OEM Metal CNC Machined Parts Service: How to Pick a Supplier

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  The essentials of buying machined metal parts
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  What to ask vs. what a good answer looks like
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  What an OEM metal CNC machined parts service actually covers
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  Reading tolerance and surface finish claims
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  Which parts suit 3-axis, 4-axis or 5-axis work
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  Material and finish choices that change the outcome
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  Lead time, MOQ and quote details to pin down
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  Certifications and inspection that hold up in an audit
• 9
  7 steps to qualify an OEM metal CNC machined parts supplier

An OEM metal CNC machined parts service is contract manufacturing of metal components built to your drawings and material callouts. The supplier does not sell a catalog item. It converts your CAD files, tolerance stack and finish requirements into a production process, then repeats that process across however many pieces you order.

That covers more than cutting metal. A complete service includes DFM review before the first chip, material sourcing, fixturing, machining on the right machine class, deburring, surface finishing, inspection and packing. If any of those steps sit outside the supplier's walls, you inherit a coordination problem.

The job is judged on repeatability, not on a single beautiful first article. A prototype that measures perfectly on Monday tells you little about piece 4,000 on a Friday shift. Ask how the process is held: spindle health, fixture rigidity, temperature, tool wear and in-process gauging.

GreatLight has run this kind of work since 2011 from Dongguan, with a second plant in Singapore. The shop holds 127 high-precision CNC machines across 7,600 m², including 16 simultaneous 5-axis centers and a Ø400 mm rotary table for parts that need multi-face access in one setup.

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  Built to drawingYour geometry, your material, your finish spec.
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  Repeatable processThe same setup and inspection logic on every run.
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  Single ownerMachining, finishing and inspection under one roof.

A tolerance number on a website is a promise about the shop's best day. What you need is the tolerance it can hold on your part, on your machine class, in your quantity. A 300 mm aluminum bracket and a 20 mm stainless steel fitting do not live in the same tolerance world, even from the same vendor.

At GreatLight, the working figure is ±0.005 mm (±0.0002 in) on features the process can support. Whether your part reaches that depends on geometry, material and how the datum is set. Deep pockets in 17-4PH fight you. Thin walls in 6061 move when you unclamp them.

Surface finish follows the same logic. Ra 0.2–0.8 μm is a fine finish, often reached by fine boring or polishing. Ra 0.8–1.6 μm is the common high-quality machined finish. Ra 1.6–3.2 μm is as-machined and perfectly acceptable for most internal brackets.

Do not specify a tighter finish than the function needs. A Ra 0.4 μm callout on a hidden mounting face adds cost and cycle time and buys nothing. Put tight tolerances and fine finishes only on the features that touch, seal, slide or are measured.

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  Tolerance is per featureNot a single number for the whole part.
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  Finish costs moneyTighter Ra means slower passes or extra operations.
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  Datum choice mattersA clean datum makes the tolerance achievable.

Machine class is a cost decision as much as a capability decision. A 3-axis mill handles flat plates, pockets, slots and parts you can reach from one direction. If your part is a bracket with features on two faces, a 4-axis machine with a rotary table cuts it in fewer setups and holds position better.

Five-axis simultaneous machining pays off when the part has compound angles, deep contoured pockets, or features on five faces that must stay aligned to each other. Engine parts, impellers, medical instruments and aerospace housings fall into this group. GreatLight runs 16 simultaneous 5-axis centers for exactly this reason.

Size drives the choice too. The shop's largest travel is 4,000 × 400 × 150 mm, with medium platforms at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm. Long, slender parts and large plates need the big platform.

Turning and mill-turn work is a separate track. Shafts, bushings, fittings and parts with concentric diameters belong on a lathe or mill-turn center, not on a mill with a rotary table. GreatLight runs 16 mill-turn centers so a part can be turned and milled without a second setup.

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  3-axisPrismatic parts, one accessible face.
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  4-axisTwo or more faces, moderate complexity.
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  5-axisCompound angles, tight multi-face alignment.

Material selection is often fixed by the drawing, but the grade still matters. In aluminum, 6061-T6 is the general-purpose choice, 7075 gives higher strength, and 2024 machines well but has weaker corrosion resistance. In stainless, 303 machines freely, 304 and 316 resist corrosion, and 17-4PH gives strength with heat treatment.

Steel grades such as 1018, 1045, 4130, 4140 and 4340 cover the range from easy-machining low carbon to high-strength alloy. Titanium TC4 (Ti-6Al-4V) and Inconel are used where weight or temperature demands it, but they cut slowly and wear tooling, so expect a longer cycle and a higher price.

Finishing is where outsourcing hurts most. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting and laser marking all change the part after machining. If those steps leave the building, your schedule now depends on a third party's queue.

Laser marking has a practical limit worth noting: minimum character height is 1.5 mm. If your drawing calls for smaller text or a dense data matrix, raise it at the DFM stage rather than discovering the limit after finishing.

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  Aluminum6061-T6 general purpose; 7075 for strength.
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  Stainless303 free-cutting; 316 for corrosion; 17-4PH for strength.
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  Titanium and InconelSlow cutting, higher cost, use only when required.
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  In-house finishingKeeps the schedule under one roof.

Quote speed tells you something about a shop's load. GreatLight returns a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours of approval. Parts typically ship in 3–5 days. Those windows assume the drawing is released and the material is in stock.

The historical late-delivery probability at GreatLight is below 2%. Treat that as a process indicator, not a guarantee for your specific order. Ask any supplier how they track on-time delivery and what happens when a machine goes down mid-run.

MOQ is the other commercial lever. GreatLight sets no minimum order quantity, so a single prototype and a 10,000+ piece production run both go through the same shop. That matters when you are still validating a design and do not want to fund a large first order.

Get the quote structured by operation: material, machining, finishing, inspection and freight. A single lump sum hides where the cost sits and makes it hard to compare two suppliers fairly. It also makes it hard to see which feature is driving the price.

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  Quote window12 hours including DFM notes.
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  Production startWithin 24 hours of approval.
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  ShippingParts ship in 3–5 days.
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  MOQNone — one piece to 10,000+.

Certifications are a filter, not a ranking. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive and EV work. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security, which matters when your drawings are confidential.

Match the certificate to your industry. A medical device program run by a shop with only ISO 9001 will create paperwork problems at your own audit. Automotive work without IATF 16949 raises the same flag. GreatLight holds all four, which lets one supplier cover several product lines.

Inspection is where claims meet evidence. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection. Reports are available on request. If a drawing calls for a first article inspection report or a specific sampling plan, say so at the quote stage.

Ask how non-conformance is handled. A supplier should be able to describe containment, root cause analysis and corrective action without reading from a script. If the answer is vague, the inspection process is probably paperwork.

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  ISO 9001:2015Baseline quality management.
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  IATF 16949:2016Automotive and EV programs.
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  ISO 13485:2016Medical device components.
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  ISO 27001:2022Information security for sensitive files.