The first step in a consideration of the mechanics of chip formation is to identify correctly the type of chip involved (steady state, BUE, discontinuous, wavy, or sawtooth). If, at the outset of a metal cutting analysis, an incorrect chip type is assumed, the results obtained may be misleading. It is extremely difficult to predict the type of chip that will form and the best way of determining this is experimentally. A very few cuts under the conditions of interest will clearly indicate the type of chip involved and hence the type of analysis to be applied.
The friction between the chip and the tool plays a significant role in the cutting process. This friction may be reduced by
1. improved tool finish and sharpness of the cutting edge
2. use of low-friction work or tool materials
3. increased cutting speed (V)
4. increased rake angle (α)
5. use of a cutting fluid
When tool face friction is decreased there is a corresponding increase in shear angle and an accompanying decrease in the thickness of the chip. As shown in Fig. 1.6, the plastic strain in the chip decreases as the shear angle increases. The length of the shear plane is seen to be significantly decreased as the shear angle increases. The force along the shear plane will increase as the area of the shear plane increases, assuming the shear stress on the shear plane remains constant.
The temperature of a cutting tool may reach a high value particularly when a heavy cut is taken at high speed. This is evident when the work or tool is touched; by the presence of temper colors on the chip, work, or tool; or may even be evident due to the loss of hardness of the tool point with an attendant loss of tool geometry and failure by excessive flow at the cutting edge.
The operational characteristics of a cutting tool are generally described by a single word—machinability. There are three main aspects of machinability:
1. tool life
2. surface finish
3. power required