China CNC Milling Service Guide

A milling quote is only meaningful once you know which spindle will cut the part. A 3-axis vertical mill handles prismatic work: pockets, slots, flat faces, bolt patterns, all approached from one direction. Add a fourth axis and you can index the part to four sides without a second setup, which matters when hole positions on two faces must hold a tight relationship. Five-axis simultaneous motion goes further. It tilts the tool or the table while cutting, so undercut walls, deep cavities and compound angles come off in one operation.

What a shop actually owns tells you what it can quote well. A supplier with 127 high-precision CNC machines spread across 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers can route a job to the right spindle instead of forcing it onto whatever is free. That routing decision is where cost and accuracy are won or lost.

Size drives the choice too. The largest travel here reaches 4,000 mm, with work envelopes down through 750 × 1,150 × 550 mm, 600 × 600 × 600 mm, and compact 500 × 500 × 450 mm and 500 × 310 × 200 mm machines. A Ø400 mm rotary table covers round and index work. If your part fits a compact envelope, you should not be paying for a large gantry.

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  3-axisFlat plates, covers, brackets, housings open from one side.
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  4-axisShafts, manifolds and parts needing indexed features on multiple faces.
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  5-axisImpellers, medical instruments, aerospace brackets, organic shapes.
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  Mill-turnParts combining turned diameters and milled features in one cycle.

A general milling tolerance of ±0.005 mm is achievable on critical features when the setup, tooling and temperature are controlled. It is not a blanket number that applies to every dimension on a drawing. Datum structure, feature size and material all shift what a shop can hold. On a 300 mm aluminum plate, thermal expansion alone moves the part more than 0.005 mm across a normal shop day.

Surface finish is the same story. As-machined surfaces typically land at Ra 1.6–3.2 μm. A high-finish cut reaches Ra 0.8–1.6 μm. Fine finishing with a small stepover and sharp tooling can reach Ra 0.2–0.8 μm on the right features. If your drawing calls for a mirror finish over a large area, expect a secondary operation rather than a single pass.

The practical move is to tolerance only what functions. Mark the features that mate, seal, slide or locate. Leave the rest at general tolerance. This lowers cost and reduces the chance that a shop spends cycle time chasing a number that does not matter. Every tight call should have a reason on the drawing.

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  Critical featuresCall out ±0.005 mm only where fit or function depends on it.
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  General surfacesLeave at standard tolerance; they rarely need more.
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  Fine finishRa 0.2–0.8 μm on seals and sliding faces, not whole parts.

Aluminum is the default for prototypes and housings. Grades like 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 cut fast and hold good finish. The trap is thin walls. A 1 mm aluminum wall will deflect under cutting force and chatter. Adding a temporary rib or leaving stock for a finishing pass usually solves it.

Stainless steel is where feeds and speeds decide the outcome. Grades 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630) all work harden. A tool that rubs instead of cutting raises surface hardness and kills the next pass. Rigid setups and constant feed keep the cut under the work-hardened layer.

Steel, titanium and nickel alloys demand more. Grades 1018, 1045, 4130, 4140, 4340, A36 and tool steel are common. TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B / AZ91D need sharp tooling, lower cutting speeds and often more coolant. Copper and brass grades such as C101, C103, C110, beryllium copper, C27400, C28000 and C36000 machine cleanly but can be gummy at high speed. Plastics including ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre need sharp tools and care with clamping pressure.

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  AluminumFast, good finish, watch thin walls and chatter.
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  StainlessWork hardens; never let the tool rub.
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  Titanium and InconelLow speeds, rigid setup, generous coolant.
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  PlasticsSharp tools, light clamping, control heat.

Certificates are a starting point, not proof. A shop holding ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022 has documented systems for quality, automotive, medical and information security. Ask which certificate applies to the plant that will run your parts. A group certificate does not always cover every site.

Inspection is the stronger signal. Look for raw material check, in-process monitoring and final inspection, with reports available on request. A shop that inspects 100% of parts before shipment and can send dimensional reports on request is easier to work with than one that only promises quality. Ask what measurement equipment sits on the floor, and whether it is calibrated.

The commercial terms matter as much as the machines. No minimum order quantity means a single prototype and a 10,000-part run can both be quoted. Quotation and free DFM analysis within 12 hours, production starting within 24 hours, and parts shipping in 3–5 days set a realistic rhythm for most projects. Historical late-delivery probability below 2% is a useful benchmark to compare against. Secure, confidential uploads and an NDA on request protect your drawings.

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  CertificatesConfirm which plant and scope they cover.
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  InspectionRaw material, in-process, final; reports on request.
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  TermsNo MOQ, 12-hour quote, NDA available.