CNC Machining Parts China: How Capability and Inspection Shape the Part

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  How CNC machining parts China programs handle part geometry
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  Where ±0.005 mm is realistic and where it is not
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  Material choice, surface finish and what the pair costs you
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  Inspection evidence: what you should receive with the parts
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  Lead time, MOQ and the checks that keep a program on schedule
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  Matching part features to machine and tolerance band

The first question is not who owns the machines, but which machine will touch your part. In a 127-machine shop, the split usually looks like this: 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. Each group carries a different cost and a different tolerance ceiling.

Three-axis work suits flat plates, covers, brackets and any part where all features are reachable from one direction. It is the cheapest way to remove metal and the easiest to inspect. The limit is setup count: every new face adds a fixture, a re-zero, and a small stack-up error.

Four-axis and mill-turn centers add a rotary axis, so holes and pockets on four sides cut in one setup. When a part has a cylindrical body plus cross-drilled ports, mill-turn removes it from the lathe queue and cuts concentricity risk. The Ø400 mm rotary table covers most of these bodies.

Simultaneous 5-axis is reserved for parts you cannot reach otherwise: contoured impellers, deep angled pockets, thin ribs on a curved surface. Cutting in one setup holds true position between features. It also costs more per hour, so we do not move simple parts onto those machines.

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  3-axisFlat and prismatic parts, one face per setup
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  4-axisFour-sided holes and slots in a single setup
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  Mill-turnCylindrical bodies with cross ports and threads
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  5-axisContoured, undercut or deep angled features

±0.005 mm (about ±0.0002 in) is achievable on a CNC machining parts China program, but not on every feature of every part. Tolerance is a system result: machine rigidity, tool wear, thermal growth, fixturing and material all move the number. A shop that quotes the same tolerance for a 20 mm bore and a 900 mm bolt circle is not being careful.

Size matters. A tight band on a short, well-supported feature behaves differently from the same band across a long span. As length grows, thermal drift and tool deflection take over. For large parts we often hold the critical mating features at ±0.005 mm and let non-critical edges sit at ±0.05 mm.

Material matters just as much. Aluminium 6061 and 7075 cut clean and hold band well. Stainless 316L and 17-4PH work-harden, so light passes and sharp tooling are required. Titanium TC4 and Inconel move more under heat, which means rough, cool, then finish. The tolerance you can hold follows that sequence, not the drawing alone.

The practical rule: put the tight tolerance only on the features that touch another part. Every extra tight dimension adds inspection time and cost with no gain in function.

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  Mating fitsHold ±0.005 mm where parts assemble
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  Free edges±0.05 mm is usually enough
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  Long spansExpect drift to grow with length

Material and finish are usually decided together, because the finishing step can move dimensions. Anodizing builds a layer on aluminium and changes a press fit. Hardcoat anodizing builds more. If a bore is already at the top of its band, anodize after machining can push it out.

Aluminium grades 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 cover most housings, brackets and heat sinks. 7075 gives strength but is less weldable and more prone to stress corrosion. 6061 is the default when you have no strong reason to change.

Stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH cover food, medical and marine hardware. 303 machines freely. 316L resists chloride. 17-4PH reaches high strength after aging, which is a heat-treatment step, not a machining step.

Surface finish follows the same logic. As-machined Ra 1.6–3.2 μm suits most functional surfaces. Ra 0.8–1.6 μm is a standard fine finish. Ra 0.2–0.8 μm is for sealing faces and sliding contact, and it costs extra because it needs slower passes and a separate sequence.

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  As-machinedRa 1.6–3.2 μm, functional surfaces
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  Fine finishRa 0.8–1.6 μm, mating and visible faces
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  Sealing facesRa 0.2–0.8 μm, slower passes required

A finished part is only half the deliverable. The other half is the record that shows it was made to the drawing. A working inspection chain has three points: raw material check on arrival, in-process monitoring during cutting, and final inspection before shipment. Reports are available on request.

In-process checks matter most on long runs. When a tool wears, the bore grows gradually. Measuring at fixed intervals catches that drift before a batch is finished, instead of after. On a 10,000-part run this is the difference between a rework lot and a scrap lot.

Final inspection covers the drawing dimensions, thread gauges, and any surface or marking callouts. Laser marking has a minimum character height of 1.5 mm, so keep part numbers and traceability codes at or above that size or they will not read cleanly.

A 99.99% qualification rate is a target across the process, not a promise that every drawing will pass first time. If a feature is at the edge of capability, say so during DFM review. That is cheaper than finding it at final inspection.

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  IncomingMaterial grade and condition verified
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  In-processFixed-interval checks catch tool wear drift
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  FinalDrawing dimensions, threads, finish, marking

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours once drawings and material are confirmed. Parts ship in 3–5 days for standard scope. Those numbers hold when the drawing is complete and the material is in stock.

There is no minimum order quantity. One prototype and a 10,000-part run go through the same first-article process. That is deliberate: the fixture and program you validate on the prototype are the ones that carry into production.

The common delays are not machining delays. They are unclear GD&T, missing thread callouts, a finish specified without a Ra value, or a tolerance that only one machine in the shop can hold. Each of these costs a day or more of back-and-forth.

Historical late-delivery probability sits below 2%, and the main driver is upstream: material availability and drawing clarity. Sending a complete drawing package is the single largest schedule lever a buyer controls.

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  QuoteWithin 12 hours, with DFM notes
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  StartWithin 24 hours after confirmation
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  Ship3–5 days for standard scope