Theory of cutting Tool wear. Technology II Jan Tomíček

Transcription

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 1) Simple wear o Abrasive wear o Adhesive wear o Diffusion wear o Chemical wear 2) Plastic deformation 3) Fracture

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

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

6 1.simple wear 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 1.simple wear Diffusion wear Chemical affinity of materials * SC vs. steel

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

9 Plastic deformation All cutting tools, all materials after some amount of tool wear the contact surface is too large - Massive heat generation leeds to rapid temperature increase - The hardness of material decreases (limiting temperature) - The high cutting force load leeds to loss of geometry and appearance of plastic deformation(usually together with rapid wear of combined thermo-mechanical load)

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 Property Abrasion Adhesion Difusion Chemical Plastic def. Fracture Hardness X X X Strength/ toughness Chemical stability Thermal conductivity Thermal expansion X X X X X X X X X X X Melting point X X Surface inertivity X

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

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

15 Notch type wear Tool wear forms Comb cracks on the edge Fatigue cracks Chipping

16 Tool wear types

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18 Notch type wear Caused by hard surface

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

20 Chipping Special type of fracture small particles (ceramics)

21 Tool wear flank wear(notch) Specified by Average wear-area width (VB)

22 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)

23 Tool wear crater wear Characterised by Depth of crater (KT) in mm

24 Tool life Defined tool property!!! Cutting time required to reach tool-life criterion(definite tool wear) There is relation between the tool life and cutting speed Taylor s Equation T Tool life (min) Vc cutting speed(m.min-1) n empirical exponent(tool material effect) Cvc empirical constant

25 Tool life Equation in log scale straight line Tg α values 0,2.HSS steel 0,3.carbides 0,45.ceramics

26 Tool life Experimental measurement Changing cutting speed VB measurement

27 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 range of Vc

28 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

29 Tool wear measurement Direct measuring the lost tool material Weight Isotopes method Resistance layer method Micrometric etc. Indirect measuring the effects related to wear Glossy ring method optical reflection Force measurement Temperature Vibration or sound measurement Roughness measurement Acoustic emission (ultrasonic) etc.

30 Literature: BRINKSMEIER, E. : Monitoring of grinding wheel wear, Ananals of the CIRP, vol,1/1992, pp. 373 GIUSTI, F. - SANTOCHI, M. TANTUSSI, G.: On line sensing of flank and crater wear of cutting tools, CIRP, 1/1987, pp. 41 GOMES DE OLIVEIRA, J.F. DORNFELD, D.A. - WINTER, B.:Dimensional characterization of grinding wheel surface through acoustic emission, Ananals of the CIRP, vol, 1/1994, p. 291 ISO 3685 : Tool Life Testing With Single Point Turning Tools. 1990