Chinese CNC Machining Service: What Engineers Should Check

A quotation tells you very little about how a part will actually be made. The machine list tells you a lot. Ask for the count and type of spindles: how many 3-axis, how many 4-axis, how many simultaneous 5-axis centers, how many mill-turn machines. When a shop runs mostly 3-axis work, it will quote your five-sided housing honestly, then move it between three fixtures. Each refixturing adds a datum shift and a stack-up that eats into your tolerance budget.

Travel matters as much as axis count. If the largest envelope on the floor is smaller than your part, the shop will either decline or split the job into pieces you have to join later. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. The largest travel is 4,000 × 400 × 150 mm, so long, thin profiles are machined in one setup instead of being repositioned.

Cycle time is where axis count pays you back. A part with features on five faces needs five setups on a 3-axis machine, plus fixtures and re-datuming. One 5-axis setup removes most of that. For low quantities, the setup savings usually outweigh the higher hourly rate. At high volume, a well-designed 3-axis process with dedicated fixtures can still win on unit cost, so do not assume 5-axis is always cheaper.

Spindle hours are not the same as delivery dates. A shop with 127 machines can absorb a rush order; a shop with eight spindles cannot. Ask how many machines are dedicated to prototype work versus production, and whether your job will sit in a queue behind a 10,000-piece run.

Any supplier can print ±0.005 mm on a website. The useful question is which machines, which materials, and which feature types hold that number. A ±0.005 mm callout on a 20 mm bored hole in aluminium is routine on a 5-axis center with a probing cycle. The same callout on a 900 mm long, thin wall in 316L stainless is a different job, because thermal drift and tool deflection dominate.

Hold the tight tolerance on the features that need it. Design drawings that apply a general ±0.005 mm block tolerance to every dimension force the shop to inspect and rework features that have no function. That raises cost and lengthens lead time without improving the part. Position and datum tolerances on mating interfaces matter more than the overall envelope.

Surface finish and tolerance interact. A Ra 0.2–0.8 μm finish usually needs a separate finishing pass with a smaller stepover, so it adds cycle time. Ra 0.8–1.6 μm is a normal machined finish for most sealing faces and bearing bores. Ra 1.6–3.2 μm is fine for brackets, covers, and non-contact surfaces. Specify the range per face, not one number for the whole part.

Ask what the shop does before inspection: raw material check, in-process monitoring, final inspection. GreatLight inspects 100% of parts before shipment and can supply reports on request. Reports are only useful if they name the instrument, the datum scheme, and the sampling plan.

Aluminium is the default for prototypes and most enclosures. 6061 and 6061-T6 machine cleanly and take anodizing well. 7075 gives higher strength for structural brackets but is less weldable and more prone to stress corrosion. 2024 is strong and fatigue resistant, though it needs a protective finish. Cast aluminium like ADC12 is common in die-cast housings, but machining castings calls for different fixturing than machining billet.

Stainless steels separate suppliers quickly. 303 machines freely but is not ideal for corrosive service. 304 and 316L are the workhorses for medical and food-contact parts. 17-4PH (SUS630) can be heat treated to high strength after machining, which is useful for shafts and valve bodies. 316L and 17-4PH both work-harden, so rigid setups and correct feeds matter more than spindle speed.

Titanium and nickel alloys are a smaller club. TC4 (Ti-6Al-4V), TA1, and TA2 are available, along with Inconel and magnesium AZ31B / AZ91D. Titanium cuts slowly, generates heat at the edge, and can ignite as fine chips, so chip evacuation and coolant strategy drive the process. Magnesium needs dedicated handling because of fire risk. Not every shop should quote these jobs.

Plastics cover a wide range. POM and PA are stable and easy to machine. PEEK holds up at temperature and in chemical contact but costs far more than the aluminium part it replaces. Carbon fibre composites machine with high tool wear and require dust control. Finishing options include anodizing (clear, colour, hardcoat, conductive), electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing, and polishing.

Certificates are scope documents. ISO 9001:2015 covers a general quality management system. IATF 16949:2016 is the automotive standard and adds traceability and change control expectations. ISO 13485:2016 applies to medical device manufacturing. ISO 27001:2022 covers information security, which matters when you send CAD files and drawings across borders. Read the scope statement, not just the logo.

Ask which certificate applies to your part. A shop may hold four certificates while your job runs in a plant covered by only one of them. The relevant question is which site machines your parts and what that site is certified for. This matters most for automotive and medical work, where the customer audit follows the part.

Documentation should match the drawing. A first article inspection report, material certificates, and a certificate of conformity are normal requests. For critical features, ask for a capability study or a dimensional report with actual values rather than pass or fail. If the shop cannot name the CMM model and the calibration interval, the report is decoration.

Confidentiality is part of quality. Uploads should be secure and confidential, and an NDA should be available on request. For defence-adjacent, medical, or unreleased consumer products, confirm in writing who can see the files and how long they are retained. GreatLight offers an NDA on request and treats uploads as confidential.

Lead time has three parts: quoting, production start, and shipping. A clear quotation and free DFM analysis within 12 hours is a reasonable benchmark for a responsive shop. Production can start within 24 hours once the drawing, material, and finish are confirmed. Parts then ship in 3–5 days for typical jobs. These are working targets, not contracts, and complex geometry or exotic material extends them.

Quantity changes the right process. There is no minimum order quantity here, so one prototype and a 10,000+ part run both fit. At one to fifty pieces, setup dominates and 5-axis machining is usually the fastest route. At several thousand pieces, dedicated fixtures, pallets, and possibly die casting or vacuum casting become cheaper per part. The break-even depends on geometry, not on a fixed number.

Do not optimize on unit price alone. Freight, duty, currency, and the cost of a failed lot all land on your side of the ledger. A shop with a historical late-delivery probability below 2% is worth more than one quoting ten percent less with vague dates. Ask for the on-time figure and what it covers.

Send a complete data package. A 3D model, a 2D drawing with datums and tolerances, material specification, finish callouts, and quantity. Missing information is the single largest cause of quotation delays and of parts made to the wrong revision.