CNC Machining Services China: 7 Checks Before You Send a PO

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  What matters most when comparing suppliers
• 2
  Fit criteria by part and order type
• 3
  Tolerance: what ±0.005 mm really covers
• 4
  Part size and machine travel
• 5
  Order quantity, setup cost, and no-MOQ sourcing
• 6
  Certifications that actually match your industry
• 7
  Lead time, quoting speed, and the real schedule risk
• 8
  Inspection and documentation before shipment
• 9
  Quotes, confidentiality, and total landed cost
• 10
  How to vet a supplier in seven steps

Tolerance is the first number buyers compare, and the easiest to misread. A supplier quoting ±0.005 mm is describing the tightest it holds on a controlled feature, not a blanket promise across every dimension on the drawing. On a milled aluminum housing, the bore for a bearing might sit at ±0.005 mm while the outer profile sits at ±0.1 mm. Both are correct calls.

Ask which features carry the tight callout and how they will be measured. A CMM report on the critical bore tells you more than a general statement. If the supplier cannot name the gauge or the method, the tolerance number is marketing.

Metrology also has a floor. Thermal drift moves a 300 mm aluminum part by several microns between morning and afternoon in an unconditioned shop. Suppliers that hold ±0.005 mm reliably control temperature near the machine and let parts stabilize before final inspection.

For most industrial parts, ±0.05 mm is enough and costs far less. Reserve ±0.005 mm for fits, sealing surfaces, and mating bores. Putting it everywhere slows production and inflates the quote.

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  Tight callouts belong on mating features onlyBearing bores, dowel holes, seal grooves, spigots.
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  Ask for the inspection methodCMM, bore gauge, or optical comparator, named in writing.
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  Check the temperature storyUncontrolled shops struggle below ±0.01 mm on long parts.

A part that fits the machine envelope in one setup is cheaper and more accurate than the same part split across two operations. Machine travel is the honest limit here. GreatLight runs 16 simultaneous 5-axis centers, 16 mill-turn centers, and a large-format machine with 4,000 × 400 × 150 mm travel, which covers most frame, plate, and manifold work up to 4,000 mm.

Small parts have their own constraints. A 20 mm bracket with a 3 mm wall needs a small tool and light depth of cut, and the fixture matters more than the spindle. Shops that quote small precision work without asking about wall thickness are guessing.

Five-axis capability changes the design conversation. If a part has features on five faces, or undercut geometry a three-axis machine cannot reach, simultaneous 5-axis removes extra fixtures and the positional error that comes with them. That is a real cost saver on complex housings.

Send the 3D model, not just a PDF. The CAM programmer can see stock, tool reach, and fixture access in minutes and will usually flag a cheaper orientation.

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  One setup beats twoFewer fixtures means less stack-up error.
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  Know the envelope before you design4,000 mm maximum processing size on the large machine.
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  Five-axis pays off on complex geometryUndercuts and five-face features drop fixture count.

Minimum order quantity is where small teams get stuck. A supplier that wants 500 pieces before it turns on a machine is not the right partner for a design that is still changing. GreatLight has no minimum order quantity and runs from one prototype to 10,000+ part runs on the same floor.

The economics are simple. Setup, programming, and fixturing are fixed costs. On one part, those dominate the price. On 5,000 parts, material and cycle time dominate. A supplier that quotes both honestly will show you where the step changes are, and where a design tweak removes a fixture.

Watch for a quote that only gives a unit price. You want the setup line, the material line, and the finishing line separated. That structure lets you compare two suppliers on the same basis and see who is padding which part of the job.

Volume also changes process choice. Below a few hundred parts, CNC milling is usually right. Above that, die casting or vacuum casting can cut unit cost if the geometry allows it, and the tooling cost amortizes.

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  No MOQ removes the guessingOne piece to 10,000+ on the same equipment.
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  Ask for a cost breakdownSetup, material, machining, finishing as separate lines.
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  Revisit the process at volumeCNC first, casting once the design freezes.

