Tool materials and tool wear

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  • 1.
  • 2. Desirable properties of tool material
    SAM, VJTI
  • 3. Desirable properties of tool material
    Hot hardness
    • High hot hardness means higher speeds and feed rates (higher production rates and lower costs).
    Toughness and Impact strength
    • Tool does not chip or fracture
    Thermal shock resistance
    Wear resistance
    • Tool does not have to be replaced as often
    Chemical stability and inertness
    • To minimize adverse reactions
  • Development of Cutting Tool Materials
  • 4. Characteristics of Tool Materials
  • 5. Factors affecting selection of Tool Materials
    Hardness and condition of the workpiece material
    Operations to be performed
    Amount of stock to be removed
    Accuracy and finish requirements
    Type, capability, and condition of the machine tool to be used
    Rigidity of the tool and workpiece
    SAM, VJTI
  • 6. Factors affecting selection of Tool Materials
    Production requirements influencing the speeds and feeds selected
    Operating conditions such as cutting forces and temperatures
    Tool cost per part machined, including initial tool cost, grinding cost, tool life, frequency of regrinding or replacement, and labor cost—the most economical tool is not necessarily the one providing the longest life, or the one having the lowest initial cost
    No Tool Material Satisfies All These Criterion
    SAM, VJTI
  • 7. High Speed Steel
    High alloy steel
    They are either molybdenum or tungsten based but necessarily contains 4% chromium
    M = Molybdenum
    T = Tungsten
    M >40 = Super HSS materials; capable of treating to high hardness
    SAM, VJTI
  • 8. High Speed Steel
    Advantages of HSS
    • Heat treated to high hardness within the range of Rc 63–68
    • 9. M40 series of HSSs is normally capable of being hardened to Rc70, but a maximum of Rc68 is recommended to avoid brittleness
    • 10. HSSs also possess a high level of wear resistance
    • 11. HSS tools possess an adequate degree of impact toughness and are more capable of taking the shock loading of interrupted cuts than carbide tools
    • 12. When HSSs are in the annealed state they can be fabricated, hot worked, machined, ground, and the like, to produce the cutting tool shape
    • 13. Toughness in HSSs can be increased by adjusting the chemistry to a lower carbon level
    SAM, VJTI
  • 14. High Speed Steel
    Limitations of HSS
    • Tendency of the carbide to agglomerate in the centers of large ingots
    • 15. Improved properties and grindability are important advantages of powdered metal HSSs
    • 16. hardness of these materials falls off rapidly when machining
    • 17. temperatures exceed about 538–593°C
    • 18. use of lower cutting speeds than those used with carbides, ceramics
    SAM, VJTI
  • 19. High Speed Steel
    Applications of HSS
    • Most drills, reamers, taps, thread chasers, end mills, and gear
    • 20. Cutting tools are made from HSSs
    • 21. HSS tools are usually preferred for operations performed at low cutting speeds and on older, less
    • 22. Rigid, low-horsepower machine tools
    Powder metallurgy HSS
    • Uniform structure with fine carbide particles and no segregation
    • 23. Lower in cost because of reduced material, labor, and machining costs, compared to those made from wrought materials
    • 24. Near net shape
    • 25. more design flexibility
    • 26. Applications : Milling cutters
    SAM, VJTI
  • 27. Cemented Tungsten Carbide
    Most carbide grades are made up of tungsten carbide with a cobalt binder
    Advantages of WC
    • Hardness of softest WC is higher than hardened steel
    • 28. High hot hardness
    Grades of WC
    Straight WC
    • Co as a binder
    • 29. Best suited for material having abrasion as a primary tool wear e.g. cast iron, non ferrous materials, non metals
    Complex WC
    • Comprises carbides : TiC, TaC, NbC with Co as a binder
    • 30. ferrous materials, non metals
    SAM, VJTI
  • 31. Cemented Tungsten Carbide
    Tungsten carbide is extremely hard and offers the excellent resistance to abrasion wear
    The most significant benefit of TiC is a reduction in the tendency of the tool to fail by cratering.
    The most significant contribution of TaC is that it increases the hot hardness of the tool, which in turn reduces thermal deformation
    Effect of Co as a binder
    Co is more sensitive to heat, abrasion and welding
    The more cobalt present, the softer the tool, making it more sensitive to thermal deformation, abrasive wear and chip welding
    Cobalt is stronger than carbide. Therefore, more cobalt improves the tool strength and resistance to shock
  • 32. Cemented Tungsten Carbide
    Classification system
    ISO classification number ranges from 05 to 50 : e.g. P20, K35, M40; 05 is most wear resistance whereas 50 is most fracture resistance
    Coated carbide tools is the most significant advance in cutting tool materials since the development of WC tooling
    Various single and multiple coatings of carbides and nitrides of titanium, hafnium, and zirconium and coatings of oxides of aluminum and zirconium, as well as improved substrates better suited for coating, have been developed to increase the range of applications for coated carbide inserts.
