Types of Chip Formation During Machining
As every one knows, chips are formed during the machining of
workpieces. The side of the chip in contact with the cutting
tool is normally shiny, flat and smooth while the other side,
which is the free workpiece surface, is jagged due to shear.
It is important to study the formation of chips during the
machining process as the former affects the surface finish,
cutting forces, temperature, tool life and dimensional
tolerance. Understanding the chip formation during the machining
process for the specific materials will allow us to determine
the machining speeds, feed rates and depth of cuts for efficient
machining and increased tool life in the specific actual
machining operation. During the machining process, three basic
types of chips are formed. They are discontinuous chips,
continuous chips, and continuous chips with built-up edge.
Discontinuous chip formation normally occurs during machining of
brittle work material. This type of chips also occus in
machining operation with small rake angles on cutting tools,
coarse machining feeds, low cutting speeds. Discontinuous chip
formation results in poor workpiece surface finish.
During continuous chip formation, a continuous "ribbon" of metal
flows up the chip-tool zone. This is considered to be the ideal
condition for efficient cutting action.
Continuous chip with built-up edge formation is basically the
same process as continuous chip formation, except that as the
metal flows up the chip-tool zone, small particles of the metal
begin to adhere or weld themselves to the edge of the cutting
tool. As the particles continue to weld to the tool, it affects
the cutting action of the tool.
This type of chip formation is common in machining of softer
non-ferrous metals and low carbon steels. Common problems are
the built-up edges breaking off and being embedded in the
workpiece during machining, decrease in tool-life and final poor
surface finish of the workpiece.
Studies on the built-up edges have shown that the chip material
is welded, deformed and then deposited onto the rake face of the
tool layer by layer. It is thus possible to observe the presence
of built-up edges by studying the back face of the chip during
the machining process. This is normally used in micro or ultra
precision machining operation.
To reduce built-up edges, improve the lubrication conditions,
use sharp tools and better surface finish tool and also apply
ultrasonic vibration during the machining process.