CNC Machining in China: A Design & Sourcing Guide for Engineers and Buyers

1. CNC processes and what each is used for

Milling and turning cover most machined parts. What separates one supplier from another is the number of controlled axes on the machines and, with it, the geometry that can be produced in a single setup.

3-axis milling

The spindle moves in X, Y and Z while the workpiece stays clamped. Three-axis work suits prismatic parts — plates, brackets, housings, manifolds — whose features are reachable from one direction, or from two with a single flip of the part. It is the most widely available configuration and the easiest to quote.

The design consequence is practical: features on all four sides of a block require multiple setups, which adds handling and introduces the risk of datum shift between operations.

4-axis milling

A fourth, rotary axis (usually turning about X) indexes the part between operations, or machines cylindrical surfaces continuously. Typical uses include shafts with flats or slots, cylindrical housings with ports drilled at several angles, and parts where several faces must be machined from the same datum without re-clamping.

5-axis milling

Two rotary axes are added to the three linear ones. Machining can be simultaneous — the tool stays normal to a curved surface — or positional (“3+2”), where the table indexes to an angle and locks before cutting. Five-axis work is common for complex contours, impellers and blades, deep cavities, and undercut features that a three-axis machine cannot reach.

In production the practical benefit is fewer setups and less custom fixturing, not automatically tighter accuracy. Five-axis capacity, programming skill and post-processor quality vary between shops, so confirm capability before designing a part that depends on it.

CNC turning

Turning produces parts that are essentially rotational: shafts, bushings, spacers, fittings and threaded connectors. Bar-fed lathes with live tooling (turn-mill centres) can also mill flats, cross-holes and slots, so a part that would otherwise need two processes can often be finished on one machine.

Drilling and tapping

Holes are usually produced on the mill or the lathe, but the dedicated operations still matter: reaming for size and finish, counterboring for bolt heads, countersinking and tapping. General-purpose metric threads should be called out with their standard designation — the tolerance system for ISO metric screw threads is specified in ISO 965-1, and the basic thread profile in ISO 68-1. Custom thread forms should be avoided unless there is a functional reason for them.

Wire EDM

Wire electrical discharge machining cuts conductive material with a thin wire electrode in a dielectric, removing metal by spark erosion. It is used for through-profiles that need sharp internal corners, for punch and die work, and for cutting hardened tool steel that would be impractical to mill. The workpiece must be conductive, and the process is normally limited to through-features rather than three-dimensional cavities.

Sinker EDM

Sinker (die-sinking) EDM burns a cavity with a shaped electrode, typically after milling has removed the bulk of the material. It is chosen for deep ribs, sharp internal corners and cavities in hardened steel.

Because a single part often needs several of these operations, a supplier that can run turning, milling, EDM and finishing in one plant reduces part handling and keeps responsibility for the final tolerances in one place.

2. Material selection for CNC parts

Material choice drives machinability, achievable finish, corrosion behaviour and the finishing options that remain open. The grades below are the ones most commonly specified for machined parts; drawings should name the grade and the governing material specification so the shop orders the correct stock form (bar, plate or extrusion).

Aluminium 6061 and 7075

6061 is the general-purpose wrought aluminium alloy: it machines cleanly, has good corrosion resistance, anodises well and is widely stocked. Typical parts are enclosures, brackets, fixtures, panels and general mechanical components. Extruded bar stock is covered by ASTM B221; plate and sheet by ASTM B209.

7075 is a high-strength aluminium alloy used where strength-to-weight ratio governs the design — highly loaded brackets, structural fittings, and some aerospace and sporting applications. It is less commonly stocked than 6061 and is generally selected for strength rather than for appearance or for joining.

Stainless steel 304 and 316

Both are austenitic grades covered for bar and shape stock by ASTM A276.

304 is the default stainless for general corrosion resistance — architectural fittings, food-contact and dairy hardware, and general industrial parts.

316 contains molybdenum, which improves resistance to chlorides and to some chemical environments; it is the usual choice for marine, chemical, pharmaceutical and coastal applications.

Machining stainless steel is more demanding than aluminium: the material work-hardens, cutting speeds are lower, tool wear is higher and rigid setups matter. Where corrosion performance is critical, machined stainless parts are frequently specified with a passivation step after machining (see Section 4).

Carbon steel

Carbon steels span a wide range, and the machining decision usually comes down to the grade family. Free-machining low-carbon grades are used for high-volume turned parts where chip control and surface finish matter. Medium-carbon grades are used for shafts and load-bearing parts that will be heat treated after machining. Cold-finished carbon steel bar stock is specified under ASTM A108.

Untreated carbon steel corrodes readily, so a protective finish — zinc plating, painting or another barrier coating — is normally part of the specification rather than an optional extra.

