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Theory of cutting – Surface integrity

Theory of cutting – Surface integrity. Technology II – Jan Tomíček. Why?. Why are we so much concerned about the influence of T, Vc, Q and others? All parametres have great influence on final surface. What is the surface? Surface layer. Surface layer properties. Main factors.

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Theory of cutting – Surface integrity

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  1. Theory of cutting – Surface integrity Technology II – Jan Tomíček

  2. Why? • Why are we so much concerned about the influence of T, Vc, Q and others? All parametres have great influence on final surface. What is the surface? Surface layer

  3. Surface layer properties

  4. Main factors • Primarydeformationzone Effectof stress on material. Smallrakes smalleffect Big(negative)rakesgreat influence • Built-upedge Changeoftoolformprecision, dimensions, tollerances

  5. Main factors • Cuttingedgeradius Materialgoingunderedgeisworkhardened. A – slowlygoingdown B – Verysteep drop down C – cracksofsurface.

  6. Main factors • Tool wear (tool sharpness) • Shrap becomes worn • Radius, smoothness, straghtness Residual stress, high temperature, phase transform. oxidation

  7. Main factors • Tool geometry Corner radius, feed marks Clearance angle, cutting edge angle

  8. Factors… • Vibration – toughnes of the system MTPE (Machine-Tool-Piece-Enviroment) • Cutting liquids • Material isotrophy

  9. TOOL MATERIALS • carbon and low alloy steel • high-speed steel (HSS) – highly alloyed steel – alloying ingredients: W, Mo, Cr, V + Co • sintered (cemented) carbide – powder metalurgy: WC, TiC, TaC + Co (as the binder) • coated sintered carbide- thin layer of wear resistant material (TiC, TiN or Al2O3) • ceramics– pure (Al2O3) or mixed (+ TiC, TiN) • diamond (synthetic) • cubic boron nitride (cBN)

  10. TOOL MATERIALS

  11. Steel tool materials • Carbon content 0,7-1,5% • Hardenable – quenching • Hand tools -axes, pickaxes, qarrying tools • Stamping dies, metal cutting tools • Hand tools (file, chisel…)

  12. Tool materials hardness

  13. Typical HSS tool materials

  14. Sintered carbides(ISO grades) • K= WC + Co tough → non-steel cutting (non-ferrous metals, cast iron) – discontinuous chip • P= WC +TiC + TaC + Co hard, wear resistant, brittle → steel cutting– continuous chip • M= WC + TiC + TaC + Cr2O3 + Co→ for materials difficult to machine or universal use Cobalt content influence: hardness ← low …Co… high → toughness

  15. Coated sintered carbide

  16. Coated sintered carbide insert

  17. Cermets • Collective name Materials where hard particles are: TiC, TiCN, TiN instead of WC Ceramic bonding material: MgO, BeO(berylium)Al2O3 CERamic/METal

  18. Ceramics • Two basic groups • Alumium oxide based • Pure (low strength, toughness, fracture and chipping) • Mixed (up to 20-45% of TiC or TiN – thermal conductivity) • Re-inforced (whisker) – crystal fibres made of SiC • Silicon nitride based

  19. Diamond • Monocrystalic – natural – highesthardness • Polycrystalicdiamond (PCD) • MONO – fine machining – precision (glassmaking) dressing ofgrindingwheels • PCD – thinlayer(0,5-1,5 mm) ofgrainsbonded to a basic SC material. !!!Hightemperature (600°C)

  20. Cubic boron nitride(CBN) • Synthetic material (not natural) • Very hard (next to diamond) • Best known abbrasive • Cheaper then diamond for large pieces

  21. TOOL MATERIALS USE

  22. Cutting materials properties

  23. MACHINABILITY OF MATERIAL Machinability denotes the relative ease with which a material can be machined using appropriate tooling and cutting conditions. Influencing factors: • tool wear and tool life; • chip formation; • cutting forces and power; • surface finish

  24. Machinability testing and rating Testing – a comparison of work materials Measuring– the machining performance of a test material is measured relative to that of a base (standard) material Criterion used: varying cutting speed by identical other cutting conditions: cutting speed vc of tested vs. standard (etalon) material Rating – machinability index (machinability rating):

  25. CUTTING FLUIDS Functions: • removing heat generation at the shear and friction zone – the coolingeffect; • reducing friction at the tool – chip and tool – workpiece interfaces – the lubricatingeffect; • washing away chips – the cleaningeffect. Fluid types: • cutting oils – mineral oils - lubricants; • emulsified oils – oil droplets suspended in water (1:30) , the cheapest, both cooling and lubricating; • chemical fluids – chemicals (eg. S, Cl, P) in a water solution – good coolants, mostly for high cutting speeds and high cutting temperatures (grinding).

  26. Cutting fluids – the lubricant effect Two forms of tool – workpiece contact: • roughness peaks contact • surface contact

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