Certificates are a filter, not a score. ISO 9001:2015 covers general quality management and is the baseline for most industrial work. If your part goes into a vehicle or engine assembly, IATF 16949:2016 is the one that matters, because it adds process control and traceability requirements a general shop does not maintain.

Medical device work runs on ISO 13485:2016. The difference is documentation depth: material traceability, cleaning records, and change control that a regulator or a notified body may ask to see. A shop without it can still cut the part, but you carry the compliance work yourself.

ISO 27001:2022 covers information security. It is relevant when your drawings and models are the sensitive asset, not the metal. For teams worried about design leakage, it is a better indicator than a general quality certificate.

Ask for the scope statement on the certificate, not just the logo. A certificate that excludes the process you need, or the site you are buying from, does not cover your order.

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  ISO 9001:2015Baseline for general industrial parts.
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  IATF 16949:2016Automotive and engine components.
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  ISO 13485:2016Medical devices and instruments.
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  ISO 27001:2022Design and data confidentiality.

Lead time quotes are only useful if you know what starts the clock. At GreatLight, quotation and free DFM analysis come back within 12 hours, production can start within 24 hours of a released order, and parts ship in 3–5 days for standard work. Those windows assume clean drawings and available material.

The schedule risk is rarely machining. It is material delivery, finishing queues, and unanswered questions on the drawing. A supplier that asks three sharp questions in the first day is faster overall than one that starts cutting immediately and stops on day two.

Historical late-delivery probability below 2% is a useful number because it is measured, not promised. Ask any supplier how they track on-time delivery and what their last quarter looked like. Vague answers mean they do not measure it.

For hard deadlines, agree the ship date in writing and name the finishing step in the schedule. Anodizing and plating add days that a machining-only quote often omits.

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  12-hour quote and DFM returnIncludes manufacturability notes, not just price.
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  Production start within 24 hoursOnce drawings and material are confirmed.
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  Parts ship in 3–5 daysStandard geometry, material in stock.

100% inspection before shipment sounds like a slogan until you ask what is measured. A useful inspection plan names the features, the instruments, and the sampling. Raw material certificates arrive with the stock, in-process checks catch drift during the run, and final inspection confirms the finished part against the drawing.

The qualification rate across production is 99.99%. That number only means something alongside a clear definition of a defect and a process for handling one. Ask how non-conforming parts are segregated and whether you get a report.

Reports are available on request. For a prototype, a dimensional report on critical features is usually enough. For a production run, first article inspection plus in-process data gives you something to file. Specify what you need in the PO so nothing is assumed.

Finish inspection is separate from dimensional inspection. Anodizing thickness, plating adhesion, and bead blast uniformity are checked visually or with a gauge, and they should appear on the inspection list if the surface is functional or visible.

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  Name the features and instrumentsA plan without instruments is not a plan.
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  Raw material check firstCertificates confirm grade and heat.
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  Ask for reports in the POFAI, dimensional, and finish as needed.

A quote that lists only a unit price hides the real cost. Freight, duty, packaging, and payment fees all land on your desk. Compare total landed cost per good part, not the headline number. A slightly higher unit price with no rework and a clean inspection report is usually cheaper.

Confidentiality deserves its own line. Uploads are handled as secure and confidential, and an NDA is available on request. If your design is the competitive asset, sign the NDA before you send the model, not after the quote.

Payment terms and currency matter for cash flow. Agree them up front along with the Incoterm. Ambiguity here causes more delay than machining ever does.

Finally, test the supplier with a small order. One prototype tells you how they quote, how fast they respond, and how they handle a change request. That is a cheap way to decide before a production commitment.

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  Compare landed cost per good partInclude freight, duty, and packaging.
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  Sign the NDA before the modelNDA available on request.
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  Run a prototype firstResponse speed and change handling are visible fast.