China CNC machining accuracy: what holds and what drifts

A drawing tolerance is a limit on a single feature. It says nothing about the rest of the part. This is where most overseas buyers lose time on a Chinese quote. Meeting ±0.005 mm on one bored hole is possible while the position of that hole still misses by 0.05 mm, because position depends on how the part was located, not on how small the cutting step was.

So the first question is not what the shop can hold. It is which features the tolerance applies to. Bore diameters, bearing seats, seal grooves and mating faces usually need the tight callout. Bolt clearance holes, chamfers, fillets and non-functional edges almost never do. Put tight limits only where function needs them, and the part gets cheaper without losing anything that matters.

There is a second layer: datum choice. A tolerance is only repeatable if the datum on the drawing matches the surface the part is clamped on. When the two disagree, the machine can be perfect and the inspection report still fails. We ask for the datum callout before quoting, because it decides the fixture design and the number of setups.

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  Functional featuresBores, seats, seal grooves, mating faces. Tight limits belong here.
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  Non-functional featuresClearance holes, chamfers, fillets. Standard limits are enough.
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  Datum matchDrawing datum must match the clamping surface, or accuracy is not repeatable.

A three-axis machine cuts one face at a time. Complex parts then need several setups, and every re-clamp adds a small position error. Two or three setups can stack up to 0.02–0.05 mm of lost position before the tool touches metal.

Simultaneous five-axis work removes most of that stacking. Adding two rotary axes, typically A and B, lets the tool reach compound angles and contoured surfaces in one setup. The reference stays the same from the first cut to the last, so position error does not accumulate across setups.

On our floor, 16 simultaneous five-axis machining centers run alongside 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Five-axis is not automatically the right answer for every part. A simple bracket with two flat faces and four holes is faster and cheaper on a three-axis machine with a good fixture. Five-axis earns its cost when the part has compound angles, deep pockets with limited tool access, or features that would otherwise need three or more setups.

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  One setupFewer re-clamps, less stacked position error.
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  Compound anglesReachable without tilting the part by hand.
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  Contoured surfacesTool stays normal to the surface, so finish is more even.
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  When to skip itSimple flat parts run cheaper on three-axis.

Thin walls are the classic problem. Below about 1 mm in aluminium, cutting force pushes the wall away from the tool, then it springs back. The cut looks fine on the machine and measures oversize on the CMM. The fix is usually a support fixture plus lighter finishing passes, not a tighter machine.

Deep holes are the second one. A drill or boring bar deflects as length-to-diameter ratio grows. Past roughly 5:1 in aluminium, and 4:1 in stainless, diameter and straightness both drift. Gun drilling or peck cycles help, but the extra time shows up in the price.

Heat is the third. Aluminium moves about 23 μm per metre per degree Celsius. A part that measures on size at 25 °C can be out of tolerance at 20 °C if it was cut hot. For long parts, we let the workpiece settle before final inspection rather than measuring straight off the machine.

The fourth is surface finish next to a tight tolerance. Ra 0.2–0.8 μm and ±0.005 mm on the same face means slower feeds and a finishing tool change. It is doable, and it costs more than either requirement alone.

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  Thin wallsBelow about 1 mm in aluminium, deflection pushes the cut oversize.
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  Deep holesPast roughly 5:1 in aluminium, tool deflection moves the diameter.
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  Long partsThermal growth of 23 μm/m per °C shows up on warm parts.
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  Finish plus toleranceRa 0.2–0.8 μm with ±0.005 mm needs slower finishing passes.

A tolerance claim is only as good as the measurement behind it. Calipers and micrometers read a single dimension, and they read it at one point. They cannot tell you whether a bore is round or whether a face is flat. For a drawing with a position callout, that gap matters.

CMM work covers the 3D side. A coordinate measuring machine reports position, roundness, flatness and perpendicularity against the drawing datums. For first articles, we run the full drawing. For production runs, we sample the critical features and keep the raw data.

Material certificates travel with the parts. Mill test reports tie the heat number to the chemistry and mechanical properties, which is what an auditor asks for when the part goes into a regulated assembly. Process documentation and PPAP packages are available when the customer's quality system needs them.

Every order gets 100% inspection before shipment: raw material check on receipt, in-process monitoring during cutting, and a final inspection before packing. Inspection reports are issued on request rather than by default, because not every buyer wants the paperwork. Our recorded qualification rate across these steps is 99.99%.

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  Hand toolsGood for a single dimension, blind to form and position.
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  CMMReports position, roundness, flatness against datums.
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  Material certsMill test reports link heat number to chemistry.
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  Final gate100% inspection before shipment, reports on request.

Value in China CNC machining rarely comes from the hourly rate alone. It comes from fewer setups, fewer scrapped parts and less back-and-forth on the drawing. A shop that catches a datum conflict during DFM saves the buyer a full revision cycle. That is worth more than a few percent off the piece price.

The equipment mix matters too. Running a simple part on a three-axis machine and a contoured part on a five-axis center keeps the hourly load matched to the job. Putting every part on a five-axis center would raise the price without improving anything the customer can measure.

Volume has a real effect. No minimum order quantity applies here, so one prototype and a 10,000-part run both go through the same shop. Unit cost drops as fixtures amortize and programming spreads over more parts. The accuracy spec, though, does not change with volume.

What value does not include is a promise about price or delivery dates. Those depend on the drawing, the material and the current load. We quote from the file, not from a rate card.

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  Fewer setupsEach removed setup removes a position error source.
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  DFM feedbackA caught datum conflict saves a drawing revision.
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  Right machineMatching machine to part keeps the price honest.
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  No MOQPrototype and production run the same route.

Ask for the inspection plan, not just the tolerance. A supplier who can name the instrument and the sampling rate for each critical feature has thought about the part. A supplier who answers with a general tolerance range has not.

Ask how many setups the part needs. That number drives both accuracy and cost, and it tells you whether the quoted process matches the geometry. If the answer is four setups on a part with compound angles, the position error will stack.

Ask what happens when a feature fails. Rework, scrap, or a deviation request? The answer tells you whether the shop measures in-process or only at the end. In-process checks catch a drift before a whole batch is cut.

Then check the certifications against the industry. ISO 9001:2015 covers general quality systems. IATF 16949:2016 applies to automotive work, ISO 13485:2016 to medical devices, and ISO 27001:2022 to information security. A shop that holds the one your industry needs has already been audited on the process you are buying.

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  Inspection planInstrument and sampling rate named per critical feature.
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  Setup countTells you whether position error will stack.
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  Failure pathRework, scrap or deviation request, decided in advance.
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  Certification fitMatch the certificate to your industry, not to the logo wall.