China CNC processing factory: what engineers should check first

A quotation is only meaningful when the shop owns the machine that can cut your geometry. Ask for the spindle count by axis configuration. A factory with 127 high-precision CNC machines is not automatically a match for a deep 5-axis pocket; what matters is how many of those machines are simultaneous 5-axis centers and how large their travels are.

Travel size decides which parts you can send. On a large gantry platform, GreatLight machines up to 4,000 mm with a working envelope of 4,000 × 400 × 150 mm. Medium platforms cover 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact cells run 500 × 500 × 450 mm and 500 × 310 × 200 mm, usually for small brackets and connector housings.

Count the axes you actually need. A part with features on five faces in one setup needs a simultaneous 5-axis center, and 16 of those are available here. A shaft with cross-holes needs a mill-turn center; 16 are on the floor. Flat plates with pockets on one face run faster and cheaper on a 3-axis machine.

Ask what the shop does when a part exceeds the envelope. Tilting a part on a rotary table is normal, but every re-clamp adds setup error. A Ø400 mm rotary table is a practical limit for parts that must stay rigid during roughing.

Tolerance numbers on a website are usually the best case on a small part in aluminum. The useful question is what the shop holds on your material, at your size, across a production run. GreatLight works to ±0.005 mm (±0.0002 in) on critical features when the drawing, fixturing and material allow it.

Surface finish follows the same logic. Ra 0.2–0.8 μm is a fine finish and normally needs a separate finishing pass or a post-process. Ra 0.8–1.6 μm is a high finish that suits most sealing faces and bearing bores. Ra 1.6–3.2 μm is as-machined and is fine for brackets, covers and internal structure.

Be careful with thin walls and long bores. A 0.5 mm wall in 6061 will move after clamping, so the tolerance you can hold drops. Deep bores smaller than Ø6 mm need long reach tooling that deflects, and that deflection shows up as taper.

Tighter than ±0.005 mm is possible on select features, but it usually means grinding, temperature control or extra inspection. Say on the drawing which two or three dimensions actually matter. Shops quote better when the critical callouts are explicit.

Material coverage separates a general job shop from a shop that machines your alloy every week. Aluminum grades are the easy case: 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 all cut cleanly with the right feeds.

Stainless is where tool life and cycle time change. Grades 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630) are all machinable, but 316L and 17-4PH work-harden, so light passes and constant coolant matter. Expect a higher unit price than 303.

Titanium and nickel alloys need a different conversation. TA1, TA2 and TC4 (Ti-6Al-4V) cut at low surface speed with heavy coolant; Inconel is slower again and eats inserts. Magnesium AZ31B and AZ91D machine fast but require chip control because fine chips are a fire risk.

Plastics round out the list: ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre. PEEK and carbon fibre need sharp tooling and dust extraction. Soft plastics are cheap to cut but hard to hold to tight flatness after the material relaxes.

Certificates tell you which management systems the plant runs, not how good your part will be. Still, they matter for supplier approval. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The first covers general quality, the second covers automotive, the third covers medical devices and the fourth covers information security.

Ask how inspection is done, not only whether it exists. At this factory every part is inspected before shipment: incoming raw material check, in-process monitoring and final inspection. Reports are available on request, including dimensional data and material certificates. That is what you send to your own customer when they ask for traceability.

The reported qualification rate is 99.99%. Read that as a process number, not a promise on any single order. For a first article, ask for the full dimensional report, then compare two or three critical dimensions against your own CMM results before you release the run.

For regulated work, match the certificate to the industry. A medical device supplier needs ISO 13485 in the approval file, and an automotive Tier 1 will want IATF 16949 plus PPAP-style documentation. A general ISO 9001 shop can still make the part, but your paperwork gets harder.

Quotation speed is a real signal. GreatLight returns a quotation plus a free DFM analysis within 12 hours, and production can start within 24 hours once the drawing and PO are clear. Parts normally ship in 3–5 days for standard work. The historical late-delivery probability is below 2%, which is a record of past runs, not a guarantee on your order.

Confirm what the lead time includes. Tooling, first article inspection, anodizing and laser marking all add days. A part that needs hardcoat anodizing after machining cannot ship on the same clock as a bare aluminum bracket, so ask for the sequence in writing.

There is no minimum order quantity. Runs go from one prototype to 10,000+ parts, and the price curve is driven by fixturing and setup, not by a minimum buy. For a one-off prototype, expect to pay for setup time rather than material.

Uploads are treated as secure and confidential, and an NDA is available on request before you send files. If your drawing package is controlled, sign the NDA first, then release the STEP file and the 2D drawing with GD&T callouts.

One practical note on cost. The cheapest quote often comes from a shop that plans to single-block the part and skip the finishing pass. Compare cycle time assumptions and inspection plans, not just the number at the bottom of the PDF.