    C- Classification
    C1 to C4 for Cast iron
    C5 to C8 for Steel
    ISO- Classification
    P = Stainless Steel
    M = Steel
    K = Cast Iron
  • 33. Ceramics
    Ceramics are primarily aluminum oxides
    Inconsistent and unsatisfactory results during initial periods of development
    Improvements :better control of microstructure (primarily in grain size refinement) and density, improved processing, the use of additives, the development of composite materials, and better grinding and edge preparation methods. Tools made from these materials are now stronger, more uniform, and higher in quality
    Types of ceramics
    Plain ceramics, which are highly pure (99 percent or more) and contain only minor amounts of secondary oxides (produced by powder metallurgy)
  • 34. Ceramics
    Types of ceramics
    Plain ceramics, which are highly pure (99 percent or more) and contain only minor amounts of secondary oxides (produced by powder metallurgy)
    Composite ceramics : are Al203-based materials containing 15–30 percent or more titanium carbide (TiC) and/or other alloying ingredients
  • 35. Ceramics
    Advantages
    Increased productivity: Ceramic cutting tools are operated at higher cutting speeds than tungsten carbide tools
    Good hot hardness, low coefficient of friction, high wear resistance, chemical inertness, and low coefficient of thermal conductivity
    Most of the heat generated during cutting is carried away in the chips, resulting in less heat buildup in the workpiece, insert and toolholder
    Better size control by less tool wear
    Machining of many hard materials
  • 36. Ceramics
    Limitations
    Brittle than carbides
    Less mechanical and thermal shock resistance
    Less interchangeability with the carbide tool holders
    Applications
    High speed machining of steel and cast iron requiring continuous machining
    Most suitable for machining of chemically active materials
    Face milling and turning applications
  • 37. Single crystal and polycrystalline diamonds
    Best suited for precision machining with very high surface finish and to increase productivity by reducing downtimes
    Diamond is the cubic crystalline form of carbon that is produced in various sizes under high heat and pressure. Natural, mined single-crystal stones of the industrial type used for cutting tools are cut (sawed, cleaved, or lapped) to produce the cutting-edge geometry required for the application.
    Advantages
    Hardest material known. Indentation hardness is five times than carbide.
    Extreme hardness and abrasion resistance can result retaining their cutting edges virtually unchanged throughout most of their useful lives
    Because of the diamond’s chemical inertness, low coefficient of friction, and smoothness, chips do not adhere to its surface or form built-up edges when nonferrous and nonmetallic materials are machined.
  • 38. Cubic Boron Nitride
    Super abrasive crystal that is second in hardness and abrasion resistance only to diamond
    CBN crystals are used most commonly in super abrasive wheels for precision grinding of steels and super alloys
    Advantages
    Greater heat resistance than diamond tools
    High level of chemical inertness
    Compacted CBN tools are suitable, unlike diamond tools, for the high speed machining of tool and alloy steels with hardness to Rc70, steel forgings and Ni-hard or chilled cast irons with hardness from Rc45–68, surface-hardened parts, and nickel or cobalt-based super alloys
    They have also been used successfully for machining powdered metals, plastics, and graphite.
  • 39. Tool Wear
  • 40. Temperature in Primary and Secondary Machining Regions
  • 41. Cubic Boron Nitride
    Heat
    Control all the mechanisms of tool failure so tool life is limited only by abrasion wear
  • 42. Tool Failure Mechanisms
    Abrasive wear
    Built-up edge
    Rake surface
    Flank surface
    Thermal/mechanical cracking/chipping
    Cratering
    Thermal deformation
    Chipping
    Mechanical
    Thermal expansion
    Notching
    Fracture
  • 43. Comparison of Catastrophic and Progressive Failure
  • 44. Comparison of Crater and Flank Wear
  • 45. Locations of Tool Wear
  • 46. Abrasive Wear (Abrasion)
    Abrasive wear occurs as a result of the interaction between the workpiece and the cutting edge.
    The width of the wear land is determined by the amount of contact between the cutting edge and the workpiece.
    Rapid failure
    Constant Period
    Break In Period
  • 47. Heat Related Tool Failure Mechanisms
    Cratering(Chemical Wear)
    The chemical properties of the tool-material and the affinity of the tool-material to the workpiece material determine the development of the crater wear mechanism
    Hardness of the tool-material does not have much affect on the process. The metallurgical relationship between the materials determines the amount of crater wear.
    Tungsten carbide and steel have an affinity to each other
    The mechanism is very temperature-dependent, making it greatest at high cutting speeds. Atomic interchange takes place with a two-way transfer of ferrite from the steel into the tool. Carbon also diffuses into the chip.
  • 48. Heat Related Tool Failure Mechanisms
    Built-up Edge (Adhesion)
    It occurs mainly at low machining temperatures on the chip face of the tool. It can take place with long chipping and short-chipping workpiece materials—steel and aluminum.
    This mechanism often leads to the formation of a built-up edge between the chip and edge.
    It is common for the build-up edge to shear off and then to reform.
    At certain temperature ranges, affinity between tool and workpiece material and the load from cutting forces combine to create the adhesion wear mechanism.
    Machining work-hardening materials, such as austenitic stainless steel, this wear mechanism can lead to rapid build-up at the depth of cut line resulting in notching as the failure mode.
  • 49. Heat Related Tool Failure Mechanisms
    Built-up Edge (Adhesion)
    Increased surface speeds, proper application of coolant, and tool coatings are effective control actions for built-up edge
    Thermal Cracking (Fatigue wear)
    Thermal cracking is a result of thermo mechanical actions
    Temperature fluctuations plus the loading and unloading of cutting forces lead to cracking and breaking of the cutting edge
    Carbide and ceramics are relatively poor conductors of heat which leads to fatigue wear
    Thermal Deformation
    As the cutting edge loses its hot hardness the forces created by the feed rate cause the cutting edge to deform
  • 50. Mechanical Failure Mechanisms
    Chipping (Mechanical)
    Small chipping of tool material
    Cutting force should be less than shearing force. Chipping is larger on flank surface than on a face
    Rake Surface
    Flank Surface
  • 51. Mechanical Failure Mechanisms
    Insert Fracture
    When the edge strength of an insert is exceeded by the forces of the cutting process the inevitable result is the catastrophic failure called fracture.
    Excessive flank wear land development, shock loading due to interrupted cutting, improper grade selection or improper insert size selection are the most frequently encountered causes of insert fracture
  • 52. Heat Related Tool Failure Mechanisms