How to Vet a CNC Machined Service Manufacturer

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  Key takeaways
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  What to check against what it tells you
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  Match the machine to the part geometry before anything else
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  Tolerance and finish claims need a measurement method behind them
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  Material range shows how wide the supplier can actually work
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  Lead time, MOQ and quote discipline decide the project timeline
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  Finishing and secondary operations belong in the same quote
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  Step by step: how to vet a supplier in one week

The first question is not price. It is whether the part fits a machine the supplier actually owns. A shop can quote anything; it can only cut what sits on its floor. Ask for the machine list with axis counts and travels, then check your largest envelope against it.

At GreatLight the list is 127 high-precision CNC machines. That includes 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Travels range from 500 × 500 × 450 mm on the compact side up to 4,000 × 400 × 150 mm for long parts, with a Ø400 mm rotary table for round work.

Geometry decides the machine class. A housing with features on five faces and a true position callout between them belongs on 5-axis, because every refixture adds stack-up error. A shaft with a cross-hole and a threaded end is mill-turn work. A simple bracket with one flat face is a three-axis job, and paying 5-axis rates for it is waste.

Watch for the mismatch case. If a supplier quotes a 900 mm long part but lists no machine with that travel, the quote is either outsourced or wrong. Both cost you weeks.

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  5-axisComplex housings, impellers, parts with angular features in one setup.
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  Mill-turnShafts, fittings and parts that need turning and milling without refixture.
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  3-axisPlates, brackets and simple pockets where cost matters more than setup count.
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  Large travelFrame rails, structural beams and long extrusions up to 4,000 mm.

Every supplier page says precision. The useful question is how they prove it. Ask which instrument measures the tightest callout on your drawing, and what report ships with the parts. If the answer is a caliper and a pass or fail note, the ±0.005 mm claim is not supported.

A workable quality chain has three gates. Raw material is checked on arrival against the mill certificate. In-process monitoring catches drift before a batch is finished. Final inspection happens on 100% of parts before shipment, with reports on request. That structure is what keeps a process stable between the first article and the ten thousandth part.

Surface finish is where drawings are most often under-specified. Ra 1.6–3.2 μm is fine for as-machined non-critical faces. Ra 0.8–1.6 μm is the normal target for sealing surfaces, bearing bores and sliding fits. Ra 0.2–0.8 μm needs a deliberate plan: finer stepover, sharp tooling, and often a finishing pass after heat treatment.

Certifications tell you which industries the quality system was built for. ISO 9001:2015 is the baseline. IATF 16949:2016 covers automotive and EV work. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you upload proprietary CAD. GreatLight holds all four.

One more check. Ask what happens to a non-conforming part. A supplier that can describe its quarantine and disposition process has a real system. One that says it will just remake it has a habit.

A narrow material list is a warning sign. It usually means the shop has cutting parameters dialed in for two or three alloys and will struggle with anything else. Aluminum and mild steel are easy. Titanium, Inconel and 17-4PH are not, and they behave differently on the same machine.

Ask for specific grades, not families. Aluminum 6061 and 7075 machine very differently. Stainless 303 is free-cutting while 316L work-hardens and needs lower surface speed and heavier feed to get under the hardened layer. Titanium TC4 (Ti-6Al-4V) needs sharp tooling, high pressure coolant and a conservative depth of cut, or you burn the tool and scrap the part.

The practical test is to name your alloy and grade, then ask how many jobs like it they ran last quarter. A supplier with 15 years in the trade should answer without checking. GreatLight has worked aluminum 6061, 2024, 5052, 5083, 6082, 7075 and ADC12, stainless 303 through 440C and 17-4PH, steel 1018 to 4340 and tool steel, copper and brass grades, titanium TA1 to TC4, Inconel, magnesium AZ31B and AZ91D, plus plastics from ABS and POM to PEEK and carbon fibre.

Plastic parts deserve their own note. PEEK and carbon fibre are abrasive and need carbide or diamond tooling. POM moves with temperature, so a shop that measures right off the machine will report the wrong number. Ask how long the part rests before final inspection.

Lead time is a chain, not a single number. It runs from RFQ to quote, quote to production start, and production start to shipment. A supplier can be fast at one link and slow at another. Ask for all three.

The benchmark to compare against: quotation and free DFM analysis within 12 hours, production start within 24 hours, parts shipping in 3–5 days. Those numbers only hold if the shop reads your drawing properly at the quote stage. A cheap quote that ignores a tight tolerance becomes a change order later, which is the most expensive kind of delay.

MOQ is the other commercial filter. A supplier with a high minimum is built for volume, and a prototype will either be rejected or priced to discourage you. No minimum order quantity means the same floor runs one prototype and a 10,000+ part run. That matters when you need a fit check before committing tooling.

Confidentiality is a commercial term too. Uploads should be secure and confidential, and an NDA should be available on request without a fight. If the supplier hesitates on an NDA for a prototype, think about what that says for a production program.

Ask about late delivery history. A supplier that tracks it and reports a figure below 2% is managing the schedule. One that has no number is not measuring.

A machined part is rarely finished when it leaves the spindle. Anodizing, plating, powder coating and black oxide all change dimensions slightly, and some change them enough to break a tolerance. If finishing is quoted separately by a third party, nobody owns the final dimension.

The finishes that matter most for fits are the thin ones. Electroless nickel, zinc, silver and gold plating add microns. Hardcoat anodizing adds more and builds unevenly on edges. Bead blasting and tumbling round sharp edges, which can erase a small chamfer callout. Laser marking needs a minimum character height of 1.5 mm to stay legible.

The cleanest arrangement is one supplier that machines, finishes and inspects. The tolerance is then held against the finished part, not the as-machined one. When you split those steps, write the post-finish dimension on the drawing and say so explicitly.

Cosmetic requirements deserve the same treatment. Brushed and polished surfaces show handling marks, so specify the direction and the acceptable scratch limit. Vague cosmetic notes are the most common source of arguments at incoming inspection.