China CNC Machining Precision: What the Process Can Actually Hold

A drawing that says ±0.005 mm is a request, not a promise. On a machined feature, that band has to absorb thermal growth, tool wear, spindle error, workholding deflection and the resolution of whatever measured the part. We quote ±0.005 mm on features that are reachable in a single setup on a machine that can hold it. If a hole sits 300 mm from the datum and the tolerance is tighter than the machine's positioning accuracy over that distance, we say so before cutting metal.

The practical split matters more than the headline number. Bore diameters, slot widths, step heights and face flatness on a small part are usually repeatable at ±0.005 mm. Position of one hole to another across a 1,000 mm casting is a different job, and a shop that quotes the same band for both is telling you it did not read the drawing.

Thermal drift is the quiet one. A spindle that runs for three hours grows; aluminum grows faster than steel. On a ±0.005 mm bore, a 2 °C shop swing over a 100 mm steel part moves the dimension by roughly 2 μm, which is 40% of the band. That is why tight work gets roughed, cooled and finished in a separate pass.

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  Reachable in one setupSmall bores, slots, steps and faces hold ±0.005 mm reliably.
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  Not reachable across a castingHole-to-hole position over 1,000 mm is a different tolerance class.
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  Temperature is a variableA 2 °C swing can consume 40% of a ±0.005 mm band.

Five-axis machining is not automatically more accurate than three-axis. It earns its place when a part has features on five sides, deep pockets with drafted walls, or contoured surfaces that would need four fixtures on a three-axis machine. Every refixturing step adds a new datum error. Cutting those features in one setup removes the stacking.

We run 16 simultaneous five-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, 127 machines in total across three plants. The mix matters because a shop that owns only five-axis machines will quote five-axis work at five-axis rates. A simple bracket with two holes belongs on a three-axis mill, and the price should reflect that.

Five-axis also helps surface finish on curved geometry. A ball nose tool held at a fixed angle leaves scallops; tilting the tool so the contact point stays on the tool's effective radius keeps the stepover even. That is how a contoured surface reaches Ra 0.8–1.6 μm without hand polishing.

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  Use five-axis forFive-sided parts, drafted deep pockets, contoured surfaces, one-setup datums.
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  Stay on three-axis forFlat plates, simple brackets, prismatic parts with two or three faces.
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  Finish benefitTool tilt keeps stepover even, so curved surfaces need less polishing.

Aluminum 6061, 7075, 2024 and 6082 cut clean and hold tight tolerances well. They also move with temperature, so a part measured straight off the machine at 30 °C will read differently in a 20 °C inspection room. For ±0.005 mm work in aluminum, we let the part stabilize before final inspection.

Stainless 303, 304, 316L and 17-4PH work-harden and push the tool away, which shows up as taper in deep bores and chatter on thin walls. Titanium TC4 (Ti-6Al-4V) and Inconel are worse: low thermal conductivity keeps heat in the cut, so the tool edge wears fast and the dimension drifts through a long run. These materials can still hold tight bands, but the cycle takes longer and the tool changes more often.

Plastics are a separate problem. POM, PEEK, ABS and PC deflect under clamping pressure and spring back after the cut. A ±0.005 mm callout on a thin PEEK wall is often not a machining problem but a metrology problem, because the part moves when you touch it.

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  AluminumHolds tight bands well; watch thermal expansion before inspection.
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  Stainless and titaniumWork-hardening and heat cause taper, chatter and tool wear.
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  PlasticsClamping and springback dominate; tight callouts may be unmeasurable.

A tolerance nobody measured is a guess. We inspect 100% of parts before shipment, with raw material verification on incoming stock, in-process checks during the run, and a final inspection before packing. Reports are available on request, and a first article report on a new part tells you whether the process is stable before you commit to a run.

The measurement method has to be finer than the tolerance. A ±0.005 mm bore cannot be signed off with a caliper that reads to 0.01 mm. That work goes to a micrometer, a bore gauge or a CMM. If a supplier quotes a tight band and then hands you a caliper reading, the number on the report means little.

Process control is what keeps a run consistent. On a 10,000-part order, the first part and the last part should land in the same band. That comes from checking the tool every set number of parts, replacing it on a wear limit rather than on failure, and logging the offsets. Our historical qualification rate sits at 99.99%, which reflects that routine more than any single machine.

We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The first three govern how we control process and product quality. The fourth covers how we handle your drawings and models.

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  100% inspectionRaw material check, in-process monitoring, final inspection before shipment.
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  Right instrument±0.005 mm needs a micrometer, bore gauge or CMM, not a caliper.
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  Run consistencyTool changes on a wear limit keep part one and part 10,000 aligned.

Ask which machine will run the part and what its positioning accuracy is. A supplier that cannot name the machine class before quoting is guessing. The answer also tells you whether the price is built on the right process or on a generic assumption.

Send the drawing with the critical dimensions marked. A good shop will come back with a DFM note flagging features it cannot hold, datum suggestions, or a tolerance it would relax to cut cost. We return a quotation and free DFM analysis within 12 hours, and production can start within 24 hours of approval.

Ask how the part will be inspected and what the report will contain. If the answer is a caliper and a handwritten note, the ±0.005 mm callout will not survive the trip. Ask for the instrument and the report format before the first cut, not after the parts arrive.

Finally, check the confidentiality terms. Uploads are secure and confidential, and an NDA is available on request. If a supplier treats your drawings casually during the quoting stage, expect the same during production.

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  Name the machineAsk for the machine class and its positioning accuracy before quoting.
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  Mark critical dimensionsA DFM reply should flag features that cannot hold the callout.
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  Agree the reportFix the instrument and report format before the first cut.