Milling Cutter Deflection and Its Solutions

1. What is Deflection?

Deflection refers to the displacement of the cutting tool under load, which can lead to tool bending or breakage. This phenomenon may cause reduced tool life, poor surface finish, dimensional inaccuracies, or even complete workpiece failure.

A useful analogy is a diving board: when unoccupied, the board remains straight, but as a diver moves toward the end, it bends progressively. Similarly, cutting tools experience deflection under cutting forces.

Common signs of deflection include:

• Reduced tool life or tool breakage

• Insufficient surface finish quality

• Dimensional inaccuracies in the workpiece

2. Solutions

2.1 Minimizing Tool Overhang

Tool overhang refers to the length of the tool extending beyond the tool holder. The longer the overhang, the greater the risk of deflection, reducing tool rigidity.

Excessive overhang shortens the clamping area of the tool shank, weakening overall rigidity and increasing the likelihood of tool vibration, which may lead to breakage. To achieve optimal cutting conditions, it is crucial to clamp the tool as securely as possible to minimize overhang.

2.2 Long Flute End Mills vs. Long Reach End Mills

Another key strategy to minimize tool deflection is understanding the differences between long flute end mills and long reach end mills and selecting the appropriate tool for the specific machining application.

• Tool rigidity depends on core diameter: A larger core diameter provides greater rigidity and longer tool life.

• Flute length and stability: Shorter flute lengths offer higher rigidity, reducing the risk of deflection.

The diagram below illustrates the relationship between tool tip force and flute length, showing that deflection is minimized when only the tool tip is engaged in cutting. To extend tool life, the best practice is to choose the largest possible tool diameter and reduce tool overhang and cutting length to maximize rigidity.

2.3 When to Use Long Reach End Mills?

Long reach end mills are typically used when the tool holder cannot reach deep into a feature, but the tool’s non-cutting portion can. The shank behind the cutting edge is often slightly smaller in diameter than the main tool diameter to prevent rubbing against the workpiece (also known as “tool rubbing”).

Due to their versatility and durability, long reach end mills are among the most commonly used tools in a machining setup.

2.4 When to Use Long Flute End Mills?

Long flute end mills have extended cutting lengths and are primarily used for seamless sidewall machining or finishing operations inside deep slots. Their core diameter remains consistent throughout the cutting length, making them more prone to bending when machining deep cavities.

If the engaged cutting edge is too short at high feed rates, tapering may occur. Long flute end mills are highly effective for deep slot milling and are particularly useful in high-efficiency machining techniques, where their superior chip evacuation capability outperforms standard end mills.

3. The Effect of Core Diameter on Tool Deflection

Diameter is a critical factor when calculating tool deflection. For long flute tools, core diameter rather than cutting diameter should be considered when determining deflection.

This is because the cutting edges do not provide structural support at the bottom of the flute valleys, meaning that bending resistance mainly depends on the core diameter. For tools with significant overhang, deflection calculations should consider the core diameter up to the overhang length and transition to the neck diameter beyond that point.

Optimizing these parameters can help keep tool deflection within a negligible range, improving machining accuracy and stability.