CNC Machining Services Range: What to Check Before You Send an RFQ

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  Key takeaways
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  How to Match Part Features to a Supplier's Range
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  What the Phrase CNC Machining Services Range Actually Covers
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  Tolerance, Finish and Inspection: Where Quotes Diverge
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  Order Quantity, Lead Time and Certifications as Selection Filters
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  Materials and Finishes: The Part of the Range Buyers Underuse
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  Industry Requirements That Change the Shortlist
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  Six Steps to Qualify a Supplier's Range

Suppliers use the same phrase for very different things. For some it means the list of processes they offer: milling, turning, EDM, 3D printing, sheet metal. For others it means the physical envelope they can hold on a machine table. Both matter, but they answer different questions. Before you compare quotes, separate the two, because a shop with a long process list may still be unable to reach the far side of your part.

The process side is easier to check. A full range usually includes 3-axis, 4-axis and 5-axis milling, mill-turn work, wire and sinker EDM, surface grinding, and a prototyping route for parts that are not ready for hard tooling. At GreatLight the floor carries 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix is what lets one shop quote a simple bracket and a five-face housing without sending work out.

The envelope side is where surprises appear. Maximum processing size is 4,000 mm, with a large travel of 4,000 × 400 × 150 mm. Medium machines cover 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, while compact machines handle 500 × 500 × 450 mm and 500 × 310 × 200 mm. If your part is 900 mm long and needs work on four sides, the compact machines are irrelevant and you should be asking about the gantry.

A useful habit: sketch the blank, mark the faces that need machining, and note the longest distance between any two features. That single number rules in or rules out most suppliers in under a minute.

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  Process rangeWhat operations the shop can perform in house.
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  Envelope rangeWhat part size and shape fits on the machines.
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  Both must clear your drawingA gap in either one stops the job.

Two quotes for the same drawing can differ by a factor of three, and tolerance is usually the reason. A shop quoting ±0.05 mm is not being generous; it is pricing a different process plan. Tight limits need temperature-stable finishing passes, sharp tooling, and often a separate inspection step. If your drawing shows ±0.005 mm only on a bearing bore and general limits elsewhere, say so. The shop can then concentrate effort where it pays off.

Finish works the same way. As-machined surfaces sit around Ra 1.6–3.2 μm, which is fine for brackets, covers and most structural parts. Sealing faces, sliding surfaces and optical mounts usually need Ra 0.8–1.6 μm, and some call for Ra 0.2–0.8 μm. Each step down adds time. Do not specify a fine finish across an entire part unless the function demands it.

Inspection is the third lever. A supplier that checks raw material, monitors in-process dimensions and inspects 100% before shipment will catch a drifting setup before the whole batch is scrap. Ask what gets recorded and whether reports come with the shipment. At GreatLight, inspection reports are available on request, and the historical qualification rate is 99.99%. That number is only meaningful if you know what was measured, so ask for the inspection plan.

One more point on finish: coating changes size. Anodizing builds a few microns; plating can add more. If a bore must stay within ±0.005 mm after coating, the machinist has to cut it undersize on purpose. Tell the shop the function, not just the nominal dimension.

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  Split your tolerancesMark only functional features as tight; leave the rest general.
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  Match finish to functionRa 1.6–3.2 μm for structure, Ra 0.8–1.6 μm for sealing.
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  Ask for the inspection planKnow what is measured, how often, and what is reported.

Minimum order quantity is a quiet deal breaker. Some shops will not quote a single part, so you buy ten and shelve nine. Others quote prototypes at a prototype price and then re-quote the production run with new tooling and a new setup. Neither is wrong, but both cost you time. A supplier with no minimum order quantity can take one prototype and the same part number at 10,000+ pieces on the same equipment, which keeps the process history intact.

Lead time should be split into stages so you can compare like with like. Quotation and DFM feedback within 12 hours is a reasonable target for a shop with real engineering capacity. Production start within 24 hours after drawing release matters when the part is a rebuild item. Shipping in 3–5 days after that applies to standard materials and simple finishing. If your part needs hard anodizing, laser engraving below 1.5 mm character height, or a special alloy, add time and ask the shop to confirm.

Certifications are not interchangeable. ISO 9001:2015 is the baseline for general industrial work. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical devices, where traceability and process control are audited. ISO 27001:2022 covers information security, which matters if your drawings and CAD files are confidential. Match the certificate to your industry, not to the longest list.

Confidentiality deserves its own line. Uploads should be handled as confidential by default, and an NDA should be available on request. If a supplier hesitates on that, move on.

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  Check MOQ before design freezeIt affects whether you can iterate cheaply.
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  Compare lead time in stagesQuote, start, ship. One blended number hides risk.
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  Pick certificates by programAutomotive, medical and general industry differ.

Material availability often decides the schedule more than machining does. Aluminium grades such as 6061, 6061-T6, 7075, 2024, 5052 and 6082 are common and machine quickly. Stainless 303, 304, 316L, 17-4PH and 440C cover most corrosion and wear needs, though 316L and 17-4PH cut slower and cost more per hour. Steel grades 1018, 1045, 4130, 4140 and 4340 suit shafts and structural parts, while titanium TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B are specialty choices with their own tool wear and fixturing rules.

Plastics are part of the same conversation. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre all appear in prototype work, and each behaves differently. POM holds tolerance well and machines cleanly; PEEK needs sharp tooling and slower feeds; carbon fibre eats cutters and needs dust control. If your part is a functional prototype, the material choice is a machining decision, not just a purchasing one.

Finishing options decide the last 0.1 mm of the job. Anodizing comes in clear, colour, hardcoat and conductive versions. Electroless nickel, zinc, silver and gold plating serve electrical and wear duties. Powder coating and black oxide handle appearance and mild corrosion. Bead blasting, tumbling, brushing and polishing change surface texture, and laser marking or engraving can add part numbers, with a minimum character height of 1.5 mm.

The trap is combining steps without telling the shop the order. A part that is machined, then anodized, then laser marked has three chances for a dimension to drift. List the sequence on the drawing and let the shop confirm it before cutting metal.

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  Name the grade, not just the family6061-T6 and 7075 machine differently.
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  Call out post-processing orderMachining, coating and marking interact.
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  Check marking size limitsLaser engraving works down to 1.5 mm character height.

Aerospace work tends to involve thin walls, tight true position and material traceability. Five-axis capability and stable fixturing matter more than raw spindle speed, because a re-clamped part loses accuracy. Prototype programs in this sector often run parallel with test hardware, so a shop that can move from one part to a small batch without retooling saves a full cycle.

Automotive and EV programs bring volume, repeatability and documentation. Here the IATF 16949:2016 certificate is a starting point, not a bonus. The relevant question is whether the shop can hold a process capability across 10,000+ parts and still report dimensional data at the end. Mill-turn centers help because they reduce the number of setups per part, and every setup removal is a variance source.

Medical devices push inspection and traceability further. ISO 13485:2016 signals a quality system built for that sector, but the practical points are clean handling, material certificates and the ability to inspect small features without damaging them. Robotics, electronics and new energy work sit between these extremes: usually fewer regulatory demands, but short lead times and fast design changes.

Industrial machinery is the broadest category, from one-off replacement parts to production runs. It is also where a wide CNC machining services range pays off most, because the same shop can quote a large welded frame and a small precision bushing in the same week.

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  AerospaceThin walls, true position, traceability, few setups.
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  Automotive and EVVolume, capability data, IATF 16949:2016.
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  MedicalISO 13485:2016, clean handling, material certs.