China Custom CNC Machining Service: What Engineers Should Check

Send a 3D CAD file or a 2D drawing with a defined datum scheme. The shop reviews it for wall thickness, tool reach, thread depth and surfaces the cutter cannot reach in one setup. That review is a DFM pass, and it is where most cost is removed or added. A good supplier comes back with specific questions instead of a silent yes.

Then comes process planning. A part may be roughed on a three-axis machine, finished on a five-axis center, and turned on a mill-turn lathe to keep concentricity between a bore and an outer diameter. Fixture design matters more than spindle speed on thin walls. Clamp the part the wrong way and a 0.1 mm wall will move before the cutter touches it.

Material sourcing sits before all of that. Bar stock, plate and castings arrive with mill certificates that name heat number, chemistry and mechanical properties. If your drawing or your industry requires traceability, ask for those documents with the quote. They are routine paperwork for an ISO 9001 shop, not a special favor.

Finishing and inspection close the loop. Anodizing, plating, powder coating or bead blasting change dimensions by a few microns, so the shop decides which features to mask and which to machine after coating. Final inspection reports travel with the parts when you ask for them.

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  Datums firstA drawing without clear datums forces the shop to guess, and guessing costs you tolerance.
• 2
  Wall thicknessBelow 1 mm on aluminum, expect to discuss support and cut depth.
• 3
  Thread calloutsGive thread class and depth, not just a nominal size.
• 4
  CertificatesAsk for the mill cert list at quote time, not after shipment.

Five-axis machining pays off when a part has free-form surfaces, deep pockets on several faces, or tight angular relationships that would otherwise need three or four separate setups. An impeller, a turbine blade or a housing with angled ports fits this group. Each additional setup adds fixture error, and those errors stack up against your tolerance budget.

A three-axis mill still handles the majority of prismatic parts: brackets, plates, manifolds, covers. If every feature can be reached from one or two directions, five-axis adds cost without adding precision. We keep 27 three-axis machines for exactly this reason.

Turning and mill-turn cover shafts, bushings, fittings and parts that need a bore and an outer diameter to stay concentric. Mill-turn centers cut both in one program, which removes a re-chuck step. For rotary work up to Ø400 mm, a rotary table on a mill is often enough.

Size decides the rest. Our largest travel is 4,000 × 400 × 150 mm for long, slim parts. Medium work covers 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Small precision parts run on 500 × 500 × 450 mm and 500 × 310 × 200 mm machines, where thermal drift over a short cycle is easier to control.

• 1
  Free-form or multi-faceChoose five-axis; fewer setups means fewer stacked errors.
• 2
  Prismatic and simpleThree-axis is faster and cheaper for the same result.
• 3
  Concentric boresMill-turn or a lathe with a sub-spindle beats two separate ops.
• 4
  Long and thinCheck travel and support before quoting the 4,000 mm class.

We hold ±0.005 mm on features that need it, which is ±0.0002 in. That number applies to a defined feature under stable temperature, not to every dimension on the print. Applying it everywhere multiplies inspection time and rejects good parts for no functional reason.

Surface finish follows the same logic. As-machined surfaces sit in the Ra 1.6–3.2 μm band. A good finish from a finishing pass reaches Ra 0.8–1.6 μm. Fine finishing at Ra 0.2–0.8 μm is for sealing faces, optical seats and bearing journals. Each step down adds cycle time.

Material choice drives tooling and speed. Aluminum 6061 and 7075 cut fast and hold thin walls well. Stainless 304 and 316 work-harden, so lightradial cuts and sharp tools matter. Titanium TC4 (Ti-6Al-4V) and Inconel need slow speeds, rigid setups and more tool wear, which shows up in the price.

Hardened or difficult alloys should be machined before heat treatment where the geometry allows it. When a part must be ground or EDM after hardening, plan that into the routing from the start. Adding it later usually means a new fixture and a longer lead time.

Every order gets a raw material check, in-process monitoring and a final inspection before shipment. That is our baseline, not an upgrade. What changes between projects is how much of it is documented and which instruments are used.

For a first article, ask for a First Article Inspection report that lists every dimension on the drawing with the measured value next to the nominal and the tolerance. For running production, statistical process control charts on the critical features show whether the process is drifting or stable. Both are available on request.

Metrology equipment includes coordinate measuring machines, optical comparators, surface roughness testers and height gauges. A CMM report is the usual evidence for position and profile tolerances. Surface roughness testers cover the Ra callouts. Height gauges handle simple step dimensions faster than a full CMM program.

Our qualification rate on shipped parts is 99.99%, and inspection covers 100% of parts before they leave. Those numbers only mean something if the inspection plan matches your drawing. Send the drawing with the critical dimensions marked, and the plan follows from there.