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Chip Formation and Tool Life 2x1

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  17/07/20081 Manufacturing Processes: MODULE  Machine Tools II Lecture NotesJoeetGhose Chip Formation Email: joyjeetghose@gmail.com Senior Lecturer, Department of Production Engineering,Birla Institute of Technology, Ranchi   Chip Formation ã Regardless of the tool being used or the metal being cut, the chip formingrocess occurs b a mechanism called lastic deformation. Thisdeformation can be visualized as shearing. That is when a metal issubjected to a load exceeding its elastic limit.ã This action, shown in Figure is similar to the action that takes place whena deck of cards is given a push and sliding or shearing occurs between theindividual cards. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005  17/07/20082 Types of chip ã Discontinuous or segmented ã Continuousã Continuous with a built-up edge. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005 Source: George Schneider,Jr. CMfgE, Cutting Tool Applications Types of chip FIGURE:Variouschipsproducedinturning:(a)tightlycurledchip;(b)chiphitsworkpieceandbreaks;(c)continuouschipmovingawayfromworkpiece;and(d)chiphitstoolshankandbreaksoff. Source :G.Boothroyd, FundamentalsofMetalMachiningandMachineTools . Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005  17/07/20083 Types of chip (video) Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005 Video showing different types of chip formation Continuous Chip ã During cutting of ductile materials a continuous ribbon type chip is produced.ã The pressure of the tool makes the material ahead of cutting edgedeformed lasticall .ã It generally suffers compression and shear. The material then slides over the tool rake face for some distance and then leaves the tool.ã Factors favorable to its formation are ductile metals, such as mild steel,copper etc., fine feed, high cutting speed, large rake angle, keen cuttingedge, smooth tool face and efficient lubrication system. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005  17/07/20084 As discussed in continuous chips, the temperature is higher at interface between chip and the tool during cutting. Also, the work material; slidesunder heavy pressure on the rake face before being transformed into a freechip. Therefore in these conditions some portion of the chip may stick to the Continuous Chip with a Built-up Edge (BUE) rake face of the tool. Because of such closed contact, it discharges its heat tothe tool and thus becomes stringer than the rest of the material following over it. Naturally it attracts more of the deforming work material and thus the sizeof the BUE goes on increasing. When it reaches a certain “critical” size, it becomes unstable and portion of it may disengage and break up. These broken portions and BUE get embedded in the machine surface or get attached to theunderside of the following chip.Favorable condition are low cutting speed, excessive feed, small rake angle,and lack of lubricant.   Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005 Discontinuous Chip ã Discontinuous or segmented chips are produced when brittle metal such as castiron and hard bronze are cut or when some ductile metals are cut under poor cutting conditions.ãof ductile materials. Even a slight plastic deformation produced by a smalladvance of the cutting edge into the job leads to a crack formation in thedeforming zone. With further advance of the cutting tool, the crack travels and asmall lump of material starts moving up the rake face. The force and constraintsin the motion acting on the lump make the crack propagate towards the surface,an us a sma ragmen o e c p ge s e ac e . s e oo moves ur er,this sequence is repeated.ã Conditions tending to promote discontinuous chip formation include: brittlemetal, greater depth of cut, low cutting speed and small rake angle. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005
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