How to Evaluate a CNC Machining Service Factory

• 1
  Key takeaways
• 2
  What to check and what a good answer looks like
• 3
  Match the machine mix to your part geometry
• 4
  Test the tolerance claim against the process
• 5
  Read the inspection and documentation routine
• 6
  Map certifications to your industry
• 7
  Weigh MOQ, lead time, and quote turnaround together
• 8
  Check material and finishing coverage before you commit
• 9
  Step by step: screening a CNC machining service factory

A CNC machining service factory is only as flexible as its spindle mix. A shop with 27 three-axis machines can run simple plates all day, but it will struggle with a part that needs five faces in one setup. Ask for the count by axis type, not the total. 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.

Geometry decides the axis count. A bracket with holes on three perpendicular faces is a three-axis job if you accept two setups. The same bracket with a curved mounting pad and a tight bore-to-bore tolerance is a four-axis or five-axis job, because re-fixturing will stack error. If the factory cannot tell you which machine your part will run on, the quote is a guess.

Size matters too. Maximum processing size is 4,000 mm, with travels of 4,000 × 400 × 150 mm for long parts and 750 × 1,150 × 550 mm or 600 × 600 × 600 mm for medium work. Compact travels of 500 × 500 × 450 mm and 500 × 310 × 200 mm cover small, high-volume parts. Send the envelope with the drawing.

A rotary table of Ø400 mm tells you the fourth-axis work envelope. If your part needs to swing past that, ask how they plan to hold it. The answer should mention the fixture, not just the machine.

Every factory advertises tight tolerance. Fewer explain how they reach it. A capability of ±0.005 mm (±0.0002 in) is a process result, not a spec sheet line. It comes from a rigid setup, a controlled temperature floor, a probe on the machine, and a metrology step that repeats.

Ask two questions. Which machine will run the critical feature? How will you inspect it? If the answer is a caliper on the bench, the ±0.005 mm claim will not survive a first article. If the answer names a CMM or an in-process probe with a documented routine, the claim has a floor under it.

Surface finish follows the same logic. Ra 0.2–0.8 μm is a fine finish that usually needs a finishing pass with a small stepover or a secondary operation. Ra 0.8–1.6 μm is a typical high-quality machined finish. Ra 1.6–3.2 μm is as-machined and fine for most brackets. Tell the factory which faces need which grade instead of calling out the whole part.

Tolerance and finish drive cost more than material does. A part held at ±0.005 mm on two bores but ±0.1 mm elsewhere is cheaper than a drawing that blankets the whole part at ±0.005 mm. A good quoting engineer will flag that. A weak one will just price it.

Inspection is where a CNC machining service factory proves the parts match the model. The routine should cover three stages: raw material check on receipt, in-process monitoring during the run, and final inspection before shipment. 100% inspection before shipment is the baseline for anything with a critical feature.

Ask what a report contains. A useful first-article report lists the nominal, the actual, and the deviation for each controlled dimension. A certificate of conformance alone says the parts were made; it does not say they were measured. If you need dimensional data, say so at the RFQ stage, not after the parts ship.

For medical and automotive buyers, traceability is part of the requirement. Material certificates, heat lot numbers, and revision control need to live in the same file as the inspection data. A 99.99% qualification rate sounds good in a brochure, but it only means something if you can see how it was measured.

One more point. In-process monitoring catches drift. If a tool wears through a 500-part run, the parts made at hour six will not match the parts made at hour one. A shop that checks only the first and last part is guessing about everything in between.

Certifications are not decoration. Each one answers a different buyer question. ISO 9001:2015 covers the quality management system and applies to almost any industrial customer. IATF 16949:2016 adds automotive-specific requirements for production and service parts. ISO 13485:2016 covers medical device quality. ISO 27001:2022 covers information security, which matters when you send proprietary CAD.

Match the certificate to the program. If you build automotive brackets or engine hardware, ask for IATF 16949 and check the scope on the certificate. If you build a surgical instrument housing, ISO 13485 is the relevant one. Aerospace work usually needs a different set of controls than any of these, so ask what the shop does for first-article and material traceability on that side.

Do not accept a certificate scan without the scope and the expiry date. A certificate for a different legal entity or a different plant does not cover the parts you are buying. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022 across its three wholly-owned plants.

Information security is the one most buyers skip. If your drawings and models are your product, ISO 27001:2022 is a reasonable filter. An NDA on request is a second layer, not a replacement.

These three numbers describe how a factory actually works with you. No minimum order quantity means you can order one prototype and a 10,000+ part run under the same terms. That is a real signal, because it means the shop has a setup for small lots and a plan for volume, not just one or the other.

Quote speed tells you about the front office. A quotation and free DFM analysis within 12 hours means an engineer looks at the file and flags problems before the quote goes out. Slower quotes often come back with vague assumptions about tolerances and finishes, and you pay for those assumptions later.

Production start within 24 hours and parts shipping in 3–5 days covers standard work with released drawings and available material. It is not a promise for every part. Titanium, Inconel, and hardcoat anodizing add steps. Ask which items in your BOM fall outside the standard window.

A historical late-delivery probability below 2% is more useful than a slogan about on-time delivery. Ask how the shop tracks it. If the number comes from a system that logs promised date versus ship date, it is worth something.

Material range is a quiet filter. Aluminum 6061, 7075, and 6082 are routine. Stainless 303, 304, 316L, 17-4PH, and 440C are also routine, but 17-4PH in the H900 condition behaves differently than annealed stock. Titanium TA2 and TC4 (Ti-6Al-4V) need different feeds, and Inconel will slow any shop down. Copper alloys like C101, C110, and beryllium copper add their own tool wear.

Plastics are a separate conversation. POM and PEEK machine cleanly with sharp tooling and air blast. ABS and PC are softer and prone to burrs and melted edges. Carbon fibre needs diamond tooling and dust control. A factory that treats all plastics the same way will hand you fuzzy edges.

Finishing is where quotes differ most. Anodizing in clear, color, hardcoat, and conductive types are not interchangeable. Electroless nickel, zinc, silver, and gold plating all have different thickness controls. Powder coating and black oxide change dimensions slightly. Bead blasting, tumbling, brushing, and polishing change the surface and sometimes the edge break.

Laser marking has a floor: minimum character height 1.5 mm. If your part number is smaller than that, plan for a different marking method. Also decide the finish callouts per face. Masking a threaded hole for anodizing costs less than re-tapping it later.