China CNC Precision Processing Service: 7 Checks Before You Order
This guide is for engineers and sourcing managers comparing a China CNC precision processing service. It covers the tolerance, inspection, certification, and lead-time questions that decide whether a supplier can hold your print. Read it to know which checks to run and when a quote should be rejected.
In this article
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Key takeaways
What to Compare Across Suppliers
Use the left column as your question list. Ask each supplier for the number, not a description.
| Criterion | What to ask for | Good sign | Red flag |
|---|---|---|---|
| Achievable tolerance | Tightest tolerance held on a similar feature | ±0.005 mm with a stated datum | "As per drawing" with no number |
| Inspection | What is measured and on which instrument | 100% inspection, reports on request | Final check only, no report |
| Certification | Certificate scope and issuer | ISO 9001, IATF 16949, ISO 13485, ISO 27001 | Expired or unstated scope |
| Lead time | Quote turnaround, production start, ship date | Quote in 12 hours, start in 24 hours | Vague "2-4 weeks" for everything |
| MOQ | Smallest order accepted | From 1 prototype to 10,000+ parts | High MOQ with no prototype path |
| Quote detail | Operation list, material spec, finish spec | Line-by-line with DFM notes | One lump price, no notes |
| Confidentiality | NDA and file handling | NDA on request, secure uploads | No NDA before file transfer |
| Communication | Named engineer on the account | Direct engineer contact | Sales-only channel |
Pick the supplier that answers technical questions
The right China CNC precision processing service gives you a number, a machine group, an inspection method, and a DFM note in the first reply. If a supplier cannot name the tolerance, the machine, or the report, the price is not comparable.
Tolerance: What a China CNC precision processing service can actually hold
Tolerance is the first filter. A shop that advertises ±0.005 mm is stating a capability, not a promise for every feature on your print. The number applies to specific features, on specific machines, with a defined datum. Before you accept it, ask which machine would run your part and which feature would be measured to that band.
On a 5-axis machine, positional accuracy comes from the rotary table and the controller, not from the operator's hand. GreatLight runs 16 simultaneous 5-axis machining centers with a Ø400 mm rotary table, 27 three-axis machines, 12 four-axis mills, and 16 mill-turn centers. That mix matters: a simple turned bushing and a contoured housing do not belong on the same machine.
Feature size sets the limit too. Maximum processing size is 4,000 mm, and the large travel is 4,000 × 400 × 150 mm. As parts get longer, thermal growth and fixture deflection eat into the tolerance budget. A ±0.005 mm callout on a 2 m part is a different job from the same callout on a 60 mm bracket.
Surface finish and tolerance travel together. As-machined surfaces sit around Ra 1.6–3.2 μm. If your print calls for Ra 0.8–1.6 μm or Ra 0.2–0.8 μm, the shop adds a finishing pass or a secondary operation. That changes cycle time, so it should appear in the quote as a separate line.
- 1Ask for the datumA tolerance without a datum is not measurable.
- 2Separate the featuresList critical features; not every dimension needs the tight band.
- 3Watch lengthLong parts lose tolerance to heat and deflection.
Material and finish: where cost quietly moves
Material choice changes machinability, tool wear, and lead time more than most buyers expect. Aluminum 6061-T6 and 7075 cut fast and hold tight tolerances well, which is why they dominate prototypes. Stainless 316L and 17-4PH need slower speeds and more tool changes. Titanium TC4 (Ti-6Al-4V) and Inconel sit at the other end: low thermal conductivity, high tool wear, longer cycle times.
The quote should name the grade, not just the family. "Aluminum" is not enough. 6061 and 7075 differ in strength and anodizing response. On the stainless side, 303 machines freely but 316L resists corrosion better. A shop that lists grades is a shop that keeps them in stock and tracks heat numbers.
Finishing is a second operation with its own tolerance impact. Anodizing adds a thin oxide layer that shifts dimensions slightly. Hardcoat anodizing shifts them more. If a bore must stay within ±0.005 mm after coating, say so before the quote, not after. The same applies to electroless nickel and plating.
Bead blasting, tumbling, and polishing change surface texture but not dimensions in a meaningful way. Laser marking is different: minimum character height is 1.5 mm, so fine text on a small part may need a different marking method. Put the marking requirement on the drawing.
- 1Name the grade6061, 7075, 316L, 17-4PH — not just the alloy family.
- 2Flag post-machining tolerancesCoating moves dimensions; state which ones must survive.
- 3Check marking sizeLaser marking needs 1.5 mm minimum character height.
Inspection and certification: reading the paperwork correctly
Certificates are a starting point, not a result. ISO 9001:2015 covers a general quality system. IATF 16949:2016 applies to automotive production. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send CAD files across borders. Match the certificate to your program before you compare prices.
The more useful question is what gets measured. A supplier running 100% inspection before shipment should be able to describe three stages: raw material check, in-process monitoring, and final inspection. Ask what instrument measures your critical feature and what the report contains. A CMM report with nominal, actual, and deviation columns is usable. A stamped "passed" sheet is not.
Inspection reports on request are normal. Build the request into the purchase order rather than asking after the parts ship. If your program needs first article inspection, say so at quote stage so the shop can plan the setup and the time.
Qualification rate is another number worth asking about. A 99.99% qualification rate reflects the shop's own historical output, not a guarantee for your part. Treat it as a signal that the process is under control, then verify with your own incoming inspection on the first batch.
