3 February 2016

What is chip formation

In all machining process removal of excess material from the preformed blanks in the form of a chip.

In chip formation tool is considered stationary and workpiece moves to the right thus the metal is severely compressed in the are in front of a cutting tool. This causes shear and plastic flow at high temperatures when the stress in the workpiece just before a cutting tool reaches a value that exceeds the ultimate strength of the metal, thus forming a chip.

Chip formation typically occurs where the edge of the material being cut is touched by a machine tool. Now let you know the chip thickness ratio. 

Chip thickness Ratio = a2 / a1

where a2 = thickness of a chip

           a1 = uncut thickness

Types of Chip :

The form and dimension of the chip in the machining process indicate the nature and quality of a particular machining process, but the type of chip formed depends on material cut properties and different cutting conditions.

1. Discontinuous Chip ( Segmental Form )

This type of chip material can't withstand high shear forces and therefore the shear formed cleanly away from the chips. The chips formed, however, may be attached loosely to each other.

These chips consist of elements fractured into fairly small pieces ahead of the cutting tool.

This type of chip obtained in mostly brittle material such as cast iron and bronze.

On some ductile metals, this type of chip can also be formed only under certain conditions, especially at very low speeds and if the friction coefficient is low.

2. Continuous Chip ( Ribbon Type )

This type of chip consists of elements bonded firmly together without being fractured.

For low friction at the tool-chip interface, low power consumption, long tool life, and good surface finish, the continuous form of a chip are considered most desirable.

A disadvantage of continuous chips is the fact that they can become very long and become entangled with the machine it causes a safety hazard.

3. Continuous chip with built-up edge

The term built-up edge implies the construction of a metal ridge above the cutting edge on the top surface of the tool. 

A pile of compressed and highly stressed metal forms at the extreme edge of the tool when cutting is started in ductile metals. This piled up metal is welded to the cutting tip with high heat and pressure generated there and it forms built-up edges.

Condition for the formation of this type of chip :

  • Low cutting speed
  • Low rake angle 
  • High feed
  • Lack of cutting fluid
  • Large depth of cut