Introduction:
Cemented carbide tools are widely used in machining operations due to their high hardness, wear resistance, and thermal stability. These tools are made by combining tungsten carbide particles with a metallic binder, typically cobalt. The geometry of the cutting tool, including the rake angle, plays a crucial role in determining the performance and efficiency of the machining process.

Negative back rake angle:
The correct answer to the question is option 'B' - negative back rake angle. The back rake angle refers to the angle between the cutting edge and a line perpendicular to the base of the tool. In the case of cemented carbide tools, a negative back rake angle is provided. Let's understand why negative rake angle is preferred for cemented carbide tools.

Advantages of negative back rake angle:
1. Increased tool life: The negative rake angle helps in distributing the cutting forces more evenly across the cutting edge. This reduces the stress concentration on the tool and improves its resistance to wear, resulting in increased tool life.

2. Enhanced cutting edge strength: The negative rake angle provides a stronger cutting edge by increasing the thickness of the tool near the cutting edge. This improves the tool's resistance to chipping and fracture, especially when machining hard or brittle materials.

3. Improved chip control: The negative rake angle promotes chip breaking and evacuation. It creates a shearing action that leads to the formation of shorter and more manageable chips. This helps in preventing chip clogging, reducing built-up edge formation, and improving the chip flow during the machining process.

4. Lower cutting forces: The negative rake angle reduces the cutting forces acting on the tool. This is beneficial for minimizing power consumption, reducing machining vibrations, and improving surface finish.

5. Suitable for difficult-to-machine materials: Cemented carbide tools with a negative back rake angle are particularly effective when machining materials with low machinability, such as stainless steel, superalloys, and hardened steels. The negative rake angle helps in reducing the cutting forces and heat generation, making the machining process more stable and efficient.

Conclusion:
In conclusion, cemented carbide tools are usually provided with a negative back rake angle. The negative rake angle offers various advantages, including increased tool life, enhanced cutting edge strength, improved chip control, lower cutting forces, and suitability for difficult-to-machine materials. These factors make cemented carbide tools with a negative rake angle highly effective for a wide range of machining applications.