What effect does tool radius have on cutting forces during milling?

The tool radius in milling plays a significant role in influencing cutting forces, and its effect can be understood in terms of the geometry of the tool and the nature of the cutting process. Here's how the tool radius affects cutting forces during milling:

1. Larger Tool Radius:

• Reduced Cutting Forces: A larger tool radius, such as in a corner radius end mill, generally reduces cutting forces. This is because the larger radius increases the contact area between the tool and the workpiece, distributing the cutting forces over a larger surface. This can help reduce the intensity of the forces acting on any specific part of the tool, leading to lower peak cutting forces.
• Lower Cutting Depth per Tooth: With a larger radius, the cutting edge engages the material at a more gradual angle, leading to a lower chip load and reduced cutting force per tooth. This is especially beneficial in heavy cuts or when working with tough materials, as it can reduce tool wear and prolong tool life.
• Reduced Vibration (Chatter): Larger radius tools are less prone to vibrations or chatter because the broader contact area provides more stability during the cutting process. This is particularly important in finishing operations where surface finish and stability are critical.

2. Smaller Tool Radius:

• Higher Cutting Forces: A smaller tool radius (or a sharp-edged tool) tends to concentrate cutting forces on a smaller contact area, which increases the local intensity of cutting forces. This can result in higher peak forces, especially in high feed-rate or high-speed machining scenarios.
• Higher Chip Load: With a smaller radius, the tool may engage the material at a steeper angle, which increases the depth of cut per tooth (chip load). This leads to higher cutting forces per tooth, which can cause increased tool wear and higher power consumption.
• More Prone to Tool Deflection: The sharper cutting edge of a smaller-radius tool is more susceptible to deflection under high cutting forces, especially in deep or aggressive cuts. This deflection can lead to poor surface finish, dimensional inaccuracies, or tool breakage in extreme cases.

3. Tool Radius and Cutting Force Direction:

• The tool radius also affects the direction of the cutting forces. Larger radius tools tend to produce more radial (sideways) forces, whereas smaller radius tools may result in a higher axial (vertical) force component. This affects the overall force balance on the spindle and the machine, influencing the overall stability of the milling operation.

4. Effect on Power Consumption:

• The larger the tool radius, the less power is typically needed to complete the cut because the larger cutting edges tend to reduce the cutting force per tooth. In contrast, smaller radius tools require more power to achieve the same material removal rate because they generate higher cutting forces.

5. Impact on Surface Finish and Tool Wear:

• Larger Radius: The smoother engagement with the workpiece due to the larger tool radius generally results in better surface finish. The gradual cutting action reduces the likelihood of surface imperfections and tool wear.
• Smaller Radius: Although smaller radius tools can produce finer details in machining (e.g., in profiling or intricate cuts), they tend to wear faster and may result in a poorer surface finish due to higher cutting forces and more significant tool wear.

Conclusion:

The tool radius directly affects the cutting forces in milling by influencing the contact area, the cutting depth per tooth, and the overall stability of the cutting process. Larger radius tools typically reduce cutting forces and improve tool life, while smaller radius tools can increase cutting forces, potentially leading to greater tool wear and power consumption. In practice, selecting the optimal tool radius for a given operation depends on factors such as material properties, cutting conditions, required surface finish, and machine capabilities.