Custom CNC Machine Service: What to Verify Before You Send a PO

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  Key takeaways
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  What to compare across three supplier types
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  Tolerance: what the custom CNC machine service can hold in production
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  Material and geometry limits of a custom CNC machine service
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  Lead time, MOQ and quote quality
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  Inspection and certification: what the paperwork covers
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  Red flags when you audit a custom CNC machine service
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  Step by step: qualifying a custom CNC machine service

Every shop quotes a tolerance. The useful question is whether that number holds across the whole lot, in the material you specified, on the feature that matters. A ±0.005 mm callout on a 3 mm pin hole in 6061 is a different problem from the same callout on a 200 mm bolt circle in 17-4PH stainless. Heat, tool wear and fixturing all move the result.

Ask the shop to name the machine and the setup. A tolerance of ±0.005 mm usually means a temperature-controlled room, a probe or CMM check, and a finishing pass after roughing. If the answer is only a machine model number, the tolerance is an aspiration, not a process.

Where you should relax the callout: non-critical clearance holes, cosmetic pockets, and any surface that gets a coating afterward. Tightening a drawing you do not need raises cost, adds inspection time and slows the schedule. Send the critical features, not the whole model, at the tight class.

Finish is part of tolerance. Ra 0.8–1.6 μm is a normal machined finish; Ra 0.2–0.8 μm needs a separate operation and sometimes hand work. Specify finish only on sealing faces, sliding surfaces and bearing bores.

Material choice drives tooling, cycle time and risk. Aluminum grades like 6061, 7075 and 6082 cut fast and hold tight tolerances well. Stainless 304 and 316L work harden, so light passes and sharp tooling matter. Titanium TC4 (Ti-6Al-4V) and Inconel need slower speeds, more coolant and a shop that plans for tool wear.

Ask what the shop stocks and what it orders per job. A supplier who keeps common aluminum and stainless on the floor can start production within 24 hours after drawing approval. Material bought per job adds days and sometimes a minimum purchase you did not plan for.

Geometry sets the machine. A part with features on five faces, deep pockets at an angle, or a bore that must stay coaxial with an outside diameter belongs on a 5-axis center. Splitting it across three setups adds stacking error and handling damage.

Size matters too. GreatLight runs a 4,000 mm maximum processing size with travels of 4,000 × 400 × 150 mm, plus medium and compact cells for smaller work. If your part is longer than a shop's largest travel, no amount of skill fixes it. Ask for the envelope before you send the file.

Compare schedules by milestone, not by a single delivery date. Ask when material arrives, when the first article is cut, and when inspection happens. GreatLight returns a quotation and free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days for standard work.

Minimum order quantity tells you how the shop is built. A high MOQ means setup cost is spread across many parts, which is fine for production but painful for a redesign. No minimum order quantity lets you run one prototype, test it, then scale to 10,000+ parts on the same process.

Read the quote line by line. Good quotes separate material, machining, finishing, inspection and freight. They list the tolerance class and the finish. They name the material grade, not just "aluminum". If the quote is one lump number, you cannot tell what changed when the revision comes back.

Watch for the missing line items. Deburring, anodizing masking, laser marking and certificate packages are common add-ons that show up after you compare prices. Ask now, not at invoice time.

Inspection is where a custom CNC machine service proves the claim. Ask what is measured, on which features, and with what equipment. First article inspection on the critical dimensions plus in-process checks on a defined frequency is normal good practice. Visual checks alone are not inspection.

GreatLight inspects 100% of parts before shipment, covering raw material check, in-process monitoring and final inspection, with reports on request. For a regulated program, ask for the report format up front so your quality team can accept it without a second round of emails.

Certifications have scope. ISO 9001:2015 is the general quality baseline. IATF 16949:2016 applies to automotive production and brings stricter change control and traceability. ISO 13485:2016 is for medical devices. ISO 27001:2022 covers information security, which matters if your CAD files are sensitive.

Match the certificate to the program, not to the website banner. A medical housing does not become compliant because the shop also serves automotive customers. Ask which process, plant and line the certificate covers.

The first red flag is a quote with no DFM feedback. A shop that will machine anything you send, without commenting on wall thickness, tool reach or datum strategy, is not reviewing the part. Free DFM analysis within 12 hours is a normal deliverable, not a favor.

The second is refusal to name the machine or the setup count. Five-axis work quoted as three-axis pricing usually means the shop plans to re-fixture the part, and that is where position error enters.

The third is vague inspection language. "Quality guaranteed" is not a measurement plan. Ask for the dimension list, the gauge type and the sampling rule.

The fourth is a schedule that only exists as a final date. Without material and machining milestones, you cannot tell whether a delay started last week or will start tomorrow.

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  No DFM commentsThe part was priced, not reviewed.
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  No setup countYou cannot predict position error.
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  No inspection planFinal inspection becomes a visual check.
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  No certificate scopeThe banner and the program do not match.