CNC Machining Service Aluminum: How to Choose a Supplier

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  Key takeaways
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  What to check before you award the job
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  Start with the alloy and the part geometry
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  Tolerance and finish: what is realistic
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  Lead time, MOQ and the documents that matter
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  How to read an aluminum machining quote
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  Step by step: sourcing an aluminum CNC job

Aluminum is not one material. 6061-T6 is the default for brackets, plates and housings because it cuts fast, holds a good finish and welds well. 7075-T6 is roughly twice the strength but galls more easily, so it needs sharp carbide, lower feed per tooth and more coolant. 2024-T4 machines well but has poor corrosion resistance without a coating. If your drawing says only "aluminum," the shop picks the cheapest grade that fits, and that may not be the one your FEA assumed.

Geometry decides the machine. A part with features on five faces, deep pockets or undercuts will need either multiple setups on a 3-axis mill or one setup on a 5-axis center. Multiple setups add fixture error: each re-clamp can shift the datum by 0.02–0.05 mm. A 5-axis center holds one datum and reaches the back side without moving the part, which is why it is the safer route for thin walls and complex contours.

Wall thickness matters more than overall size. Below about 1.5 mm in aluminum, cutting forces start to deflect the wall and chatter shows up as a rippled surface. The fix is not a slower spindle; it is a lighter radial cut, a higher helix tool and sometimes a support wax or sacrificial rib. Ask the shop how they plan to hold thin sections before the first chip is cut.

Quantity changes the plan too. One prototype can be machined from billet with soft jaws and no fixture. A 10,000-part run justifies a dedicated fixture, probing cycles and SPC sampling. A supplier that quotes both the same way is not thinking about your cost curve.

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  6061-T6General purpose. Fast cutting, good finish, easy anodizing.
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  7075-T6High strength. Slower feeds, sharper tools, watch for stress cracks.
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  2024-T4Fatigue resistant. Needs coating for corrosion protection.
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  ADC12Die-cast grade. Use when the part starts as a casting and is only finished by CNC.

A tolerance callout is only meaningful if the shop can measure it. General machining on aluminum holds ±0.1 mm without special effort. Moving to ±0.025 mm requires stable tooling, temperature control and a machine that repeats. The tightest band we quote is ±0.005 mm, and that applies to specific features, not the whole part. If every dimension on your drawing carries the same tight tolerance, expect a higher price and a longer first-article review.

Surface finish follows the same logic. As-machined aluminum typically lands at Ra 1.6–3.2 μm. A fine carbide finish pass gets you to Ra 0.8–1.6 μm. Below Ra 0.8 μm you are looking at polishing or a lapping operation, and the geometry may not allow it in corners. Specify finish only where it matters: sealing faces, optical mounts, sliding surfaces. A blanket Ra callout on every face adds cost with no functional gain.

Anodizing and other coatings change dimensions. Type II anodizing builds roughly 0.005–0.015 mm per surface, hardcoat can add more, and the growth is not perfectly uniform on edges. If a bore must stay at a press-fit size, mask it or cut it undersize before coating. Tell the shop which features are critical after finishing, not before.

Inspection is where tolerance claims become real. Ask what gets measured, on what machine, and whether the report travels with the parts. For a first article, a CMM report on the datum and the critical features is standard practice. For production, in-process probing plus a final check on sampled parts is enough for most aluminum work.

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  ±0.1 mmStandard milling, no special setup.
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  ±0.025 mmNeeds stable tooling and temperature control.
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  ±0.005 mmFeature-specific. Confirm the shop can measure it.

Lead time is easier to judge by how fast the shop responds than by the number printed on a quote. A supplier that returns a quote plus a DFM note within 12 hours has already looked at your model. If the DFM flags a thin wall or an unreachable corner, that is a good sign. A quote with no comments usually means nobody opened the file.

MOQ is a common blocker for prototype work. Some shops will not quote a single aluminum part because the setup cost exceeds the part price. Ask directly whether they run one-offs and what the minimum charge is. No minimum order quantity means you can test a design before committing to a production run, which is often the whole point of an aluminum prototype.

Certifications tell you which industries the shop is set up to serve. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and implies traceability and change control. ISO 13485:2016 is for medical devices and adds process validation and clean handling. ISO 27001:2022 covers information security, which matters if you are sending CAD files under NDA.

Get the paperwork list in writing before the PO. For most aluminum parts that means a material certificate, a dimensional report on request, and a finish certificate if anodizing is subcontracted. If your end customer audits the supply chain, ask whether the shop can provide a first-article inspection report and a control plan. These are normal requests, not favors.

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  Material certConfirms the alloy grade and heat lot.
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  Dimensional reportCMM or hand-tool data on critical features.
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  First-article reportFull layout on the first part of a new run.
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  NDASign before CAD files are uploaded.

Aluminum quotes are usually built from four lines: material, machining time, finishing and inspection. Material is priced by weight plus a cut charge, so a part nested from a standard plate is cheaper than one cut from a thick billet. Machining time depends on the number of setups and the volume of metal removed. A quote that lists only a total price hides where the cost sits, which makes it hard to compare two suppliers fairly.

Setup count is the biggest lever you control. A design that can be machined from three sides needs two or three setups; one that can be reached from five sides in a single 5-axis setup may cost less overall even at a higher hourly rate. If two quotes differ by more than 30 percent, ask each shop how many setups they planned. The answer usually explains the gap.

Finishing is often quoted separately, and it should be. Anodizing is priced by surface area and color, hardcoat costs more than Type II, and masking adds labor. Bead blasting before anodizing gives a matte look but can round sharp edges, so specify whether edges must stay crisp. Laser marking is priced per character or per part, and character height below 1.5 mm is hard to read after coating.

Watch for quotes that assume a different alloy, omit finishing, or exclude inspection. Those are not cheaper quotes; they are incomplete ones. A fair comparison puts the same alloy, the same finish and the same report on every line.

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  MaterialGrade, form and nesting plan.
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  MachiningNumber of setups and cycle time.
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  FinishingAnodize type, color, masking.
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  InspectionWhich features get measured and reported.