Mechanics of metal cutting
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Mechanics of metal cutting

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    Mechanics of metal cutting Mechanics of metal cutting Presentation Transcript

    • MECHANICS OF METAL CUTTING
    • Topics to be covered
      • Inroduction to Machining Technology
      • Cutting Models
      • Turning Forces
      • Merchants Circle
      • Power & Energies
    • Elements of Metal Cutting
    • Heat Generation Zones (Dependent on sharpness of tool) (Dependent on  ) (Dependent on  10% 30% 60%
    • Tool Terminology Side relief angle Side cutting edge angle (SCEA) Clearance or end relief angle Back Rake (BR),+ Side Rake (SR), + End Cutting edge angle (ECEA) Nose Radius Turning Cutting edge Facing Cutting edge
    • Cutting Geometry
    •  
    • Material Removal Rate
    • Cutting Models ORTHOGONAL GEOMETRY OBLIQUE GEOMETRY Tool workpiece Tool workpiece
    • Assumptions (Orthogonal Cutting Model)
      • The cutting edge is a straight line extending perpendicular to the direction of motion, and it generates a plane surface as the work moves past it.
      • The tool is perfectly sharp (no contact along the clearance face).
      • The shearing surface is a plane extending upward from the cutting edge.
      • The chip does not flow to either side
      • The depth of cut/chip thickness is constant uniform relative velocity between work and tool
      • Continuous chip, no built-up-edge (BUE)
    • Orthogonal Cutting
    • ‘ Turning’ Forces For Orthogonal Model End view section 'A'-'A' Note: For the 2D Orthogonal Mechanistic Model we will ignore the radial component F t 'A' 'A' c F
    • ‘ Facing’ Forces For Orthogonal Model End view Note: For the 2D Orthogonal Mechanistic Model we will ignore the Longitudinal component
    • 'Turning' Terminology
      • N is the speed in rpm
      • D is the diameter of the workpiece
      • is the feed (linear distance/rev)
      • d is the depth of cut
      • is the surface speed
      • =  DN
      Standard Terms Beware , for turning: In the generalized orthogonal model depth of cut (to) is f (the feed), and width of cut (w) is d (the depth of cut)
    • Orthogonal Cutting Model (Simple 2D mechanistic model) Mechanism: Chips produced by the shearing process along the shear plane  t 0  + Rake Angle Chip Workpiece Clearance Angle Shear Angle depth of cut Chip thickness Tool Velocity V tool t c
    • tool Cutting Ratio (or chip thicknes ratio)  t c t o  A B Chip Workpiece
    • Experimental Determination of Cutting Ratio Shear angle  may be obtained either from photo-micrographs or assume volume continuity (no chip density change): i.e. Measure length of chips (easier than thickness) w t L 0 0 0 w c L c c t
    • Shear Plane Length and Angle  or make an assumption, such as  adjusts to minimize cutting force: (Merchant)  t c t o  A B Chip tool Workpiece
    • Velocities (2D Orthogonal Model) Velocity Diagram (Chip relative to workpiece) V = Chip Velocity (Chip relative to tool) Tool Workpiece Chip V = Cutting Velocity (Tool relative to workpiece) Shear Velocity c     V s V V s V c
    • Cutting Forces ( 2D Orthogonal Cutting) Free Body Diagram Generally we know: Tool geometry & type Workpiece material and we wish to know: F = Cutting Force F = Thrust Force F = Friction Force N = Normal Force F = Shear Force F = Force Normal to Shear c t s n Tool Workpiece Chip Dynamometer R R R R F c F t  s F F n N F
    • Force Circle Diagram (Merchants Circle) R F t F c Tool F N      F s    F n
    • Results from Force Circle Diagram (Merchant's Circle)
    • Forces on the Cutting Tool and the workpiece
      • Importance: Stiffness of tool holder, stiffness of machine, and stiffness of workpiece must be sufficient to avoid significant deflections (dimensional accuracy and surface finish)
      • Primary cause: Friction force of chip up rake face + Shearing force along shear plane
      • Cutting speed does not effect tool forces much (friction forces decrease slightly as velocity increases; static friction is the greatest)
      • The greater the depth of cut the greater the forces on the tool
      • Using a coolant reduces the forces slightly but greatly increases tool life
    • Stresses On the Shear plane: On the tool rake face:
    • Power
      • Power (or energy consumed per unit time) is the product of force and velocity. Power at the cutting spindle:
      • Power is dissipated mainly in the shear zone and on the rake face:
      • Actual Motor Power requirements will depend on machine efficiency E (%):
    • Material Removal Rate (MRR)
    • Specific Cutting Energy (or Unit Power) Energy required to remove a unit volume of material (often quoted as a function of workpiece material, tool and process:
    • Specific Cutting Energy Decomposition 1. Shear Energy/unit volume (Us) (required for deformation in shear zone) 2. Friction Energy/unit volume (Uf) (expended as chip slides along rake face) 3. Chip curl energy/unit volume (Uc) (expended in curling the chip) 4. Kinetic Energy/unit volume (Um) (required to accelerate chip)
    • Specific Cutting Energy Relationship to Shear strength of Material SHEAR ENERGY / UNIT VOLUME FRICTION ENERGY / UNIT VOLUME APPROXIMATE TOTAL SPECIFIC CUTTING ENERGY
    • Relation between Pressure and Cutting velocity
    • Effect of Rake angle on Cutting Force
    • Average Unit Horsepower Values of Energy per unit volume
    • Typical Orthogonal Model Violations
      • Geometry and form Violations (i.e. non zero angles of inclination, not sharp - radiused end)
      • Shear takes place over a volume (not a line or plane)
      • Cutting is never a purely continuous process (cracks develop in chip; material not homogeneous)
      • 'Size Effect' - larger stresses are required to produce deformation when the chip thickness is small (statistical probability of imperfection in the shear zone)
      • BUE - some workpiece material 'welds' to the tool face (cyclic in nature)