China's Large CNC Processing: A Buyer's Guide for Engineers

Large CNC processing is defined by the work envelope, not by the part name. A supplier can run a large machine and still not be able to hold your part, because the limiting number is the travel on the axis that carries the longest feature. At GreatLight the largest envelope is 4,000 × 400 × 150 mm, which suits long extrusions, rails, structural beams and long housing profiles. Everything above that envelope has to be split, welded, or moved to a different process.

Most oversized work does not need the biggest machine. The medium group covers 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and handles most plate work, manifolds, brackets and frames. The compact group at 500 × 500 × 450 mm and 500 × 310 × 200 mm covers the majority of production volumes. Putting a 300 mm part on a 4,000 mm machine wastes setup time and ties up capacity you may need later.

A useful first step before requesting a quote is to write down the maximum X, Y and Z extents plus the largest single feature that must be machined in one setup. That one line tells a supplier whether the part goes on a 3-axis bed, a 4-axis tombstone, or a 5-axis center. It also tells you where the risk sits: long thin parts deflect, deep pockets need long tools, and tall parts need a machine with enough Z to clear the fixture.

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  Envelope firstTravel limits decide feasibility before price does.
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  Long parts4,000 mm travel suits rails, beams and long housings.
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  Plate work750 × 1,150 × 550 mm covers most manifolds and frames.
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  Fixture heightZ travel must clear the vise or tombstone, not just the part.

Simultaneous 5-axis matters when a part has features on multiple faces that must share one datum. Angle housings, impeller-like geometry, contoured pockets and parts with compound angles are the classic cases. Machining them in one setup removes the re-fixturing error that builds up when you rotate a part three times on a 3-axis mill. GreatLight runs 16 simultaneous 5-axis machining centers alongside 12 four-axis mills and 27 three-axis machines, so the choice is made on geometry, not on what happens to be free.

Five-axis is not automatically better. Prismatic parts with features on two or three orthogonal faces are usually faster and cheaper on a 4-axis mill with a rotary table. A Ø400 mm rotary table lets the tool reach four sides in one program while the part stays clamped to a rigid indexer. For simple plate work, a 3-axis machine with good fixturing will beat a 5-axis center on cycle time every time.

The decision point is usually tolerance stack-up. If a drawing calls for a bore and a mating face to be coaxial within ±0.005 mm, one setup is the safest route. If the faces are independent and each carries its own datum, multi-setup machining is fine and often cheaper. Bring the drawing and the datum scheme to the conversation, not just the model.

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  One setup winsUse 5-axis when faces must share a single datum.
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  Four-axis is enoughOrthogonal features on a tombstone or rotary table.
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  Three-axis is fineFlat plates and open pockets with simple datums.

Material choice changes the machining plan more on large parts than on small ones. Aluminium 6061-T6 and 6082 cut fast and hold size well, but thin walls move after clamping is released. Stainless 304 and 316 work-harden, so deep pockets need constant feed and sharp tooling; 17-4PH adds a heat-treat step that can shift dimensions unless the sequence is planned. Steel grades such as 4140 and 4340 are common for structural and tooling parts where strength matters more than weight.

Titanium TC4 (Ti-6Al-4V) and Inconel are used for high-temperature and high-strength applications. Both cut slowly, generate heat at the edge, and require more tool changes. On a large frame, that translates into longer cycle times rather than a different machine. Magnesium AZ31B and AZ91D machine quickly but need chip control because fine magnesium chips are a fire risk, so coolant and chip evacuation are planned up front.

Plastics behave differently again. PEEK and POM hold tight tolerances but move with temperature; ABS and PC are usually specified for enclosures and covers where surface finish matters more than precision. We machine carbon fibre as well, with tooling and dust extraction set for abrasive composite dust. If your part mixes materials, for example a steel insert in an aluminium housing, say so early because the fixturing and the inspection plan change.

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  Aluminium6061, 6082, 7075 and ADC12 for fast, stable cuts.
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  Stainless303, 304, 316L and 17-4PH; watch work hardening.
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  Titanium and InconelSlow speeds, high heat, more tool wear.
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  PlasticsPEEK and POM hold size; ABS and PC are for covers.

GreatLight works to ±0.005 mm (±0.0002 in) on qualified features and holds a 99.99% qualification rate across production. Those numbers only mean something when the inspection plan matches the drawing. We check incoming raw material, monitor dimensions in process, and inspect 100% of parts before shipment. Reports are available on request, and for large parts the report usually matters more than the part, because it is the evidence your own quality team will file.

Surface finish is quoted separately from dimensional tolerance. As-machined surfaces sit at Ra 1.6–3.2 μm, a high-finish pass reaches Ra 0.8–1.6 μm, and fine finishing goes to Ra 0.2–0.8 μm. On large faces, achieving a fine finish across the whole surface takes more passes and more time, so it is worth specifying finish only where it functions: sealing faces, sliding surfaces, and cosmetic exterior panels.

Finishing options include anodizing in clear, colour, hardcoat and conductive types, electroless nickel, zinc, silver and gold plating, powder coating and black oxide, plus bead blasting, tumbling, brushing and polishing. Laser marking is available down to 1.5 mm character height. If a large part is going to be anodized after machining, mention it before the process plan is fixed, because anodizing adds a thin build-up on dimensions and can change how a bore fits.

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  Dimensional±0.005 mm on qualified features, 100% inspected.
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  FinishRa 0.2–0.8 μm for sealing and sliding faces.
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  CoatingAnodizing adds build-up; flag it before machining.
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  MarkingLaser marking down to 1.5 mm character height.

China's large CNC processing market includes both factories and intermediaries. The difference shows up in the questions they ask. A plant will ask about datums, workholding and the largest single setup. A trader will ask for a drawing and return a number. Neither answer tells you the whole story, but the first one tells you the part has been thought about.

Ask which machine will run the job and what its travels are. Ask how the part is clamped and whether the fixture already exists. Ask who inspects it and what instrument is used on the critical dimension. A plant with 127 high-precision CNC machines across 3 wholly-owned plants and 7,600 m² of floor space can answer those questions with specifics, and can usually start production within 24 hours of a confirmed order.

Certifications are a filter, not a guarantee. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The 27001 scope is the one buyers forget: it covers how your drawings and models are stored and who can open them. If your part is proprietary, ask about file access, NDA availability and how uploads are handled. We treat uploads as secure and confidential and sign an NDA on request.

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  Ask for travelsWhich machine, and what is its actual envelope?
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  Ask for the fixtureClamping plan reveals whether the part was studied.
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  Ask for the instrumentCMM, height gauge or bore gauge on the key feature.
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  Ask about filesISO 27001 scope covers drawing confidentiality.