Past CNC Machining Service: What Still Matters When You Choose a Supplier

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  Key takeaways
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  Five checks to run on any past CNC machining service quote
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  Tolerance and geometry decide whether the process fits
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  Material and finish choices change the price more than the machine
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  Lead time, MOQ and quotingscope: what to pin down in writing
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  Certification and inspection are the buyer's safety net
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  Six pitfalls that show up after you place the order
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  Step by step: how to vet a past CNC machining service in one week

A past CNC machining service can hold ±0.005 mm, but that number only applies where the drawing asks for it. If you tolerance a bolt hole pattern at ±0.05 mm and a bearing bore at ±0.005 mm, the shop plans two different operations. The tight bore is often bored on a mill-turn center after the soft jaw is cut, not drilled in the same pass as the holes. Ask which feature carries the tight tolerance before you compare prices.

Geometry matters just as much. A part with undercuts, deep pockets, or contoured faces usually needs 5-axis work or a mill-turn cycle. GreatLight runs 16 simultaneous 5-axis centers and 16 mill-turn centers for this reason. A flat bracket with drilled holes is cheaper on a 3-axis machine, and we will say so. The goal is not to sell the biggest machine; it is to pick the process that reaches the tolerance with the fewest setups.

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  Name the tight featureMark it on the drawing so the quote reflects the real operation.
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  Check wall thicknessThin walls under 1 mm can deflect during clamping and finishing.
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  Watch deep pocketsA depth-to-diameter ratio above 4:1 needs a longer, thinner tool and slower feed.

Aluminium 6061 machines fast and takes anodizing well. It is the default for enclosures, brackets and heat sinks. Stainless 304 and 316L cut slower and work-harden if the feed is too light, so a 316L part can cost two to three times the same part in 6061. Titanium TC4 (Ti-6Al-4V) and Inconel need lower cutting speeds and more tool changes. If the part does not need corrosion resistance or high temperature strength, switching to 6061 or 7075 often cuts cost without losing function.

Surface finish is quoted as Ra. As-machined parts sit around Ra 1.6–3.2 μm. A sealing face or a bearing fit may need Ra 0.8–1.6 μm, and optical or medical sealing surfaces can reach Ra 0.2–0.8 μm, which usually means a separate finishing pass. Bead blasting, anodizing, and electroless nickel all hide tool marks, but they also add days. Put the finish callout on the drawing before you request a quote, not after.

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  Cost ladder6061 < 304 < 316L < Ti-6Al-4V for the same geometry.
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  Finish timingAnodizing and plating sit outside the machining cycle, so they extend lead time.
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  Laser markingMinimum character height is 1.5 mm; smaller text will not read cleanly.

A past CNC machining service page may promise fast delivery, but you need dates in the quote. At GreatLight, quotation and free DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Those numbers apply to the parts we quote, not to every possible geometry. A 4,000 mm long part on the large travel machine has a different schedule than a Ø400 mm rotary table job.

MOQ is the other lever. We take one prototype and run 10,000+ pieces on the same route, so you can validate a design before committing to tooling. Watch for suppliers who quote a low unit price but add a setup charge per part number, or who fold first-article inspection into a separate invoice. Ask for the setup fee, the unit price, and the inspection cost as three separate lines.

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  Ask for a date, not a range'3–5 days after drawing release' is a date you can plan around.
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  Split the quoteSetup, unit price, finish, and inspection should be separate.
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  Prototype firstOne piece catches drawing errors before a 10,000-piece run.

ISO 9001:2015 covers quality management for general machining. Automotive buyers usually need IATF 16949:2016, medical device buyers need ISO 13485:2016, and customers sending sensitive CAD files often ask about ISO 27001:2022 for information security. GreatLight holds all four. A certificate is only useful if it is current and scoped to the process you are buying, so ask for the certificate number and the expiry date.

Inspection is where a supplier proves the tolerance claim. We check raw material on arrival, monitor dimensions during the run, and inspect 100% of parts before shipment, with reports on request. If your part is safety-critical, ask for the inspection plan before the first cut: which features, which gauges, and how often. A supplier who cannot describe that plan in one page is a risk.

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  Match the certIATF 16949 for automotive, ISO 13485 for medical.
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  Ask for the planFeature list, gauge type, and check frequency.
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  Keep recordsReports on request protect you during incoming inspection.

The first pitfall is a hidden second setup. A quote says three setups, then the shop adds a fourth for deburring or a thread that cannot be reached. The second is tolerance creep: the drawing shows ±0.005 mm on one bore, and the shop reports the part as 'within tolerance' using a looser callout. Ask for the measured value on the tight feature, not a pass/fail stamp.

The third is material substitution. A 304 part arrives with a mill cert for 303, which machines faster. The fourth is finish delay. Anodizing and plating are outside processes, and a two-day queue at the plater becomes a two-day delay for you. The fifth is confidentiality. Uploads should be secure, and an NDA should be available on request. The sixth is a missing first-article report on a new part number. Any one of these can cost more than the price difference between two quotes.