be five to ten times that of aluminum for the same volume removed. Magnesium AZ31B and AZ91D machine quickly with the right coolant and chip control, though fine chips need careful handling.

Plastics behave differently again. POM and PA hold good dimensions but move with moisture. PEEK holds strength at temperature and costs accordingly. Carbon fibre composites are abrasive and need diamond tooling. Tell the shop the material and temper up front, because it changes the toolpath, the fixture, and whether the part needs stress relief before finishing.

Quoting is fast. A clear 3D model with tolerances and material gets a quotation and a DFM analysis within 12 hours at most shops that run an engineering desk. DFM feedback is worth reading: a small radius change or a relocated datum can remove an operation. Production can often start within 24 hours once the drawing is frozen.

Cutting is not the long pole. Fixture design, first-article inspection, and finishing or coating usually set the date. Simple parts ship in 3–5 days. Parts that need anodizing, heat treatment, or a CMM report add a few days, and a first article always takes longer than the tenth piece. Plan the first build separately from the production run.

Ask for the schedule in stages: material in, first article, finishing, final inspection. A supplier that gives one date and no breakdown is guessing. Historical late-delivery probability below 2% comes from shops that track each stage, not from shops that promise everything in three days.

Volume changes the answer. A single prototype and a 10,000-part run go through different routes. There is no minimum order quantity at most Chinese shops now, so a prototype and a small batch can run on the same process. That is useful when you want to validate the design before committing tooling.

Ask three questions before you send a file. Which machine will run this part? Who programs it, and do they see the model or only the drawing? What is inspected, and what report do you get? The answers tell you whether the shop quotes from a process plan or from a price list.

Certifications are a filter, not a verdict. ISO 9001:2015 covers general quality systems. IATF 16949:2016 means automotive process control: PPAP, control plans, traceability. ISO 13485:2016 applies to medical devices and brings validation records. ISO 27001:2022 covers information security, which matters when your drawings are confidential. A shop with all four has been audited on more than one axis.

Confidentiality needs a process, not a promise. Uploads should be access-controlled, files tied to a project, and an NDA available before you share the model. If a supplier hesitates on an NDA, that is the answer.

Price still matters, but compare the same scope. Two quotes look different when one includes material certification, first-article inspection, and finishing, and the other does not. Break the quote into material, machining, finishing, and inspection before you compare the totals.