Tool Wear and Tool Life
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Tool life is measured by the time period from when a tool starts cutting until failure or until it needs resharpening. Tool life can be measured in units of time, number of pieces cut, volume of material removed, or length of cut. Tools typically fail due to high temperatures, mechanical impacts, or gradual wear. Wear occurs on the flank and crater faces of tools and is caused by abrasion, diffusion, electrochemical reactions, and other mechanisms. Factors like cutting speed, workpiece properties, tool geometry, and cooling influence tool life.
Tool Wear and Tool Life
- 1. Presented by:- Mobashir Kashani (121116048) Shubham Khandelwal (121116054)
- 2. Tool Life Useful cutting life of tool expressed in time Time period measured from start of cut to failure of the tool Time period between two consecutive resharpenings or replacements.
- 3. Ways of measuring tool life No. of pieces of work machined Total volume of material removed Total length of cut. Limiting value of surface finish Increase in cutting forces Dimensional accuracy Overheating and fuming Presence of chatter
- 4. Modes of tool failure 1. Temperature failure a. Plastic deformation of Cutting Edge (CE) due to high temp b. Cracking at the CE due to thermal stresses. 2. Rupture of the tool point a. Chipping of tool edge due to mechanical impact b. Crumbling of CE due to Built up Edge (BUE) 3. Gradual wear at tool point a. Flank wear b. Crater wear
- 5. Tool wear Tool wear causes the tool to lose its original shape- ineffective cutting Tool needs to be resharpened
- 6. Causes of Tool Wear 1. Attrition wear 2. Diffusion wear 3. Abrasive wear 4. Electrochemical wear 5. Chemical wear 6. Plastic deformation 7. Thermal cracking
- 7. Attrition wear At low cutting speeds Flow of material past cutting edge is irregular and less stream lined BUE formed and discontinuous contact with the tool Fragments of tool are torn from the tool surface intermittently High Slow and interrupted cutting Presence of vibrations Found in carbide tools at low cutting speeds
- 8. Diffusion wear Diffusion of metal & carbon atoms from the tool surface into the w/p & chip. Due to High temp High pressure Rapid flow of chip & w/p past the tool Diffusion rate depends on the metallurgical relationship Significant in carbide tools.
- 9. Abrasive wear Due to Presence of hard materials in w/p material. Strain hardening induced in the chip & w/p due to plastic deformation. Contributes to flank wear Effect can be reduced by fine grain size of the tool & lower percentage of cobalt
- 10. Electrochemical wear When ions are passed b/w tool & w/p Oxidation of the tool surface Break down of tool material @ chip tool interface
- 11. Chemical wear Interaction b/w tool and work material Plastics with carbide tools Cutting fluid
- 12. Plastic Deformation When high compressive stresses acts on tool rake face- tool deformed downways – reduces relief angle Modifies tool geometry and accelerates other wear processes
- 13. Thermal cracking Due to cyclic thermal stresses at cutting edge Comb cracks Transverse cracks Chipping of tool
- 14. Geometry of tool wear Flank wear (edge wear) Crater wear (face wear)
- 15. Flank Wear Tool slides over the surface of the work piece and friction is developed Due to Friction and abrasion. Adhesion between work piece & tool- BUE Starts at CE and starts widening along the clearance face Independent of cutting conditions and tool / work piece materials Brittle and discontinuous chip Increases as speed is increased.
- 16. Primary stage rapid wear due to very high stress at tool point Wear rate is more or less linear in the secondary stage Tertiary stage wear rate increases rapidly resulting in catastrophic failure.
- 17. Crater wear Direct contact of tool and w/p Forms cavity Ductile materials – continuous chips Initiates rapid rupture near to nose Leads to weakening of the tool Increase in cutting temp Cutting forces & friction
- 18. Measurement of tool life Time for Total destruction in case of HSS or time to produce 0.75 mm wear for carbide tools Tool life expressed by Taylor’s eqn VTb = C V = cutting speed in cm/min T= tool life in min b= const= 0.1 for HSS C= 50 for HSS Cemented carbide : b=0.125, C=100 Tool life expressed in volume of metal removed L = TVfd
- 19. Measurement of tool life Diamond Indentor technique Radioactive techniques Test at elevated cutting speeds Facing tests Test with low wear criterion
- 20. Factors affecting tool life 1. Cutting speed 2. Physical properties of w/p 3. Area of cut 4. Ratio of feed to depth of cut 5. Shape and angles of tool 6. Tool material and its heat treatment 7. Nature and quantity of coolants 8. Rigidity of tool and wp