Lecture 3  theory of metal cutting
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Lecture 3 theory of metal cutting

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Lecture 3  theory of metal cutting Lecture 3 theory of metal cutting Presentation Transcript

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