Lecture 3 theory of metal cutting

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Lecture 3 theory of metal cutting

  1. 1. Theory of Metal Machining<br />
  2. 2. ContentCommon features of machining processes, geometry of single point tool and tool signature, concept of speed, feed and depth of cut applicable to various machining processes.<br />SAM, VJTI<br />
  3. 3. Mechanics of Metal Cutting<br />Tool must be sharp (what do you mean by sharp?)<br /> Relative velocity<br /> Interference<br />Tool material shall be harder than the work piece material<br />Physical Phenomenon in Machining<br /> Plastic flow<br /> Fracture<br /> Friction<br /> Heat<br /> Molecular diffusion<br /> Chatter<br />At extreme condition<br />• Sticking friction at tip<br />• Deformation at high strain<br />and strain rate<br />• Nascent surface exposed<br />after deformation is very<br />active<br />SAM, VJTI<br />
  4. 4. Objectives During Machining<br />Contradicting<br />High Material Removal Rate (MRR)<br />Good accuracy and Surface finish<br />Long tool life<br />Cost<br />SAM, VJTI<br />
  5. 5. Processing Parameters in Machining<br />Cutter Related<br />Material<br /> Geometry<br /> Mounting<br />Machine Related<br />Cutting fluid type and<br />application method<br /> Depth and Width of cut<br />Spindle speed<br /> Feed rate<br />Workpiece Related<br />Material (composition, homogeneity) Geometry (bar, block, casting etc.)<br />Depth of cut<br /> Spindle speed<br /> Feed rate<br />Others <br />– Cutting fluid type and application method<br /> Depth and Width of cut<br /> Spindle speed<br />Feed rate<br />SAM, VJTI<br />
  6. 6. Effects of Processing Parameters<br />Work hardening<br />Thermal softening<br /> Hot spots on the machined surface<br />Deflection and diameter variations<br />Tool life<br />Surface finish<br />Cutting forces and<br />Torques and power<br />Tool temperature<br />Frictional effects on tool face<br />Built up edge<br />Formation<br />Chatter, noise and<br />Vibrations<br />SAM, VJTI<br />
  7. 7. Theories of Chip Formation<br />Chip formation studies helps in understanding mechanics of metal cutting or physics of machining<br /> They lead to equations that describe the interdependence of the process parameters such as depth of cut, relative velocity, tool geometry etc. These relations help us in selecting optimal process parameters.<br />SAM, VJTI<br />
  8. 8. Theories of Chip Formation – Theory of Tear<br />A crack propagates ahead of the tool tip causing tearing similar to splitting wood [Reuleaux in 1900]<br />SAM, VJTI<br />
  9. 9. Theories of Chip Formation – Theory of Tear<br />Against the traditional wisdom, the tool was observed to wear, not at the tip, but a little distance away from it. Therefore, this theory was subscribed by many researchers for a long time.<br />SAM, VJTI<br />
  10. 10. Theories of Chip Formation – Theory of Tear<br />Further studies attributed the wear away from the tip to the following:<br /> Chip velocity w.r.t. the tool is zero at the tip.<br /> The tip is protected by BUE.<br /> Temp is also high a little away from the tip due to the<br />frictional heat.<br />Subsequent studies proved the chip formation as shear and not tear. Thus the theory of tear was rejected.<br />SAM, VJTI<br />
  11. 11. Theories of Chip Formation – Theory of Compression<br />The tool compresses the material during machining.<br />This was based on the observation that the chip length was shorter than the uncut chip length.<br /> Later it was established that this shortage in length corresponds to the increase chip thickness. <br />Thus this theory too was wrong<br />SAM, VJTI<br />
  12. 12. Theories of Chip Formation – Theory of Shear<br />The excessive compressive stress causes shear of the chip at an angle to the cutting direction [Mallock in 1881].<br />SAM, VJTI<br />
  13. 13. Theories of Chip Formation – Theory of Shear<br />Mallock’s other contributions<br />Emphasis on the influence of friction at chip-tool interface<br />Studied the effect of cutting fluids<br />Studied the influence of tool sharpness<br />Studied chatter<br />His observations on the above studies still hold good although he could not explain all of them at that time.<br />SAM, VJTI<br />