Brass

Free-cutting brass, specified under ASTM B16 for rod, bar, wire and shapes, is among the most machinable metals used in CNC work: it produces short chips, takes a fine finish at high feed rates and is well suited to small turned parts such as fittings, valves and electrical components.

Brass is usually selected for machinability, electrical conductivity and appearance rather than for high mechanical strength or heavy marine service.

Engineering plastics

Plastics are machined where corrosion resistance, electrical insulation, low weight or low friction matter more than stiffness:

• POM (acetal) — good dimensional stability and low friction; gears, bushings, rollers, valve bodies. Classification and specification of POM moulding and extrusion materials is covered by ASTM D6778.
• PA (nylon) — tough and wear-resistant, but it absorbs moisture, which changes dimensions; best for parts where tolerance is not critical or where moisture exposure is controlled.
• PEEK — retains strength at high temperature and resists many chemicals; used in medical, semiconductor and aerospace parts.
• PTFE — chemically inert and very low friction, but it creeps under sustained load.
• PC and ABS — used for covers, guards and non-structural parts where toughness or cost matters more than strength.

Plastics machine easily with sharp tooling, but heat build-up and swarf evacuation need attention, and finishes such as anodising or plating do not apply.

3. Tolerances and how to specify them correctly

Tolerance strategy is where most quotation disputes start. A drawing that tolerances every dimension individually invites both higher inspection effort and ambiguity about how the part will be measured.

General tolerances

When dimensions are not individually toleranced, drawings reference a general tolerance standard. ISO 2768-1 defines general tolerances for linear and angular dimensions without individual tolerance indications, and organises them into tolerance classes so that a single title-block note covers all non-critical dimensions. A revised edition of ISO 2768 is in preparation at the time of writing, so confirm with your supplier which edition and which class a drawing is being worked to.

Practical points:

• State the general tolerance standard and the class in the drawing title block; do not leave it to the shop’s default.
• General tolerances are not a licence to ignore fit-critical features — those still need individual tolerances.
• Where a hole is intended to receive a shaft, pin or bearing, define the fit using the ISO code system for linear size tolerances rather than a rounded-off decimal. ISO 286-1 defines that system, including tolerance grades and limit deviations for holes and shafts.

Geometric dimensioning and tolerancing (GD&T)

GD&T controls form, orientation, location and run-out — the characteristics that linear dimensions cannot express. ISO 1101 defines the symbol language for geometrical specification and the rules for interpreting it; ASME Y14.5 defines the equivalent system used widely in North American drawings and is regarded as the authoritative guideline for GD&T.

Apply GD&T where geometry carries function:

• Flatness or profile where a face seals or mates against another part.
• Position at hole patterns, so the pattern is controlled relative to datums rather than to individual dimensions.
• Run-out or concentricity on rotating parts — the definition matters, and ISO and ASME treat some characteristics differently.

Two cautions. First, state on the drawing which system governs (ISO 1101 or ASME Y14.5); the two are close but not identical, and mixing conventions creates interpretation risk. Second, every geometric control adds inspection time, so specify only the controls your function actually needs.

Surface roughness

Surface texture requirements should be indicated using the drawing symbols defined in ISO 1302, with the parameters defined in ISO 4287.

• Roughness parameters describe the fine-scale irregularities left by the cutting tool; they are distinct from waviness and from dimensional tolerance, and a part can be dimensionally correct and still fail a texture requirement.
• Call out texture only on functional surfaces — sealing faces, bearing seats, sliding surfaces, cosmetic faces. Applying a fine requirement across an entire part adds finishing operations without benefit.
• Give the parameter (for example Ra or Rz) and the value, and note the measurement direction where the lay of the surface matters.

When a requirement cannot be verified economically in production, agree with the supplier how it will be measured before the part is made.

4. Surface finishing options

Finishing is where a machined part becomes a product: it sets corrosion resistance and appearance for the life of the part. Two rules apply to every finish below. First, state the finish on the drawing with a specification or standard, not a colour word alone. Second, remember that coatings and conversion layers add or remove material at the surface, so mask or allow for features that are dimensionally critical — threads, bores and mating faces in particular.

Anodising (aluminium only)

Anodising converts the aluminium surface into an oxide layer, so it applies only to aluminium and its alloys — never to steel. Two families matter:

• Decorative and protective anodising — the method for specifying these coatings on aluminium and aluminium alloys is set out in ISO 7599.
• Hard anodising — a thicker, harder anodic layer used for wear resistance on sliding and abrasive-contact surfaces; requirements and test methods are specified in ISO 10074.

Anodised parts are usually sealed to support corrosion performance, and colour is achieved with dyes that sit within the porous oxide layer. Because the layer grows both outward and into the surface, fine tolerances on anodised features should be discussed with the finisher rather than assumed.

