China Precision CNC Processing Factory: How Tolerances Actually Hold

Axis count is a geometry decision, not an upgrade path. A prismatic bracket with holes on one face runs best on a three-axis mill, where the part stays clamped and the tool comes from one direction. Adding rotary axes to that job only adds setup risk. We keep 27 three-axis machines for exactly this kind of work.

A part earns four-axis time when features wrap around a single rotational axis: shaft flats, cross-drilled holes, cam profiles, slotted cylinders. One rotary table holds the part while the tool reaches several sides, so the operator does not re-fixture between operations. Our four-axis mills carry a Ø400 mm rotary table, which fits most shaft and housing work.

Five-axis matters when the feature normal changes direction in more than one plane, or when a single setup removes stack-up error. Impellers, turbine blades, medical bone plates with compound curves, and deep cavities with undercut walls all fall here. We run 16 simultaneous five-axis machining centers, and the tool reaches the surface at an angle instead of rubbing with the tip.

The trade-off is real. Five-axis programming and simulation cost more hours, and small simple parts do not get cheaper by running on a bigger machine. A well-quoted shop will tell you when three axes is the right answer.

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  Three-axisFlat plates, covers, housings, hole patterns on one face
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  Four-axisShafts, cams, cross-drilled cylinders, wrapped slots
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  Five-axisImpellers, blades, compound-curve medical parts, deep cavities
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  Mill-turnTurned parts with milled features, one setup, 16 centers

We quote a general tolerance of ±0.005 mm (±0.0002 in) where the drawing calls for it. That number is not a shop-wide default; it is a capability we hold on specific features, and it needs to be marked on the drawing. A hole position, a bore diameter, and a flat surface all reach that band differently.

Thermal drift is the first thing that breaks tight tolerances. A spindle running for hours grows, and a part measured hot will not match the same part measured at 20 °C. We rough, let the part rest, then finish. For aluminum, this gap is small. For Inconel and titanium, it is not.

Fixture rigidity decides the second half. Thin walls deflect under cutting force, so a 2 mm wall on a 6061 housing will spring back during a heavy pass. The fix is usually process, not machine: lighter radial engagement, a support fixture, or a change in the order of operations. That is why we ask for a 3D model, not only a PDF, before quoting.

Surface finish travels with tolerance. If you need Ra 0.2–0.8 μm on a sealing face, the finishing pass is planned from the start, because a tool path that holds size will not automatically hold that finish.

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  Size tolerance±0.005 mm (±0.0002 in) on marked features
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  Fine finishRa 0.2–0.8 μm, sealing faces and bearing bores
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  High finishRa 0.8–1.6 μm, most mating surfaces
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  As-machinedRa 1.6–3.2 μm, non-critical faces

Aluminum is the easy case until it is not. 6061 and 7075 cut fast and hold size well, but thin 7075 webs crack if you take an aggressive finishing pass. 2024 machines cleanly and holds a better fatigue life, which is why aerospace brackets often call for it. We stock 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, and ADC12.

Stainless steels split into free-machining and not. 303 turns and mills easily, while 304 and 316 work-harden under a rubbing cut, so the tool must stay engaged and the feed cannot drop. 17-4PH (SUS630) machines in the annealed state and then ages to high strength, which changes the planning order: rough, heat treat, finish, then inspect.

Titanium and nickel alloys are where feed and speed decisions show up in the price. TC4 (Ti-6Al-4V) has low thermal conductivity, so heat stays in the cutting zone and the tool edge wears quickly. Inconel is worse. Both need slower surface speeds, more coolant, and shorter tool life, which adds machine hours rather than material cost.

Plastics behave in the opposite direction. PEEK and ULTEM hold tight tolerances but absorb moisture, so a part measured right after cutting can shrink later. ABS, PC, POM, PMMA, PP, HDPE and carbon fibre are all in scope, with the cutting strategy set by how the chip breaks.

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  Aluminum6061, 7075, 2024, 5052, 5083, 6063, 6082, ADC12
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  Stainless303, 304, 316L, 420, 440C, 17-4PH
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  Titanium and nickelTA1, TA2, TC4, Inconel, magnesium AZ31B / AZ91D
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  PlasticsPEEK, ULTEM, POM, PC, ABS, PA, carbon fibre

A single good part does not prove a process. We inspect 100% of parts before shipment, and that runs through three stages: incoming raw material check, in-process monitoring during cutting, and final inspection before packing. Reports are available on request with the shipment.

For tight-tolerance work, the first article is measured and compared against the model before the run continues. If a bore comes out 0.008 mm over, the correction happens in the tool offset, not in the next batch. Catching drift at part three is cheaper than catching it at part three hundred.

The quality system covers ISO 9001:2015, IATF 16949:2016 for automotive, ISO 13485:2016 for medical devices, and ISO 27001:2022 for information security. Those certificates set the documentation and traceability level, but they do not replace a measurement plan on your drawing.

Our historical qualification rate is 99.99%, and late delivery has stayed below 2%. We publish those numbers because buyers ask, not because they replace a schedule discussion.

A quote and a free DFM analysis go back within 12 hours of receiving the files. Production can start within 24 hours after that, and parts typically ship in 3–5 days. Those are standard timelines for typical work, and complex five-axis parts with tight finish calls will need their own schedule.

The DFM review is where cost is decided. We check wall thickness, tool reach, corner radii, thread depth, and whether a feature can be reached without a second setup. A radius smaller than the tool can cut, or a pocket deeper than four times the tool diameter, will drive the price up before any metal is removed.

There is no minimum order quantity. One prototype and a 10,000+ part run go through the same first-article process, though the tooling and fixture plan differs. For one-off prototypes we often machine from billet; for repeat runs we look at fixtures and inspection gauges that cut cycle time.

Uploads are handled as confidential, and an NDA is available on request. If your drawing package needs a controlled-data agreement before release, say so at the start and it gets signed before files move.

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  Quote and DFMWithin 12 hours of files
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  Production startWithin 24 hours
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  ShippingTypically 3–5 days
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  Order sizeOne prototype to 10,000+ parts