OEM aluminum CNC processing: how the process actually works

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  What OEM aluminum CNC processing changes about the part
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  Which aluminum alloy suits your OEM aluminum CNC processing job
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  How 3-axis, 4-axis, and 5-axis setups split the work
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  Tolerance, surface finish, and what actually drives cost
• 5
  Finishing options and their effect on fit
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  What to verify before you place an OEM aluminum order
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  Choosing alloy and process route by part type

OEM aluminum CNC processing is subtractive. A rotating cutter removes material from a solid billet, plate, or extrusion until the geometry matches the CAD model. Nothing is molded, so no tooling is cut and no draft angle is needed. That is why the process fits low and mid volume work, engineering changes, and parts that must be functional before anyone commits to a die.

The aluminum side of the job matters as much as the spindle. Aluminum cuts fast, moves heat away from the edge quickly, and can be machined dry or with minimal lubrication. But it also has a low modulus, roughly a third of steel. Thin walls deflect under cutting force, and a part that measures correctly on the bench can spring out of tolerance once it is unclamped.

So the real work of OEM aluminum CNC processing sits in three places: choosing the alloy, controlling the setup and workholding, and holding size while the material releases internal stress. A supplier who only talks about spindle speed is describing half the job.

The output you receive is a finished part, not a near-net shape. Holes, bores, threads, pockets, and sealing faces are all cut in the same setup where possible, which is what keeps hole-to-hole position accurate. Post-processing such as anodizing happens after machining, and it adds a thin oxide layer that can shift a tight fit.

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  No tooling costGeometry changes cost programming time, not a new mold.
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  One setup, many featuresFewer re-fixturings means less stack-up error.
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  Stress releaseRough, rest, then finish for thin or asymmetric parts.

6061-T6 is the default answer for most housings, brackets, fixtures, and manifolds. It machines cleanly, welds, anodizes well, and holds ±0.005 mm on stable features without drama. If you have no strong reason to pick something else, 6061-T6 is the alloy to start from.

7075 is roughly twice the strength of 6061 and machines to a sharper edge, which is why it shows up in aerospace brackets and high-load fixtures. It also costs more, welds poorly, and anodizes to a slightly different color than 6061, so mixing the two in one visible assembly is a mistake. 2024 is strong and fatigue resistant but has weak corrosion resistance unless it is clad or coated.

For corrosion resistance and formability, 5052 and 5083 are the marine and tank alloys, though they are gummy to machine and better suited to sheet work than to billet parts. 6082 sits close to 6061 with slightly better strength in Europe-sourced stock. ADC12 is a die-casting alloy; it appears in machined castings rather than in parts cut from plate.

Two practical rules. First, specify temper, not just alloy. 6061-T6 and 6061-O behave nothing alike on the machine. Second, tell us the function. A bracket loaded in bending and a cosmetic bezel may share an alloy but need completely different finishing and inspection.

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  6061-T6General purpose, anodizes well, good tolerance stability.
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  7075-T6High strength, sharp edges, higher cost, poor weldability.
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  2024Fatigue resistance, needs coating for corrosion.
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  5052 / 5083Corrosion resistance, gummy to cut, better as sheet.

A 3-axis machine moves the cutter in X, Y, and Z while the part stays still. It is the cheapest way to make a plate with pockets, holes, and a flat back. The limit is reach: any feature on a side wall needs a second setup, and each new setup adds a locating error you cannot remove later.

A 4-axis mill adds rotation about one axis, usually A. That solves parts that are prismatic around a single centerline, such as a shaft with cross holes, a manifold with ports on four faces, or a long extrusion with features along its length. One rotary setup replaces three or four flat setups.

5-axis machining tilts the tool or the table so the cutter can approach a face at the correct angle. For aluminum this matters in two ways. Short, rigid tools reach deep pockets without long overhangs, so chatter drops and surface finish improves. And undercut geometry, angled ports, and compound curved surfaces can be cut in a single continuous pass.

The choice is not about prestige. If a part has three orthogonal faces and no undercuts, 3-axis with good fixtures is faster and cheaper. If it has angled faces, deep cavities, or tight position between features on different planes, 5-axis usually wins once you count the setups it removes. GreatLight runs 16 simultaneous 5-axis centers, 12 four-axis mills, and 27 three-axis machines, so the setup plan can follow the geometry rather than the other way round.

