CNC Aluminum Cost-Saving Design: GD&T, Finish, and Features

Fast time-to-market and tight budgets don’t have to fight your machining bill. The biggest, most controllable levers in CNC aluminum often hide in plain sight: how you tolerance the drawing, which surface finish you actually need, and how you shape features for tool access. Get those three right and you’ll cut cost and lead time without sacrificing function.

Key takeaways

• Default to general tolerances (ISO 2768) and apply tight GD&T only to function-critical features; over-tolerancing inflates both cycle and inspection time.

• Match finish to function: as‑machined Ra ≈ 3.2 µm is fine for many non-cosmetic parts; bead blast for matte; anodize Type II for appearance/corrosion, Type III for wear.

• Design features for machinability: avoid deep, narrow pockets; keep walls ≥ 1.0 mm; add generous internal fillets; minimize undercuts.

• State inspection intent in RFQs (gauge vs. CMM) and limit formal reports to truly critical features.

• Consider 5‑axis to reduce setups and use shorter, stiffer tools on complex, multi-face parts.

  
  

GD&T for cost control without compromising function

Here’s the big idea: tolerance only what the part’s function truly needs. Let non-critical dimensions ride on general tolerances and use GD&T where it clarifies function (datums, position, form) instead of spraying ± numbers everywhere. Using ISO 2768 as your base defaults keeps quotes reasonable and focuses attention on what matters.

For hole patterns, prefer true position at MMC rather than very tight bilateral size limits. It expresses function and often enables functional-gauge checks at moderate tolerances, reducing CMM dependency. Reserve tight positional tolerances (<0.10 mm) and form controls (flatness/parallelism) for sealing, precision alignment, and bearings, and be explicit about which features need inspection data.

Inspection time and cost scale with tolerance tightness. Trade publications and metrology vendors consistently note that moving from moderate true position (around 0.20 mm) to very tight (around 0.05 mm) shifts verification from simple gauges to CMM programs—minutes per part instead of seconds in volume scenarios. Because per-part timing depends on probing strategy and volume, treat the savings as a range and validate with your supplier’s quality team.

Micro-example (tolerance relaxation and inspection scope)

• Situation: A 6‑hole pattern used for a cover plate alignment was held at true position 0.05 mm with CMM reports on all parts.

• Change: Relax true position to 0.20 mm at MMC, keep two dowel holes at 0.10 mm with report, leave the rest to ISO 2768 base.

• Expected effect: In many shops, the relaxed holes can be verified by functional gauges or spot checks while only the dowel holes require CMM data. This commonly trims inspection time per part from minutes to seconds and reduces fixturing complexity, aligned with supplier guidance and CMM usage trends highlighted by metrology vendors and machining outlets.

Surface finish and roughness (Ra): choose only what you need

Finish is a classic over-spec area. “As machined” on aluminum often looks better than teams expect, and you can always step up to bead blast or anodize when the function or cosmetics require it.

• Typical as‑machined Ra: About 1.6–6.3 µm (63–250 µin) depending on toolpath, cutter, and setup.

• Bead blasting: Produces a uniform matte and can equalize visual texture, but it doesn’t create a mirror finish; Ra often sits roughly in the 0.8–>3.2 µm band depending on media and pressure.

• Anodizing: Anodize preserves the underlying texture. Type II (sulfuric) is commonly 10–25 µm average for decorative/corrosion resistance; Type III (hardcoat) is thicker (≈35–50 µm typical) for wear.

Procurement-ready spec phrases (copy/paste into RFQs)

• “Unless noted, as‑machined Ra ≤ 3.2 µm (126 µin).”

• “Bead blast prior to anodize: Glass bead #70–120, 80–100 psi, uniform matte; mask threads and Ø holes labeled critical.”

• “Anodize Type II per MIL‑PRF‑8625, clear, average thickness 10–15 µm (8–12 µm local).”

• “Hardcoat anodize Type III, Class 1 (undyed), thickness 35–50 µm; mask sealing faces; verify thickness by eddy-current if required.”

