Mechanism of Chip formation, Types of chip, Chip Breaker

Mechanism of Chip formation:-

·       
The tool is considered stationary, and the workpiece
moves to the right.
The metal is severely compressed in the area
in front of the cutting
tool
·       
This
causes high temperature shear and plastic
flow if the metal is ductile.
·       
When the stress in the workpiece just
ahead of the cutting tool reaches a value exceeding the ultimate strength of
the metal, particles will shear to form a chip element which moves up along the face of work.
·       
The outward or shearing movement of each successive element is arrested by work hardening and the movement
transferred to the next element.
·       
The process is repetitive and a continuous chip is formed
having a highly
compressed and burnished underside, a minutely serrated top side caused
by the shearing action.
·       
The place along which the element shears
is called the shear plane.
·       
Thus the chip is formed by plastic
deformation of the grain structure of the metal along the shear








Types
of chip
·       
The form and dimension of a chip in
metal machining indicates the nature and quality of a particular machining
process, but the type of chip formed is greatly influenced by the properties of material cut and various
cutting conditions.
·       
In engineering manufacture particularly in metal machining processes
hard brittle metals have
a very limited use, and ductile metals
are mostly used.
·       
 Chips of ductile
metals are removed by varying
proportions of tear, shear and flow. This results in three
general types of shapes.

Ø
The discontinuous ( segmental form)
Ø
The continuous or ribbon type.
Ø
The continuous with built up edge.
·       
Discontinuous or segmental chips consist
of elements fractured into fairly small pieces
ahead of the cutting tool.
·       
This
type of chip is obtained
in machining most
brittle material, such as cast
iron and bronze. These
materials rupture during plastic deformation, and form chips as separate small pieces.
·       
As these chips
are produced, the cutting edge smoothes over
the irregularities and fairly good surface finish is obtained.
Tool life is also reasonably good and power consumption is low.
·       
Discontinuous chips can also be formed
on some ductile metals only under certain conditions particularly at very
low speeds and if the coefficient of friction is low. With ductile
metals, however the surface finish
is bad and the tool life is short.
·       
Conditions tending to promote
its formation include:
brittle metal, greater
depth of cut, low cutting speed and small rake angle.

·       
Continuous chips consist of element
bonded firmly together without being fractured.
·       
Underside of continuous chip has small notches while the lower
side, which slides over the tool face, is smooth and shiny.

·       
The continuous form is considered most
desirable for low friction at the tool chip interface, lower power consumption, long tool life
and good surface
finish.
·       
Factor
favorable to its formation are: ductile metal,
such as mild steel, copper,
etc., fine feed, high cutting speed,
large rake angle,
keen cutting edge,
smooth tool face and
an efficient lubrication system.
·       
The term built
up edge implies
the building up of a ridge of metal on the top surface of the tool and above the cutting edge.
·       
It appears that, when the cut is started in ductile metals,
a pile of compressed and highly stressed metal forms
at the extreme edge of the tool.

·       
Owing
to the high heat and pressure generated
there, this piled up metal is welded to
the cutting tip and forms a ‘false’
cutting edge to the tool. This is usually referred
to as the ‘built up edge’.
·       
Conditions tending to promote
the formation of built up edges include:
low cutting speed, low rake angle,
high feed, lack of cutting
fluid and large
depth of cut.










            Chip
Breaker
·       
A continuous type chip form a long cut is usually quite troublesome.
·       
Such chips foul the tools; clutter up
the machine and workplace, besides being extremely difficult to remove
from the swarf
tray.
·       
They should be broken into comparatively small pieces for ease of handling and to
prevent it from becoming a work hazard,
hence the chip
breakers are used
to reduce the swarf into small pieces as they are formed.

·       
The fact that the metal is already
work hardened helps the chip breaker to perform
effectively.
·       
Various
types of chip breakers are made, but all of them consist
mainly of a step or groove ground into the leading edge of
the tool or a piece of cutting tool material clamped on top of the cutting
tool.
·       
In normal shop
practice common methods
of breaking the chips are
summarized as follows:
Ø
By clamping a piece of sheet metal in the path of the coil.
Ø
By a stepped
type breaker in which a step is ground on the face
of the tool
along the cutting edge.
Ø
By a groove type breaker
in which a small groove
is ground behind the cutting edge.
Ø
By a clamp type breaker in which a thin
carbide plate or clamp is brazed or screwed
on the face of the tool.




                                 

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