CNC Machining Services for OEM Projects: A Buyer's Checklist

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  Key takeaways
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  What to check before you award an OEM machining contract
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  Tolerance: what a number on a website does not tell you
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  Machine capacity: the check that kills projects late
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  Certifications: read the scope, not the logo
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  MOQ, quoting and lead time: where budgets slip
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  When CNC machining is the wrong choice for an OEM project
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  Step by step: qualify an OEM machining partner in 7 moves

Every shop advertises a best-case tolerance. The useful question is whether that tolerance survives your part. A thin-walled aluminium housing at 6061-T6 behaves differently from a 17-4PH shaft, and a five-axis cut on a curved surface holds size differently than a flat face milled on a three-axis machine. Ask for the tolerance on the specific feature, not for the shop's floor number.

GreatLight works to ±0.005 mm (±0.0002 in) on turned and milled features, with surface finish options from Ra 0.2–0.8 μm for sealing faces up to Ra 1.6–3.2 μm for as-machined brackets. Those ranges are not a promise for every part. Long slender parts, deep pockets and thin webs push you toward the looser end unless you accept extra setups and more inspection time.

The practical test is the first article. Before a full run, we measure the critical dimensions, record the actual values and send the report with the parts. If a dimension sits at 80% of the tolerance band on the first piece, it will drift. A shop that shows you real numbers is easier to work with than one that only repeats a spec sheet.

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  Tight featuresBores, bearing seats and sealing faces justify ±0.005 mm and Ra 0.2–0.8 μm.
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  Loose featuresCovers, brackets and mounting holes are fine at Ra 1.6–3.2 μm.
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  Geometry mattersDeep pockets and thin walls are harder than the tolerance alone suggests.

An OEM project usually starts small and grows. The shop that cut your prototype must be able to cut part number 8,000 on the same process, or you will pay for a second qualification. That means looking at the machine list, not the brochure. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous five-axis centers, 16 mill-turn centers, 27 three-axis machines, 12 four-axis mills and a Ø400 mm rotary table.

Size limits decide more projects than most buyers expect. The largest travel is 4,000 × 400 × 150 mm. Medium work fits on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm tables. Compact parts run on 500 × 500 × 450 mm or 500 × 310 × 200 mm machines. If your part is 4,200 mm long, no amount of five-axis skill helps. Check the envelope before you send a purchase order.

Scope matters too. A shop that only mills will send your turned parts elsewhere, and you lose control of the schedule. Look for milling and turning under one roof, plus finishing and assembly if the drawing calls for anodizing or laser marking. GreatLight offers five-axis, four-axis and three-axis machining, mill-turn, rapid prototyping, sheet metal fabrication, die casting, vacuum casting, 3D printing and surface finishing.

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  Ask for travel limitsGet the X, Y and Z numbers for the machine that will run your part.
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  Check the second operationTurning, finishing and marking should stay in the same supply chain.
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  Plan for volume growthConfirm the same process can scale from 1 piece to 10,000+.

Certificates are only useful when they cover the work you are buying. ISO 9001:2015 is a general quality system and fits most industrial machinery, robotics and electronics parts. Automotive and EV programs usually require IATF 16949:2016. Medical device work needs ISO 13485:2016. If your drawings or data are sensitive, ISO 27001:2022 covers information security.

GreatLight holds all four: ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The point is not the wall of logos. It is that the audit trail, document control and traceability procedures are already in place, so you are not paying a supplier to build a quality system on your project.

Two questions cut through most certification claims. First, which site is certified? A group certificate may not cover the plant that will run your parts. Second, what does the inspection record look like? Ask for a sample report. If the shop cannot show you one, the certificate will not help when a dimension drifts.

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  Industrial partsISO 9001:2015 is the baseline requirement.
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  Automotive and EVIATF 16949:2016 is normally required by the customer's quality team.
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  Medical devicesISO 13485:2016 applies to the manufacturing process, not only the product.
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  Sensitive designsISO 27001:2022 governs how files and data are handled.

Minimum order quantity is a hidden cost. A shop that demands 500 pieces before it will quote forces you to buy inventory before the design is proven. GreatLight has no minimum order quantity, so you can start with one prototype and scale to a 10,000+ part run on the same process. That is useful when a design is still moving and you need real machined parts for a test fixture.

Lead time should be split into stages. Ask how long the quote takes, when production starts and when parts ship. GreatLight returns a quotation and free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days. The historical late-delivery probability is below 2%. Those numbers apply to normal jobs, not to parts that need a new mold or an unusual material.

Quoting is the last place to check. A price without a material grade, finish and inspection line is not a price. Ask whether the quote includes material certification, first-article inspection and packaging. If the answer changes after you place the order, the original quote was incomplete.

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  No MOQOne prototype is a valid order, and volume pricing comes later.
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  Quote in 12 hoursIncludes a free DFM analysis so you can fix problems before cutting.
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  Ship in 3–5 daysProduction can start within 24 hours of a released PO.

CNC machining is the right call for tight tolerances, low to medium volume and parts that must be functional, not just shaped. It is the wrong call when the part is a large thin shell with no critical dimensions, when the annual volume is high enough that tooling pays back, or when the geometry is a hollow form that only molding can produce.

Die casting and vacuum casting usually beat machining on cost once volume passes the tooling break-even point. Sheet metal fabrication is faster for flat brackets and enclosures. 3D printing wins when you need a form-and-fit check in days and the material properties are secondary. A shop that offers all of these can tell you which one fits, instead of pushing the process it owns.

There is also a middle path. A machined prototype can validate the design, then the same shop can quote a die-cast or vacuum-cast version for production. That keeps the interface dimensions consistent and avoids a second round of tolerance stack-up work.

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  Choose CNCTight tolerance, functional parts, low to medium volume.
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  Choose castingHigher volume where tooling cost is recovered by piece price.
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  Choose sheet metalFlat or folded parts with generous tolerances.
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  Choose 3D printingFast form-and-fit checks where material strength is not critical.