China CNC Processing Service Guide

Most shops advertise tight numbers. Ask instead what they hold on your part shape. A 16-station five-axis cell cuts a prismatic block and a thin-walled housing in very different ways. On rigid fixtures, a competent shop holds ±0.005 mm (±0.0002 in) on critical features. On a 300 mm thin wall, thermal drift and tool deflection move that number, so we flag it during DFM rather than after the first article.

Machine travel decides whether your part fits at all. Large gantry work reaches 4,000 × 400 × 150 mm. Mid-size five-axis handles 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact cells cover 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table sets the practical limit for parts that need continuous rotation. If your part crosses two of these classes, say so early. The quote changes.

Surface finish follows the same logic. As-machined faces land around Ra 1.6–3.2 μm. A high-quality cut reaches Ra 0.8–1.6 μm with a clean toolpath and a stable setup. Fine finishing down to Ra 0.2–0.8 μm is possible, but it costs time and needs the right insert grade. Do not call out a blanket finish on every face. Mark only the sealing faces, bearing bores, and optical seats.

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  Fits the machine envelope firstConfirm travel and rotary capacity before quoting a complex part.
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  Tolerance is per featureA tight callout on one bore is normal. Tight callouts everywhere raise cost and risk.
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  Finish is per faceSpecify Ra only where function needs it.

Aluminum 6061-T6 is the default for prototypes and fixtures. It machines fast, holds threads well, and anodizes cleanly. Grade 7075 gives higher strength for structural brackets but chips harder and can stress-crack if the toolpath is rough. 2024 is strong and fatigue-resistant, though it needs a protective finish. ADC12 is a die-casting alloy, so we use it when the part will later be cast.

Stainless 303 is the free-machining choice for shafts and fittings. 304 and 316L resist corrosion but work-harden, so light radial cuts and sharp tools matter. 17-4PH (SUS630) takes heat treatment to high strength and is common in aerospace and medical hardware. On titanium TC4 (Ti-6Al-4V), heat builds at the cutting edge. Low speed, high feed, and flood coolant keep the tool alive. Inconel is slower again, and we price it that way.

Plastics behave differently from metals. POM and PA are stable and hold tolerance on small parts. PEEK handles heat and chemicals but costs more than most stainless. ABS and PC are fine for covers and housings. Carbon fibre cuts cleanly only with diamond tooling, and the dust needs extraction. Tell us the end-use temperature and load. That single answer narrows the material list fast.

Five-axis pays off on parts with angled faces, deep pockets, or features that would need three separate setups on a three-axis mill. Each setup adds a datum error and a queue. Cutting a hydraulic manifold in one cycle keeps the port positions true to each other. That is the real gain, not the axis count. If your part is a flat plate with holes, a three-axis machine does it faster and cheaper.

Mill-turn centers handle shafts with flats, slots, and cross holes. One machine turns the diameter and mills the features, so concentricity stays tight without re-chucking. Good candidates are motor shafts, valve spools, and threaded fittings. Long slender shafts still need a steady rest or tailstock. Without support, the part pushes away from the tool and the diameter drifts.

Some geometry is better cast, printed, or fabricated. A large bracket with uniform walls is often cheaper as a weldment. A complex internal channel may suit 3D printing. A hundred identical covers may suit die casting. We will say so when that is true. A shop that quotes every job as machining is not giving you an engineering answer.

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  Choose five-axis for datum consolidationFewer setups means fewer stacked tolerances.
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  Choose mill-turn for round parts with featuresConcentricity holds without re-chucking.
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  Skip both for flat, simple geometryThree-axis is faster and cheaper.

Machining leaves burrs and tool marks. Bead blasting gives a uniform matte surface and hides light marks. Tumbling deburrs small parts in bulk. Brushing and polishing produce directional or mirror finishes. Anodizing comes in clear, color, hardcoat, and conductive types. Hardcoat adds wear resistance on sliding surfaces, but it builds thickness, so mask the bores that must stay on size.

Plating options include electroless nickel, zinc, silver, and gold. Electroless nickel gives even coverage on complex shapes and good corrosion resistance. Zinc protects steel at low cost. Silver and gold are used for conductivity and RF hardware. Powder coating and black oxide round out the common choices. Laser marking handles part numbers and logos, with a minimum character height of 1.5 mm so the text stays legible.

Heat treatment should be planned before the first cut, not after. Hardening a 17-4PH part after machining can move dimensions and require a finish grind. Stress relief between roughing and finishing keeps thin parts flat. If your drawing calls for a hardness range, tell us the temper condition. That decision changes the tool path, the stock allowance, and the price.

Inspection is where a quote earns trust. Ask what is measured, on what equipment, and how often. A sound process starts with a raw material check against the mill certificate. In-process monitoring catches drift before a batch is ruined. Final inspection covers critical dimensions, and a report goes out on request. We inspect 100% of parts before shipment, not a sample.

Certifications tell you which systems are audited. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard, and it forces traceability and change control. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your files are proprietary. Match the certificate to your industry. A medical buyer needs 13485, not just 9001.

First article inspection is the cheapest risk control you have. Send a drawing with GD&T, a material callout, and the finish spec. Review the FAI report before you approve a production run. If a shop resists an FAI on a tight part, that is your answer. Uploads stay secure and confidential, and we sign an NDA on request.

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  Match certification to industryAutomotive, medical, and IT security each have their own standard.
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  Ask for the FAI reportCheck critical dimensions before approving production.
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  Confirm material traceabilityMill certificates link the part to the heat number.

Start with the engineering response, not the price. A useful quote names the process, the machine class, the stock size, and the inspection plan. It flags the features that drive cost. A one-line price with no notes tells you the shop has not looked closely. Quotation and free DFM analysis come back within 12 hours at our shop, and production can start within 24 hours once you approve.

Check the equipment list against your part. A shop with only three-axis mills cannot cut a five-sided part in one setup. A shop without a CMM cannot verify position tolerances with confidence. Ask how many machines run, how many shifts, and who programs the job. Experienced programmers save more money than a lower hourly rate ever will.

Lead time is a planning number, not a promise. Standard parts ship in 3–5 days. Complex parts with finishing and heat treatment take longer. No minimum order quantity means one prototype or a 10,000+ part run both fit. Keep the communication loop short. A supplier who answers technical questions in plain language will handle your change requests the same way.