CNC Parts From China: A Sourcing Checklist for Engineers

• 1
  Key takeaways
• 2
  What to check, what good looks like, what to ask
• 3
  Tolerance and surface finish: what the number actually costs
• 4
  Certifications that matter for CNC parts from China
• 5
  MOQ, volume and the prototype-to-production path
• 6
  Lead time, quoting and communication habits
• 7
  Traps that slip past a low price
• 8
  Seven steps to qualify a supplier

Every quote carries a tolerance number. The question is whether the shop can hold it across the whole batch, not on the first part. A ±0.005 mm callout on a thin-wall aluminium housing is a different job from the same callout on a 40 mm steel shaft. Wall thickness, part rigidity and the number of setups decide the outcome more than the machine's spec sheet.

Surface finish moves the price in the same way. As-machined at Ra 1.6–3.2 μm covers most brackets, plates and housings. Ra 0.8–1.6 μm needs a finishing pass and a sharper tool, and it is the normal range for sealing faces and bearing bores. Ra 0.2–0.8 μm means slower feed, careful tool holding and often a polishing step. Ask which range the quote assumes.

A practical rule: if a feature does not touch another part, do not tighten it. GD&T callouts cost money on every part in the run. Engineers who mark only the functional datums usually cut 10 to 20 percent off the machining time without losing anything the assembly needs.

Geometry matters too. A 5-axis setup reaches undercuts and angled faces in one clamping. Splitting the same part across three 3-axis setups adds two datum transfers and two chances to stack error. When a part has features on five sides, ask whether it runs on a simultaneous 5-axis center. Ours include 16 of them, with a Ø400 mm rotary table for round work.

• 1
  As-machined (Ra 1.6–3.2 μm)Brackets, covers, mounting plates.
• 2
  Fine (Ra 0.8–1.6 μm)Sealing faces, bearing bores, sliding surfaces.
• 3
  Polished (Ra 0.2–0.8 μm)Optical and sealing-critical surfaces; expect extra operations.

Certificates are not decoration. Each one tells you which management system the shop runs and which customers it already serves. ISO 9001:2015 covers general quality management and is the baseline. IATF 16949:2016 adds automotive traceability, PPAP discipline and change control. ISO 13485:2016 brings medical device documentation and process validation. ISO 27001:2022 covers information security, which matters when you upload proprietary CAD.

Match the certificate to the part, not to the brochure. A prototype bracket for an industrial machine does not need IATF paperwork. A brake component does. A surgical instrument housing does need ISO 13485, and the audit trail will be requested by your own quality team later. Buying more certification than the program needs slows the quote down.

Ask for the scope statement, not the logo. A certificate with a narrow scope may not cover the process your part needs, such as heat treatment or surface finishing done outside. The scope should name the site and the activities. If the finishing step is subcontracted, ask who does it and how it is inspected.

Then ask for the paperwork you will actually receive. For critical parts that means a first article inspection report and, in automotive programs, PPAP. For production runs it means material certificates and dimensional reports. A shop that can show you a redacted sample report in the first email is usually a shop that generates them as routine.

Minimum order quantity is the fastest way to learn who a supplier serves. A shop built around prototypes will machine one piece and quote 10,000 with the same process plan. A shop built around volume will push you toward tooling and may not want a single unit on the floor at all. Neither is wrong, but only one fits your stage.

The expensive mistake is switching suppliers between prototype and production. Tolerances, fixtures and inspection plans get rebuilt, and the first production batch becomes a second learning curve. Working with one shop from one piece to 10,000+ parts keeps the process knowledge in place. We run no minimum order quantity for exactly this reason, so a design can be validated before volume is committed.

Volume also changes the process choice. Below a few hundred parts, machining usually beats tooling on total cost and lead time. Above that, die casting or vacuum casting can win on unit price, but only if the geometry allows draft angles and the surface finish demand is moderate. Ask the supplier to quote both routes and show the crossover point rather than assuming one.

Material availability sets a quiet limit. Common alloys like 6061-T6, 304 stainless and 7075 are stocked widely. Inconel, beryllium copper and Ti-6Al-4V often need to be ordered in, which adds days before the first chip is cut. If the schedule is tight, confirm the material is on the shelf before the order is released.

Lead time is a process question. A shop that answers a quotation and returns a DFM analysis within 12 hours has already done the engineering review. A shop that needs a week to price a bracket is telling you how it will handle a drawing change at 5 pm on a Friday.

Speed after the order matters as much. Production can start within 24 hours once the drawing and material are confirmed, and parts ship in 3–5 days on standard work. That pace only holds when the drawing is complete: thread callouts, radii, tolerances and finish notes all present. Incomplete drawings are the most common cause of a stalled start, not machine capacity.

Communication style is a leading indicator. Good suppliers answer technical questions with numbers and ask about function. They will tell you when a tolerance is unnecessary, and they will flag a feature that cannot be inspected. Suppliers that only confirm price and delivery are quoting, not engineering.

Language matters less than you might expect. A written DFM note with a marked-up drawing is worth more than a fluent sales email. Ask for the feedback in writing so your own team can review it, and keep one thread per project so nothing is lost between shifts.

The first trap is a tolerance quoted without a process. If the price assumes a 3-axis setup but the drawing needs five-sided access, the shop discovers the gap after the order. The fix is simple: send the STEP file with the drawing and ask which operations the quote includes.

The second is undocumented material substitution. A 6061 part becomes a cheaper alloy with similar machinability, and the difference only shows up in anodizing color or in strength testing. Require mill certificates for the specified grade on every order, not just the first.

The third is inspection that happens only when the customer asks. A supplier that inspects at the end of the run cannot correct a drift that started on part 20. Ask when measurements are taken: raw material check, in-process monitoring and final inspection. All three should be standard, with reports on request.

The fourth is data handling. Uploads should move through a secure channel, and an NDA should be available on request before drawings are shared. If a supplier asks for CAD files over a personal messaging app, that tells you how your intellectual property will be stored.

The fifth is the delivery promise with no history behind it. Ask how often the shop ships late. A figure below 2 percent on historical orders is a process result, not a slogan, and it is a fair question to put to any supplier you are considering.