Mechanism of Chip formation, Types of chip, Chip Breaker ~ MECHTECH GURU

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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