Choosing Aluminum for Small-Quantity CNC Parts
Aluminum is often the first material engineers reach for—and for good reason. It machines efficiently, is widely available, and offers an excellent balance of strength, weight, and cost. But not all aluminum alloys behave the same, and for small-quantity parts, availability and machinability matter as much as mechanical properties.
This overview focuses on aluminum alloys that are commonly stocked, machine well in low volumes, and integrate cleanly with typical finishing processes. It also highlights where designers often run into trouble when specifying aluminum for real-world parts.
What “Commonly Available” Actually Means
In small quantities, you are typically limited to alloys that are:
- Stocked by local or regional metal suppliers
- Available in bar and plate without mill minimums
- Supplied in consistent tempers suitable for machining
If an alloy requires a mill run or special extrusion, it is usually not practical for prototypes, one-offs, or short production runs.
Common Aluminum Alloys for CNC Machining (Small Quantities)
6061-T6 — The Default Choice
- Availability: Excellent (bar, plate, tube)
- Machinability: Very good
- Strength: Moderate
- Corrosion resistance: Good
- Anodizing: Excellent
6061-T6 is the most commonly used aluminum alloy in CNC machining. It is widely stocked, machines predictably, and anodizes consistently.
Where it fits well:
- General machine components
- Brackets, frames, plates
- Housings and enclosures
- Prototypes and low-volume production
Design notes:
- Good balance of strength and machinability
- Stable during machining
- Anodizes uniformly with predictable color
- Not as strong as 7075, but often strong enough
Design takeaway:
If you don't have a specific reason to choose another alloy, 6061-T6 is usually the right starting point.
7075-T6 — High Strength, Higher Cost
- Availability: Good (mostly bar and plate)
- Machinability: Very good
- Strength: High (comparable to some steels)
- Corrosion resistance: Fair
- Anodizing: Acceptable but less cosmetic
7075-T6 is chosen when strength-to-weight ratio is critical.
Where it fits well:
- High-load machine components
- Pins, shafts, and structural parts
- Aerospace-style applications
Design notes:
- Machines cleanly with a bright finish
- More expensive than 6061
- Anodizing color can be darker or inconsistent
- Less corrosion resistant than 6061
Design takeaway:
Use 7075 when strength matters—not by default. It's a performance alloy, not a general-purpose one.
2024-T3 / T351 — Fatigue-Resistant, Specialty Use
- Availability: Moderate (mostly plate)
- Machinability: Good
- Strength: High
- Corrosion resistance: Poor
- Anodizing: Limited
2024 is strong and fatigue-resistant but has drawbacks.
Where it fits well:
- Fatigue-loaded parts
- Aerospace-inspired designs
- Applications where anodizing is not required
Design notes:
- Poor corrosion resistance without coating
- Not ideal for cosmetic parts
- Less forgiving for general shop use
Design takeaway:
2024 is rarely the best choice for small-quantity industrial parts unless fatigue performance is explicitly required.
5052 — Formability Over Machinability
- Availability: Excellent (sheet, plate)
- Machinability: Fair
- Strength: Low-moderate
- Corrosion resistance: Excellent
- Anodizing: Acceptable
5052 is often used for sheet metal but occasionally appears in machined parts.
Where it fits well:
- Formed or bent components
- Corrosion-resistant parts
- Marine or outdoor applications
Design notes:
- Softer and gummier to machine
- Poor choice for tight tolerances
- Not ideal for complex CNC features
Design takeaway:
5052 is a forming alloy first, machining alloy second.
Cast Aluminum (e.g., MIC-6 / ATP-5)
- Availability: Good (plate)
- Machinability: Excellent
- Strength: Lower than wrought alloys
- Stability: Excellent
- Anodizing: Limited / inconsistent
Cast tooling plate is stress-relieved and extremely flat.
Where it fits well:
- Fixture plates
- Precision bases
- Large flat components
Design notes:
- Very stable during machining
- Not suitable for high-stress parts
- Anodizing may be uneven or cosmetic only
Design takeaway:
Cast plate is about stability, not strength.
Machining Considerations for Aluminum
Surface Finish
- Aluminum generally machines to a bright finish
- Tool sharpness and chip evacuation matter
- Softer alloys (5052) appear duller
Tolerances
- Aluminum allows tight tolerances, but:
- Thin walls can deflect
- Large flat parts may move after machining
- Stress-relieved stock improves stability
Thermal Expansion
- Aluminum expands more than steel
- Important for assemblies, bearing fits, and temperature-sensitive designs
Finishing Compatibility
| Finish | Compatibility |
|---|---|
| Anodizing | Excellent for 6061, acceptable for 7075 |
| Electroless nickel | Excellent for all wrought alloys |
| Powder coating | Good; thickness must be considered |
| Paint | Good |
| Black oxide | Not applicable |
Design note:
If anodizing appearance matters, 6061 is the safest choice.
Common Design Mistakes
- Specifying obscure aluminum alloys with long lead times
- Using 7075 when 6061 is sufficient
- Forgetting anodizing growth on tight features
- Designing thin walls without considering stiffness
- Assuming all aluminum anodizes the same
Practical Selection Guidance
- General CNC parts: 6061-T6
- High strength, low weight: 7075-T6
- Flat, stable plates: Cast tooling plate
- Formed parts: 5052
- Fatigue-critical designs: 2024 (with caution)
Design Takeaway
For small-quantity CNC machining, aluminum selection is less about chasing datasheet performance and more about availability, predictability, and finish compatibility. Using commonly stocked alloys—especially 6061-T6—keeps lead times short, costs controlled, and results consistent.