Powder coating

Powder coating applies a dry polymer powder electrostatically and cures it into a continuous film. It is typically used on steel and aluminium fabricated parts and enclosures where a durable, decorative, opaque finish is required.

Compared with anodising, powder coating produces a thicker film, which makes it less suitable for tight-tolerance or fine-detail features and means threads usually need masking. The coating powders themselves are specified and tested under the ISO 8130 series, and corrosion-protection paint systems for steel structures are addressed by ISO 12944.

Electroplating

Electroplating deposits a metal layer for corrosion protection, conductivity, wear resistance or appearance. The most common specification in machined general engineering is zinc plating on steel: ISO 2081 specifies electroplated zinc coatings with supplementary treatments on iron or steel, including heat-treatment requirements before and after plating, and ASTM B633 covers electrodeposited zinc coatings applied to iron or steel for corrosion protection.

Electroplating is normally a rack or barrel process, so parts need contact points and should be designed with that in mind. On hardened or high-strength steel parts, the plating supplier should confirm the pre- and post-plating heat treatment, since that requirement is part of the zinc-plating specification.

Passivation (stainless steel)

Passivation is a chemical treatment that removes free iron and other surface contamination from stainless steel and helps the natural passive oxide layer re-form after machining. ASTM A967 specifies chemical passivation treatments for stainless steel parts and the tests used to verify them.

Passivation is not a coating — it adds no measurable thickness — and it is therefore the usual finishing step for machined 304 and 316 parts where corrosion resistance of the machined surface matters.

Other common finishes

The following are frequently specified in practice but should always be tied, on the drawing, to a written supplier specification rather than to a name alone:

• Bead blasting / glass bead finishing — a uniform matte surface, often used before anodising or as a cosmetic finish.
• Brushing / linishing — directional satin finish for decorative metal parts.
• Black oxide — a conversion coating used mainly on steel for appearance and mild in-process protection; it offers limited corrosion resistance without a supplementary oil or wax.
• Laser marking / engraving — for part numbers, traceability codes and logos on machined parts.

5. DFM: design for manufacturability

Design for manufacturability is the practice of removing features that are expensive to make but add nothing to function. On CNC work the recurring issues are simple, and most are fixed at the drawing stage.

1. Tolerances match function. Specify the loosest tolerances that the part’s function allows. A general tolerance class per ISO 2768 covers non-critical dimensions; individual tolerances and geometric controls should be reserved for the features that need them.
2. Give internal corners a radius. A cutter is round, so a square internal corner can only be produced by leaving a radius or by adding a secondary EDM operation. Design the radius into the part and specify it on the drawing.
3. Keep pockets open enough to reach. Deep, narrow pockets force the use of long, slender tools that deflect, cut slowly and break. Widening a pocket or reducing its depth usually improves both accuracy and surface finish.
4. Use standard sizes. Standard drill, reamer and tap sizes are cheaper and faster than specials. For threaded features, use standard metric thread designations consistent with ISO 965-1 and ISO 68-1.
5. Design for clamping. A part needs surfaces the machine can hold. Irregular parts that must be machined all over may require soft jaws or custom fixtures, which is design work the buyer effectively pays for in time even when it is not itemised.
6. Minimise the number of setups and machined faces. Every additional setup adds the need for a new datum and re-clamping error. Where the geometry genuinely requires access from many directions, 4- or 5-axis machining may reduce setups — but only if the chosen supplier has the machines.
7. Supply a complete data package. A native CAD file plus a neutral exchange format (for example STEP), a controlled drawing with material grade and standard, general tolerance class, GD&T scheme per ISO 1101 or ASME Y14.5, surface texture per ISO 1302, thread callouts and the finishing specification. Missing information is the most common cause of slow or inaccurate quotations.
8. Avoid unproducible detail. Very small text, sharp-edged lettering, and features narrower than the smallest practical cutter should be removed or enlarged at the design stage.

6. How to evaluate a CNC machining supplier

Work through supplier evaluation in a fixed order: management system, measurement capability, documentation, then process evidence. Certificates are the entrance ticket, not the verdict.

Quality management system. ISO 9001 specifies the requirements for a quality management system and is the baseline certificate to ask for. The current edition is ISO 9001:2026, which has replaced the 2015 edition, so check which edition a supplier’s certificate actually names before treating old and new certificates as equivalent. A certificate is only as good as the body that issued it: ISO/IEC 17021-1 sets out requirements for the competence, consistency and impartiality of bodies providing audit and certification of management systems. Validate the certificate in IAF CertSearch, the global database of accredited management-system certifications, rather than accepting a scanned PDF at face value. Read the scope statement as well — a certificate can cover a different site, product range or process from the one quoting your job.

