Material removal rate in grinding operation is small due to:

1. Only a small portion of abrasives are involved in cutting:
In grinding, the material is removed by the cutting action of abrasive particles. However, only a small portion of the abrasive particles actually participate in cutting. This is because the grinding wheel is composed of many abrasive particles that are distributed over the surface of the wheel. During the grinding operation, only a few of these particles come into contact with the workpiece at any given time. As a result, the material removal rate is relatively low.

2. Positive rake angle:
The rake angle is the angle between the rake face of the cutting tool and a reference plane perpendicular to the workpiece surface. In grinding, a positive rake angle means that the cutting edge of the abrasive particle is inclined in the direction of the cutting motion. This helps in reducing the cutting forces and improving the cutting action. However, a positive rake angle does not necessarily lead to a higher material removal rate. It mainly affects the cutting forces and the quality of the surface finish.

3. Zero rake angle:
A zero rake angle means that the cutting edge of the abrasive particle is perpendicular to the workpiece surface. This can result in a more aggressive cutting action and higher material removal rate. However, in grinding, a zero rake angle is not commonly used because it can lead to higher cutting forces and increased heat generation, which may cause thermal damage to the workpiece.

4. Material removal rate does not depend on the rake angle:
The material removal rate in grinding primarily depends on factors such as the hardness of the workpiece material, the grit size of the abrasive particles, the grinding wheel speed, and the grinding depth. The rake angle, on the other hand, affects the cutting forces and the surface finish, but it does not have a significant influence on the material removal rate.

In conclusion, the material removal rate in grinding is small mainly because only a small portion of the abrasives are involved in cutting. The rake angle, whether positive or zero, does not have a direct impact on the material removal rate but affects other aspects of the grinding process.