China CNC Precision Processing: How to Pick a Provider

Precision is a number, not a claim. When a shop in China quotes ±0.005 mm, ask which machines hold it, on which materials, and over what part size. A tight tolerance on a 30 mm aluminum bracket is routine. The same tolerance on a 900 mm stainless housing is a different problem, because thermal drift and fixturing deflection scale with part length.

The useful question is how tolerance travels across the shop floor. A provider with 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 16 mill-turn centers and 27 three-axis machines, can route a job to the machine that suits it instead of forcing every part onto the same spindle. That routing decision is where most of your cost and lead time sit.

Surface finish matters as much as size. As-machined aluminum typically lands around Ra 1.6–3.2 μm. A high-quality cosmetic or sealing surface usually needs Ra 0.8–1.6 μm, and optical or bearing seats can go to Ra 0.2–0.8 μm with the right tool path and finishing pass. Specify the finish where it functions, not on the whole part.

One more constraint engineers forget: maximum processing size. GreatLight machines up to 4,000 mm, with travels of 4,000 × 400 × 150 mm on the large frame, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on mid frames, and 500 × 500 × 450 mm or 500 × 310 × 200 mm on compact machines. If your part exceeds the envelope, it becomes two parts and an assembly.

Five-axis machining is not automatically better. It earns its cost when a part has features on multiple faces, deep pockets with steep walls, or organic geometry that would need three or four separate setups on a 3-axis machine. Every extra setup adds a re-clamp, a datum shift and an opportunity for error, so reducing setups is really a quality decision.

Simultaneous 5-axis also lets the cutter stay normal to the surface and use a shorter, stiffer tool. That improves accuracy on deep features and reduces hand polishing on curved surfaces. For parts with a single flat face and simple holes, a 3-axis machine with a good fixture will be faster and cheaper. Say so in the RFQ if you want the lower price.

Mill-turn centers handle parts that need both turning and milling without moving between machines. Shafts with cross-holes, bushings with milled flats and small housings with bores fall into this group. A Ø400 mm rotary table covers most of these parts, and keeping them on one machine removes a concentricity risk that is hard to inspect afterward.

The right answer usually comes out of a DFM review rather than a machine list. Send the 3D model and the drawing early, and a competent provider will tell you which features drive the price and where a small design change removes a whole operation.

Most China CNC precision processing work is aluminum, and that is for good reason. Grades like 6061, 6061-T6, 7075, 6082 and 2024 machine cleanly, hold tolerance and anodize well. 7075 gives higher strength but machines with more spring and needs sharper tooling. Cast aluminum such as ADC12 behaves differently from wrought stock because porosity shows up at the surface after anodizing.

Stainless is where quotes spread out. 303 and 304 are common and predictable. 316L and 17-4PH (SUS630) are tougher on tooling and often need more passes, which shows up in price. Thin-wall stainless parts are the classic difficult job: the wall moves as the material is removed, so the sequence of operations matters more than the machine.

Titanium and nickel alloys sit at the top of the difficulty scale. TC4 (Ti-6Al-4V) and Inconel generate heat at the cutting edge, so tool life is short and cycle times are long. They are the right choice when weight or temperature demands it, and the wrong choice when aluminum would survive. Magnesium AZ31B and AZ91D machine quickly but need careful chip handling.

Plastics are a separate discipline. POM and PEEK hold dimensions well but move with temperature. ABS, PC, PMMA, PA and HDPE are usually chosen for prototypes and covers. Carbon fibre is abrasive and delaminates if the tool path is wrong, so it needs diamond-coated tooling and a slower feed.

A tolerance claim is only worth the measurement behind it. Ask what gets inspected, at which stage, and what report you receive. GreatLight runs raw material checks, in-process monitoring and a final inspection on 100% of parts before shipment, with reports available on request. A first article inspection report plus a material certificate is the normal package for a new part number.

Look at the calibration trail, not just the certificate wall. CMMs and micrometers need current calibration records, and the shop should be able to show them. ISO 9001:2015 covers the general quality system. IATF 16949:2016 applies to automotive work, ISO 13485:2016 to medical devices and ISO 27001:2022 to information security. Match the certification to your industry and your audit requirements.

For a process indicator, ask for the qualification rate on comparable parts. GreatLight reports 99.99%, and a historical late-delivery probability below 2%. Treat these as process data from one supplier, not a guarantee for your specific part, and ask how they are measured.

If your drawings are sensitive, settle confidentiality before you upload. Secure handling and an NDA on request are standard at a serious provider, and the review should cover sub-suppliers for finishing operations as well as the machine shop itself.