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- 1. MECHANICS OF METAL CUTTING
- 2. Topics to be covered <ul><li>Inroduction to Machining Technology </li></ul><ul><li>Cutting Models </li></ul><ul><li>Turning Forces </li></ul><ul><li>Merchants Circle </li></ul><ul><li>Power & Energies </li></ul>
- 3. Elements of Metal Cutting
- 4. Heat Generation Zones (Dependent on sharpness of tool) (Dependent on ) (Dependent on 10% 30% 60%
- 5. 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
- 6. Cutting Geometry
- 8. Material Removal Rate
- 9. Cutting Models ORTHOGONAL GEOMETRY OBLIQUE GEOMETRY Tool workpiece Tool workpiece
- 10. Assumptions (Orthogonal Cutting Model) <ul><li>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. </li></ul><ul><li>The tool is perfectly sharp (no contact along the clearance face). </li></ul><ul><li>The shearing surface is a plane extending upward from the cutting edge. </li></ul><ul><li>The chip does not flow to either side </li></ul><ul><li>The depth of cut/chip thickness is constant uniform relative velocity between work and tool </li></ul><ul><li>Continuous chip, no built-up-edge (BUE) </li></ul>
- 11. Orthogonal Cutting
- 12. ‘ 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
- 13. ‘ Facing’ Forces For Orthogonal Model End view Note: For the 2D Orthogonal Mechanistic Model we will ignore the Longitudinal component
- 14. 'Turning' Terminology <ul><li>N is the speed in rpm </li></ul><ul><li>D is the diameter of the workpiece </li></ul><ul><li>is the feed (linear distance/rev) </li></ul><ul><li>d is the depth of cut </li></ul><ul><li>is the surface speed </li></ul><ul><li>= DN </li></ul>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)
- 15. 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
- 16. tool Cutting Ratio (or chip thicknes ratio) t c t o A B Chip Workpiece
- 17. 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
- 18. 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
- 19. 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
- 20. 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
- 21. Force Circle Diagram (Merchants Circle) R F t F c Tool F N F s F n
- 22. Results from Force Circle Diagram (Merchant's Circle)
- 23. Forces on the Cutting Tool and the workpiece <ul><li>Importance: Stiffness of tool holder, stiffness of machine, and stiffness of workpiece must be sufficient to avoid significant deflections (dimensional accuracy and surface finish) </li></ul><ul><li>Primary cause: Friction force of chip up rake face + Shearing force along shear plane </li></ul><ul><li>Cutting speed does not effect tool forces much (friction forces decrease slightly as velocity increases; static friction is the greatest) </li></ul><ul><li>The greater the depth of cut the greater the forces on the tool </li></ul><ul><li>Using a coolant reduces the forces slightly but greatly increases tool life </li></ul>
- 24. Stresses On the Shear plane: On the tool rake face:
- 25. Power <ul><li>Power (or energy consumed per unit time) is the product of force and velocity. Power at the cutting spindle: </li></ul><ul><li>Power is dissipated mainly in the shear zone and on the rake face: </li></ul><ul><li>Actual Motor Power requirements will depend on machine efficiency E (%): </li></ul>
- 26. Material Removal Rate (MRR)
- 27. 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:
- 28. 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)
- 29. Specific Cutting Energy Relationship to Shear strength of Material SHEAR ENERGY / UNIT VOLUME FRICTION ENERGY / UNIT VOLUME APPROXIMATE TOTAL SPECIFIC CUTTING ENERGY
- 30. Relation between Pressure and Cutting velocity
- 31. Effect of Rake angle on Cutting Force
- 32. Average Unit Horsepower Values of Energy per unit volume
- 33. Typical Orthogonal Model Violations <ul><li>Geometry and form Violations (i.e. non zero angles of inclination, not sharp - radiused end) </li></ul><ul><li>Shear takes place over a volume (not a line or plane) </li></ul><ul><li>Cutting is never a purely continuous process (cracks develop in chip; material not homogeneous) </li></ul><ul><li>'Size Effect' - larger stresses are required to produce deformation when the chip thickness is small (statistical probability of imperfection in the shear zone) </li></ul><ul><li>BUE - some workpiece material 'welds' to the tool face (cyclic in nature) </li></ul>

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