- 1Match certificate to industryISO 9001 general; IATF 16949 automotive; ISO 13485 medical.
- 2Ask for the report formatNominal, actual, deviation — not a pass stamp.
- 3Request FAI up frontPut it in the PO, not in a later email.
Lead time, MOQ, and quote clarity
A quote is a technical document. If it arrives as a single number with no operation breakdown, you cannot tell where the cost sits or what happens when a feature changes. Ask for the material spec, the machine group, the finish, and the inspection line as separate items. That structure also makes re-quotes faster when you revise the design.
Lead time has three parts: quote turnaround, production start, and shipping. GreatLight provides a quotation and free DFM analysis within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Those numbers apply to typical jobs, not to every geometry. A part needing a custom fixture or a specialty material will take longer, and an honest supplier says so at quote stage.
MOQ is a setup question. A supplier with no minimum order quantity can run one prototype and then a 10,000+ part run on the same process. That flexibility matters when you are still validating a design. A shop that only quotes at high volume is optimized for production, which is fine later but expensive during development.
Confidentiality should be settled before files move. Uploads are secure and confidential, and an NDA is available on request. If your program involves controlled drawings or a customer's proprietary design, get the agreement signed first. It costs a day and removes a recurring worry.
- 1Demand a line-item quoteMaterial, machining, finish, inspection — separate.
- 2Ask about the exceptionWhat would push this job past 3–5 days?
- 3Sign the NDA earlyBefore the first file transfer, not after.
How to Run a Supplier Check in 7 Steps
Run these in order. Each step produces something you can compare across suppliers.
- 11. Split critical from cosmetic featuresMark the dimensions that carry function and list them separately. A print where every dimension is critical gets quoted defensively, and the price reflects that.
- 22. Send STEP plus a 2D drawingThe 3D model defines geometry; the 2D drawing carries tolerances, datums, and notes. Sending only a STEP file forces the shop to guess the tolerance band.
- 33. Ask for DFM feedback with the priceA useful response points at thin walls, deep pockets, or features that need a second setup. If the reply is only a number, ask again.
- 44. Confirm the machine groupAsk which machine runs your part: 3-axis, 4-axis, 5-axis, or mill-turn. The answer tells you whether the tolerance and the geometry are a match.
- 55. Fix the finish spec before quotingState Ra 1.6–3.2 μm, Ra 0.8–1.6 μm, or Ra 0.2–0.8 μm. Also state whether anodizing or plating follows, because coating changes dimensions.
- 66. Define the inspection deliverableSay what report you need and which features it covers. For first articles, request full dimensional reporting before production continues.
- 77. Run a small batch before the full orderOrder one to five pieces, measure them in-house, then release the production quantity. This catches datum disagreements while the fix is still cheap.
Questions Buyers Ask Before Ordering
Can a China CNC precision processing service hold ±0.005 mm on every feature?
No supplier can hold the tightest tolerance everywhere on a part, and you should be suspicious of one that says yes. The ±0.005 mm band applies to defined features with a clear datum, measured on a specific instrument.
A practical approach is to tolerance only what functions. Housings, bearing bores, and mating faces usually need the tight band. Clearance holes and outer profiles rarely do. Narrowing the tight features lowers cost and makes inspection meaningful.
What does the quote include, and what costs extra?
A clear quote lists material grade, machining operations, surface finish, and inspection. Programming and standard fixturing are normally inside the piece price for repeat work.
Extra costs usually come from three places: custom fixtures, specialty materials with long mill lead times, and secondary operations such as hardcoat anodizing or plating. Ask for these as separate lines so you can decide whether to keep them.
How do I know the parts I receive match the print?
Ask what was measured, on which instrument, and what the report shows. A dimensional report with nominal, actual, and deviation columns lets you compare directly against your drawing.
Then inspect incoming parts yourself on the critical features. The first batch is the real test of whether the supplier read your datum the same way you did. If a feature is off, the report tells you whether the cause was setup or measurement.
Is a low price a warning sign?
Not by itself, but a price far below the others usually means something was left out: a looser tolerance band, a cheaper material grade, no finishing step, or no inspection report.
Compare quotes line by line. If one quote has fewer lines, ask what was excluded. A slightly higher quote with a complete breakdown is easier to defend to your own customer than a low quote that grows after the order.
Which certifications matter for my industry?
ISO 9001:2015 is the baseline for general industrial work. Automotive programs generally require IATF 16949:2016. Medical device work generally requires ISO 13485:2016. ISO 27001:2022 covers information security, which is relevant when you share proprietary CAD data.
Ask for the certificate scope, not just the logo. A certificate covering a different plant or a narrower process does not apply to your order.
Can the same supplier run a prototype and then production?
Yes, if the shop has no minimum order quantity and keeps the process the same across volumes. Running one piece and later 10,000+ parts on the same machine group avoids a second qualification cycle.
Confirm this at quote stage. Some suppliers prototype in one facility and produce in another, which means your validated process does not carry over automatically.
Send Your Drawing and Get a Quote in 12 Hours
Upload your STEP file and 2D drawing. You get a quotation and free DFM analysis within 12 hours, with the tolerance, material, finish, and inspection lines separated.
12-hour quote100% inspectionNo MOQNDA on request
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