Chips formation and type:

In any machining operation, the material is removed from the workpiece in the form of chips and its nature differs from operation to operation. chips are formed due to tearing and shearing.

        In the process of chip formation by tear , the workpiece material adjacent to the tool face is compressed and a crack runs ahead of the cutting tool and toward the body of the workpiece.

        In chip formation by shear, there is the general movement of the chip over the tool face.

        Mainly three different types of chips are formed during the metal cutting :

v     Continuous chip

v     Discontinuous chip

v     Continuous chip with a build-up edge.

 

Continuous chip: 

continuous chip

    A continuous chip is obtained while cutting ductile material such as low carbon steel, aluminum, and copper. It consists of elements bonded firmly together without being fractured.

    Under the best conditions, the metal flows by means of plastic deformation and gives a condition, the metal flows by means of plastic deformation and gives a continuous ribbon of metal which under the microscope, shows no signs of tears or discontinuities. This chip is severely deformed and either comes off in the form of a long string or curls into a tight roll.

Discontinuous chip :

Discontinuous chip

    

    A discontinuous chip is obtained while cutting more brittle such as gray cast iron, bronze. It consists of elements fractured into fairly small pieces ahead of the cutting tool. brittle material rupture during plastic deformation and form chips as a separate small piece. As these chips produce the cutting edge smoothen over the irregularities and a fairly good finish is obtained. Tool life is also reasonably good and the power consumption is low.

    Discontinuous chips can also be formed on some ductile metals only under certain conditions, particularly at very low speed, and if the coefficient of friction is low.

Continuous chip with a build-up edge:

Continuous chip with a build-up edge

    While during cutting the temperature and pressure is quit high, it causes the chip material to weld itself to the tool face near the nose. this is called “built-up-edge”. It appears that when the cut is started in ductile metals, accumulated build-up of chip material will then break away part adhering to the underside of the chip and part to the work-pice. this process gives rise to a poor finish on the machined surface and accelerated wear on the tool face. Build-up edge chip adheres to the workpiece due to high heat, so a rough surface finish is produced. Due to the high heat produced, rake angle and nose radios reduce and therefore tool life reduces.