Question 5.

As we have seen, chips carry away the majority of the heat generated during machining. If chips did not have this capacity, what suggestions would you make in order to be able to carry out machining process without excessive heat. Explain?

Solutions:

When the chips are formed they have long grain structures, traverse to the direction of cutting. However, as the chip rubs the surface of the workpiece, the grain structure on the lower surface bends to form a "long tail" due to secondary deformation and the heat build up. This lower surface then recrystallizes due to the grain structure and intense heat. This recrystallized area is highly undesirable since it is more difficult to have smaller, broken chips.

We can improve the chip breaking by

• increasing the brittleness of the chip material by the low temperature application of the cryogenic coolant;
• reducing the secondary deformation that occurs at the bottom surface of the chip which rubs on the tool face;
• reducing the long tail in the grain structure of the chips, which is a major cause of the difficulty in breaking ductile chips;
• avoiding the welding or recrystallizing of the bottom layer of the chip (to avoid the strong bonding of chip structure which adds to the difficulty in chip breaking) by the coldness and reduction of the secondary deformation;
• bending and curling the chip for breaking by the use of both a mechanical chip breaker and the pressure from the cryogenic fluid

Explanation:

The build-up edge formation is automatically prevented by injecting the super cold cryogenic fluid to the cutting tip area. This reduces the adhesion of the chip material to the tool tip which lowers the temperature in the cutting zone and prevents the welding phenomenon. Also, this invention can clean and remove the possible build-up edge with the cryogenic jet. Because the build-up edge is the main cause of poor surface quality, the method of the present invention also indirectly improves the finish of the machined surface. Therefore, it is an object of the present invention to provide a cutting tool which injects liquid nitrogen or other liquified gas as close as possible to the highest heat affected area of the cutting tool, not the workpiece, through a nozzle, between the chip and the tool face. This provides a fluid cushion which reduces the friction of the chip rubbing on the tool face, and reduces the cutting force involved, as well as the abrasive wear.