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Theory of cutting – Tool wear

Theory of cutting – Tool wear. Technology II – Jan Tomíček. Tool wear. Cutting tools undergone wear during the cutting. The wear means that the tool loses its volume and geometrical properties. There are different mechanisms of tool wear. Tool wear mechanisms. Wear Abrasive wear

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Theory of cutting – Tool wear

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  1. Theory of cutting – Tool wear Technology II – Jan Tomíček

  2. Tool wear Cutting tools undergone wear during the cutting. The wear means that the tool loses its volume and geometrical properties. There are different mechanisms of tool wear

  3. Tool wear mechanisms • Wear • Abrasive wear • Adhesive wear • Diffusion wear • Chemical wear • Plastic deformation • Fracture

  4. Abrasive wear • Related to hardness • Caused by hard particles in workpiece material • Caused by particles of the built-up-edge • Caused by the transformed surface (hardened)

  5. Adhesicve wear Appears at low cutting temperatures (or cutting speeds) High pressure causes pressure welds on tops of the surface irregularities.

  6. Diffusion wear • For diffusion is necessary certain temperature approx. 600°C • NO Diffusion using carbon steels and HSS steel (low temperature) • Diffusion using SC, ceramics, PCD and CBN. • Diffusion is exchange of chemical elements between tool and material

  7. Diffusion wear • Chemical affinity of materials * SC vs. steel

  8. Chemical wear • New comounds or composition of surafce layer is created (Oxidation, cuttig fuild reaction etc.)

  9. Plastic deformation Allcuttingtools, allmaterials – aftersomeamountoftoolwearthecontactsurfaceistoolarge • Massiveheatgenerationleeds to rapid temperatureincrease • Thehardnessofmaterialdecreases (limitingtemperature) • Thehighcuttingforceloadleeds to lossof geometry andappearanceofplasticdeformation(usuallytogetherwith rapid wearofcombinedthermo-mechanicalload)

  10. Hardness to temperature rel.

  11. Fracture • Usually from overloading the tool – heavy forces from heavy cutting condition • Hard particles, particles with high strength • Increase of cutting forces due to tool wear

  12. Important tool properties

  13. Tool wear forms Basic mechanisms are combined to create a tool wear form for different cutting cases. Flank wear Crater wear

  14. Tool wear forms • Complex tool wear forms • Notch(groove) wear • Thermal cracks • Plastic deformation • Fracture • chipping

  15. Tool wear types

  16. Tool wear – flank wear(notch) • Specified by „Average wear-area width“ (VB)

  17. Tool wear – flank wear(notch) • Three regions • AB – initial tool wear(rapid but decreasing) • BC – uniform wear(uniform, slower) • CD – rapid wear (fast and increasing)

  18. Tool wear – crater wear • Characterised by „Depth of crater (KT) in mm

  19. Notch type wear • Caused by hard surface

  20. Thermal cracking • Thermal expansion due to temperature change (discontinual cutting e.g.milling)

  21. Chipping • Special type of fracture – small particles (ceramics)

  22. Tool life • Definedtoolproperty!!! „Cuttingtimerequired to reachtool-lifecriterion(definitetoolwear) Thereisrelationbetweenthetoollifeandcutting speed  Taylor‘s Equation T…Toollife (min) Vc…cutting speed(m.min-1) n…empirical exponent(toolmaterialeffect) Cvc…empiricalconstant

  23. Tool life Equation in log scale • straight line n…values 0,2….HSS steel 0,3….carbides 0,45….ceramics

  24. Tool life Experimental measurement Changing cutting speed – VB measurement

  25. Tool wear vs. cutting speed Left – typical relationship Right – effect of built-up edge (wear of BUE instead of the tool) irregular!!! Out of normaly used Vc

  26. Taylor‘s equation – general form Cv…emipirical constatn x,y…empirical exponents typical values (x=0,15, y=0,5, n=0,3) for SC cutting Influeace: highest – cutting speed, feed, depth of cut -- lowest

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