CNC processing services: how to pick a supplier without guessing

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  Key takeaways
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  What to compare before you send a PO
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  What CNC processing services actually cover
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  Tolerance, surface finish and the limits of the process
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  Certifications, inspection and traceability
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  Lead time, order size and how quotes are built
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  When CNC processing services are the wrong choice
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  Step by step: qualifying a supplier for your part

CNC processing services is a broad label. In practice it means milling, turning, mill-turn work, drilling and the finishing steps that follow. A shop that only mills will send your turned shaft to another vendor, and the tolerance stack moves with it. Before you compare prices, list the operations your part needs and check that one supplier covers them.

At GreatLight the work is split across 127 high-precision CNC machines: 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters because a 5-axis center can cut an angled port in one setup, while a three-axis machine needs two fixtures and adds position error each time the part is moved.

Size decides which machine is even possible. The largest travel is 4,000 × 400 × 150 mm for long parts such as rails and extrusion profiles. Medium work runs in 750 × 1,150 × 550 mm or 600 × 600 × 600 mm envelopes, and compact parts use 500 × 500 × 450 mm or 500 × 310 × 200 mm. There is also a Ø400 mm rotary table for round parts that need indexed features.

Ask which machine will run your part before the quote is issued. If the answer is vague, the price is probably based on a general assumption rather than the drawing in front of you.

±0.005 mm (±0.0002 in) is achievable, but not everywhere on every part. Tolerance applies to specific features the shop can reach and measure. A deep bore with a 10:1 depth-to-diameter ratio will drift, and a thin wall under 1 mm will move when the clamps release. Mark the critical dimensions on the drawing instead of applying a blanket tolerance to the title block.

Surface finish follows the same logic. Ra 0.2–0.8 μm needs a fine finishing pass, sharp tooling and often a dedicated setup. Ra 0.8–1.6 μm covers most sealing faces and bearing seats. Ra 1.6–3.2 μm is the normal as-machined result and is fine for brackets, housings and covers.

Material choice sets the practical ceiling. Aluminium 6061-T6 and 7075 cut fast and hold tight tolerances well. Stainless 316L and 17-4PH work-harden, so light passes and constant feed are needed. Titanium TC4 (Ti-6Al-4V) and Inconel generate heat at the cutting edge, which shortens tool life and pushes cost up. None of this is a reason to avoid the material, but it is a reason to expect a different price and a different cycle time.

If a feature cannot be machined, say so early. A shop that reviews the model before quoting will flag it. A shop that quotes blind will find it after the first part is scrapped.

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  Mark critical featuresTolerance on the title block costs more than tolerance on three dimensions.
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  Match finish to functionRa 0.8–1.6 μm for sealing faces, Ra 1.6–3.2 μm for general parts.
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  Expect material effectsStainless and titanium cut slower than aluminium and need different tooling.

Certificates are not interchangeable. ISO 9001:2015 is the baseline quality system. IATF 16949:2016 adds automotive requirements such as PPAP discipline and change control. ISO 13485:2016 covers medical devices and brings its own documentation rules. ISO 27001:2022 is about information security, which matters if you are sending CAD files and drawings to an outside shop.

Ask which certificate applies to your project, then ask what records come with the parts. Inspection before shipment covers raw material check, in-process monitoring and final inspection. Reports are available on request. For a first article, request the dimensional report and the material certificate together so you can tie the part back to the heat number.

Confidentiality is part of sourcing, not a side issue. Uploads are handled as secure and confidential, and an NDA is available on request. If your drawings carry export-controlled or unreleased geometry, sign the NDA before the first file transfer, not after the quote.

A qualification rate of 99.99% sounds like a marketing line until you do the math. On a 10,000-piece run it means roughly one rejected part. Ask how rejects are handled, whether the shop replaces them at its own cost, and how it records the cause.

Quotation and free DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. That timeline assumes the drawing is released and the material is in stock. If the part needs a special alloy or a hardcoat anodize, add time and confirm it in writing rather than assuming the standard clock applies.

There is no minimum order quantity. A single prototype and a 10,000+ part run both go through the same shop, but they do not go through the same process. Prototypes are often machined from billet with soft jaws. Production runs move to dedicated fixtures, sometimes to die casting or vacuum casting, where the geometry allows it. Ask what changes between the two.

Quote structure is where comparisons go wrong. Two numbers can differ by 40% simply because one includes anodizing and the other does not, or because one prices 100% inspection and the other prices sampling. Ask for the quote broken into material, machining, finishing, inspection and freight. Then you can compare like with like.

Watch the historical late-delivery figure. A probability below 2% is a useful benchmark, but ask what causes the misses. Usually it is material delays or a finish step outsourced to a third party. A shop that keeps anodizing, plating and bead blasting in house removes one handoff from the schedule.

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  Break the quote downMaterial, machining, finishing, inspection and freight as separate lines.
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  Ask about the transitionWhat changes from prototype tooling to production fixtures.
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  Check the finishing chainIn-house anodizing and plating remove an outside handoff.

CNC machining is subtractive and priced by cycle time. A part with a large hollow volume and thin walls can take longer to cut than to cast. If the annual volume is above roughly 5,000 pieces and the geometry is stable, die casting or vacuum casting usually wins on unit cost. CNC still makes the tooling and the first articles.

Very simple parts are also worth a second look. A flat plate with four holes and a bend may be faster and cheaper as sheet metal fabrication. A bracket with no tight tolerance may be a better fit for 3D printing in the prototype stage, especially if the design is still changing weekly.

Plastics change the calculation too. POM, PEEK, ABS and PC all machine well, but a part with a deep rib and a uniform wall may be a better injection molding candidate once the design is frozen. The rule is straightforward: use CNC when the geometry is complex, the tolerance is tight, or the volume is low. Use another process when all three point the other way.

None of this is a defect of CNC processing services. It is a boundary. A supplier that tells you when to use a different process is more useful than one that quotes everything.