Cost/lead-time sensitivity of finish choices

Surface Finish decisions often add secondary handling, masking, QA checks, and queue time. Use the following as a directional guide when scoping budget and schedule.

Notes: Ranges reflect supplier pages describing finish processes and queue times; validate with your finisher’s current load and color availability.

Structural and feature design: geometry that machines faster

Think of the cutter like a stiff pencil: short, fat, and with a smooth path, it moves fast and clean. Long reach, sharp internal corners, and thin walls slow everything down.

Rules of thumb for CNC aluminum

• Thin walls: Aim for ≥ 1.0 mm as a general baseline; thinner walls (≈0.6–0.8 mm) are possible for short spans with careful fixturing but increase chatter and deformation risk.

• Pocket depth-to-width: Avoid deep, narrow pockets. Efficient ranges are often about 1:4 to 1:6 depth:width; deeper cavities need long tools and slower step-downs, raising cycle time.

• Internal fillets: Use large, consistent radii. As a starting point, target at least ~1/3 of pocket depth or match readily available end mill radii. Larger radii reduce cutting forces and allow faster, more stable toolpaths.

• Tool access and setups: Minimize reorientations. For complex multi-face features, 5‑axis can cut total time by enabling a single setup with shorter, stiffer tools—despite a higher hourly rate.

Micro-example (fillet radius and cycle time)

• Situation: A rectangular pocket 12 mm deep with 0.5 mm internal corner radii forces a small, long-reach tool and slow step-downs.

• Change: Increase corner radii to 1.5 mm and allow a larger, stiffer end mill with shorter overhang.

• Expected effect: Toolmaker guidance indicates reduced cutting forces, less deflection, better tool life, and the ability to increase feed per tooth. In practice, this can cut roughing/pocketing time significantly while improving surface quality—validate parameters on your machine and material.

Kaierwo Surface Treatment Projects

• Scenario: A startup’s enclosure required cosmetic black anodize on two outside faces and had 12 internal holes with tight true position callouts. During quoting, the vendor’s DFM review suggested narrowing the cosmetic scope to the visible faces only, switching non-visible areas to “as‑machined Ra ≤ 3.2 µm,” and relaxing eight non-critical holes to ISO 2768 defaults while keeping four datum-related holes at true position 0.10 mm with reports.

• Result: One secondary step (bead blast) was removed, anodize masking was reduced, and inspection reporting was limited to truly critical holes. Outcome: shorter queue time at the finisher and faster in-process checks on the floor. For context on services relevant to this workflow, see Kaierwo’s CNC aluminum machining capabilities; when parts demand single-setup access or multi-face features, consider 5‑axis machining to reduce setups and tool overhang.

Quick CAD‑to‑RFQ checklist

• State base tolerances: “ISO 2768‑f unless otherwise noted.”

• Identify critical features; use GD&T for those only; specify which require a CMM report.

• If holes are positional: “True position 0.20 mm @ MMC on pattern; two datum holes at 0.10 mm require report; others per ISO 2768.”

• Choose finishes by function: start with “as‑machined Ra ≤ 3.2 µm”; add bead blast only for matte; anodize Type II for appearance/corrosion; Type III for wear.

• If blasting/anodizing: specify media or thickness and call out masking for threads and precision bores.

• Structural DFM: walls ≥ 1.0 mm; internal fillets generous and consistent; avoid undercuts; confirm tool reach and setup count; consider 5‑axis for multi-face parts.

• Provide STEP + fully dimensioned 2D drawing; mark cosmetic faces.

CNC aluminum cost-saving design in practice: what to try first

If you need a single place to start, choose one part and run three edits in CAD: 1) push non-critical features to ISO 2768, 2) swap any “default bead blast + anodize” stack to “as‑machined + Type II” unless a matte look is mandatory, and 3) increase all internal corner radii to match common end mills. Then request two quotes—one with legacy specs and one with the edits. Which gets you closer to your budget and date?

 Next steps

Pilot these changes on your next prototype build and compare quotes and inspection plans. For a deeper dive into prototyping and validation workflows, see the prototype CNC machining for design validation guide or click here to contact us.