Metal CNC Processing Service: How to Pick the Right Shop

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  Key takeaways
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  What to check before you award a metal CNC processing service
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  Start with the tolerance the metal CNC processing service can hold
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  Surface finish and why Ra 0.2–0.8 μm costs more
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  Material choice drives machining strategy
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  Inspection evidence is what you actually buy
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  Lead time, MOQ and the prototype-to-production gap
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  How to qualify a shop in seven steps

Every RFQ should begin with the tightest tolerance on the drawing and the feature that carries it. A bearing bore at Ø20 H7 (±0.021 mm) and a mounting face flat to 0.05 mm are different problems. One needs a boring cycle with a warm spindle; the other needs a stable fixture. When a shop says it holds ±0.005 mm, ask where. That number is realistic on a 5-axis machining center with a Ø400 mm rotary table and a short tool holder. It is not realistic on the same part clamped once in a three-jaw chuck with 200 mm of tool overhang.

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  Tolerance is per featureA blanket ±0.005 mm on every dimension raises cost with no functional gain. Mark only the features that matter.
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  Datums decide repeatabilityState A-B-C datums clearly. Shops that re-clamp across three setups lose stack-up accuracy.
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  Thin walls moveBelow 1.0 mm wall thickness in aluminium, expect spring passes and possibly a stress-relief step.

Finish is where quotes diverge fastest. As-machined surfaces land at Ra 1.6–3.2 μm on most aluminium and steel work. A general high finish at Ra 0.8–1.6 μm needs controlled feed, a sharp cutter and often a finishing pass with a smaller stepover. Fine finish at Ra 0.2–0.8 μm is a different job: it usually means a dedicated finishing operation, sometimes polishing, and it is hard to hold inside deep pockets where the tool chatters.

Tell the shop where the finish matters. A sealing face, a sliding surface or an optical mount may genuinely need Ra 0.4 μm. A bracket that bolts to a frame does not. If you spec fine finish across the whole part, you pay for it everywhere and you still may not get the one surface that matters.

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  Anodizing changes sizeType II clear anodizing adds roughly 5–10 μm per surface. Account for it on tight fits.
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  Blasting hides tool marksBead blasting gives a uniform matte look but can round sharp edges and mask micro-cracks.
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  Laser marking needs roomMinimum character height is 1.5 mm for legible marking on machined surfaces.

Aluminium 6061-T6 cuts fast and holds tolerance well, which makes it the default for prototypes and fixtures. 7075 is stronger but more prone to distortion after heavy material removal; rough it, let it rest, then finish. Stainless 304 work-hardens if the cutter rubs, so feeds should stay aggressive enough to cut under the hardened layer. 17-4PH (SUS630) machines well in the solution-treated condition and is often aged after machining.

Titanium TC4 (Ti-6Al-4V) and Inconel need lower cutting speeds, more coolant and rigid tooling. Inconel in particular punishes light, fast passes. Copper and brass families machine easily but burr heavily, so deburring is part of the process, not an afterthought. Magnesium AZ31B and AZ91D cut cleanly but require chip control because fine magnesium chips are a fire risk.

The practical question for a buyer is stock. A metal CNC processing service that keeps common grades on the floor can start production within 24 hours. A shop that orders every grade per job adds days before the first chip.

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  Ask for the grade list6061, 7075, 304, 316L, 17-4PH, TC4 and Inconel should be normal, not special orders.
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  Stress relief matters on thin partsOn long, thin aluminium plates, a rough-and-rest cycle prevents bowing after the last cut.

A machine can produce a good part by accident. Inspection is how the shop knows it is good. A workable flow is raw material check on arrival, in-process monitoring on critical dimensions, and final inspection before shipment. That final step should cover 100% of parts, not a sample, when the part is functional.

Ask what the report contains: which dimensions, which instrument, which datum setup. A CMM report on the first article plus a dimensional report on the production lot is normal for tight work. For medical and automotive programs, the paperwork matters as much as the part, which is why ISO 13485:2016 and IATF 16949:2016 scopes are worth checking on the certificate itself.

A shop reporting a 99.99% qualification rate is telling you it tracks first-pass yield. Ask how that is measured. If the answer is vague, treat the number as marketing.

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  First article is not enoughTool wear shifts dimensions over a run. In-process checks catch the drift.
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  Reports on requestSay in the PO that you want dimensional reports. It is easier to ask upfront than to chase later.

Lead time breaks into three parts: quoting, material, and machining. A quotation and free DFM analysis within 12 hours is a reasonable benchmark for a well-staffed shop. Production can start within 24 hours when material is on the shelf. Parts ship in 3–5 days for typical runs. The historical late-delivery probability below 2% is the kind of figure you can ask to see evidence for.

MOQ is the other half of the story. No minimum order quantity, from one prototype to 10,000+ part runs, means the shop can quote a single part without a penalty structure. That matters because the prototype phase is where design changes land. A shop that forces a 500-piece minimum on a first article will slow your program down.

Watch the transition point. A process tuned for one-off prototypes is not the same as one tuned for 10,000 pieces. Ask how the shop handles fixture design and tool life when the order scales.

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  Quote speed is a signalA 12-hour turnaround usually means the shop has a DFM engineer reading drawings, not just a sales desk.
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  Scaling changes the planSoft jaws work for ten parts. Steel fixtures and preset tooling make sense past a few hundred.