CNC Machining Services for Sale: What to Check Before You Buy

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  Key takeaways
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  Seven checks, and what a good answer looks like
• 3
  Tolerance: what ±0.005 mm actually covers
• 4
  MOQ, materials and the real cost of small runs
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  Lead time: separate the quote clock from the production clock
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  Certifications: match the scope, not the logo
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  Finishing and inspection: the two places quotes go wrong
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  Machine selection: why 5-axis is not always the answer
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  Step by step: how to qualify a supplier before you place the order

Every shop quotes a tolerance number. Few explain what it applies to. On a turned shaft 30 mm long, ±0.005 mm is routine on a good lathe. On a 4,000 mm frame with six setups, that same number needs temperature control, a stable fixture and a CMM large enough to check it. The number is not wrong. The question is whether the shop can hold it on your feature.

A practical test: send a drawing with two or three tight features and the rest open. See how the supplier splits the tolerance. A shop that marks only the bearing bore and the locating face as tight is reading the drawing. A shop that stamps ±0.005 mm on the whole title block is selling a slogan.

Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal machined finish on aluminum and mild steel. Ra 0.2–0.8 μm needs a finer stepover, sharper tooling and sometimes a second operation. If your sealing face needs Ra 0.4 μm, say so on the drawing. It changes cycle time and price, and nobody should discover it at final inspection.

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  Datums firstTolerances without clear datums get re-interpreted at the machine.
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  One tight face beats tenConcentrated tolerance costs less than tolerance spread everywhere.
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  Ask for the inspection methodCMM, micrometer or optical comparator. Each has a range.

Minimum order quantity is the fastest way to read a supplier's business model. A shop that sets MOQ at 500 pieces is built for production. Your ten-piece prototype will be scheduled after every larger job, and the setup cost gets spread over ten parts instead of five hundred. That is fine if you only need production. It is painful if you are still iterating.

GreatLight runs no minimum order quantity, from one prototype to 10,000+ part runs. The same 127 machines cover both. That matters because the second iteration of a design usually arrives three days after the first, and switching suppliers between revisions wastes a week on requalification.

Material choice drives both price and lead time. Aluminum 6061-T6 machines fast and is the default for housings and brackets. 7075 gives higher strength but costs more and chips differently. Stainless 316L and 17-4PH are common in medical and marine parts; 17-4PH in the H900 condition is hard on tooling. Titanium TC4 (Ti-6Al-4V) and Inconel need slower feeds, more tool changes and a shop that has cut them before. If a supplier lists titanium but cannot tell you its cutting parameters, they are brokering the job.

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  Aluminum6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, ADC12.
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  Stainless303, 304, 316, 316L, 420, 430, 431, 440C, 17-4PH.
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  Titanium and specialTA1, TA2, TC4, Inconel, magnesium AZ31B / AZ91D.
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  PlasticsABS, PC, PMMA, POM, PA, PEEK, PP, HDPE, carbon fibre.

Suppliers quote lead time in different units. Some count from purchase order. Some count from drawing approval. Some count from material arrival, which is the number that actually moves. Ask which one they mean, then ask what happens if the material is not in stock.

A workable split looks like this: quotation and free DFM analysis within 12 hours, production start within 24 hours, parts ship in 3–5 days. That covers standard aluminum and steel stock sizes. It does not cover a special titanium grade that has to be ordered, or a part that needs hardcoat anodizing after machining.

For anything with an outside process, add the finishing step in writing. Anodizing, electroless nickel and powder coating each add their own queue. A shop that owns the finishing line controls that queue. A shop that subcontracts it does not, and you will hear about the delay after it happens.

Historical late-delivery probability at GreatLight is below 2%. That number is only useful if you know what it covers, so treat it as a baseline for standard work, not a promise on every exotic job.

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  12 hoursQuotation and free DFM analysis.
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  24 hoursProduction start after approval.
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  3–5 daysTypical shipment on standard materials.

Certificates are cheap to print and expensive to hold. Any serious supplier will send you a PDF with a scope statement and an expiry date. Read the scope. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and comes with traceability and PPAP expectations. ISO 13485:2016 is for medical devices and adds validation and record retention. ISO 27001:2022 covers information security, which matters when you upload CAD files.

The practical question is which one your program needs. A bracket for an industrial machine does not need ISO 13485. A surgical instrument housing does, and the auditor will ask for the certificate during supplier qualification, not after. If your customer requires IATF 16949, a supplier holding only ISO 9001 will fail the audit even if the parts are perfect.

ISO 27001 is the one buyers overlook. Every quote request involves sending drawings. Ask where files are stored, who can open them, and whether an NDA can be signed before the first upload. Confidentiality language in a quote email is not the same as a controlled file system.

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  ISO 9001:2015General manufacturing quality baseline.
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  IATF 16949:2016Automotive, with traceability requirements.
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  ISO 13485:2016Medical devices, validation and records.
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  ISO 27001:2022Information security for uploaded files.

A machined part is rarely a finished part. Anodizing changes dimensions slightly, especially hardcoat, which can build 20–50 μm per surface. If your bore is already at the top of tolerance, hardcoat will close it. Plating adds its own thickness. Powder coating hides scratches but adds 60–100 μm and softens sharp edges. Tell the supplier which surfaces are masked and which are coated.

Laser marking has a hard limit: minimum character height 1.5 mm. Engineers often design a 0.8 mm serial number and find out at the marking station. Check that before you release the drawing.

Inspection is where a low price usually hides. GreatLight inspects 100% of parts before shipment, with raw material check, in-process monitoring and final inspection, and reports on request. That is a different promise from sampling a few parts per lot. On a 200-piece run, sampling can miss the one bore that drifted after the tool wore. Ask for the inspection record format before you order, so you know what you will receive.

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  AnodizingClear, colour, hardcoat, conductive.
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  PlatingElectroless nickel, zinc, silver, gold.
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  Coating and blastingPowder coat, black oxide, bead blasting, tumbling, brushing, polishing.
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  MarkingLaser marking and engraving, minimum 1.5 mm character height.

A shop with 16 simultaneous 5-axis machining centers will quote your simple plate on a 3-axis mill, because that is cheaper and faster. If every quote you receive comes back as 5-axis work, the supplier is either showing off or has no 3-axis capacity free. Ask which machine will run the job.

The useful split is by geometry. A flat bracket with holes on one face belongs on a 3-axis machine. A part with features on four sides, like a manifold or a gearbox cover, saves setups on a 4-axis mill. Impellers, turbine blades, medical implants and complex housings with undercuts need simultaneous 5-axis, because the tool axis tilts continuously to reach the surface.

Size decides the rest. GreatLight runs a 4,000 mm maximum processing size, with travels of 4,000 × 400 × 150 mm on the large machines, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on the medium frame, and 500 × 500 × 450 mm and 500 × 310 × 200 mm on the compact ones. A Ø400 mm rotary table handles round parts that need indexing. If your part is 3.5 m long and you send it to a shop whose biggest travel is 1 m, you will get a polite email, not a part.

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  16 five-axis centersSimultaneous 5-axis for contoured and undercut geometry.
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  12 four-axis millsMulti-face parts, fewer setups.
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  27 three-axis machinesPrismatic parts, lowest cost per feature.
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  16 mill-turn centersShafts and housings that need turning and milling in one setup.