The behaviour of chip on rake face of the tool caught the attention of many investigators because it was found that the value of coefficient of friction at tool chip interface is very high. It has been established that the normal stress on the interface due to the cutting force reaches such a high value that yielding occurs and the chip material flows into the aspirates on the ground face of the tool.
The frictional force on the rake face over the areas where yielding occurs is then equal to the force necessary to cause shearing of either the body of the chip or the surface contaminants, and accordingly the coulomb’s laws of friction cannot hold.
Zorev in 1948 established that the sliding of chip on tool face behaves differently in different zones. Probably two zones called sticking region and sliding region as shown in Fig. 22.29 could be considered.
The texture of stacking zone is different from sliding zone which is composed of longitudinal scratches. Due to two different types of contacts, viz. sticking and sliding, the laws for the variation of coefficient of friction principally differ very much.
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Columb’s laws of friction holds good for sticking region and accordingly shear (tangential) contact stress increases in this region and on reaching a maximum value (shear flow stress), it remains constant in the sliding region. The measured value of coefficient of friction is average of the coefficient of friction for the two zones. In actual experiments normal stress near tool point in the sliding region is found to decrease in some cases.