Every machining operation involves the formation of chips. The chip is formed by deformation of the metal lying ahead of the cutting edge by a process of shear. The chips produced during machining of various metals can be broadly classified into the following three types.
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Discontinuous or segmental chip
This type of chips are produced during machining of hard and brittle metals like bronze, brass and cast-iron. Sometimes, cutting of ductile metals at very low feeds with small rake angle of the cutting tool and high speeds and high friction forces at the chip tool interface also result in the production of discontinuous chips. Discontinuous chips in ductile materials are formed when the hydrostatic pressure near the cutting edge is tensile or the shear energy reaches a critical value. The formation of this type of chip in brittle materials imparts good finish, increases tool life and consumes less power. Presence of discontinuous chips in ductile-materials results in poor-finish and excessive tool-wear. Smaller chips are easier to dispose off.
If discontinuous chips are produced from the brittle materials, then surface finish is fair, power consumption is low and tool life is reasonable. However when these are produced with ductile materials, then finish is poor and tool wear is excessive. Other factors responsible for promoting the production of Discontinuous Chips are smaller rake angle on the tool and too much depth of cut.
As the name itself implies it is continuous, means the presence of separated segmental elements is totally eliminated in this case. This type of chip is produced while machining a ductile material, like mild steel, under favorable cutting condition such as high cutting speed and minimum friction between the chip and the tool face. Otherwise, it will break and form the segmental chip. The friction at the chip-tool interface can be minimized by polishing the tool face and adequate use of coolant. Also, with diamond tool the friction is less. The basis of the production of a continuous chip is the continuous plastic deformation of the metal ahead of the tool, the chip moving smoothly up the tool face.
Sometimes, continous chips are produced at low cutting speed if effective cutting fluid is used because this type of chip is associated with low friction between the chip and the tool. Since finish is best, power consumption is low and tool life high with this type of chip, this is most preferred type. Other factors responsible for promoting its production are bigger Rake angle, finer Feed and Keen cutting edge of the tool.
Continuous chip with built up edge
This type of chip is very similar to that of continuous type, with the difference that it is not as smooth as the previous one. This type of chip is obtained by machining ductile material, When high friction exists at the chip tool interface. The upward flowing chip exerts pressure on the tool face. The normal reaction Nr of the chip on the tool face is quite high, and is maximum at the cutting edge or nose of the tool. This gives rise to an excessively high temperature and the compressed metal adjacent to the tool nose get welded to it. The chip is also sufficiently hot and gets oxidized as it comes off the tool and turns blue in colour. The extra metal welded to the nose or point of the tool is called Built up Edge. This metal is highly strain hardened and brittle. With the result, as the chip flows up the tool, the Built-up edge is broken and carried away with the chip while the rest of it adheres to the surface of the work piece, making it rough. Due to the Built-up-Edge the rake angle is also alterned and so is the cutting force. The common factors responsible for promoting the formation of Build-up Edge are low cutting speed, excessive feed, small rake angle lack of lubricant.