Customized CNC machining in China: a guide

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  How customized CNC machining in China turns a CAD file into a part
• 2
  What tolerances and surface finishes are realistic
• 3
  Materials that machine well, and the ones that surprise you
• 4
  How to judge a Chinese CNC supplier before you commit
• 5
  Which supplier profile fits which part

Customized CNC machining removes material with computer-controlled cutters. You send a 3D model, the shop writes toolpaths from it, and a spindle follows those paths through aluminum, steel, titanium or plastic. Nothing about the physics changes when the machine sits in Dongguan instead of Detroit. What changes is how the work is scheduled, inspected and documented.

The chain starts with DFM. A machinist reads your model and looks for features that will chatter, need a custom cutter, or cannot be reached at all. Wall thickness under 0.5 mm on a 100 mm aluminum plate is a flag. So is a 3 mm deep slot that is only 1 mm wide. Good shops return a marked-up drawing within hours, before any metal is cut.

Next comes fixturing. A 5-axis center can reach five faces in one setup, which removes stacking error. On a 3-axis machine the same part may need four setups and four datums, and each re-clamp adds positional drift. This is the single biggest reason a part that holds ±0.005 mm on one supplier's 5-axis machine drifts out of tolerance on a cheaper 3-axis quote.

Finally, inspection closes the loop. A part is only as good as the measurement that proves it. First articles should be checked against the drawing on a CMM, with the report attached. Runs should carry in-process checks at defined intervals, not just a final look.

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  DFM firstFree analysis before cutting; it catches the features that drive cost.
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  Fewer setups5-axis work reduces datum stack-up on complex geometry.
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  Measured, not assumedAsk for the report, not a verbal 'it's good'.

A general machining tolerance of ±0.005 mm is achievable, but it is not a blanket number. It applies to features with a stable datum, a rigid setup and a short reach. A 300 mm long bore held in a vise will move when the material relaxes. Ask what the tolerance applies to, and on which features.

As-machined surfaces land around Ra 1.6–3.2 μm. That is fine for brackets, housings and most functional faces. Sealing surfaces, bearing bores and sliding fits usually need Ra 0.8–1.6 μm, which means a finishing pass with a smaller stepover. Optical and vacuum applications may call for Ra 0.2–0.8 μm, and that is a different cost tier because it consumes machine time and often needs polishing after cutting.

The tolerance you ask for should match the function. Tightening a non-critical dimension from ±0.1 mm to ±0.01 mm buys nothing and adds scrap risk. Engineers who send a fully tight drawing usually get a higher quote and no better part. Mark the critical few dimensions and let the rest run general.

Material choice interacts with all of this. 6061-T6 cuts clean and holds tolerance. 304 stainless work-hardens and pulls. Titanium Ti-6Al-4V needs sharp tooling, low cutting speed and attention to heat. Inconel is slower still. A shop that quotes all four at the same price is not quoting the same process.

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  General±0.005 mm where the setup supports it.
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  Functional facesRa 0.8–1.6 μm for seals and fits.
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  Optical / vacuumRa 0.2–0.8 μm, higher cost tier.

Aluminum covers most prototyping work. 6061 and 6061-T6 are the default for enclosures, brackets and fixtures. 7075 gives higher strength for aerospace brackets but machines with more spring and needs sharper tools. 2024 has good fatigue behavior and poor corrosion resistance unless it is anodized or coated. 5052 and 5083 are chosen for welded frames and marine parts.

Stainless is where quotes spread out. 303 is free-machining and cheap to run. 304 is the general-purpose grade but galls and work-hardens if the feed is too light. 316 and 316L add corrosion resistance for medical and food equipment. 17-4PH can be heat treated to high strength after machining, which is useful for shafts and valve parts.

Steels behave predictably: 1018 and 1045 for general parts, 4130 and 4140 for stressed components, 4340 where toughness matters. Tool steel shows up in dies and wear plates. Titanium and Inconel are slow, expensive and worth it only when the service temperature or strength-to-weight ratio demands them.

Plastics are not a soft option. POM machines cleanly and holds tolerance. PEEK is dimensionally stable at high temperature but costly. ABS and PC are common for enclosures. Carbon fibre reinforced grades eat tooling and need diamond-coated cutters, so expect a different price band.

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  Default aluminum6061-T6 for most functional prototypes.
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  Watch stainless304 work-hardens; 303 is easier and cheaper.
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  Plastic is not cheap by defaultPEEK and carbon-filled grades cost more to cut.

Start with the certifications that map to your industry. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters if your drawings are sensitive. A supplier holding all four has been audited on process, not just on paperwork.

Then look at the machine list against your part. A shop with 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and 16 mill-turn centers, can cover a range of geometries. Maximum processing size of 4,000 mm handles long parts that a 500 mm machine cannot. Ask which machine will run your job and how many setups it needs.

Ask about inspection. A shop that inspects 100% of parts before shipment and offers reports on request is telling you the measurement is part of the process. A shop that says 'we check visually' is telling you something else. For medical and aerospace work, request the dimensional report with the first article.

Finally, treat communication as a technical variable. A quote and free DFM analysis within 12 hours means an engineer read your file. Production starting within 24 hours means the schedule is real. Parts shipping in 3–5 days is a normal pace for simple geometry. If any of these numbers is far off, ask why before you release the PO.

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  Match certifications to industryIATF 16949 for automotive, ISO 13485 for medical.
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  Match machines to geometry5-axis and mill-turn change setup count.
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  Ask for the inspection reportIt should be normal, not a special request.