CNC machining factory guide

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  Key takeaways
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  What to check against what it tells you
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  How a CNC machining factory guide should read machine capability
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  Tolerance, finish and the inspection that proves both
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  Certifications, confidentiality and what each one covers
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  MOQ, lead time and how the quote is built
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  Finishing, secondary operations and where quality slips
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  Seven checks before you place the order

Machine count is the easiest number to publish and the least useful. A shop with 127 high-precision CNC machines can still struggle with a 600 mm 5-axis housing if none of those machines has the travel. Read the list by geometry instead. Ask which machine will run your part, what its travel is, and how many setups it needs.

Setup count drives cost and error. Every refixture adds a datum, and every added datum adds stack-up. A part that fits a 5-axis center in one setup behaves differently from the same part spread across three 3-axis operations. If the shop cannot tell you the setup plan at quote stage, the tolerance number is guesswork.

Travel data narrows the field fast. Work up to 4,000 × 400 × 150 mm suits long extrusions, rails and frame members. Medium envelopes around 750 × 1,150 × 550 mm and 600 × 600 × 600 mm cover most enclosures and plates. Compact envelopes near 500 × 500 × 450 mm and 500 × 310 × 200 mm handle small precision parts where spindle speed matters more than size.

Mill-turn and rotary capability decide the round parts. A Ø400 mm rotary table lets the shop cut features on multiple faces without releasing the part. Mill-turn centers handle shafts and fittings that would otherwise need a lathe plus a mill plus a fixture. When a factory lists 16 mill-turn centers, that tells you turning work stays in-house.

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  Ask for the setup planOne setup on 5-axis is worth more than three on 3-axis.
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  Match travel to part envelopeAdd stock and fixture height, not just finished size.
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  Check turn capacityShafts and threaded fittings need lathe or mill-turn hours.

±0.005 mm is a real number on the right feature, on the right machine, with the right fixture. It is not a shop-wide promise. On a 20 mm bore in aluminium held in a collet, it is routine. On a 900 mm frame rail with thin walls, thermal drift and clamping distortion will eat that budget before the cutter touches metal. Ask which features the shop expects to hold at ±0.005 mm and which ones it will quote looser.

Surface finish follows the same logic. Ra 0.2–0.8 μm needs a fine finishing pass, sharp tooling and often a dedicated operation. Ra 0.8–1.6 μm is a normal turned or milled finish on most materials. Ra 1.6–3.2 μm is as-machined and perfectly acceptable on non-sealing, non-sliding faces. Specifying fine finish everywhere adds cycle time for no functional gain.

Inspection is where a factory shows its hand. 100% inspection before shipment, with raw material check, in-process monitoring and final inspection, is the baseline. Reports on request are normal. What matters is the instrument. A CMM report on a critical bore is evidence. A caliper reading written on a packing slip is not.

Material choice interacts with all of this. Aluminium 6061-T6 and 7075 cut cleanly and hold tight tolerances. Stainless 316L and 17-4PH move more and need slower feeds. Titanium TC4 and Inconel hold strength at temperature but punish small tools. If your print calls ±0.005 mm in Inconel, expect the shop to push back, and take that as a good sign.

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  Tolerance per featureGlobal tolerance callouts hide the hard features.
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  Finish only where neededSealing and sliding faces yes, cosmetic faces no.
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  Instrument, not just reportCMM data beats a number on a slip.

Certifications are a filter, not a ranking. ISO 9001:2015 covers general quality management and is the floor for any serious factory. IATF 16949:2016 is the automotive standard and matters if your part ends up in a vehicle program with PPAP expectations. ISO 13485:2016 is the medical device standard and points to traceability and process control that device makers need. ISO 27001:2022 covers information security, which is what you want when your CAD files are the product.

Match the certificate to the industry you sell into, not to the longest list. A factory with all four is useful if your program spans automotive and medical, but the daily question is whether the relevant system is actually running. Ask for the certificate scope and expiry date, and ask who signs the inspection records.

Confidentiality is a separate concern from quality. Uploads should be secure and confidential by default, and a non-disclosure agreement should be available on request rather than treated as a special favor. If your design is the competitive advantage, get the NDA in place before you release CAD.

Traceability matters more than most buyers expect. Material certificates, heat lot numbers and inspection records should be retrievable by part number months after shipment. If the factory cannot describe how it stores that data, assume it does not.

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  Ask for scope and expiryA named certificate with a date is a working system.
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  NDA before CAD releaseAvailable on request, not negotiated from scratch.
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  Traceability by part numberMaterial and inspection records must be retrievable.

MOQ is the fastest way to tell who a factory is set up to serve. No minimum order quantity, from one prototype to 10,000+ part runs, means the shop has a quoting and setup process that tolerates small jobs. A high minimum usually means the opposite: the floor is optimized for long runs and your prototype will wait behind production orders.

Lead time only means something with a start point. Quotation and free DFM analysis within 12 hours is a response commitment. Production starting within 24 hours is a scheduling commitment. Parts shipping in 3–5 days is a delivery window. Ask which of those three you are being promised, because they fail for different reasons.

Late delivery risk is worth asking about directly. A historical late-delivery probability below 2% is a number you can hold a supplier to. If a factory will not discuss past performance, plan a buffer into your build schedule.

Read the quote line by line. Material grade, stock size, setup hours, machine hours, finishing and inspection should be visible or at least explainable. A single lump-sum price hides where the cost sits, which makes it hard to decide whether a design change is worth it. DFM comments at quote stage are the clearest signal that an engineer, not a salesperson, looked at your file.

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  Prototype plus productionOne shop for both avoids a second qualification cycle.
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  Pin down the promiseQuote, start of production and shipment are three dates.
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  Ask for DFM notesComments on thin walls and deep pockets save money later.

Finishing is where tight-tolerance parts most often come back wrong. Anodizing adds a coating that changes dimensions on tight bores and threads. Hardcoat anodizing is worse. Electroless nickel and plating add their own thickness. If a bore is held at ±0.005 mm, the shop must mask it or compensate the pre-plate size. Ask which one they do.

Bead blasting, tumbling, brushing and polishing change surface texture and can round a sharp edge you specified. Laser marking and engraving need a minimum character height of 1.5 mm to stay legible on a machined face, and marking on a sealing surface is a leak path waiting to happen.

Powder coating and black oxide are cosmetic or corrosion barriers, not dimension control. Keep them off mating faces unless the print says otherwise. The same applies to colour anodizing, which is a dye process with batch-to-batch variation.

Subcontracted finishing is the common failure point. If the factory sends parts out for anodizing, it loses control of schedule and of handling damage. Ask whether finishing is done in-house or managed through a qualified partner, and who inspects the parts on return.

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  Mask tight boresCoating thickness will close a ±0.005 mm fit.
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  Keep marking off seals1.5 mm minimum character height, non-sealing faces.
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  Ask who finishesIn-house control beats a subcontracted chain.