How to Vet a Custom CNC Processing Manufacturer in China

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  Key takeaways
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  What to Verify Before You Award a Job
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  Check 1: Does the Machine List Match the Part Geometry
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  Check 2: Tolerance Claims Need a Named Feature
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  Check 3: Certifications Only Count Inside Their Scope
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  Check 4: Lead Time, MOQ and Quote Data
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  Check 5: Finishing and Materials In House
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  Step by Step: How to Qualify a Supplier in Two Weeks

Start with geometry, not price. A part with undercuts, deep pockets on five faces or a 20° compound angle cannot be made on a three-axis mill without extra fixtures, and each extra setup adds stack-up error. Ask the supplier to name the machine that will run your part. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers.

Travel matters as much as axis count. A 4,000 mm maximum processing size with 4,000 × 400 × 150 mm travel covers long structural parts such as EV battery tray rails and machine frames. Medium work sits in the 750 × 1,150 × 550 mm and 600 × 600 × 600 mm envelope, while compact parts run on 500 × 500 × 450 mm or 500 × 310 × 200 mm machines. A Ø400 mm rotary table handles round parts that need milling and drilling on multiple faces.

Here is the failure mode to watch. A supplier quotes your part, then subcontracts it to a shop you never vetted. The price looks fine and the first article may even pass. Problems show up at volume, when the second-tier shop changes a fixture or a programmer. Ask directly who cuts the metal, and ask whether the work stays inside the quoting company's own plants.

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  Ask for the machine modelNot the category. Model numbers let you check spindle speed, torque and control.
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  Count the setupsEach extra setup is another chance for datum error to enter the part.
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  Confirm the travelA part that overhangs the table cannot be held to tight flatness.

±0.005 mm is a real capability, but it is not a blanket promise across every dimension of a part. It applies to specific features on specific materials under stable thermal conditions. When a supplier states that number without asking which features matter, the number is marketing. When they ask which hole, which bore, which face, they are planning the process.

Machining tolerance and surface finish interact. A bore held to ±0.005 mm and finished to Ra 0.2–0.8 μm usually needs a separate finishing pass or a reaming operation, not a single light cut. As-machined surfaces sit at Ra 1.6–3.2 μm, and a high-finish range of Ra 0.8–1.6 μm covers most functional sealing and bearing seats. If your drawing calls for both a tight tolerance and a fine finish on the same wall, expect the quote to reflect an extra operation.

Material behavior drives the rest. Aluminium 6061-T6 and 7075 cut cleanly and hold tolerance well. Stainless 316L and 17-4PH work-harden, so heavy radial cuts and dwelling in the cut both push dimensions around. Titanium TC4 (Ti-6Al-4V) and Inconel need lower cutting speeds, more coolant and a tool path that keeps the cutter moving. Tell the supplier the material and heat treatment before they quote the tolerance.

The practical test is simple. Send one drawing with two features marked: one at ±0.005 mm and one at ±0.05 mm. See whether the reply treats them differently. A shop that quotes the same process for both is not reading the print.

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  Datum strategyAsk which datum the inspection report will reference.
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  Thermal controlTight work needs a temperature-stable measuring room, not the shop floor.
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  Gauge capabilityA gauge with 0.01 mm resolution cannot prove a ±0.005 mm result.

Four certificates appear on most Chinese machining websites. They are not interchangeable. ISO 9001:2015 is the baseline quality management system and says little about your specific part risk. IATF 16949:2016 adds automotive-specific requirements such as traceability, change control and production part approval discipline. If you buy engine brackets, inverter housings or EV structural parts, that scope matters more than the ISO 9001 logo.

ISO 13485:2016 covers medical device manufacturing. It brings documented process validation, clean handling expectations and stricter records for anything that touches a patient or a diagnostic instrument. ISO 27001:2022 covers information security, which is what protects your CAD files, drawings and production volumes from being reused or leaked.

Read the scope statement on each certificate, not the front page. A valid IATF certificate lists the site address and the product categories covered. If the address on the certificate is not the address of the plant that will machine your parts, the certificate does not apply to your order. Ask for the certificate number and check the issuing body's public register.

For regulated programs, pair the certificate with a quality agreement. State the inspection plan, the sampling rate, the record retention period and the change notification window in writing. That document does more work during an audit than the certificate on the wall.

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  Match scope to riskAutomotive, medical and IT security risks need different certificates.
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  Check the site addressCertificates are issued per site, not per company group.
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  Ask about change notificationProcess or supplier changes should reach you before parts ship.

Compare lead times on the same basis. Ask three questions: when does the quote arrive, when does production start, and when do parts ship. GreatLight returns a quotation and free DFM analysis within 12 hours, can start production within 24 hours of approval, and ships parts in 3–5 days. If a supplier quotes 30 days for a simple aluminium bracket, either their queue is full or the work is going out the door.

MOQ is the next filter. No minimum order quantity means you can run one prototype, check the fit and finish, then scale to 10,000+ part runs on the same process. That removes the tooling decision from your prototype budget, which matters when a design is still moving. Ask whether the prototype and the production run use the same machine class. If the prototype comes off a 5-axis center and production moves to a three-axis line, expect a dimensional shift.

Then look at what the quote includes. A useful quote lists material grade and condition, machine type, machining time, finishing steps, inspection scope and packaging. A single lump-sum number hides the assumptions, and those assumptions become change orders later. Ask for the material certificate with the first article so you can confirm the grade that was actually cut.

Price per part falls with volume, but not in a straight line. Setup, programming and fixturing spread across the run. When you push for a lower unit price on a 50-piece order, you are usually asking the shop to skip an operation, not to work smarter.

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  Quote basisAsk what machining time and setup count the price assumes.
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  Prototype to productionConfirm the same machine class runs both stages.
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  PackagingSpecify surface protection for cosmetic and anodized parts.

A part is not finished when the spindle stops. Anodizing, plating, powder coating, black oxide, bead blasting, polishing and laser marking all change dimensions or appearance, and some of them change both. Hardcoat anodizing builds a layer that can move a tight bore out of tolerance if the masking and allowance are not planned. Ask who controls that allowance.

Material coverage tells you how wide the shop's experience runs. A supplier that stocks or routinely cuts 6061, 7075, 304, 316L, 17-4PH, 4140, TC4, Inconel, C36000 brass, POM, PEEK and carbon fibre will not learn on your job. A shop that has only cut aluminium will struggle with the tool wear and chatter behavior of Inconel, and your first article will show it.

Laser marking has a hard limit worth knowing early. The minimum character height is 1.5 mm. If your drawing needs a 0.8 mm serial number, plan for a different marking method or accept a larger data field. Small details like this cause most of the back-and-forth during first article review.

For cosmetic parts, agree on a visual standard before production. Send a sample or a photo with the areas that matter marked. Bead blasting and brushing hide tool marks, but they also change gloss, and two batches can look different under factory lighting.

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  Anodizing allowanceHardcoat builds thickness. Plan the pre-plate dimension.
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  Material certificatesAsk for the mill cert with the first article.
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  Marking heightLaser marking needs at least 1.5 mm character height.