Comprehensive Aluminum Processing FAQ

A practical, engineer‑friendly FAQ for designers, engineers, and buyers sourcing custom aluminum parts—from prototype to low-volume production.

1) Which aluminum alloys should I choose for CNC vs. sheet metal?

Short answer: 5052-H32 for bending/formed brackets; 6061-T6 (or 6082-T6) for general CNC and mixed fab; 7075-T6 or 2024-T3 only when you truly need high strength and accept trade-offs.

Why: 5052-H32 has excellent formability and corrosion resistance, making it a top sheet metal choice. 6061-T6 balances machinability, strength, and cost—great for CNC and many bent parts (with larger bend radii than 5052). 7075-T6 offers very high strength but poor formability/weldability; 2024-T3 has strong fatigue performance but weaker corrosion resistance. See high-level overviews in the 2024–2025 period from Kloeckner Metals on the strength/formability trade-offs in 7075 vs 6061 and Protolabs’ aluminum alloy comparison for machining implications.

You might also want to consider how anodizing appearance varies by alloy; Cu-containing grades (e.g., 2xxx/7xxx) can show color differences.

Internal resource: For machining specifics by alloy, see Aluminum CNC Machining.

2) What tolerances are realistic for CNC aluminum parts?

Short answer: If not otherwise stated, many shops use general tolerances per ISO 2768; a common default is ISO 2768‑m (medium). For critical features, specify individual tolerances or GD&T.

Reference ranges: For linear dimensions, ISO 2768‑1 tabulates typical permissible deviations by size and class; for example, in the >6–30 mm range, class m is ±0.2 mm and class f is ±0.1 mm, per accessible engineering tables summarizing the ISO standard in 2025 (Amesweb — ISO 2768-1 table). Use tighter, feature-level callouts only where function demands—it reduces cost and lead time.

Internal resource: General capability and DFM guidance: CNC Machining.

3) What surface roughness (Ra) should I call out?

Short answer: Ra 3.2 μm is a standard as‑machined finish; Ra 1.6 μm is common for mating faces; Ra 0.8–0.4 μm requires special finishing passes or processes.

Context: These typical ranges for aluminum are widely cited by reputable manufacturing references in 2024–2025 (e.g., Xometry EU — selecting surface roughness and Protolabs’ guidance on finish vs. function). Indicate surface texture using ISO 1302 symbols on drawings; Keyence provides a clear primer on the symbol set and parameters.

4) Why did my 6061‑T6 sheet crack when bending, and what bend radius should I use?

Short answer: 6061‑T6 is less formable than 5052‑H32 and often needs a larger inside bend radius—commonly around 2–4× material thickness (or more as thickness increases). Bending with grain increases cracking risk.

Details: Minimum bend radii depend on alloy, temper, thickness, and grain direction. Industry tables (regularly referenced by fabricators through 2025) show 5052‑H32 tolerating tighter radii than 6061‑T6 across thicknesses; see Protocase’s bend radii tables for concrete values by gauge (Protocase — bend radii). If you must bend 6061, consider larger radii, bending transverse to grain, or specifying a softer temper for forming (with re‑heat‑treat as applicable).

Internal resource: Bending guidelines and typical gauges: Sheet Metal Fabrication.

5) Will anodizing change my dimensions or hole sizes?

Short answer: Yes. Type II sulfuric anodize typically adds on the order of 5–25 μm thickness, while Type III hardcoat is often 40–60 μm. Roughly half the coating thickness can build outward, affecting fits.

Practical allowance: A common rule-of-thumb is to undersize precision bores or specify masking. Industry explainers note Type III hardcoat commonly at 40–60 μm in production contexts (Precision Coating, 2024–2025) and describe dimensional growth effects that can add several micrometers per side depending on the process and alloy (Precision Coating — Type III hardcoat; BWC Profiles — dimensional impact). Align on thickness, sealing, masking, and fit strategy with your anodizer before production.

Internal resource: Finishing options and prep (bead blast, masking): Surface Finishing.

6) How do I maintain color consistency for black anodized parts across batches?

Short answer: Control what you can control—use the same alloy/temper and supplier, specify pre‑treatment (etch, bead‑blast), define anodize thickness, sealing method, and accept/reject criteria with a physical master sample.

Context: Color shifts arise from alloy chemistry, surface prep, thickness, dye chemistry, and sealing. The Aluminum Anodizers Council’s materials explain how specification choices (e.g., MIL‑PRF‑8625 Type II vs III, Class 1 vs 2) impact outcomes and why visual masters matter in procurement (AAC specification overviews, 2024–2025).

7) What’s a sensible DFM checklist for CNC and sheet aluminum parts?

CNC: Avoid blanket tight tolerances; call out only functional features. Maintain wall thicknesses ≥ 1–1.5 mm (more for large spans), add fillets matching tool sizes, and provide workholding flats. Specify Ra targets only where needed.
Sheet: Minimum hole diameter ≈ material thickness; hole‑to‑bend and bend‑to‑bend distances ≥ 2–3×T; use appropriate bend radii per alloy/temper charts; bend transverse to grain when possible. Fabrication references like Dayton Rogers codify these rules for aluminum in 2024–2025.

