How to Choose a CNC Metal Factory Wisely

Request the machine list before you request a price. A supplier with 27 three-axis machines and no fourth or fifth axis will quote your part anyway, then run it in three setups. Every extra setup adds a datum shift. On a bracket with angular faces, that shift shows up as a true-position error you cannot inspect your way out of.

Five-axis work is not automatically better. It pays off when a part has features on multiple faces, deep pockets with contoured floors, or short rigid tools are the only way to reach a corner without chatter. A 16-machine five-axis group running simultaneous motion can hold an angular face and its mating bore in one setup. The trade-off is cycle time on simple parts, where a three-axis machine is faster and cheaper.

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  Prototype quantitiesOne to 50 parts: three-axis plus a fourth axis for access is usually enough.
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  Multi-face featuresAngular holes, compound angles, deep contoured pockets: five-axis removes setups.
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  Long partsUp to 4,000 mm travel on our large-frame machines; check the envelope first.
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  Small tight-tolerance partsCompact travels of 500 × 500 × 450 mm give the best rigidity per cut.

A tolerance number on a website means nothing without the method behind it. ±0.005 mm is achievable on a 100 mm aluminum housing held in a temperature-stable shop with a calibrated CMM. The same claim on a 900 mm steel weldment is a different conversation, and any factory that quotes the same number for both is guessing.

Ask which features the tight tolerance applies to. On most parts, two or three dimensions carry the function and the rest are general tolerance. If a supplier treats every dimension as critical, the price climbs for no reason. If they treat none of them as critical, you find out at assembly.

Material matters too. Aluminum 6061 and 7075 cut clean and hold size well. Stainless 316L and 17-4PH work-harden, so feeds and speeds have to be right or the surface tears and the final pass moves. Titanium TC4 and Inconel need slower parameters and more tool changes, which shows up in both cost and lead time.

Surface finish follows the same rule. Ra 0.8–1.6 µm is a normal machined finish. Ra 0.2–0.8 µm usually needs a separate finishing pass or a polishing operation, and that step has to be quoted.

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  ±0.005 mmFine work on small to medium parts, verified on a CMM, not a caliper.
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  Ra 1.6–3.2 µmAs-machined finish for most functional surfaces.
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  Ra 0.8–1.6 µmStandard high-quality finish after a controlled finishing pass.
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  Ra 0.2–0.8 µmFine finish, often needs polishing or lapping after machining.

Certificates are a starting filter, not proof. ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022 each cover a different scope. An IATF certificate matters if your part goes into a vehicle program. ISO 13485 matters for medical devices. ISO 27001 covers how your drawings and data are handled, which is a separate risk from machining quality.

Then look at the inspection chain. Raw material check, in-process monitoring, and final inspection before shipment are the three points where defects get caught. A factory that only inspects at the end finds a bad batch after 400 parts are already made. In-process checks catch a drifting dimension at part 12.

Ask what happens when a dimension is out. The answer tells you more than the certificate. A shop with a clear nonconformance process will tell you how they quarantine, rework, or scrap, and how they tell you. A shop that says it never happens is telling you they do not measure.

For critical parts, request the inspection report with the shipment. On our side, every order gets 100% inspection before it ships, with reports available on request. That is a process statement, not a guarantee that a specific dimension will always land in the middle of the band.

Packaging belongs in the same conversation. A 0.005 mm bore can be ruined by a loose part in a cardboard box. Ask how finished parts are separated, protected, and labeled before you approve the shipping method.

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  Material certsAsk for the mill certificate on the heat number, not just a generic sheet.
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  First articleRequest a first-article inspection report before the full run starts.
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  In-processFrequency should be defined: every 10 parts, every hour, or after a tool change.
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  Final reportDimensional report plus visual and finish notes, shipped with the parts.

Quoted unit price is only part of the number. Setup cost, fixture cost, and programming time are real. For a 10,000-part run they disappear into the piece price. For a 5-part prototype they dominate it, and a supplier who hides them in a low unit price will recover them somewhere else.

Tooling ownership is worth clarifying. If the factory builds a fixture for your part, who keeps it, and can you move it to a second source later? A factory that owns the fixture controls your lead time. A factory that documents the fixture and shares the drawing gives you options.

Lead time should be split into stages. Quotation and DFM feedback, material purchase, machining, finishing, and shipping are separate clocks. A 12-hour quote and a 24-hour production start mean nothing if the finishing step adds a week. Ask for the full sequence with a date on each step.

Minimum order quantity is another term to check early. If you are at prototype stage, you do not want to commit to 1,000 pieces to get a fair price. A supplier that runs from one prototype to 10,000+ parts lets you validate before you scale.

Confidentiality is a commercial term too. Uploads and drawings should be treated as confidential, and an NDA should be available on request if your design is not public. Ask how files are stored and who can open them.

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  Setup and fixtureAsk for these as separate line items so you can compare quotes fairly.
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  Volume price breaksRequest pricing at 1, 100, 1,000, and 10,000 pieces.
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  Lead time by stageGet a date for material, machining, finishing, and ship.
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  Data handlingConfirm file storage and access rules, and sign an NDA if needed.