How to choose a precision CNC machining factory

• 1
  What matters most before you send an RFQ
• 2
  Judging a precision CNC machining factory by part type
• 3
  What ±0.005 mm really requires from a precision CNC machining factory
• 4
  Machine mix, travel and the 4,000 mm ceiling
• 5
  Material range and finish options that change the quote
• 6
  Certifications, inspection and what they cover
• 7
  Lead time, MOQ and confidentiality
• 8
  Seven checks to run on a precision CNC machining factory

A tolerance callout is a claim about the whole process, not just the machine. To hold ±0.005 mm, the factory needs a controlled room temperature, a spindle that repeats, and a setup that does not move the part between operations. Fixture design usually decides the outcome. A perfect machine with a loose vise still produces a part that drifts.

This is where axis count matters. A 5-axis center tilts the tool to the feature, so angled holes, undercuts and contoured pockets come off in one setup. Each re-fixture adds a new datum and a new stack-up. On a part with six toleranced features, moving from four setups to one often removes more error than any machine upgrade.

The limit is not the axis count. Thin walls, deep slots and tall ribs deflect under cutting force. A 0.8 mm wall in aluminium will move even on a good machine unless the programmer lightens the radial depth of cut and supports the wall. Ask how they control chatter before you ask about spindle speed.

Finish follows the same logic. Ra 1.6–3.2 μm is normal as-machined output. Ra 0.8–1.6 μm needs a finishing pass and a sharp tool. Ra 0.2–0.8 μm usually means a separate finishing operation, and it should be quoted as one.

• 1
  Datum strategyAsk which face is the primary datum and how many times the part is re-chucked.
• 2
  In-process checksCMM or optical checks between operations catch drift before the part is finished.
• 3
  Material behavior7075 and 17-4PH move differently than 6061; the program should reflect that.

A factory's machine list tells you what it can quote without subcontracting. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants in Dongguan and Singapore, covering 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix is not accidental. It lets the shop route a part to the cheapest machine that still holds the tolerance.

Travel sets the physical limit. Maximum processing size reaches 4,000 mm, with large travels of 4,000 × 400 × 150 mm, medium platforms of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact sizes of 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table handles the round work. If your part falls outside these envelopes, the factory either subcontracted it or declined it.

For turned parts, mill-turn centers matter more than lathe count. Combining turning and milling in one chucking removes the concentricity error that appears when a shaft moves to a second machine. It also shortens the route, which shows up as a lower price and a shorter lead time.

Ask for the specific machine assigned to your part. A quote that names the axis count and the work envelope is a quote you can audit. A quote that only says "CNC machining" is not.

Material choice drives tool wear, cycle time and scrap risk. Aluminium 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 cut fast and are the default for housings and brackets. Stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630) cut slower and work-harden; tool paths must avoid dwelling in the cut.

Steel grades 1018, 1045, 4130, 4140, 4340, A36 and tool steel cover structural and wear parts. Copper and brass families such as C101, C103, C110, beryllium copper, C27400, C28000 and C36000 machine cleanly but need sharp tooling to avoid burrs. Titanium TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B / AZ91D sit at the difficult end: low cutting speeds, high heat, and a real risk of distortion on thin sections.

Plastics are a separate discipline. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre need different feeds and often a different fixture. PEEK and carbon fibre wear tools quickly; POM and PA are dimensionally stable but can burr at edges.

Finishing is where quotes diverge most. Anodizing (clear, colour, hardcoat, conductive), electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing are all available. Laser marking and engraving is possible down to a minimum character height of 1.5 mm. Confirm whether the finish tolerance is included in the drawing or added after machining.

A certificate tells you which management system the factory runs. ISO 9001:2015 is the baseline for general machining. IATF 16949:2016 adds automotive process control and traceability. ISO 13485:2016 covers medical device quality. ISO 27001:2022 addresses information security, which matters when you upload proprietary CAD.

Inspection practice matters more than the certificate on the wall. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection. Reports are available on request. Ask for the inspection plan before the first article, not after.

A qualification rate of 99.99% is a process claim, not a marketing line. It depends on the feature mix and the tolerance band. On a simple bracket it is easy. On a 0.8 mm wall with a ±0.005 mm bore, it requires the setup discipline described above.

For regulated work, ask how non-conforming parts are handled and whether the first-article inspection report is included or quoted separately. That one question separates a factory that documents from one that improvises.

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours. Parts ship in 3–5 days. Historical late-delivery probability is below 2%. Those numbers describe a process, so treat them as the plan and confirm the schedule against your drawing when you order.

There is no minimum order quantity. One prototype and a 10,000+ part run go through the same process. That matters because it removes the re-qualification step between prototype and production, which is where tolerances usually shift.

Confidentiality is a purchasing decision, not a detail. Uploads are secure and confidential, and an NDA is available on request. If your part is patentable or under a customer agreement, request the NDA before you send STEP files.

Ask what the quote includes and excludes. Material certification, first-article report, finish and packaging are the four lines most often assumed. Get them written into the quote so the comparison between factories is apples to apples.