Core takeaway: For most custom aluminum parts, 6061-T6 offers the best balance of strength (tensile strength ~310 MPa), machinability, and cost, while 7075-T6 (tensile strength ~570 MPa) is the better choice when higher strength-to-weight ratio is required. Most custom aluminum CNC machining work is held to ISO 2768-m (medium) general tolerances, with tighter tolerances down to ±0.01 mm possible on critical dimensions at higher cost. This guide breaks down the alloy, tolerance, and finishing decisions that determine both the performance and the price of your custom aluminum parts.
For product designers, mechanical engineers, and procurement managers sourcing parts overseas, the alloy and process decisions made at the design stage directly affect lead time, unit cost, and part performance. This guide is written from a manufacturing floor perspective — based on our processing experience across thousands of aluminum CNC machining orders — rather than as a generic materials overview.
Before tolerances, finishes, or fixturing are discussed, the alloy determines what is physically achievable in aluminum CNC machining. Choosing the wrong alloy at the design stage is the single most common cause of late-stage redesigns we see from overseas buyers.
| Alloy | Tensile Strength | Yield Strength | Hardness (Brinell) | Machinability Rating | Typical Use Case |
|---|---|---|---|---|---|
| 6061-T6 | ~310 MPa | ~276 MPa | ~95 HB | 90 (good) | General structural parts, enclosures, brackets |
| 6082-T6 | ~340 MPa | ~310 MPa | ~95 HB | 85 (good) | Structural frames, EU-standard equivalent of 6061 |
| 7075-T6 | ~570 MPa | ~503 MPa | ~150 HB | 70 (moderate) | Aerospace fittings, high-load fixtures, molds |
| 7050-T7451 | ~510 MPa | ~455 MPa | ~140 HB | 70 (moderate) | Thick-section aerospace parts, stress-corrosion sensitive designs |
| 2024-T351 | ~470 MPa | ~325 MPa | ~120 HB | 65 (moderate) | Fatigue-critical aerospace structures |
| 5052-H32 | ~228 MPa | ~193 MPa | ~60 HB | 70 (moderate, gummy) | Sheet-formed and welded assemblies |
| 5083-H111 | ~300 MPa | ~145 MPa | ~75 HB | 60 (poor, gummy) | Marine housings, welded pressure enclosures |
A frequent design mistake is applying tight tolerances across an entire drawing instead of only on functional features. This inflates cost without improving part performance.
Unless a drawing calls out otherwise, ISO2768-m governs every dimension without an explicit tolerance:
| Nominal Size Range | ISO 2768-m Permitted Deviation |
|---|---|
| 0.5 – 3 mm | ±0.1 mm |
| Over 3 – 6 mm | ±0.1 mm |
| Over 6 – 30 mm | ±0.2 mm |
| Over 30 – 120 mm | ±0.3 mm |
| Over 120 – 400 mm | ±0.5 mm |
| Over 400 – 1000 mm | ±0.8 mm |
| Feature Type | Standard Tolerance | Precision Tolerance | Cost Impact |
|---|---|---|---|
| General linear dimensions | ±0.05 mm | ±0.02 mm | Baseline |
| Hole diameters (reamed/bored) | ±0.025 mm | ±0.01 mm | +15–30% |
| Flatness (per 100 mm) | ±0.05 mm | ±0.02 mm | +10–20% |
| Thread fit (tapped) | 6H/6g standard | 4H/4g precision | +10–25% |
| Surface roughness (as-milled) | Ra 1.6 µm | Ra 0.4 µm (post-polish) | +20–40% |
Surface treatment affects corrosion resistance, appearance, and — in electrical or medical applications — functional insulation. Below is a comparison based on the finishes most frequently requested for custom aluminum parts.
| Finish | Corrosion Resistance | Typical Thickness | Appearance | Relative Cost |
|---|---|---|---|---|
| Type II Anodizing | Good | 5–25 µm | Matte to satin, dyeable in most colors | Low–Medium |
| Type III Hard Anodizing | Excellent | 25–100 µm | Grey to black, dense, limited color range | Medium–High |
| Electrophoresis (E-coating) | Excellent | 10–30 µm | Even gloss or matte film, covers recesses uniformly | Medium |
| Electroplating (Ni / Cr / Zn) | Good | 5–30 µm | Bright metallic, electrically conductive | Medium–High |
| Sandblasting | None (surface prep only) | N/A | Uniform matte texture, hides tool marks | Low |
| Polishing | None (surface prep only) | N/A | Mirror or high-gloss, Ra down to 0.1 µm | Medium–High |
| Brushing | None (surface prep only) | N/A | Directional linear grain | Low–Medium |
Mechanical treatments always precede chemical or electrochemical ones. Sandblasting, polishing, and brushing are surface preparations, not standalone protection — a bead-blasted part left unanodized will oxidize and stain within weeks of handling. When specifying on a drawing, state the full sequence (for example, “bead blast #120 grit → Type II anodize, black, 15 µm”) rather than a single finish name.
Procurement managers evaluating quotes benefit from understanding what drives cost, rather than comparing unit prices in isolation.
Unit Cost ≈ (Material Cost + Machine Time Cost) ÷ Batch Quantity + Fixed Setup Cost ÷ Batch Quantity + Finishing Cost per Part
This means that for low-volume prototypes, setup cost often dominates the quote, while for production runs, material and machine time dominate. Designers can reduce prototype cost significantly by minimizing unique setups and non-standard tolerances, even before volume increases.
Related reading: Aluminum vs. Stainless Steel CNC Machining: Choosing the Right Material
Selecting the right alloy, applying tolerances only where they add functional value, and matching surface finish to the part’s operating environment are the three decisions that most influence both performance and cost in custom aluminum parts. Based on our processing experience, most overseas buyers reduce cost by 15–30% simply by reviewing tolerance callouts against actual functional requirements before quoting. Whether the project calls for standard aluminum CNC machining or precision-tolerance custom aluminum parts, aligning these design choices early leads to more predictable lead times and fewer revision cycles with your CNC supplier.
Learn about our aluminum CNC machining capabilities, tolerance ranges, and finishing services.
A: Most CNC machining shops apply ISO2768-m (medium class) as the default general tolerance for unspecified dimensions, with tighter tolerances down to ±0.01–0.02 mm available on critical features at additional cost.
A: 6061-T6 is the most common choice due to its balance of machinability, strength (~310 MPa tensile), and anodizing compatibility. 7075-T6 is preferred when higher strength-to-weight ratio is required.
A: Type II anodizing typically adds a modest, low-to-medium cost per part depending on color and batch size; Type III hard anodizing costs more due to longer processing time and thicker coating buildup.
A: Yes, on select features using precision grinding or wire EDM as secondary operations, but this significantly increases cost and is usually reserved for critical mating or sealing surfaces.
A: A practical minimum is around 0.8 mm for 6061 aluminum; thinner walls increase the risk of vibration, deflection, and dimensional inconsistency during machining.
Fiona Huang is a Sales Engineer with 16+ years of experience in the mechanical and precision manufacturing industry. She specializes in CNC machining components, custom fasteners, and engineering-driven manufacturing solutions. With strong expertise in international trade and technical project support, she focuses on transforming complex engineering requirements into high-precision, production-ready solutions for global clients.
