China's CNC prototype solutions: how to judge one before you send a PO

Most prototypes that arrive as a 3D file can be cut three ways: three-axis milling, four-axis, or simultaneous five-axis. The part geometry decides which. A flat bracket with holes on one face runs fine on a three-axis machine, and it is the cheapest route. Add features on four or five faces and you either pay for multiple setups or move to a machine that indexes the part.

Setup count is where prototype cost actually lives. Every extra fixture is another chance for a datum to shift. On a five-axis center the tool reaches the part from an angle instead of the part being re-clamped, so a housing with angled ports and a contoured pocket comes off in one operation. That matters more on a two-piece prototype than on a 10,000-part run, because you cannot amortize a bad first article across later units.

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  Three-axisPrismatic parts, one or two setups, open tolerances. Fastest to quote.
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  Four-axisCylindrical and shaft-type parts, indexed features around an axis.
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  Five-axisAngled faces, deep pockets, organic contours, single-setup accuracy.
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  Mill-turnTurned body plus milled features; avoids a second fixture.

A prototype shop in China will quote ±0.005 mm if you ask for it. The question is whether the feature needs it. General dimensions on a machined aluminum bracket usually hold ±0.05 mm without extra cost. Reserve the tight callouts for mating surfaces, bearing bores, and seal grooves. Blanket tolerances across a whole drawing push the price up and slow the first article.

Surface finish follows the same logic. As-machined aluminum lands around Ra 1.6–3.2 μm. A finishing pass takes it to Ra 0.8–1.6 μm. Below that you are into polishing or lapping, which adds a manual step and a second inspection. Write the finish on the faces that seal, slide, or get coated, and leave the rest as machined.

Material choice changes the achievable numbers more than the machine does. Aluminum 6061-T6 cuts clean and holds thin walls. Stainless 316 work-hardens and will pull if the feed is wrong. Titanium TC4 (Ti-6Al-4V) needs slower speeds and sharp tooling, so a tight corner radius costs more. Tell the shop what the part must do, not just the alloy name.

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  Ask for a DFM reviewA shop that flags an unmakeable feature before cutting saves a week.
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  Call out datumsState which faces locate the part for inspection.
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  Mark critical dimsA boxed tolerance list beats a blanket note.

Prototype work rarely needs exotic alloys, but it often needs a specific one. We machine 6061, 7075, 2024, and 6082 aluminum, plus 303, 304, 316L, 17-4PH stainless and 1018, 1045, 4140 steel. Titanium TA2 and TC4, Inconel, and magnesium AZ31B are available when the application requires them. On the plastic side, POM, PEEK, PC, ABS, and carbon fiber are common for functional mockups.

Finish is usually where a prototype earns its keep. Anodizing in clear, color, or hardcoat protects handling surfaces and hides tool marks. Electroless nickel adds wear resistance on sliding parts. Bead blasting gives a uniform matte look before anodizing. Laser marking works down to 1.5 mm character height for part numbers and traceability codes.

Pick the finish before the material when the part is cosmetic. Some coatings shift dimensions by a few microns, which matters if you are holding ±0.005 mm on the same face. Say so on the drawing and the shop will mask or compensate.

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  Aluminum6061-T6, 7075, 2024, 6082, ADC12
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  Stainless303, 304, 316L, 17-4PH (SUS630)
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  Titanium and specialTA1, TA2, TC4, Inconel, AZ31B
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  PlasticsPOM, PEEK, PC, ABS, carbon fiber

A prototype is only useful if you can trust the numbers on the report. Ask what gets measured and when. A normal sequence is incoming material check, in-process monitoring on critical features, and a final inspection before packing. For a first article, request a dimensional report with the actual readings, not just a pass or fail.

Certification matters when the part feeds a regulated program. ISO 9001:2015 covers general quality systems. IATF 16949:2016 applies to automotive work. ISO 13485:2016 covers medical devices, and ISO 27001:2022 covers information security, which is what protects your CAD files. If your program requires one of these, verify the certificate scope, not just the logo.

Confidentiality is a real concern when you send a new design overseas. Ask for an NDA before releasing files. Uploads should move over an encrypted channel, and access should be limited to the engineers who quote and program the job. We sign NDAs on request and keep drawings off shared drives.

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  First article reportActual readings on the dimensions you marked critical.
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  Material certsMill certificates traceable to the heat number.
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  NDA before filesSigned before any drawing leaves your side.