04 types of chip formation, cutting temperature,etc.,
- 1. Types of Chip formation, Cutting Temperature, Cutting fluids and Tool wear
- 2. Actual Chip Formation • More realistic view of chip formation, showing shear zone rather than shear plane • Also shown is the secondary shear zone resulting from tool-chip friction Types of chip formation • Discontinuous chip • Continuous chip • Continuous chip with Built-up Edge (BUE) • Serrated chip or segmented chip Machining operation Machining operation. Animation
- 3. VIDEO VIDEO CHIP FORMATION Discontinuous chip Continuous chip Continuous chip with Built-Up Edge Serrated or segmented chip It depends on: Workpiece material Tool geometry Cutting conditions Chip formation types
- 4. Discontinuous Chip • Brittle work materials • Low cutting speeds • Large feed and depth of cut • High tool-chip friction Discontinuous Chip Discontinuous Chip . Animation
- 5. Continuous Chip • Ductile work materials • High cutting speeds • Small feeds and depths • Sharp cutting edge • Low tool-chip friction Continuous Chip Continuous Chip . Animation
- 6. • Semicontinuous - saw-tooth appearance • Cyclical chip forms with alternating high shear strain then low shear strain • Associated with difficult-to-machine metals at high cutting speeds Serrated Chip or segmented chip Serrated Chip Serrated Chip. Animation
- 7. Continuous with BUE • Ductile materials • Low cutting speed • High feed • High depth of cut Continuous with BUE
- 8. Cutting Temperature • Approximately 98% of the energy in machining is converted into heat • This can cause temperatures to be very high at the tool-chip • The remaining energy (about 2%) is retained as elastic energy in the chip High cutting temperatures result in the following: • Reduce tool life • Produce hot chips that pose safety hazards to the machine operator • Can cause inaccuracies in part dimensions due to thermal expansion of work material Cutting Temperature. Animation
- 9. Cutting Temperature Cutting Temperature. Animation Cutting Temperature
- 10. Distribution of Heat Heat dissipation depends on cutting speed Cutting Speed, v % Heat Tool-work ---10 % Work-chip----- 10 % Chip-tool----- 80 % keep cutting zone temperature low Cutting FluidSolution
- 11. Cutting Temperature • Analytical method derived by Nathan Cook from dimensional analysis using experimental data for various work materials. ΔT – Temperature rise at tool-chip interface; U – Specific energy; Vc – Cutting speed; tu – Chip thickness before cut; C – Volumetric specific heat of work material; K – Thermal diffusivity of work material; 0.333 0.4 c uv tU T C K
- 12. Cutting Temperature • Experimental methods can be used to measure temperatures in machining -Most frequently used technique is the tool-chip thermocouple • Using this method, Ken Trigger determined the speed-temperature relationship to be of the form: T – Measured tool-chip interface temperature Vc – Cutting speed K, m – Constants m cT Kv
- 13. Cutting fluid is any liquid or gas that is applied to the chip or cutting tool to improve cutting performance. Cutting fluids serve 4 principle functions: 1.To remove heat in cutting (COOLING): The energy used in the cutting process is almost exclusively transformed into heat that goes to the workpiece, tool and chip. The effective cooling action depends on the method of application, type of fluid, fluid flow rate and pressure. 2.To lubricate the chip-tool interface (LUBRICATION): It reduces friction forces and temperatures. 3.To wash away chips (CHIP REMOVAL): This is only applicable to small and discontinuous chips. 4.To avoid part oxidation (ANTI-CORROSION): The environment humidity in combination with the high temperatures (500-900ºC) obtained during machining may cause part oxidation. Thus, the cutting fluid must contain anti-corrosion additives. Use of cutting fluids contributes to: • longer tool life. • Produce workpieces of accurate sizes (reduce thermal expansion). Achieve proper surface quality of the workpiece. • Support chip removal. • Reduce thermal stress on machine tool. CUTTING FLUIDS CUTTING FLUIDS
- 14. CUTTING FLUIDS METHODSOFAPPLICATION LUBRICATION TYPE CONTENT USED VOLUME CHARACTERISTICS Wet machining (using coolant) Manual application 10 to 100 l/min Used for manual tapping. Cutting fluids are used as lubricants. Flooding supply Lubricating system of machine tools need to be cleaned from time to time to eliminate microorganisms. Coolant-fed tooling or internal cooling Some tools (typically drills) are provided with axial holes so that cutting fluid can be pumped directly to the cutting edge. Coolant pressures up to 80 b ars. Coolant-fed tool holders Special tool holders required for milling, turning or drilling operations. Coolant pressures up to 30 bars. Reduced lubrication Minimum quantity llubrication (MQL) 50 ml/h up to 1-2 l/h Cutting fluid is deposited as drops or air-oil mix. Valid for not very demanding machining operations. It can be external or internal. Without lubrication Dry machining without It shows economic and environmental benefits. Under research. Novel cooling methods are under research: high pressure cooling (> 70bar), criogenic cooling (N2, CO2),... VIDE O VIDE O
- 15. CUTTING FLUIDS Manual application Flooding supply Coolant-fed tooling Coolant-fed tool holder Cutting oils are based on mineral or fatty oil mixtures. Commonly used for heavy cutting operations. Soluble oils is the most common (95% of the time), cheap and effective form of cutting fluid. Oil droplets suspended in water in a typical ratio water to oil 30:1. Emulsifying agents are also added to promote stability of emulsion, as well as anticorrosive additives. Chemical fluids (synthetic) consists of chemical diluted in water. They may have harmful effects to the skin. TYPES OF CUTTING FLUID
- 16. Three Modes of Tool Failure Fracture failure (Mechanical chipping) When the cutting force at tool point becomes excessive, it leads to failure by brittle fracture. Temperature failure (Thermal cracking and softening) Cutting temperature is too high for the tool material, which makes the tool point to soften, and leads to plastic deformation along with a loss of sharp edge. Gradual wear Gradual wearing of the cutting edge causes loss of tool shape, reduction in cutting efficiency and finally tool failure.
- 17. Preferred Mode of Tool Failure: Gradual Wear Fracture and temperature failures are premature failures. Gradual wear is preferred because it leads to the longest possible use of the tool. Gradual wear occurs at two locations on a tool: • Crater wear – occurs on top rake face • Flank wear – occurs on flank (side of tool) Tool Failures
- 18. Flank Wear or wear land • It occurs on the tool flank as a result of friction between the machined surface of the work piece and the tool flank. • Due to Friction and abrasion. • Increases as speed is increased. Crater wear • It consists of a concave section on the tool face formed by the action of the chip sliding on the surface. • Direct contact of tool and chip. • Forms cavity Flank wear & Crater wear
- 19. Mechanism of wear • Adhesion wear: Fragments of the work-piece get welded to the tool surface at high temperatures; eventually, they break off, tearing small parts of the tool with them. • Abrasion: Hard particles, microscopic variations on the bottom surface of the chips rub against the tool surface and break away a fraction of tool with them. • Diffusion wear: At high temperatures, atoms from tool diffuse across to the chip; the rate of diffusion increases exponentially with temperature; this reduces the fracture strength of the crystals. • Chemical wear: Reaction of cutting fluid to material of tool.
- 20. Preferred Mode of Tool Failure: Gradual Wear