China CNC Milling Parts: What Engineers Should Check Before Ordering

Milling removes material with a rotating multi-tooth cutter while the workpiece stays clamped to a table or fixture. The tool moves in X, Y and Z, and on a 4-axis or 5-axis machine it also rotates or tilts. That extra motion is what decides whether a part needs one setup or four.

The process suits prismatic shapes: housings, brackets, manifolds, plates, heat sinks, mold inserts, engine and gearbox components. It handles pockets, slots, ribs, threads, bores and flat sealing faces in the same cycle. Material choice is wide. Aluminum 6061-T6 and 7075, stainless 304 and 17-4PH, 4140 steel, titanium TC4 and plastics such as POM and PEEK all mill well with the right cutter and coolant.

Milling is the wrong process when the part is a long shaft with a single outside diameter, or when wall thickness drops below about 0.5 mm over a large area. Thin walls deflect under cutting force and the cutter leaves chatter marks. Very deep narrow slots, roughly deeper than 4 times the cutter diameter, also push cost up because the tool has to be long and slow. In those cases turning, EDM or sheet metal is usually cheaper.

A practical rule: if the part fits inside a 500 × 500 × 450 mm envelope and has at least two flat faces to clamp on, milling is a strong candidate. GreatLight machines travel sizes up to 4,000 × 400 × 150 mm for long parts, and the 16 simultaneous 5-axis centers cover work that would otherwise need three or four separate fixtures.

The setup count drives both cost and error. Every time a part is unclamped and repositioned, the datums shift slightly. On a ±0.025 mm part that is manageable. On a ±0.005 mm part it is often the largest single source of scrap.

3-axis milling cuts from one direction. It is the fastest and cheapest option for flat plates, simple brackets and any part where all features are reachable from the top, or from the top and bottom in two clean operations. GreatLight runs 27 three-axis machines for this work.

4-axis milling adds a rotary table, usually Ø400 mm, so the part can index around one axis. This fits cylindrical parts with cross-holes, slots along a diameter, or a series of faces arranged around a hub. Twelve four-axis mills handle that family.

5-axis milling tilts the cutter or the table in two axes at once. The benefit is not speed. It is reach. Undercuts, compound angles, deep pockets with drafted walls, and impeller or turbine-like surfaces can be cut without repositioning the part. With 16 simultaneous 5-axis centers, we routinely hold ±0.005 mm (±0.0002 in) on parts that would otherwise need four setups on 3-axis machines. If your drawing has more than two angled faces, ask for a 5-axis review before you accept a 3-axis quote.

• 1
  3-axisFlat plates, brackets, simple pockets. Lowest cost per part.
• 2
  4-axisCylindrical or indexed parts with radial features. One rotation axis.
• 3
  5-axisCompound angles, undercuts, contoured surfaces. Fewer setups, tighter position control.

Aluminum is the default for prototypes and most enclosures because it cuts fast and holds a good finish. 6061-T6 is the workhorse for structural brackets and fixtures. 7075 gives higher strength for aerospace fittings but is more prone to distortion after heavy material removal, so rough and finish passes are often split. 2024 and 5052 cover sheet-derived parts and marine hardware.

Stainless 303 mills more freely than 304 because of its sulfur content, which makes it a common pick for shafts and bushings with milled flats. 304 and 316L are chosen for corrosion resistance in food, medical and marine equipment, but they work-harden. Cutters must stay sharp and feeds must stay aggressive. 17-4PH (SUS630) is used where strength and corrosion resistance are both needed, such as valve bodies and pump components.

Steel grades 1018, 1045, 4130, 4140 and 4340 cover everything from simple jigs to high-load shafts. 4140 and 4340 are usually supplied pre-hardened, which means the cutter spends more time in the cut and the quote reflects that. Tool steel appears in mold inserts and dies. Titanium TC4 (Ti-6Al-4V) and Inconel are machined for aerospace and energy parts; both generate high cutting temperatures and need slower speeds, so expect a longer cycle than the same geometry in aluminum.

Plastics behave differently again. POM and PA machine cleanly but can bow if clamped too hard. PEEK holds dimensions at high temperature and is common in medical and semiconductor fixtures. Carbon fibre needs diamond-coated tooling to avoid delamination and rapid tool wear. If your part has a mix of materials, say so at quote stage. We machine aluminum, stainless, steel, copper, brass, titanium, magnesium and the plastics listed above in the same plant.

A tolerance on a drawing means nothing without a measurement method. Ask which instrument will check each critical dimension and what the gauge uncertainty is. For ±0.005 mm features, a micrometer on a bench is not enough. CMM inspection, optical comparators and surface roughness testers are the tools that matter.

GreatLight inspects 100% of parts before shipment. The flow is raw material check, in-process monitoring during cutting, and final inspection against the drawing. Inspection reports are available on request. If a feature is defined by a GD&T callout, tell us which datums are functional. We will set up the fixture to those datums rather than to a convenient edge.

Certification is a filter, not a guarantee. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive production. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send CAD files across borders. Check that the certificates cover the site doing the work, not just a trading office.

For first articles, ask for a dimensional report plus material certificates. For production runs, agree on a sampling plan and a reaction path if a dimension drifts. Our historical qualification rate is 99.99%, and the historical late-delivery probability is below 2%, but both numbers only hold when the drawing and the inspection plan are settled before cutting starts.

A milling quote depends on geometry, tolerance, material and finish, not on weight alone. Send a STEP file plus a 2D drawing with tolerances and critical features marked. We return a quotation and a free DFM analysis within 12 hours. The DFM note flags features that will be slow or unstable to cut, such as deep pockets, thin floors or threaded holes too close to an edge.

Once the drawing is frozen, production can start within 24 hours. Simple parts ship in 3–5 days. Larger runs and parts needing anodizing, plating or heat treatment take longer, and the schedule is quoted per job. We do not publish a blanket lead time because a 10-part order and a 10,000-part order do not move through the shop at the same rate.

Uploads are secure and confidential. An NDA is available on request if your program requires one before you share drawings. For medical and automotive work, we often sign the NDA first and then run the DFM review, so the engineering conversation can start early without exposing the design.

Volume is flexible. There is no minimum order quantity. One prototype and a 10,000+ part run go through the same first-article process; the difference is in fixtures, tooling and inspection sampling. If the part may scale later, mention that at quote stage so the fixture design can support both.