Tip: Share native CAD + fully dimensioned PDFs; note critical datums and inspection priorities to save iteration time.

8) Which inspection plan should I request for first builds (FAI, AQL, CMM)?

Short answer: For critical parts, request an AS9102‑style First Article Inspection (FAI) package for the first lot; for ongoing lots, define AQL sampling by defect class and require CMM for critical GD&T features.

Why: AS9102 structures first‑piece validation with three forms (part/material/process traceability and full characteristic accountability). A 2024–2025 industry guide summarizes the forms and when to apply them outside aerospace . For routine lots, ISO 2859‑1 AQL sampling helps set acceptance thresholds; Keyence’s primer explains selecting inspection levels and Ac/Re numbers from the tables.

9) What documents should I ask for (material certs, CoC, RoHS/REACH)?

Short answer: For structural/aluminum‑grade‑critical parts, ask for EN 10204 3.1 material certificates plus a Certificate of Conformance. For EU shipments of EEE, request RoHS/REACH declarations from the supplier and finisher.

Context: The Aluminum Association’s 2024 RoHS FAQ outlines substance limits and notes alloy trace elements must comply at the homogeneous material level for EEE applications. Finishing chemicals and dyes can fall under REACH obligations; ensure declarations are collected from each process step.

10) What are the biggest overseas sourcing risks (IP, QC, delays) and how do I mitigate them?

IP: Use China‑jurisdiction NNN agreements (Non‑use, Non‑disclose, Non‑circumvent), and include tooling/data ownership clauses. Share only necessary data pre‑award; watermark RFQ drawings where appropriate.
QC: Define acceptance criteria up front, request FAI on first lot, and set AQL sampling plans. Consider third‑party pre‑shipment inspections for high‑value orders.
Schedule: Finishing queues and rework are common bottlenecks; align anodize thickness/color early and build time for re‑runs. Add logistics buffer for customs and port delays.
Contracts and payments: Stage payments to milestones (e.g., deposit, mid‑inspection, balance on passed inspection). For larger orders, consider L/C. Keep clear change‑control procedures.

11) Which Incoterms 2020 should I pick (EXW, FCA, FOB, DDP)?

Short answer: For air or containerized shipments, FCA (seller delivers to your forwarder) or CIP/DAP are often clearer than EXW/FOB. Use DDP only when the seller truly manages import clearance/taxes in your country.

Context: Export credit agencies and logistics educators note that for containerized cargo, FCA better reflects who loads and handles export clearance compared with FOB. A 2024–2025 guide from Export Development Canada breaks down responsibilities by term with plain diagrams. Pick terms based on your in‑house logistics competence and risk appetite.

12) How should I package anodized aluminum parts for export shipments?

Short answer: Individually wrap parts (PE film/foam), separate layers with non‑abrasive sheets, brace parts in cartons or crates, and add desiccants/humidity control for sea freight.

Why: Rubbing and salt‑air humidity are the two big killers of anodized cosmetics in transit. Clear “Fragile/Keep Dry/Do Not Stack” labeling helps. These steps are common industry practice and echoed by anodizing associations.

13) What payment terms are common, and how _{^{do I}}reduce risk?

Short answer: For new suppliers, T/T with 30–50% deposit and balance before shipment is common; for large orders, consider irrevocable L/C. Reduce risk with staged payments tied to inspections and with escrow for prototypes when practical.

Tip: Insist on a formal quote and purchase agreement that state acceptance criteria, rework/return policies, IP clauses, and governing law.

14) What lead times should I expect, including finishing and shipping?

Manufacturing: Simple CNC or laser‑bent parts can be a few days at highly automated shops; with anodizing, plan for an added ~3–10 days depending on queue and type. Protolabs’ 2024–2025 lead‑time page gives a sense of how geometry/finish affect speed.
Transit: Air express China → US/EU can be ~3–7 days door‑to‑door; ocean is often ~18–40 days port‑to‑port depending on lane and season. Always check live quotes.

15) Can you help with extrusion + CNC + anodize workflows?

Short answer: Yes—this is common for enclosures and rails. DFM keys: define critical post‑machined datums, allow coating growth on fits, and specify color/thickness tolerance across extrusion lots.
Internal resource: Mixed‑process contexts: Aluminum Extrusion.

16) What about threads in aluminum: tap sizes, thread strength, and inserts?

Short answer: Use standard 75% thread engagement for most loads; avoid tiny threads in soft alloys. For high-cycle or high-torque joints, use steel thread inserts (Heli‑Coil/Key‑Locking) or design for bolts into steel nuts.

Practical tips:

Pre‑anodize tapped holes when possible, or re‑tap after anodize if thread fit is critical (note anodize growth on minor diameters).
For 6061‑T6, typical minimum thread engagement ≈ 1.0×D for coarse threads; up to 1.5×D for fine threads or lower‑strength alloys.
Consider forming taps in ductile alloys to improve thread strength and finish; use sharp cutting taps for brittle tempers.

