CNC Machining Service Made in China: A Buyer's Checklist

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  Key takeaways
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  What to verify before you place an order
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  Match the tolerance claim to the machine that will cut your part
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  Read the machine list like a capacity map
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  Certifications mean different things, and the scope decides which
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  Lead time, MOQ, and how the quote is written
• 7
  Five traps that cost time and money
• 8
  Step by step: screening a supplier in one week

Almost every Chinese machine shop advertises tight tolerance. The useful question is different: which machine, which fixture, and which inspection method will hold that tolerance on your specific geometry. A ±0.005 mm callout on a 30 mm aluminum bracket is routine. The same callout on a 900 mm thin-wall housing is a fixturing problem, and fixturing is where quotes quietly get rewritten after the first article.

Ask for the machine assignment before you ask for the price. On a part with angled holes and undercuts, a simultaneous 5-axis center removes the part in one setup and keeps the datums consistent. On a simple prismatic plate, a 3-axis machine with a good vise is faster and cheaper. If the shop cannot tell you which one it will use, it has not read your drawing yet.

Geometry also sets the practical floor on surface finish. As-machined surfaces land around Ra 1.6–3.2 μm. A high-quality machined finish runs Ra 0.8–1.6 μm. Fine finishes at Ra 0.2–0.8 μm usually need a separate operation, a different tool path, or hand polishing. Budget for that step instead of arguing about it after the parts ship.

One more thing. Tolerance is a distribution, not a promise about every single part. A shop that tracks its qualification rate can tell you what fraction of a run lands in tolerance. A shop that cannot is guessing. Ask for the number.

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  Simultaneous 5-axisBest for angled features, deep pockets, and parts that need one datum throughout.
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  4-axis and mill-turnGood for cylindrical parts with cross-features; fewer setups than turning plus milling.
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  3-axisFastest and cheapest for flat, prismatic work with features on one or two faces.

A machine count tells you the shop's ceiling. GreatLight runs 127 high-precision CNC machines across 3 wholly-owned plants, with a 7,600 m² main facility in Dongguan and a second site in Singapore. The mix matters more than the total. There are 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers.

Size ranges are just as important as axis counts. The largest travel is 4,000 × 400 × 150 mm for long parts. Medium work fits 750 × 1,150 × 550 mm or 600 × 600 × 600 mm envelopes. Compact work runs on 500 × 500 × 450 mm and 500 × 310 × 200 mm machines. If your part is 1,200 mm long and needs a rotary table, only part of that list applies.

This is where a lot of sourcing goes wrong. Buyers send a large frame to a shop that only runs small VMCs, or a turned housing to a shop without mill-turn capacity. The quote comes back low, and then the process plan changes. Ask for the specific machine before you compare prices.

Material also narrows the list. Machining 7075 aluminum is routine. Running Inconel or Ti-6Al-4V (TC4) needs rigid setups, slower feeds, and more tool changes, which shows up in the price and the schedule. A shop that lists titanium and Inconel in its standard material set has already solved the tool-life problem.

Certificates are not interchangeable. ISO 9001:2015 is a quality management baseline. IATF 16949:2016 is written for automotive production and adds traceability and change-control requirements. ISO 13485:2016 is the medical device standard and pushes harder on process validation and documentation. ISO 27001:2022 covers information security, which matters when your CAD files and drawings leave your network.

For most buyers, two of those are doing real work. IATF 16949 tells you the shop can handle PPAP-style documentation and lot traceability. ISO 27001 tells you there is a controlled process for handling customer data. If your parts are medical, ISO 13485 is the one that matters. If you are making brackets for a consumer product, ISO 9001 plus a clear inspection routine is usually enough.

Ask what the certificate covers. A certificate for a trading office says nothing about the shop floor. A certificate that lists the machining site and the processes in scope is the useful document. Request the current certificate and the audit scope statement, not a scanned logo.

Inspection closes the loop. GreatLight inspects 100% of parts before shipment and runs raw material checks, in-process monitoring, and final inspection, with reports available on request. A 99.99% qualification rate is only meaningful if the inspection method matches your drawing callouts. Ask which instrument measures your tightest feature, and whether it is calibrated.

Quote speed is a proxy for shop responsiveness. GreatLight returns a quotation and free DFM analysis within 12 hours, and production can start within 24 hours once the order is released. Parts typically ship in 3–5 days. Historical late-delivery probability sits below 2%, which is a useful number because it is stated as a rate, not a promise.

MOQ tells you how the shop is organized. No minimum order quantity means quoting one prototype is a normal transaction, not a favor that delays the real work. It also means the same process plan and the same inspection routine carry over from the prototype to a 10,000+ part run. That continuity is what keeps the second order from failing.

Read the quote for what is missing. A good quote lists the material grade and temper, the stock size, the machine, the finishing steps, the inspection method, and the lead time for each line. A two-line quote that says "CNC machining, aluminum, 10 days" is not a price. It is an opening position.

Confidentiality needs to be settled before files move. Uploads should be handled as secure and confidential, and an NDA should be available on request. If a supplier hesitates on a mutual NDA for a simple prototype, that tells you something about how they will handle your production drawings later.

The first trap is the precision black hole. A supplier claims extreme precision, then fails in mass production because the equipment is worn, the process is not locked, or there is no in-process check. The fix is simple: ask for the machine, the fixture, and the inspection record for a comparable part.

The second trap is comparing quotes with different scopes. One shop includes anodizing and laser marking, another prices bare machined parts. The lower number is not cheaper; it is a different deliverable. Normalize the scope before you compare.

The third trap is ignoring finishing lead time. Anodizing, plating, powder coating, and polishing are often outsourced and add days that never appear in the machining quote. Laser marking has its own constraint: minimum character height 1.5 mm. If your part number has to be readable, design for that.

The fourth trap is treating the prototype and the production run as separate projects. They should share a process plan. The fifth is weak after-sales support. A shop that will not commit to reworking a nonconforming part has not finished the job. Ask what happens when a part fails inspection, and get the answer before the PO, not after.