How to Vet a CNC Parts Company Before You Send the PO

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  Key takeaways
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  What to compare across CNC parts suppliers
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  Start with the tolerance the part actually needs
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  Check machine capacity against your part envelope
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  Confirm the material grade and finish before the quote closes
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  Read the commercial terms like an engineer
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  Match the supplier to your industry and risk level
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  Step by step: how to qualify a CNC parts company

Every quote you receive is built on a tolerance assumption. If your drawing has a general block tolerance of ±0.1 mm and one bore at ±0.005 mm, the shop prices the whole part at the tight end unless you say otherwise. Separate critical features from cosmetic ones before you send the RFQ. Mark the two or three dimensions that carry function: bearing seats, dowel holes, sealing faces, mating bores. Everything else can sit at ±0.05 mm or looser and cost less to produce.

A CNC parts company that can hold ±0.005 mm (about ±0.0002 in) normally has temperature-controlled inspection and a CMM on site. Ask how the number is verified, not just whether it is possible. A shop that says 'yes' without asking about your datum scheme has not thought about it. Send the GD&T frame with the model so the quote reflects the real inspection work.

Surface finish follows the same logic. Ra 0.2–0.8 μm needs a finishing pass and possibly polishing. Ra 0.8–1.6 μm is a normal machined finish on aluminum and steel. Ra 1.6–3.2 μm is fine for brackets and housings. If the drawing calls for Ra 0.4 μm on an internal pocket that no seal touches, you are buying time you do not need to spend.

One more check: ask what happens when a feature comes out of tolerance. A supplier with a documented rework path is different from one that ships and waits for your incoming inspection to catch it. GreatLight runs raw material checks, in-process monitoring and final inspection, with reports on request.

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  Mark critical dimensionsTwo or three functional features, not the whole drawing.
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  Send the datum schemeGD&T frames tell the shop how to set up and inspect.
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  Tie finish to functionSealing faces need Ra 0.2–0.8 μm. Covers do not.

A CNC parts company is only as useful as the machines your geometry can reach. Five-axis capability is often listed as a checkbox, but the difference between a 3-axis mill with a tilting head and a simultaneous 5-axis center shows up on undercuts, contoured ports and single-setup parts. If your part has features on five faces, ask how many setups the shop plans. Two setups mean two chances to lose datum alignment.

Size matters just as much. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The largest travel is 4,000 × 400 × 150 mm for long parts. Medium frames cover 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact work sits in 500 × 500 × 450 mm and 500 × 310 × 200 mm envelopes, with a Ø400 mm rotary table for round parts.

Mill-turn centers matter when your part is a shaft with cross-drilled holes or a fitting with a turned bore and milled flats. Doing that on two machines adds a re-chuck and a concentricity risk. One machine that turns and mills in the same setup removes both. Ask the shop which of your features would move to a mill-turn cell and why.

For prototypes, the question is different. You want the shop to cut the real geometry, not approximate it. A supplier that quotes 5-axis for a part that fits a 3-axis vise is not saving you money. Ask for a setup plan with the quote and compare it to your own read of the part.

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  Count the setupsEach additional setup adds alignment error and cost.
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  Match travel to part size4,000 mm long parts need a different cell than 500 mm ones.
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  Use mill-turn for shaftsTurning and milling in one setup protects concentricity.

Material grade is where quotes quietly diverge. 'Aluminum' can mean 6061-T6 or a generic cast alloy with different strength and machinability. 'Stainless' can mean 303 free-machining or 316L for a medical housing. Write the grade on the RFQ and ask the shop to confirm the mill certificate is available. This is a five-minute check that prevents a scrapped batch later.

Common grades we run include 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 in aluminum; 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH in stainless; 1018, 1045, 4130, 4140, 4340, A36 and tool steel; C101, C103, C110, beryllium copper, C27400, C28000 and C36000 in copper and brass; and TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B / AZ91D for special work. Plastics cover ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fiber.

Finish changes both cost and lead time. Anodizing in clear, color, hardcoat or conductive form, electroless nickel, zinc, silver and gold plating, powder coating and black oxide, plus bead blasting, tumbling, brushing and polishing. Laser marking and engraving need a minimum character height of 1.5 mm, so keep part numbers and logos above that or the mark will not read.

Ask whether finishing is done in-house or subcontracted. A single quality-controlled environment reduces the handoff risk that shows up as a scratched or mis-colored batch. It also means one party owns the final inspection.

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  Write the grade on the RFQ'Aluminum' is not a specification.
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  Keep laser marks above 1.5 mmSmaller characters lose contrast and readability.
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  Ask who owns finishingIn-house finishing shortens the handoff chain.

Quantity flexibility is a real cost driver. A supplier with no minimum order quantity can run one prototype and then a 10,000+ part production order on the same process, which keeps the transition clean. If a shop requires tooling or a setup fee before the first part, that cost should appear in the quote, not arrive as a surprise invoice.

Lead time has three numbers, not one. How fast is the quote and DFM feedback? How fast can production start? How fast do parts ship? GreatLight quotes with free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days. Ask every supplier for those three dates separately. A shop that only gives one number is hiding the other two.

Payment terms, currency and Incoterms belong in the same conversation. So does the handling of your CAD files. Uploads should be secure and confidential, and an NDA should be available on request before you send a full assembly. If a supplier hesitates on an NDA for a production part, treat that as a signal about how they treat IP.

Certifications only count inside their scope. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive production. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security. Ask which certificate covers the plant that will actually make your part, and ask for the scope statement, not a wall photo.

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  Get three lead timesQuote, production start and ship date.
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  Settle IP before filesNDA on request, secure upload, no email attachments.
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  Match cert to plantThe certificate must cover the site making your part.

Different industries carry different failure costs. An aerospace bracket and a robot end-effector plate may look similar on the drawing, but the documentation trail is not. Aerospace work needs material traceability and first-article inspection records. Automotive and EV parts fall under IATF 16949 discipline. Medical devices need process validation and clean handling under ISO 13485.

Electronics enclosures and heat sinks are usually tolerance-driven in a different way. Flatness, hole position and finish matter more than tight diametrical fits. Robotics and automation parts often combine a machined housing with a turned shaft, which is a mill-turn job. Industrial machinery parts tend to be larger and heavier, so travel and lifting capacity decide who can quote.

New energy work, including battery and power electronics hardware, often needs conductive anodizing, copper and aluminum combinations, and careful deburring around high-current paths. Ask the shop which of these it has run before and what it changed in the process. A supplier that has solved the problem once will quote it faster and more accurately.

The honest test is whether the shop pushes back. If every question gets a yes, you are probably talking to a sales desk, not a process engineer. A useful CNC parts company will tell you when a tolerance is unnecessary, when a design will chatter, or when a finish will not survive the intended environment.

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  AerospaceTraceability and first-article reports are non-negotiable.
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  Automotive and EVLook for IATF 16949 scope on the producing plant.
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  MedicalISO 13485 and documented process validation.
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  Robotics and industrialMill-turn and large travel decide the shortlist.