17) Flatness and warpage: what’s realistic for plates and sheet parts?

CNC plates: Residual stress release can cause bow after heavy material removal. Plan stress‑relief (e.g., pre‑machining + natural/thermal aging) and balanced machining from both sides. Expect practical flatness targets on medium plates around 0.1–0.3 mm depending on size and setup.
Sheet metal: Expect some angularity/flatness variation after bending. Use hems, ribs, or flanges to stiffen large panels. Specify flatness to a datum surface, not free‑state, if parts are to be clamped in assembly.

18) Weldability and joining: which alloys and what cautions?

5xxx (e.g., 5052) generally weld well (MIG/TIG). 6xxx (e.g., 6061) welds but heat‑affected zones lose T6 strength unless re‑heat‑treated.
Avoid welding 2xxx/7xxx in most cases due to hot‑cracking and property loss. Consider mechanical fastening or bonded joints instead.
If welding 6061, design for post‑weld strength reduction (~50–60% in HAZ) or specify PWHT if feasible; isolate anodized surfaces from welding zones.

19) Cost drivers: how do I keep parts affordable?

Geometry: Reduce deep pockets, tiny internal radii, narrow slots, and thin walls. Standardize hole sizes to common drills.
Quantity: Price breaks come at setup amortization thresholds (e.g., 10/50/100/500 pcs). Group parts and finishes to reduce changeovers.
Finish: Minimize cosmetic surfaces; align finish color/thickness across parts to batch together. Avoid masking unless necessary.
Material: Choose common stock sizes; use extrusions for long profiles, then machine features.

20) Drawing best practices to avoid back‑and‑forth?

Fully define units, tolerances (default + feature), finish callouts, and datums. Include thread specs (standard + class + depth) and any special deburr/chamfer requirements.
Add a general note for edges (e.g., break sharp edges 0.2–0.5 mm) and surface treatment specifications (type, class, thickness, color, sealing).
Provide inspection key: bubble critical dimensions and list gauge/measurement method where non‑obvious.

21) Sheet metal specifics: hole/slot, bend deduction, and K‑factor?

Minimum hole size ≈ material thickness; for slots near bends, keep edge‑to‑bend ≥ 1.5–2×T.
Provide bend deduction or allow shop K‑factor if you don’t have it; request a flat‑pattern confirmation for first articles to lock numbers.
For cosmetic outer faces, bend with protective film and specify grain direction of brushing/bead blast.

22) Heat treatment and machining order: what sequence?

For 6xxx/7xxx that need T6/T73 properties after forming, consider: rough machine → solution treat/age → finish machine critical features → anodize/coat.
Avoid finishing critical bores before hardcoat anodize unless you will hone/ream to size post‑coat.

23) Tolerance strategy examples to balance cost and function

Fits: Use ISO fits (e.g., H7/g6) only on true mating features; leave others at ISO 2768‑m.
Perpendicularity/parallelism: Apply GD&T only to control function‑critical relationships; default to title‑block for non‑critical faces.
Flatness: Scale with size; avoid blanket 0.05 mm across large plates unless essential.

24) Sustainability and material choices

Recyclability: Aluminum is highly recyclable; specify recycled content only if your supply chain can certify it.
Finishes: Avoid unnecessary dye steps; clear anodize instead of black reduces dye variability and chemical use.
Packaging: Right‑size packaging and use recyclable cushioning where risk allows.

25) Supplier vetting checklist (fast)

Capabilities: In‑house CNC + sheet + finishing coordination; sample gallery of similar parts.
Quality: ISO 9001 (or equivalent), sample FAI reports, gauge list, calibration records.
Communication: Clear lead‑time commitments, DFX feedback in 24–48 h, named PM/engineer.
Compliance: Material traceability, RoHS/REACH statements, export experience.

Practical Specs and Examples (quick reference)

General tolerances: Default to ISO 2768‑m unless specified; tighten only where function demands. See accessible ISO tables for size‑based bands in 2025
Roughness: Ra 3.2 μm as‑machined; Ra 1.6 μm for mating; Ra ≤ 0.8 μm requires special finishing; ISO 1302 symbols per Keyence’s explainer.
Bend radii: 5052‑H32 supports tighter bends than 6061‑T6; consult current bend tables by gauge
Anodize allowances: Type II ~5–25 μm; Type III ~40–60 μm; plan for outward growth affecting holes/bores
Inspection: First builds → AS9102‑style FAI; ongoing → ISO 2859‑1 AQL sampling; CMM for critical features (Unitek — AS9102 FAI; Keyence — sampling).
Compliance: EU EEE → RoHS/REACH declarations; importers to the US should be aware of aluminum import license requirements summarized by trade.gov in 2024–2025.
Incoterms: Prefer FCA/CIP/DAP to align risk with capability; DDP only with experienced sellers (EDC — Incoterms).