Metal Aluminum CNC Milling: How to Choose a Supplier Who Holds Tolerance

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  Key takeaways
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  Which machining route fits your aluminum part
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  Alloy selection drives the whole metal aluminum cnc milling plan
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  Axis count and work envelope set your real tolerance ceiling
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  What ±0.005 mm actually requires from you and the shop
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  Surface finish, anodizing and the marks you cannot hide
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  Certifications, order quantity and lead time: the buyer-guide checks
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  Seven steps to qualify a metal aluminum cnc milling supplier

Aluminum is not one material. A 6061-T6 bracket and a 7075-T6 bracket can look identical on a drawing and behave nothing alike in the cut. 6061 machines cleanly, welds well and takes anodizing evenly, which is why it covers most housings, plates and fixtures. 7075 runs about twice the strength but chips hard, galls on the tool and is far less forgiving of a light fixture.

For corrosion resistance near salt water or coolant spray, 5052 and 5083 are the usual picks. They form well and resist pitting, but they are gummy under a cutter and tend to leave a built-up edge on the insert. 2024 machines well and is strong, though it needs a protective finish because it corrodes quickly at bare edges.

Ask what the shop stocks as standard bar and plate. A supplier who keeps 6061, 7075, 2024 and 6082 on the floor can quote in hours. One who has to source a single billet will add days and may substitute an alloy without telling you. Substitution is the most common quiet failure in this trade.

Wall thickness matters as much as alloy. Below roughly 0.8 mm on aluminum, chatter and spring pass become real risks. If your design pushes thinner, say so before quoting so the shop can plan fixture support and lighter finishing passes.

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  General parts6061-T6 is the default for housings, plates and brackets.
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  High strength7075-T6 when weight or load matters more than machinability.
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  Marine or wet service5052 and 5083 resist pitting better than 6061.
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  Thin wallsFlag anything under 0.8 mm so the fixture can be planned.

Every extra setup adds error. A part machined on three sides of a 3-axis mill needs three datum transfers, and each transfer stacks a small offset onto the next. A 5-axis machine reaches those faces in one setup, so true position between features stays tight. That is the practical reason to move up in axis count, not the brochure language.

The trade-off is programming time. Simultaneous 5-axis tool paths take longer to prove out, and a shop with thin 5-axis capacity will quote long lead times for simple parts. If your part has no undercut and no compound angle, 3-axis is faster and cheaper. Use the table above to decide before you send the RFQ.

Work envelope matters for large parts. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, including 16 simultaneous 5-axis machining centers, 12 four-axis mills and 27 three-axis machines. The largest travel is 4,000 × 400 × 150 mm, with medium envelopes at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. A Ø400 mm rotary table covers most round work.

One detail buyers miss: on a long part, thermal growth moves the work. A 4,000 mm aluminum plate can shift more than the tolerance band between a cold morning and a warm afternoon. Good shops rough, rest and finish rather than cutting to size in one pass.

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  One setup beats threeFewer datum transfers means tighter true position.
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  Skip 5-axis when you canNo undercuts or compound angles means 3-axis is faster.
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  Check the envelopeConfirm the machine travel covers your part plus fixture.
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  Rough, rest, finishLarge aluminum parts need a stress-relief pause.

A tolerance callout is a contract between the drawing and the inspection report. ±0.005 mm (±0.0002 in) is achievable on aluminum, but only on features where the machine, the fixture and the probe all sit inside that band. It is not a blanket number you can stamp on every dimension. Apply it where function demands it and open it up everywhere else.

Tight tolerance on an unreachable feature is a common mistake. A ±0.005 mm bore at the bottom of a 60 mm deep pocket needs a long tool, and long tools deflect. Either the shop spends extra passes with a stub tool, or the dimension drifts. Design the access before you set the number.

Surface finish links to tolerance as well. A tight bore with a rough wall will not gauge cleanly. Fine finishing to Ra 0.2–0.8 μm is available, though it costs cycle time. Ra 0.8–1.6 μm covers most sealing faces and bearing bores, and Ra 1.6–3.2 μm is normal as-machined work for non-critical surfaces.

Ask how the shop measures. Calipers cannot verify ±0.005 mm. You want CMM reports or at least bore gauges with traceable calibration. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection, and reports on request.

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  Tolerate only what mattersReserve ±0.005 mm for functional features.
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  Design the accessA deep tight bore needs a stub tool or a wider opening.
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  Match finish to functionSealing faces at Ra 0.8–1.6 μm, cosmetic at Ra 0.2–0.8 μm.
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  Demand the reportCalipers cannot prove ±0.005 mm. Ask for CMM data.

Anodizing adds roughly half the oxide thickness to the surface, and it magnifies every tool mark. A part that looks fine in bare aluminum can show visible witness lines after clear anodizing. If the part is cosmetic, tell the shop up front. They will adjust stepover, feed and tool path direction so the grain reads uniformly.

Hardcoat anodizing builds a thicker, harder layer and tends to darken color slightly. Conductive anodizing keeps the surface electrically active for grounding paths, which matters on electronics housings. Each of these is a different bath, so the finish choice belongs in the quote, not in a phone call after machining.

Bead blasting and tumbling even out tool marks before coating. Brushing leaves a directional grain that reads well on panels. Polishing gets close to a mirror, though on aluminum it is slow and easy to overwork. Laser marking is a common last step, with a minimum character height of 1.5 mm for legible text.

Keep masking in mind. Threaded holes, sealing faces and electrical contact pads usually need masking before anodizing. If you do not specify them, the coating will cover the threads and the part will not assemble. Mark every masked feature on the drawing.

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  Cosmetic parts need a warningSay so at RFQ stage so the tool path is planned for appearance.
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  Mask the threadsAnodizing adds thickness and will close a tight thread.
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  Pick the right bathClear, hardcoat and conductive anodizing are not interchangeable.
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  Laser marking limitCharacters below 1.5 mm may not read cleanly.

Certification is a filter, not a ranking. ISO 9001:2015 tells you the shop runs a documented quality system. IATF 16949:2016 is the automotive standard and matters if your part ends up in a vehicle program. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you are sending proprietary CAD files. GreatLight holds all four.

Order quantity is where small teams get squeezed. Plenty of shops price a single prototype at a level that makes the second iteration impossible. No minimum order quantity changes the economics of design work, because you can test one part, revise and test again without a tooling charge. The same process should scale to a 10,000-part run without a re-qualification.

Lead time has three parts: quoting, production start and shipping. A quotation and free DFM analysis within 12 hours lets you plan. Production can start within 24 hours once files and material are confirmed. Parts ship in 3–5 days for typical work. Ask for all three numbers, not one, because a fast quote with a slow start is not fast.

Finally, ask about confidentiality. Uploads should be secure and confidential, and an NDA should be available on request if your design is sensitive. That is a process question, not a legal formality.

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  Read the certificatesMatch the standard to your industry, not to a logo wall.
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  Check the MOQ policyNo minimum order quantity keeps prototyping affordable.
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  Get three lead-time numbersQuote, production start and ship date are separate promises.
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  Ask about the NDASecure uploads and an NDA on request protect your design.