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QE 107: Workshop Technology Section 4: Metal Cutting

QE 107: Workshop Technology Section 4: Metal Cutting. Asanga Ratnaweera Department of Mechanical Engineering University of Peradeniya Peradeniya. Material Removal Processes. Basic Cutting Principle.

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QE 107: Workshop Technology Section 4: Metal Cutting

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  1. QE 107: Workshop TechnologySection 4: Metal Cutting Asanga Ratnaweera Department of Mechanical Engineering University of Peradeniya Peradeniya ACR/QE107/SEC4

  2. Material Removal Processes ACR/QE107/SEC4

  3. Basic Cutting Principle • The basic principle is the use of a cutting tool to form a chip removed from the part (by shear). • It requires the relative motion between the tool and part. • The primary motion is called speed, v, and the secondary motion is called feed, f. • The cutting tool needs to cut into the part, called the depth of cut, d. ACR/QE107/SEC4

  4. Cutting Mechanism • During machining, the material is removed in form of chips, which are generated by shear deformation along a plane called the shear plane. ACR/QE107/SEC4

  5. Chip Types • Continuous chips • Ductile material • Ex: Mild steel, Al • High speed • Low feed • Small depth of cut ACR/QE107/SEC4

  6. Chip Types • Discontinuous chips • Brittle material • Ex: Cast iron • Low speed • High feed • Large depth of cut ACR/QE107/SEC4

  7. Chip Types ACR/QE107/SEC4

  8. Chip Types • Built Up Edge (BUE) • Some of the cut material will attach to the cutting point. • This tends to cause the cut to be deeper than the tip of the cutting tool and degrades surface finish. • Also, periodically the built up edge will break off and remove some of the cutting tool. Thus, tool life is reduced. BUE ACR/QE107/SEC4

  9. Chip Types • Built Up Edge (BUE) • built up edge can be reduced by: • Increasing cutting speed • Decreasing feed rate • Increasing rake angle • Reducing friction (by applying cutting fluid) BUE ACR/QE107/SEC4

  10. Tool Geometry • The shape and orientation of the cutting tool greatly affects the chip formation mechanics ACR/QE107/SEC4

  11. Positive Rake Neutral Rake Negative Rake Cutter Velocity Cutter Velocity Cutter Velocity - + + 0 Workpiece Normal Workpiece Normal Workpiece Normal Rake Angle • Of particular importance is the rake angle that the tool makes with the workpiece normal • Effects on the chip formation ACR/QE107/SEC4

  12. Clearance angle • Clearance angle is that the tool makes with the newly machined work surface • Avoids the work-surface being rubbed by the tool ACR/QE107/SEC4

  13. Basic Cutting Geometry • Orthogonal cutting: the cutting edge of the tool is straight and perpendicular to the direction of motion. • Oblique cutting: the tool edge is set at angle. ACR/QE107/SEC4

  14. Cutting Tools • There are basically two types of cutting tools: • Single point (e.g. turning tools). • Multiple point (e.g. milling tools). turning tools drilling tools milling tools ACR/QE107/SEC4

  15. Cutting Tools ACR/QE107/SEC4

  16. Geometry of turning tool ACR/QE107/SEC4

  17. Turning operations ACR/QE107/SEC4

  18. Milling • Work is fed into the rotating cutter • Typically uses a multi-tooth cutter. • Cutting tools for this process are called milling cutters. • Capable of high material removal rate • Two basic milling operations: • Peripheral milling • Face milling ACR/QE107/SEC4

  19. Peripheral Milling • Peripheral milling is also called plain milling • axis of the tool parallel to the surface, • cutting operation is performed by cutting edges on the outside periphery of the cutter. ACR/QE107/SEC4

  20. Face milling • Axis of rotation perpendicular to workpiece surface • The milled surface is flat and has no relationship to the contour of the cutter • Combined cutting action of the side and face of the milling cutter ACR/QE107/SEC4

  21. ACR/QE107/SEC4

  22. Feed Direction • Conventional milling (up milling) • Most common method of feed • Feed work against the rotation of the cutter • Maximum chip thickness at end of cut • Climb milling (down milling) • Maximum chip thickness at beginning of cut • Suited for machining of thin and hard to hold parts • 20% less Hp than conventional milling • Machine must be very rigid to safely ACR/QE107/SEC4

  23. Geometry of milling tools ACR/QE107/SEC4

  24. Chip Thickness • f - is the feed per tooth • d - is depth of cut • N – cutter speed • D – diameter of the cutter • tc - undeformed chip thickness • v - linear speed of the workpiece • n – number of teeth in the cutter ACR/QE107/SEC4

  25. Chip Thickness f = v / Nn ACR/QE107/SEC4

  26. Drilling Operation • Basic hole making processes account for approximately 50-70% of all the metal removal processes utilized today. • Hole making methods • Casting • Punching • Flame cutting • Machining including: • Drilling, milling, EDM, etc. ACR/QE107/SEC4

  27. Cutting edges Geometry of drilling tools ACR/QE107/SEC4

  28. Drill sectioned along the cutting edge showing change of rake due to flute form. Rake angle changes from negative to positive when radius increases Rake angle ACR/QE107/SEC4

  29. Rake angle ACR/QE107/SEC4

  30. Chip thickness ACR/QE107/SEC4

  31. P Fc Ft The cutting force • Externally applied forces Fc – Cutting force, Ft – Thrust force ACR/QE107/SEC4

  32. Fn F The cutting force • Forces on the tool F– Friction force, Fn– Normal force ACR/QE107/SEC4

  33. The cutting force • Forces on the chip Fs Fco Fs – Shear force, Fco– Compressive force ACR/QE107/SEC4

  34. Fs Fco Ft R F Fn Merchant’s Circle The cutting force • Merchant’s theory α λ -α Fc λ α λ – friction angle ACR/QE107/SEC4

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