CNC Aluminum Machining Parts: How to Pick a Supplier That Holds Tolerance

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  Key takeaways
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  Which aluminum alloy for which part
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  What CNC aluminum machining parts actually cost you in tolerance
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  Match part size and features to the machine, not the other way around
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  The features that break a quote after you place the order
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  Lead time, MOQ and paperwork: what a quote should state
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  How to read a supplier before you send the PO
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  Step by step: prepare a quote that comes back usable

Aluminum is soft, light and fast to cut, which is why so much of it goes through a mill. A 6061-T6 block removes at roughly three times the rate of 304 stainless under the same tool load. That speed is the reason aluminum milled parts are cheap at quantity, and also the reason shops hold tighter numbers on aluminum than on steel.

The catch is thermal. Aluminum expands about 23 μm per meter per degree C. A 300 mm part that measures 299.97 mm on the machine at 30 °C can read 300.00 mm in a 20 °C inspection room. If your drawing calls ±0.005 mm on a long dimension and the shop does not control temperature, you will argue about a number neither side can reproduce.

So put tight tolerances on the features that matter: bearing bores, mating faces, dowel holes, seal grooves. Leave general dimensions at ±0.1 mm or per ISO 2768-m. A blanket ±0.005 mm note across every dimension on a 400 mm plate does not improve the part. It just adds inspection time and rejects good parts.

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  Bores and fitsCall out Ø tolerance and roundness separately; roundness often drives the reaming step.
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  Flatness on thin platesState flatness over the full face, not per 25 mm, or the part will be shimmed at assembly.
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  Surface finishRa 0.8–1.6 μm is a normal milled finish; Ra 0.2–0.8 μm needs extra passes or a finish operation.

A supplier's machine list is only useful if you map your part onto it. Many RFQs fail because a part needs a 5-axis simultaneous cut but was quoted on a 3-axis mill with two re-fixturings. The second setup adds position error and lead time, and the shop usually eats that cost by cutting corners elsewhere.

Ask what travel the part fits. GreatLight runs 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Large work goes up to 4,000 × 400 × 150 mm; the mid range covers 750 × 1,150 × 550 mm and 600 × 600 × 600 mm; compact cells handle 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table handles round and index work.

Five-axis is not automatically better. A prismatic plate with holes on three faces is usually faster on a 4-axis machine with a tombstone. Five-axis earns its cost when you have compound angles, undercut pockets, or a contour that would need three setups otherwise. Say so in the RFQ and let the shop quote the cheaper route.

Thin walls are where aluminum milling gets expensive. At 2 mm and above, a normal 6061 wall machines cleanly with light finishing passes. At 1 mm, chatter and spring-back appear unless the shop reduces radial engagement and adds a semi-finish pass. Below 0.8 mm, you are into fixture design and sometimes stress relief between roughing and finishing.

Deep pockets have the same problem in a different direction. A pocket 6× deeper than the tool diameter needs a long, slender cutter that deflects. The shop compensates by stepping down in small increments, which multiplies cycle time. If the pocket is for weight only, open it up or add a corner radius. A 3 mm corner radius instead of a sharp internal corner can cut cycle time noticeably on a deep cavity.

Sharp internal corners are a drawing habit, not a design need. Every sharp corner in a milled pocket requires a tool with a smaller radius than the corner, and that tool is slower and more fragile. Put the largest radius the function allows on every internal corner.

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  Wall thicknessKeep above 1 mm where possible; design in ribs instead of a thin flat panel.
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  Pocket depthStay under 4× tool diameter for a stable cut; beyond 6× expect multiple passes.
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  ThreadsSpecify thread depth and class; blind holes need clearance at the bottom for the tap.

A usable quote names the alloy and temper, the tolerance standard, the finish, the inspection level and the delivery date. If any of those is missing, you are comparing two different offers. The cheapest number often assumes commercial tolerance, as-machined finish and no inspection report, which is not the part on your drawing.

Ask about MOQ early. Some shops price a one-off prototype at a rate that makes a 500-piece run uneconomical, or hide a setup fee in the unit price. GreatLight quotes from one prototype to 10,000+ part runs with no minimum order quantity, so the same process and inspection apply to both.

Lead time follows from setup count, not from part count. A single-setup 5-axis part can ship in 3–5 days after a 12-hour quote and DFM review, with production starting within 24 hours. A part needing anodize, laser marking and a separate inspection report adds days. Build those steps into the schedule instead of discovering them at shipping.

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  Finish calloutAnodize type and color, or bead blast plus Ra, must be on the drawing.
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  Inspection levelState whether you need a dimensional report, material cert, or first-article report.
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  ConfidentialityIf the design is sensitive, ask for an NDA before you upload the model.

Certifications are a filter, not a ranking. ISO 9001:2015 covers general quality management. IATF 16949:2016 matters if the part goes into a vehicle program and you need PPAP-style documentation. ISO 13485:2016 applies to medical devices. ISO 27001:2022 is about how your files are handled, which matters when you are sending unreleased CAD.

Ask what happens when a dimension is out. A shop with in-process monitoring catches the drift during the run. A shop that only inspects at the end finds it after the parts are made. GreatLight checks raw material, monitors in process and inspects 100% before shipment, with reports on request.

Finally, test the technical response. Send a drawing with one hard feature and see whether the reply mentions it. A shop that returns a DFM note about a 0.6 mm wall or a 1.5 mm laser-marking character height is reading your file. One that only sends a price is not.