Product Precision CNC Machining: 7 Checks Before You Send a PO

• 1
  What matters most when you compare suppliers
• 2
  What to check against what a shop tells you
• 3
  Reading a tolerance claim on product precision CNC machining
• 4
  Machine mix and maximum part size
• 5
  Materials and finishes that change the quote
• 6
  Which certification your program actually needs
• 7
  MOQ, lead time and what a quote should contain
• 8
  Seven checks to run before you release a PO

Almost every supplier page lists a tight number. The useful question is narrower: on which machine, in which material, and measured how? At GreatLight we hold ±0.005 mm (±0.0002 in) on parts set up on simultaneous 5-axis centers, and we say so because the setup is what makes the number repeatable. Move the same part to a three-axis machine with four fixtures and the stack-up changes.

Material matters as much as the machine. Aluminium 6061-T6 and 7075 cut cleanly and hold position well. Austenitic stainless such as 304 and 316L work-hardens, so light passes and rigid tooling decide whether the bore stays round. Titanium TC4 (Ti-6Al-4V) and Inconel move more under heat, which pushes the finishing pass and the inspection into the same thermal window.

Surface finish is a separate specification. A functional shaft at Ra 0.8–1.6 μm machines in one pass on a mill-turn center. A sealing face at Ra 0.2–0.8 μm needs a different tool path and often a second operation. If the drawing shows one blanket finish callout, expect either an overpriced quote or a part that misses the face that mattered.

Geometry drives the machine choice more than size does. A part with features on five faces, deep pockets, or an undercut is a five-axis job in one setup, which is where the tolerance holds. A simple plate with holes belongs on a three-axis machine and should not carry a five-axis price.

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  ±0.005 mmHeld on 5-axis setups, not as a blanket shop claim.
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  Ra 0.2–0.8 μmFine finish; costs an extra pass, quote it per surface.
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  Ra 1.6–3.2 μmAs-machined; correct for most brackets and housings.
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  Ø400 mm rotary tableSets the practical limit for turned features in one setup.

A shop's machine list tells you what it turns down. GreatLight runs 127 high-precision CNC machines across 3 wholly-owned plants covering 7,600 m². The count that matters for tight work is 16 simultaneous 5-axis machining centers and 16 mill-turn centers, because those two groups handle the parts where repositioning is the main source of error.

Size range runs from compact to large. Large travel reaches 4,000 × 400 × 150 mm, which covers long extrusion profiles and frame rails. Medium machines run 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm handle small, high-volume parts where cycle time dominates.

The 4,000 mm maximum processing size is a real limit, not a marketing figure. Beyond it the part has to be split, and split parts need a joint design that survives the load. Ask early. Finding out after the fixture is built costs weeks.

For turned work, the Ø400 mm rotary table sets the envelope for features cut in one setup. Larger diameters are still possible on mill-turn centers, but the part moves between operations and the tolerance stack grows.

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  16 five-axis centersComplex geometry in one setup.
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  16 mill-turn centersShafts and housings with turned and milled features.
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  4,000 mmMaximum processing size; longer parts need splitting.
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  7,600 m² across 3 plantsRoom to run prototypes and production side by side.

Material selection is a cost and schedule decision as much as an engineering one. The common aluminium grades (6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, ADC12) are stocked and cut fast. Stainless grades from 303 and 304 up to 17-4PH (SUS630) take longer, and 316L needs attention to work-hardening. Tool steel and the 4130/4140/4340 group are routine in our shop but slow the cycle.

Plastics behave differently again. POM and PA machine cleanly. PEEK holds dimension at temperature but is expensive and abrasive on tooling. Carbon fibre needs diamond tooling and dust control. If your part is a housing, ABS or PC may do the job at a fraction of the cost of a metal version.

Finishing is where quotes often diverge. Anodizing comes in clear, colour, hardcoat and conductive types; each changes masking and lead time. Plating options include electroless nickel, zinc, silver and gold. Powder coating and black oxide cover the industrial look. Bead blasting, tumbling, brushing and polishing change surface texture without changing dimension, if the shop masks the right faces.

Laser marking is the last step and has a hard limit: minimum character height 1.5 mm. Part numbers, serials and logos below that size will not read cleanly. Plan the marking area before the drawing is released.

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  Aluminium 6061 / 7075Fast to cut, good dimensional stability.
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  Stainless 304 / 316L / 17-4PHWork-hardening risk; light passes and rigid setups.
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  Anodizing and platingMask the datum faces or the fit changes.
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  Laser markingMinimum character height 1.5 mm.

Certification is a scope question, not a badge collection. ISO 9001:2015 covers the quality system for general industrial work. IATF 16949:2016 is what automotive and EV programs expect, because it governs traceability and change control on production parts. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your drawings and CAD files sit on a supplier's server.

Match the certificate to the part, not to the company brochure. A bracket for industrial machinery does not need IATF paperwork, and paying for it slows nothing but adds audit overhead. A brake component does need it. If a supplier lists all four, ask which one governs your job and who signs the inspection record.

Industry experience shows up in the details. Aerospace work pushes on thin-wall machining and material traceability. Automotive and EV programs run on PPAP-style documentation and repeat orders at a fixed tolerance. Medical parts are often small, in 316L or titanium, with strict burr limits and full documentation.

Robotics, electronics and new energy each bring their own habits: cable routing channels, heat-sink geometry, busbar flatness. A shop that has cut those parts before will flag problems in the DFM note instead of at the first article.

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  ISO 9001:2015Baseline for general industrial parts.
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  IATF 16949:2016Automotive and EV production programs.
• 3
  ISO 13485:2016Medical device components.
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  ISO 27001:2022Drawing and file security.

Minimum order quantity is the first commercial filter. GreatLight works with no minimum order quantity, from one prototype to 10,000+ part runs. That matters when a design is not frozen: you can cut one part, test it, and change the drawing without absorbing a tooling charge that only makes sense at volume.

Lead time has two separate numbers, and suppliers often quote only the better one. Our quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours after that. Parts ship in 3–5 days for standard work. Those three numbers describe different stages, so compare them stage by stage.

Historical late-delivery probability at our plants sits below 2%. That is a record, not a promise on your specific order. Complex finishing, imported material, or an unresolved drawing question can all extend a schedule. Ask what would move your date before you release the PO.

Confidentiality should be settled before drawings move. Uploads are secure and confidential, and an NDA is available on request. If your program is under a customer NDA of its own, say so in the first message so the paperwork runs in parallel with the quote.

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  No MOQOne prototype and 10,000+ runs on the same floor.
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  12 hoursQuotation with free DFM analysis.
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  24 hoursProduction start after drawing release.
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  3–5 daysStandard parts ship; finishing may add time.