Best Feeds and Speeds for Milling Aluminum

Optimal Cutting Speeds for 6061 Aluminum

Feed Rate Adjustments by Alloy Type

When milling aluminum, feed rates must be adjusted based on the specific alloy type to optimize cutting performance and tool life. For softer alloys like 6061-T6, higher feed rates can be employed compared to harder alloys like 7075-T6. Typically, feed rates for aluminum milling range from 0.004 to 0.012 inches per tooth for most alloys. However, this can be fine-tuned based on the alloy's properties:

6061-T6

7075-T6

• •
  6061-T6: Feed rates of 0.006-0.010 inches per tooth are common, with higher rates possible for roughing operations.
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  7075-T6: Due to its higher strength, feed rates are often reduced to 0.004-0.008 inches per tooth.
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  Cast aluminum alloys: These may require lower feed rates, typically 0.003-0.007 inches per tooth, to account for potential porosity and inconsistencies in the material.

When adjusting feed rates, it's crucial to consider the relationship between cutting speed (\(V_c\)) and feed per tooth (\(f_z\)) to maintain an optimal chip load. This can be expressed as:

\( f_z = \frac{V_f}{n \times z} \)

where \(V_f\) is the feed rate, \(n\) is the spindle speed, and \(z\) is the number of flutes. Always start conservatively and incrementally increase feed rates while monitoring cut quality and tool wear.

Impact of Tool Geometry on Chip Formation

Different Geometry

7075-T6

Tool geometry significantly influences chip formation in aluminum milling, with rake angle and cutting edge angle being particularly crucial. A positive rake angle promotes chip flow and reduces cutting forces, while a sharper cutting edge angle enhances chip breaking. For aluminum, a high positive rake angle (10°-20°) is often recommended to reduce built-up edge formation.

The insert shape also plays a vital role in chip control. Diamond-shaped inserts with acute angles (30°-35°) are preferred for high-speed finishing of wrought aluminum, while round inserts are better suited for cast aluminum to prevent poor surface finish. Chip breakers, either built into the cutting face or clamped onto the tool, effectively manage chip formation by altering the chip's path and curl.

Additionally, the helix angle of end mills affects chip evacuation, with shallow angles (15°) offering excellent chip removal but lower axial tension, while steeper angles (>55°) provide better axial tension but may compromise cutting torque.

Roughing vs Finishing Parameters

Optimal feeds and speeds for rough and finish milling of aluminum vary based on the specific alloy and tooling used. Here's a comparison of recommended parameters for these operations:

For rough milling, higher material removal rates are prioritized, allowing for more aggressive cutting parameters. A larger chip load and depth of cut are used to maximize efficiency. Stepover is typically around 30-40% of the tool diameter for optimal chip evacuation.

In contrast, finish milling focuses on achieving superior surface quality. This requires lighter cuts with reduced chip loads and depths of cut. A smaller stepover of 5-10% is often used to minimize tool deflection and improve surface finish.

For best results, climb milling with a 2/3 cutter engagement (about 67% stepover) can be employed for the final pass. Always consider machine rigidity, tool geometry, and coolant application when fine-tuning these parameters for specific applications.

Surface Finish Considerations in High-Speed Milling

High-speed milling (HSM) offers significant advantages for achieving superior surface finishes in aluminum machining. The key factors influencing surface quality in HSM include cutting parameters, tool geometry, and chip formation dynamics. Here's a comparison of HSM versus conventional milling for surface finish:

HSM achieves better surface finishes through higher spindle speeds (>15,000 RPM) and reduced chip loads. This combination results in thinner chips, lower cutting forces, and reduced heat generation at the workpiece. The process also minimizes built-up edge formation, a common issue in aluminum machining that can degrade surface quality.

To optimize surface finish in HSM, consider using coarse-grain carbide tools with either diamond-like carbon (DLC) coatings or no coating, and employ climb milling techniques with minimal margin on the cutting edges.