Inspection equipment and calibration. Ask which measuring instruments the shop owns — coordinate measuring machine, optical profile projector, surface roughness tester, hardness tester, thread gauges, micrometers and bore gauges — and how those instruments are calibrated. Measurements you intend to rely on should be traceable to a recognised reference. ISO/IEC 17025 specifies the general requirements for the competence, impartiality and consistent operation of testing and calibration laboratories.

First-article inspection (FAI). For a new part, request a dimensional report of the first article measured against the drawing, together with confirmation of the material and the finish actually applied. This turns the supplier’s process into data you can accept or reject before production is released, and it is the single most useful document in the qualification file.

Material certificates and traceability. Ask for mill certificates or material test reports stating grade, governing specification and heat or lot number — and ask whether the certificate covers the stock actually used for your parts. Traceability means a delivered part can be linked back to that heat number, which requires the supplier to segregate material by lot. Specifications such as ASTM A276 for stainless bars, ASTM B221 and B209 for aluminium, and ASTM A108 for carbon steel bars give you the grade basis to check against.

Process evidence and communication. Ask about setup documentation, in-process inspection records, and the documented procedure for non-conforming parts and returns. Then test the engineering channel: send the drawing with a genuine DFM question and judge the reply. The quality of the technical response to an RFQ is the cheapest available predictor of how the order will go.

Capability match. Establish the machine list and maximum part envelope, which materials are run routinely, and whether finishing (anodising, plating, passivation) is done in-house or subcontracted. Subcontracting is normal; what matters is whether the supplier controls the subcontractor and can produce the finishing documentation.

A paid trial order at prototype quantity is more informative than any questionnaire, but judge it on measured results, not on the fact that it arrived.

7. When a Chinese supplier makes sense — and when local does

Chinese machine shops tend to be strong where labour content is high, quantities are repeatable and the design is frozen. Local or regional shops tend to be stronger where iteration speed, physical proximity and contract-driven origin requirements dominate. Neither is universally better; the deciding factor is usually the change rate of your design and how much process work the part needs.

Where overseas sourcing tends to fit well

• Stable designs whose drawings will not change between order and shipment.
• Parts with significant manual content: multiple setups, deburring, inspection, manual finishing, and assembly of machined components.
• Programmes that combine processes under one roof — machining, finishing, assembly, packaging — so fewer suppliers have to be coordinated for one delivered part.
• Buyers who already import and therefore treat freight, customs and incoming inspection as standing process steps.
• Standard engineering materials such as 6061 aluminium, 304 and 316 stainless, common carbon steels and brass, where stock availability is not the constraint.

Where a local or regional supplier tends to fit better

• Urgent spares and repairs, and development work where the design is revised every week.
• Large or heavy parts, where freight handling dominates the landed effort.
• Parts whose end market, customer contract or origin requirement calls for domestic manufacture.
• Very small quantities, where documentation, freight and inspection overhead outweigh the machining itself.
• Programmes where you need to visit the shop, witness inspection first-hand, or hold the supplier to an on-site quality agreement.

What both routes share

• Importing machined parts involves customs formalities and duty, and classification depends on the part and the importing country’s tariff schedule. Check the official tariff database for the destination market — for the EU, the official TARIC database — rather than relying on a supplier’s summary.
• Product compliance obligations sit with the importer, not with the machine shop. Declarations about finishing and material are the buyer’s responsibility to define and verify.
• The qualification work is identical: certificates, first-article inspection, material traceability and a trial order.

For a buyer new to overseas machining, a sensible default is to place a first order at prototype quantity with one supplier, measure exactly what comes back, and only then consider production volumes — whichever region is chosen.

Request a quote

Send us the drawing and the requirement, not a specification written from scratch. We work from the same data package described in Section 5 — CAD file, controlled drawing with material, tolerance class, GD&T scheme, surface texture and finish specification — and we will tell you what is missing before a shop starts quoting.

Upload your drawing, state the material and the finishing requirement, and describe how the part is used. DFM questions are welcome; the answer costs nothing and usually saves a revision.

Related pages

• Manufacturing guides: custom processes and design rules — the hub page for machining and fabrication guides.
• Sheet metal design guide: bend radius, tolerances and materials — for parts that combine formed sheet with machined components.
• Certification and compliance hub — how product compliance obligations sit with the importer.
• ASTM vs CE vs UL: standards you need before importing — mapping standards to destination markets.
• Sourcing guides — supplier qualification, factory verification and first-order inspection.
• Request a quote — upload a drawing and get DFM feedback.

Last updated: 16 September 2026.

Sources

All sources retrieved 16 September 2026. Standard numbers are cited to the ISO, ASTM, ASME, IAF or European Commission catalogue record. Standards change; always confirm the current edition of a standard with the issuing body before purchase.