CNC Machining Factory Service: What to Check Before You Order

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  Key takeaways
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  What to check against part type
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  Tolerance: what a cnc machining factory service can hold
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  Inspection: how to verify the parts you receive
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  Lead time, MOQ and quoting: the numbers to compare
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  Certifications and industries: matching scope to your program
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  Capability and material range: reading a shop's real limits
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  7 steps to vet a cnc machining factory service

Every supplier will quote a tolerance number. The useful question is which features that number applies to. A shop running 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, can hold ±0.005 mm (±0.0002 in) on a bored hole or a milled mating face when the setup is rigid and the tool is fresh. That same shop will not hold ±0.005 mm across a 900 mm weldment that moves after clamping.

Split your drawing into critical and non-critical features before you ask for a quote. Critical: bearing bores, seal grooves, dowel holes, faces that mate with another part. Non-critical: clearance holes, outer profiles, non-functional pockets. Send both lists. A factory that only sees a general tolerance block will price the whole part to the tightest callout, and you pay for precision you never measure.

Material changes the answer too. Aluminum 6061 and 7075 cut clean and hold tight tolerances well. Stainless 316L and 17-4PH work-harden and pull more, so deep bores need more passes. Titanium TC4 (Ti-6Al-4V) and Inconel move during and after cutting. For those, plan a stress-relief step or accept a wider tolerance on long features.

Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal machined finish for most mating surfaces. Ra 0.2–0.8 μm needs a finishing pass, a smaller stepover, or a different tool, and it adds cycle time. Ra 1.6–3.2 μm is fine for brackets and covers. Put the finish callout only on the faces that need it.

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  Critical features firstList bores, grooves and mating faces separately from general tolerance.
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  Match material to toleranceAluminum holds tight numbers more easily than titanium or Inconel.
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  Finish is a cost driverRa 0.2–0.8 μm needs extra passes. Use it only where it matters.

A factory that checks 100% of parts before shipment is doing something different from one that samples. The sequence matters: raw material check, in-process monitoring, final inspection. Raw material check confirms the grade you ordered, not a substitute. In-process monitoring catches a drifting dimension before the whole batch is cut. Final inspection is the last gate.

Ask what measuring equipment sits on the floor. Calipers and micrometers handle most dimensions. CMM work is what you want for position tolerances, true position on hole patterns, and profile callouts. If your part has a GD&T frame with a position tolerance, a caliper cannot verify it. Confirm the factory can measure what you specified.

Reports are the practical part. Request dimensional reports on critical features, material certificates, and finish certificates when plating or anodizing is involved. Not every job needs a full report package. High-volume brackets may need nothing beyond a first-article check. Aerospace and medical parts usually need the full set. Say which one you want at quote time so it is priced in.

A qualification rate of 99.99% sounds like a marketing line until you ask how it is tracked. The answer should be a per-batch record, not a yearly average. Ask for the inspection plan for your part and the acceptance criteria on each feature. If the answer is vague, that is the answer.

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  Three inspection stagesMaterial, in-process, final. Ask how each one is recorded.
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  Measure what you call outPosition and profile tolerances need a CMM, not a caliper.
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  Name your report packageFirst article only, or full dimensional and material certs.

Quotes are hard to compare because every factory quotes a different scope. Normalize it. Ask for a quote plus DFM analysis within a stated window. A 12-hour turnaround on quote and free DFM analysis is useful because it surfaces manufacturability problems before you commit. If the DFM notes say a wall is too thin or a pocket needs a different tool, you can change the model while it is still cheap.

Lead time splits into two clocks: time to start and time to ship. A factory that can start production within 24 hours of a released PO is set up for fast turns. Parts shipping in 3–5 days covers most prototype and small-run work. For large parts, add fixture build time, and for finishing, add the plating or anodizing queue. Ask which steps are in-house and which are subcontracted. Subcontracted finishing is where delivery dates usually slip.

MOQ decides how much risk you carry. No minimum order quantity means you can order one prototype, check it, then scale to 10,000+ parts without changing suppliers. That matters when the design is not frozen. If a factory insists on a 500-piece minimum for a part you have never held, you are buying inventory, not information.

Price per part drops with volume, but the drop is not linear. Setup, programming and fixturing are fixed costs spread across the batch. A 10-piece order carries most of that cost per unit. A 5,000-piece order spreads it thin. If you need a cost curve, ask for pricing at three quantities so you can see where the knee is.

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  Quote two clocksTime to start production and time to ship are different numbers.
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  Ask what is subcontractedExternal finishing is the usual source of delay.
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  Price at three quantitiesOne number hides the setup cost curve.

Certifications are a filter, not a score. ISO 9001:2015 covers general quality management and applies to almost any industrial buyer. IATF 16949:2016 is the automotive standard and matters if your parts go into a vehicle program with PPAP expectations. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security, which matters when you send drawings and CAD files to an outside shop.

Match the certificate to the audit you will face. A medical device customer will ask about process validation, traceability and change control. An automotive customer will ask about control plans and PPAP. A robotics or industrial machinery customer usually cares about dimensional reports and repeatability. Buying from a factory that already works inside your regulatory frame saves months of documentation.

Industry experience shows up in small decisions. Aerospace work needs material traceability and often tighter inspection. Automotive and EV work needs process control across long runs. Medical devices need clean handling and documented change control. Electronics and new energy work often involves copper, beryllium copper and heat management features. None of this is visible on a certificate, so ask what parts the factory runs regularly.

Data handling deserves its own check. Uploads are secure and confidential, and an NDA is available on request. If your drawings are controlled, sign the NDA before you send files, not after. Ask where files are stored and who can open them. A factory with ISO 27001 has a documented answer.

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  Pick the matching standardISO 9001 for general, IATF 16949 for auto, ISO 13485 for medical.
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  Ask for part examplesCertificate scope does not prove experience in your part family.
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  NDA before filesSign first, then upload. Ask who can access the data.

Machine count is a starting point, not a capability statement. A factory with 127 high-precision CNC machines, 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers can route a part to the right machine instead of forcing it onto one. That routing decision is where cost and tolerance are won or lost.

Size limits decide feasibility. Maximum processing size is 4,000 mm, with travels like 4,000 × 400 × 150 mm for long parts and 750 × 1,150 × 550 mm or 600 × 600 × 600 mm for medium work. Compact work runs on 500 × 500 × 450 mm and 500 × 310 × 200 mm travels. A Ø400 mm rotary table handles round parts that need features on multiple faces without re-fixturing.

Material range tells you how many suppliers you need. Aluminum grades from 6061-T6 to 7075 and ADC12, stainless including 303, 316L, 17-4PH and 440C, steels from 1018 to 4340 and tool steel, copper and brass including C36000 and beryllium copper, plus titanium TA1, TA2, TC4, Inconel and magnesium AZ31B or AZ91D. Plastics run from ABS and POM to PEEK and carbon fibre.

Finishing keeps the part in one supply chain. Anodizing in clear, color, hardcoat or conductive; electroless nickel, zinc, silver and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing and polishing; laser marking with a minimum character height of 1.5 mm. When these sit behind the same quality system as the machining, the finish spec does not get lost in a handoff.

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  Routing beats machine count5-axis, 4-axis and mill-turn centers solve different geometry problems.
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  Check your part envelopeCompare the largest dimension to the 4,000 mm maximum before quoting.
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  One supplier for finishKeeping finishing in-house removes a handoff and a delay.