  14. 14. Difficulties in Machining Mechanics studies<br />Several physical phenomenon such as plastic flow, fracture, friction, heat, molecular diffusion and chatter are involved. Some of them occur in extrême conditions<br /> Friction – sticking; deformation– high strain and strain rate; nascent surface exposed after deformation is very active causing diffusion<br />The cutting zone is covered by chips and coolant.<br />Typical machining is oblique, i.e., forces, torques and deflections exist in all 3 directions.<br />SAM, VJTI<br />
  15. 15. Difficulties in Machining Mechanics studies<br />The typical machining operations are too short and the stock (depth and width of cut) keeps changing. <br />Furthermore, velocity also may change along the cutting edge as well as over time. These changes further compound the difficulties to observe the process carefully.<br />Orthogonal cutting experiments were developed to overcome these difficulties.<br />SAM, VJTI<br />
  16. 16. SAM, VJTI<br />Orthogonal Cutting<br />Facing of thin pipe on a lathe with the cutting edge radial to the pipe.<br />
  17. 17. SAM, VJTI<br />Characteristics of Orthogonal Cutting<br />A wedge shaped tool is used<br />Cutting edge is perpendicular to the direction of cut. In other words, cutting edge angle and cutting edge inclination angle <br />Uncut chip thickness is constant along the cutting edge and w.r.t. time.<br />Cutting edge is longer than the width of the blank and it extends on its both sides.<br />Cutting velocity v is constant along the cutting edge and w.r.t. time<br />
  18. 18. SAM, VJTI<br />Orthogonal Cutting - Experiments<br />Quick stopping devices to freeze the chip formation<br />Cutting wax manually slowly so as to observe it<br />Marking grids on the side of the work piece and study their deformation.<br />Microscopic studies<br />Photoelastic studies (tools made of transparent material such as persbex or resin (araldite); work piece is wax. Resulting fringe patterns are observed under polarized glasses.<br /> Observation using high speed cameras<br />Force, torque and power measurements using dynamometers.<br />Temp measurements<br />
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  25. 25. Mechanics of Metal Cutting<br /><ul><li> Plastic Deformation
  26. 26. The outer surface is usually smooth due to the burnishing effect of the tool
  27. 27. Shear Plane
  28. 28. The angle formed by the shear plane and the direction of the tool travel is called the shear angle</li></ul>SAM, VJTI<br />
  29. 29. Mechanics of Metal Cutting<br /> The type of chip produced depends upon workpiece material, tool geometry, and operating conditions.<br /> Discontinuous chips <br /> Individual segments<br /> Fracture of the metal<br /> Brittle materials (cast irons)<br />No plastic deformation <br /> Continuous chips <br /> Machining ductile materials like steel and Al <br /> Continuous deformation without fracture <br /> Chip breakers are required <br /> Tool wear increases with sliding <br />SAM, VJTI<br />
  30. 30. Mechanics of Metal Cutting<br /> Compressive deformation will cause it to be thicker and shorter than the layer of workpiece material removed<br /> The work required to deform this material usually accounts for the largest portion of forces and power involved in a metal removal operation<br /> The ratio of chip thickness, to the un-deformed chip thickness (effective feed rate) is called the chip thickness ratio. The lower the chip thickness ratio, the lower the force and heat, and the higher the efficiency of the operation<br />SAM, VJTI<br />
  31. 31. Mechanics of Metal Cutting –Power Consumption<br /> Method based on the MRR<br /> Unit Horse Power <br /> The unit horsepower factor (P) is the approximate power required at the spindle to remove 1 in3/min of a certain material. <br />SAM, VJTI<br />

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