TYPES OF CHIPS
There are three types of chips:
1. CONTINUOUS CHIP
During cutting of ductile materials, a continuous chip is produced due to the presence of tool cutting edge in compression and shear. These types of chip are in the form of long coil and have the same thickness throughout the length.
This type of chip is required, since it gives a good surface finish, improving the tool life and less power consumption. However, chip disposal is not easy and the surface finish of the finished work gets affected.
The following condition favours the formation of continuous chips.
1. Smaller depth of cut.
2. High cutting speed.
3. Large rake angle.
4. Proper cutting fluid.
5. Low friction between tool face and the chips.
2. DISCONTINUOUS CHIP OR SEGMENTAL CHIP
Discontinuous chip are as shown in the figure, which are produced while machining brittle materials such as grey cast iron, bronze, high carbon steel at low cutting speed without fluids when friction exists between the tool and chip.
During machining, the brittle materials lack its density which is necessary for plastic chip formation. But, it should be less. This will result in formation of discontinuous chip. In continuous chip formation the shearing occurs at head of cutting tool continuously without fracture whereas in discontinuous chip formation, intermittently rupture occur which will produce segment of chips.
Handling of these chips is easier and it can be easily disposed off, since they are having small length. Also , it will not spoil the finished work piece surface as they do not interfere.
The following condition favours the formation of discontinuous chips.
1. Machining of brittle materials.
2. Small rake angle.
3. Higher depth of cut.
4. Low cutting speed.
5. Excess cutting fluid.
6. Cutting ductile material at very low feeds with small rake angle of the tool.
3. CONTINUOUS CHIP WITH BUILT-UP EDGE
During cutting process, the interface temperature and pressure are quite high and between tool-chip interface the friction is also high. It causes the chip material to weld itself to tool face near the nose as shown in the figure. This is called “built-up edges”.
Formation of built-up edges in continuous chip is a transient and not steady phenomenon. The collected built-up of chip material will then break away, part adhering to the undesired of the chip and part to the work piece. Thus, the process will result in a poor surface finish on the machined surface and accelerated wear on the tool face.
However, this type of chip having some advantage, the one important favour of it is that, from the wears the rake face of the tool is protected due to moving chip and the action of heat. It may result in the increasing of tool life.
The following condition favours the formation of continuous chip with built-up edges.
1. Low cutting speed.
2. Small rake angle.
3. Coarse feed.
4. Large uncut thickness.
5. Insufficient cutting fluid.