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CNC Machining Aluminum

By STG Machine

Aluminum stands out as one of the most commonly preferred raw materials for cnc machining and the processes of fabrication more broadly.  Aluminum has a number of advantages that make CNC machining aluminum, and aluminium fabrication often preferable over other metals, non aluminum alloys and plastics. 

The properties of aluminum that make aluminum CNC machining advantageous include: lightweight, cost-effective, non-magnetic, abundantly available, and exceptionally malleable. This last property concomitant with low deformation factors and high structural stability lends itself to precision CNC machining.

Further, aluminum and aluminum alloys maintain high dimensional integrity under external stressors and environmental pressures like temperature fluctuations and impact sensitivity. 

These advantages have made aluminum a popular choice for precision (close tolerance) fabrication within the supply chains of the aerospace, semiconductor and communications industries. This popularity has driven a dramatic increase in research focused on cnc machining aluminum. And as aluminium fabrication has become more popular, the limits of aluminum machining have been pushed to new heights. Understanding a few important characteristics of aluminum can help predict cnc machining outcomes.

Types and Grades of Aluminum

There are various grades that specify an alloy’s composition and are identified by a four digit number. A few examples of aluminum types and grades are:

Aluminum 2024

Aluminum 2024 is an aluminum-copper alloy with high strength and excellent fatigue resistance. Often used in aerospace application and sheet metal fabrication.

Chemical Composition:

  • Aluminum (Al): 90.7%
  • Copper (Cu): 4.5%
  • Magnesium (Mg): 1.5%
  • Manganese (Mn): 0.3%
  • Silicon (Si): 0.5%
  • Iron (Fe): 0.5%
  • Zinc (Zn): 0.1%
  • Titanium (Ti): 0.1%
  • Chromium (Cr): 0.1%

Aluminum 5052

Aluminum 5052 is typically used in sheet metal fabrication. This has excellent corrosion resistance, good formability with medium static strength. It is not heat-treatable.

Chemical Composition:

  • Aluminum (Al): 95.7%
  • Magnesium (Mg): 2.5%
  • Chromium (Cr): 0.25%

Aluminum 6061

Aluminum 6061 is an Aluminum-magnesium-silicon alloy has good extrudability and high strength. It is one of the most popular aluminum alloys for cnc machining and fabrication and it is heat treatable.

Chemical Composition:

  • Aluminum (Al): 97.9%
  • Magnesium (Mg): 1.0%
  • Silicon (Si): 0.6%
  • Iron (Fe): 0.3%
  • Copper (Cu): 0.28%
  • Zinc (Zn): 0.2%
  • Titanium (Ti): 0.2%
  • Manganese (Mn): 0.15%
  • Chromium (Cr): 0.15%

Aluminum 7075

Aluminum 7075 is an Aluminum-zinc-magnesium alloy, known for its high strength and used in aerospace and military applications. It features a lower corrosion resistance compared to 5052, 6061 and 7075; it is heat treatable.

Chemical Composition:

  • Aluminum (Al): 90.7%
  • Zinc (Zn): 5.6%
  • Magnesium (Mg): 2.5%
  • Copper (Cu): 1.6%
  • Chromium (Cr): 0.23%

Aluminum grades can be broken into further variety and specification with additional information such as 6061-T6. T6 denotes the temper that the 6061 aluminum has undergone. For example, aluminum 6061-T6 material has undergone solution heat treatment and an artificial aging process. This results in improved properties making the material stronger and harder compared to standard 6061 aluminum. Aluminum 6061-T651 material undergoes stretching after heat treatment but before being put through an artificial aging process. This further improves the mechanical properties, reduces stresses within the material, enhancing flatness and increases stability.

Aluminum Machining Bar vs Plate

When ordering from a mill or a material house, you can get aluminum in both bar and plate form. Both of these options can affect the aluminum machining process due to their inherent differences.

Aluminum Bar

When forging aluminum, it undergoes extrusion or rolling to form bars with a uniform cross-section. These bars are available in different sizes and shapes such as round, square, rectangular or hexagonal.

Aluminum Plates

Aluminum plates are always flat, rectangular sheets of aluminum with consistent thickness. They can be cut into square or rectangular sizes to then be machined or bent.

Due to the forming process, aluminum bars are generally more compact compared to aluminum plates. Their larger cross section areas can lead to more heat generated during the fabrication process and diminished chip evacuation. This can result in higher tool wear, larger cutting forces and worse surface finish. 

The grain structure between bar and plate can also differ due to the rolling process. Bars typically have longer grain structures resulting in bar material work hardening faster when machining aluminum. Due to their grain structure and higher mass, bar material may also retain more heat compared to plate. This can affect tool life and rigidity of the raw material. Lower rigidity can result in increased deflection during cnc machining which accelerates tool wear and diminishes surface finish quality.

CNC Machining Aluminum Guidelines

Aluminium fabrication is very forgiving and here are some guidelines to ensure success:

  1. Use coolant and lubricants where needed: Most operations will be sufficient with coolant but oil may be needed with tapping operations
  2. Use the Right Tools: Make sure to talk to your local tooling representative to find the right tools based on your job.
  3. Proper Speeds and Feeds: Many tooling manufacturers publish data on what to run their tools at. Follow their guidelines along with any that you have developed from experience.
  4. Minimize Vibration: Use proper workholding and tooling to minimize deflection and vibration
  5. Maintain Sharp Tools: Inspect your tooling to ensure that the tips are sharp. As soon as they start to get dull they can cause excessive heat and poor surface finish
  6. Helical Milling: Use advance milling techniques such as helical milling to help remove material quickly, efficiently and reduce work hardening
  7. Always Climb Mill: An obvious piece of advise but one that should always be followed
CNC MACHINING ALUMINUM and Aluminium Fabrication
CNC MACHINING ALUMINUM and Aluminium Fabrication

Aluminium Fabrication Benefits and Challenges

Aluminum is a relatively cheap material, easy to machine and lightweight. Post manufacturing finishes such as anodize and electroless nickel adhere to it very well. Aluminum can be machined to a surface finish of 16 Ra with optimal tooling and aluminum can be brought to a 8 Ra or lower with lapping or grinding.

However, aluminum fabrication can become difficult due to galling and deflection. Aluminum is very susceptible to deflection due to its lower modulus of elasticity. Cutting forces and some degrees of freedom within workholding can result in the material moving in unexpected ways. 

Furthermore, aluminum is not a great candidate for welding. At STG Machine we weld aluminum quite often and it is difficult to achieve air tight seals utilizing 2 pieces of aluminum. Aluminum 6061-T6 is very porous and small cracks can emerge after the fabrication process. We strongly encourage welding stainless steel or steel when possible.

Reasonable Tolerances For Aluminum Machining

Standard ToleranceTight ToleranceExtremely Tight Tolerance
Linear Dimensions0.005”0.002”0.0005”
Hole Diameters0.005”0.001”0.0002”
Angular Dimensions1 degree0.5 degree0.2 degree
Surface Finish64 Ra32 Ra8 Ra
Machining Tolerance Guidelines for Aluminium Fabrication

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