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  3-axisFlat plates, open pockets, single-face work.
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  4-axisParts that rotate around one axis, cross holes, long profiles.
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  5-axisAngled faces, undercuts, deep cavities, compound surfaces.
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  Mill-turnRound parts with milled features in one cycle.

Tolerance is a cost multiplier, not a checkbox. Going from ±0.05 mm to ±0.005 mm changes the machine, the tool, the inspection, and often the number of passes. On aluminum, ±0.005 mm is achievable on bores, journals, and mating faces when the part is rigid and the temperature is stable. It is a poor target for a thin wall in free air, because the material will move after clamping.

Surface finish follows the same logic. As-machined aluminum lands around Ra 1.6–3.2 μm, which is fine for brackets and internal parts. A finish of Ra 0.8–1.6 μm is normal for sealing faces and bearing bores. Ra 0.2–0.8 μm calls for finer stepovers, sharp tooling, and often a separate finishing pass, so it should be reserved for surfaces that seal, slide, or show.

Be specific about which surfaces carry the tight callout. Marking an entire drawing ±0.005 mm when only two bores matter forces the shop to inspect everything to that band and slows the job. Datum selection matters too: a tolerance tied to a rough cast surface is hard to hold, while one tied to a machined face is straightforward.

Anodizing adds roughly 5–25 μm per surface depending on the process. On a Ø10 H7 bore that can close the fit. If a bore must stay precise after hardcoat, tell us and we will cut it undersize or mask it. Same for threads: a masked thread runs freely, an anodized one may not.

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  ±0.05 mmGeneral machined features, low cost impact.
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  ±0.005 mmBores, journals, mating faces on rigid parts.
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  Ra 1.6–3.2 μmAs-machined, internal and non-cosmetic.
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  Ra 0.2–0.8 μmSealing and sliding surfaces, extra pass required.

Anodizing is the most common aluminum finish. Clear and colored types build an oxide layer of roughly 5–25 μm, hardcoat builds more, and conductive anodizing is used where the part must still ground. That layer is ceramic-hard, so it resists wear and scratches, but it is also brittle at sharp corners and it changes dimensions.

Plating options include electroless nickel, zinc, silver, and gold. Electroless nickel gives a uniform coating on complex geometry and good wear resistance, which suits valve bodies and tooling. Silver and gold are used for conductivity and RF hardware. None of these are aluminum-native, so they need a zincate pre-treatment to bond.

Mechanical finishes are bead blasting, tumbling, brushing, and polishing. Bead blasting evens out tool marks and gives a matte look before anodizing. Tumbling deburrs edges in volume. Brushing leaves a directional grain that hides small scratches on flat panels. Polishing gets close to a mirror but shows every handling mark afterwards.

Laser marking handles part numbers, logos, and traceability codes. The minimum character height we can hold is 1.5 mm, so plan your marking field accordingly. Marking can be done before or after anodizing; marking after anodizing gives higher contrast on dark parts.

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  AnodizingClear, color, hardcoat, conductive; builds 5–25 μm.
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  Electroless nickelUniform on complex shapes, good wear resistance.
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  Bead blastingMatte pre-anodize finish, hides tool marks.
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  Laser markingMinimum character height 1.5 mm.

Ask how the shop handles first articles. For OEM aluminum CNC processing, the first part should be inspected against the drawing and the report sent to you before the run continues. GreatLight inspects 100% of parts before shipment, covering raw material check, in-process monitoring, and final inspection, with reports on request.

Ask about material traceability. Aluminum plate and bar should arrive with a mill certificate that ties heat number to alloy and temper. Without it, a 6061 part and a 6082 part look identical on the bench and behave differently in service.

Ask what happens when the drawing is ambiguous. A shop that quotes from the model alone will guess at datum structure, thread class, and which surfaces are cosmetic. A DFM note back within 12 hours tells you the shop read the file. Ours comes back with the quotation.

Finally, check the confidentiality terms. If your part is unreleased, an NDA should be in place before files move. Uploads are treated as secure and confidential, and a signed NDA is available on request. Certification matters too: ISO 9001, IATF 16949, ISO 13485, and ISO 27001 cover different risks.

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  First article reportInspect before the full run continues.
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  Mill certificatesHeat number tied to alloy and temper.
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  DFM feedbackConfirms the file was actually reviewed.
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  NDA and certificationsMatch the paperwork to your industry.