Various CNC Machining Service: How to Judge a Supplier Before You Send a PO

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  What matters most when you compare shops
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  Matching part type to the right process route
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  Common traps and what to ask instead
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  What ±0.005 mm actually means on a drawing
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  Machine mix tells you what a various CNC machining service can really take
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  From one prototype to 10,000+ parts without changing shops
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  Certifications and inspection: read the scope, not the logo
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  Why finishing inside the same shop shortens the chain
• 9
  Step by step: how to qualify a supplier in one week

A tolerance callout is a promise about the worst part in a batch, not the best one. When a supplier advertises ±0.005 mm, ask which features that covers. On a 4,000 mm long part, thermal drift and fixture flex eat most of the budget before the cutter touches metal. On a 50 mm bracket, the same number is comfortable.

The practical question is which features need the tight tolerance and which can float. Datum structure decides this. If your drawing dimensions from three different edges, the shop has to hold all three at once. If you dimension from one datum, the machine only has to be right in one frame. That single change often moves a part from a 5-axis job to a 3-axis job and cuts setup time.

Surface finish and tolerance are separate costs. Ra 1.6–3.2 μm is as-machined and usually comes with the cut. Ra 0.8–1.6 μm needs a finishing pass, a sharper tool, or a slower feed. Ra 0.2–0.8 μm usually means a separate operation, sometimes on a different machine. Decide if the drawing really needs it, or if a bead blast would serve the function.

Ask for the inspection method, not just the number. A bore checked with a plug gauge and a bore checked on a CMM are two different claims. For anything under ±0.01 mm, the gauge matters as much as the machine.

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  Datum firstOne clean datum frame cuts setup count and cost.
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  Split the tolerancesTight only where function demands it.
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  Match gauge to toleranceA ±0.005 mm callout needs a comparable measuring tool.

A shop's machine list is the fastest way to see if your parts fit. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants covering 7,600 m². That includes 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. The 5-axis count matters for parts with angled faces or deep pockets reachable from one side.

Size range decides more jobs than spindle speed. The largest work envelope here is 4,000 × 400 × 150 mm, which covers long extrusions and rails. Medium envelopes at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm handle most housings and plates. Compact machines at 500 × 500 × 450 mm and 500 × 310 × 200 mm cover small, high-volume parts where cycle time rules.

Mill-turn centers are the quiet advantage. A part that needs turning and cross-drilling in one setup avoids a second fixture and a second datum. That is usually where concentricity errors appear. If your part has a turned body plus radial holes, ask for mill-turn, not two operations.

A Ø400 mm rotary table extends 4-axis work to larger round parts and indexable features. If your part is a ring, flange, or manifold with features around a bore, that table is doing the work.

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  5-axis for reach16 simultaneous centers reduce setups on complex geometry.
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  Mill-turn for concentricityOne setup keeps bores and cross holes aligned.
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  Check the envelope4,000 mm max length; confirm your part fits before quoting.

Most sourcing problems start when the prototype shop cannot run production. The design is validated on one set of machines, then re-quoted and re-fixtured somewhere else. Tolerances shift, the first article fails, and the schedule slips by weeks. A supplier with no minimum order quantity removes that handoff.

No MOQ does not mean every shop is set up for it. Ask how the first article is handled. A workable answer includes a DFM review before cutting, a first-article inspection report, and a clear statement of what changes between prototype and production fixturing. Soft jaws work for five parts. Dedicated fixtures and pallets show up when volumes climb.

Volume changes the cost structure. At low quantity, programming and setup dominate. At high quantity, cycle time and material yield dominate. A shop that runs both ends of that curve will tell you which lever to pull. If a quote comes back with no comment on either, the shop is guessing.

The same machine mix covers both ends here: 3-axis and compact machines for small runs, mill-turn and pallet systems for larger ones. That means your process plan survives the volume change.

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  No MOQOne prototype to 10,000+ part runs on the same floor.
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  First article definedAsk what is measured and reported before full release.
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  Fixture planSoft jaws for prototypes, dedicated fixtures for volume.

Four certificates cover four different risks. ISO 9001:2015 is the baseline quality system. IATF 16949:2016 is the automotive standard and governs PPAP-style discipline and traceability. ISO 13485:2016 applies to medical devices and puts more weight on process validation and records. ISO 27001:2022 is information security, which matters when your CAD files are the product.

Match the certificate to your part, not to your industry label. A robotics bracket does not need ISO 13485. A surgical instrument housing does. If a supplier lists all four, ask which one governs your order and what records you will receive. The answer should name a document, not a department.

Inspection scope is the other half. GreatLight inspects 100% of parts before shipment, with raw material check, in-process monitoring, and final inspection, and reports on request. For a ±0.005 mm feature, in-process probing catches drift before the batch is finished. Final inspection alone catches it too late.

Ask for the report format before the PO. A dimensional report on a CMM, a material certificate, and a first-article report are three different documents. Knowing which ones arrive with the shipment prevents a hold at incoming inspection.

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  ISO 9001:2015Baseline system for general industrial work.
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  IATF 16949:2016Automotive traceability and process discipline.
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  ISO 13485:2016Medical device records and process validation.
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  ISO 27001:2022Information security for IP-sensitive designs.

A machined part is rarely a finished part. Anodizing, plating, powder coating, bead blasting, and laser marking all change dimensions, sometimes by more than the machining tolerance. If those steps happen at a second vendor, nobody owns the stack-up. The anodizer blames the machinist and the machinist blames the anodizer.

In-house finishing keeps the numbers in one place. Hardcoat anodizing builds roughly half the coating thickness into the surface, so a bore that was on size before coating may not be after. When both steps sit on the same floor, the process plan can pre-compensate. Plating and powder coating behave the same way.

Laser marking has its own limit: minimum character height 1.5 mm. Smaller text fills in or loses contrast, especially on curved surfaces. If your part number or UDI needs to be legible after finishing, plan the mark height and the finish order together.

A single vendor for machining and finishing also removes a freight leg and a queue. On a 3–5 day delivery target, one fewer handoff is often the difference between on-time and late.

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  Coating changes sizeAnodizing and plating build thickness; plan pre-compensation.
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  Mark after finish1.5 mm minimum character height for legible laser marks.
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  One ownerSame-floor finishing keeps dimensional responsibility clear.