How to Evaluate CNC Processing Service Providers

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  What matters most
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  Five criteria that separate suppliers
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  Tolerance claims you can verify
• 4
  Machine mix tells you what a shop can actually make
• 5
  Certifications and the inspection trail
• 6
  Lead time, MOQ and how the quote is built
• 7
  Post-processing and assembly scope
• 8
  How to run the comparison in one week

Every supplier page says precision. Few say which machine holds it. A ±0.005 mm callout on a 40 mm aluminum bracket is routine on a 5-axis center with a Ø400 mm rotary table, because the part stays in one setup and the datum never moves. The same callout on a 900 mm steel housing is a different problem: thermal growth and tool deflection start to dominate, and the shop needs to tell you that before you order.

Ask what the tolerance applies to. A shop that can hold ±0.005 mm on a 60 mm feature may only promise ±0.05 mm across a 4,000 mm part. That is not dishonesty, it is physics. The useful question is which features on your drawing are critical and whether the process can hold them without a second setup.

Material changes the answer too. Aluminum 6061 and 7075 cut clean and hold tight tolerances with light finishing passes. Stainless 316 and 17-4PH work-harden, so heavy radial cuts push the part away from the tool. Inconel and titanium TC4 need slower feeds and more rigid fixturing, and the achievable tolerance window narrows.

Surface finish is the second half of the tolerance story. As-machined surfaces land around Ra 1.6–3.2 μm. A sealing face or bearing bore often needs Ra 0.8–1.6 μm, and optical or medical contact surfaces can reach Ra 0.2–0.8 μm. Those numbers come from finishing passes and sometimes from a separate finishing operation, both of which carry cost.

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  Ask for the machine typeA tolerance number without a machine class behind it is not verifiable.
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  Send the critical featuresMark the 3 to 5 dimensions that matter so the shop quotes the real process.
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  Check the finish calloutRa 0.8–1.6 μm usually needs a finishing pass, not just a slower spindle.

A shop with 127 high-precision CNC machines across 3 wholly-owned plants can absorb a mix of work. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That spread matters because different parts belong on different platforms, and a shop that only owns one type will quote everything onto it.

Use the travel envelope as a filter. A 4,000 × 400 × 150 mm work envelope covers long aerospace stringers and extrusion profiles. Envelopes of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm handle most automotive and industrial housings. Compact 500 × 500 × 450 mm and 500 × 310 × 200 mm machines are the right home for small high-volume parts where cycle time dominates.

Mill-turn centers solve a specific problem: parts with turned diameters and milled flats that would otherwise need two fixtures and two setups. Concentricity between the bore and the bolt pattern stays inside one machine, which removes a common source of scrap. If your part has both, ask whether it will run on a mill-turn center.

The practical test is a part that cannot be made on a 3-axis machine. Send a drawing with an undercut, a compound angle or a deep cavity on five faces. A shop with real 5-axis capability will answer with a setup plan. A shop without it will either decline or quote a long multi-setup route.

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  Match part to platformSmall parts on big machines waste cycle time; big parts on small machines need re-fixturing.
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  One setup beats threeEvery additional setup adds a datum shift and a chance of misalignment.

Certifications are a filter, not a ranking. ISO 9001:2015 covers general quality management and is the baseline for any serious supplier. IATF 16949:2016 is the automotive standard and is usually required for production parts in that industry. ISO 13485:2016 applies to medical devices and brings tighter traceability. ISO 27001:2022 covers information security, which matters when you are uploading proprietary CAD files to a supplier portal.

A certificate on a wall proves nothing about your part. What matters is the inspection plan behind it. The standard pattern is raw material verification on receipt, in-process monitoring at defined intervals, and a final inspection before shipment. Ask whether inspection is sampling or 100 percent, and ask for the report format so you know what you will receive.

Keep the document trail tied to the order. Mill certificates for the material, dimensional reports for the critical features, and surface finish readings when the drawing calls them out. If a supplier cannot produce those on request, the certification number is not doing any work for you.

For regulated industries, add the NDA question early. Uploads should be treated as confidential by default, and a signed non-disclosure agreement should be available on request before you send the first STEP file.

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  Match cert to industryISO 9001 is the floor; IATF 16949 or ISO 13485 may be mandatory for your sector.
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  Ask sampling or 100%The answer changes how much incoming inspection you need to do yourself.
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  Request reports up frontMaterial certs and dimensional reports should be named in the quote.

Lead time has two parts: how fast the quote comes back and how fast parts ship. A 12-hour quotation with a DFM analysis is a reasonable benchmark, and it tells you something about how the shop is organized. If production can start within 24 hours and parts ship in 3–5 days, the shop is running a scheduling system rather than a queue.

Order minimums change your development plan. A supplier with no minimum order quantity lets you order one prototype, check the fit, then scale to 10,000+ parts on the same process and fixtures. A supplier with a large minimum forces you to commit before the design is validated, which usually costs more than the setup fee you saved.

Read the quote structure, not just the total. A quote that lists material, machining time, finishing, inspection and freight separately lets you change one variable and see the effect. A single lump sum hides where the cost sits, so you cannot tell whether anodizing or a tight tolerance is driving the price.

Watch for these traps. A price that is far below the others usually means a tolerance assumption you did not agree to. A lead time quoted without a material availability check can slip in week one. And a supplier who quotes without asking about the critical features is quoting a different part than the one you designed.

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  Separate the line itemsYou cannot optimize cost if the quote hides material, machining and finishing in one number.
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  Confirm material availabilityLead time promises are meaningless if the stock is not on the floor.
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  Start with one partNo MOQ lets you validate the process before a production commitment.

Machining is rarely the last step. Anodizing in clear, color, hardcoat or conductive grades changes the surface and can shift dimensions slightly on tight features. Electroless nickel, zinc, silver and gold plating each bring their own thickness tolerance. Powder coating and black oxide are common on industrial housings but add thickness that can close a thread or a bore.

Cosmetic finishing matters for visible parts. Bead blasting, tumbling, brushing and polishing produce different surface signatures, and a customer-facing panel is judged on appearance, not on Ra alone. Laser marking and engraving need a minimum character height of 1.5 mm to stay legible after finishing, so plan the marking in the drawing stage.

Ask who handles the finishing. A shop that manages finishing in-house or through a qualified partner keeps one point of responsibility. A shop that hands you a list of vendors makes you the project manager, and if a part fails after coating, the cause can take weeks to trace.

The same question applies to assembly. If your part is one of several in a subassembly, a supplier who can machine, finish and assemble reduces the number of incoming inspections you run and the number of places a tolerance stack can go wrong.

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  Finishing moves dimensionsPlating and coating add thickness; call out the pre-plate and post-plate sizes.
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  Marking needs 1.5 mmAnything smaller may not survive the finishing step.