CNC Machining Aluminum Services: 5 Proven Checks

• 1
  What matters most when you compare quotes
• 2
  Five checks and what good looks like
• 3
  Alloy choice changes the quote, not just the part
• 4
  Tolerance: what ±0.005 mm really covers
• 5
  Surface finish and how it is specified
• 6
  Lead time, capacity and the questions behind them
• 7
  MOQ, documentation and confidentiality
• 8
  Step by step: from drawing to a decision

Aluminum is not one material. 6061-T6 is the default for brackets, housings and fixtures because it machines cleanly at high spindle speeds and holds a good finish. 2024 and 7075 are stronger but gummier; they need carbide tools, lighter depths of cut and more coolant to avoid built-up edge. If a supplier quotes all three at the same price, the quote is probably built on 6061.

Heat treatment matters too. 6061-T6 arrives stress-relieved, so a thin wall stays flat after machining. Untreated 6061 or a rough casting can move 0.05–0.15 mm after the clamps come off. For a 4,000 mm rail or a 600 × 600 mm plate, we rough, let the part rest, then finish. That extra step shows up in the price but saves a scrapped part.

Casting alloys such as ADC12 behave differently again. They are meant for die casting, not for hogging out from billet. If your design started as a casting and you now want a machined prototype, expect different wall thickness limits and porosity that a machined blank will not have.

Ask one question early: which alloy, and is it certified to the heat lot? A mill certificate costs little and answers most arguments later.

• 1
  6061-T6Best all-round choice for machined aluminum parts.
• 2
  7075 / 2024Higher strength, slower cutting, higher tool cost.
• 3
  5052 / 5083Good for sheet and formed parts, less ideal for heavy milling.
• 4
  ADC12Die casting alloy, not a billet machining grade.

A tolerance number without a feature list is marketing. On aluminum, ±0.005 mm is achievable on a bored hole, a milled pocket floor or a turned diameter when the part is small and rigid. It is much harder across a 500 mm span, where temperature alone moves the part. A 1 °C change over 500 mm of aluminum is about 0.012 mm of growth. That is physics, not skill.

So the useful question is not what tolerance the shop can hold, but on which features and at what size. Send the drawing and ask them to mark the dimensions that fall outside their standard band. A shop that answers with a marked-up PDF is telling you it read the part.

For most brackets and enclosures, ±0.05 mm is plenty and costs far less. Keep ±0.005 mm for bearing bores, mating faces and alignment pins. Tightening every dimension on a drawing adds inspection time and slows the job without improving function.

We measure on CMM and can supply reports on request. If a supplier cannot say which instrument checks your critical dimension, treat the tolerance claim as unverified.

Finish on aluminum is usually given as Ra. As-machined surfaces sit around Ra 1.6–3.2 μm, which is fine for internal brackets. Mating faces and seals often want Ra 0.8–1.6 μm. Optical and vacuum parts can go to Ra 0.2–0.8 μm with finer passes and slower feed.

The finish and the process are linked. A face milled with a 0.8 mm corner radius at 0.05 mm per tooth leaves visible steps. The same face with a wiper insert and a light finish pass looks almost polished. Neither is wrong, but they quote differently.

Anodizing sits on top of the machined surface and does not hide tool marks. Type II clear anodizing adds roughly 5–25 μm of oxide depending on bath time, so a thread or a press fit may need masking or a pre-plate allowance. Hardcoat is thicker and wears better; conductive anodizing is used where the housing must still ground.

Tell the shop the final function. A heat sink wants flatness and clean fins. A sliding guide wants low Ra and a consistent edge break. Those two parts may share a drawing but not a process.

Lead time claims are easy to make. What you can check is machine count and whether the shop runs its own second operations. We run 127 CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, plus finishing and inspection in the same group. That setup is why a quote can come back in 12 hours and parts can ship in 3–5 days.

Size drives machine choice. A part within 500 × 500 × 450 mm fits several cells and is easy to schedule. A 4,000 mm extrusion or rail needs the long-travel machines and a different fixture plan. Ask which machine your part will run on. The answer should name a travel envelope, not a brand.

One setup versus three matters for tolerance stack. A part with features on five faces may need two or three setups on a 3-axis machine, and each re-clamp adds position error. A 5-axis center reaches those faces in one setup, which is often cheaper than chasing the tolerance later.

Plan for the bottleneck, not the average. Anodizing, plating and heat treat are external steps in most shops. Ask whether the finish is in-house and how it affects the ship date.

For machined aluminum, tooling is minimal. Fixtures and programs are the real setup cost, and they do not scale with quantity the way a die does. That is why no minimum order quantity is normal for CNC work, from one prototype to a 10,000+ part run. If a shop insists on 5,000 pieces before it will quote, you are talking to the wrong process.

Documentation matters more than most buyers expect. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is what automotive programs ask for. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers how your files are handled. Match the certificate to your industry, not to a logo wall.

Confidentiality should be settled before you upload. Drawings, STEP files and test data are the design. Ask for an NDA before sending anything, and check that uploads are handled as confidential by default.

Finally, ask for inspection records. 100% inspection before shipment, with raw material checks, in-process monitoring and a final report on request, is a reasonable baseline. A shop that only offers a packing slip is asking you to trust the process instead of showing it.