Thermal aspects in machining

Thermal aspects in machining

1. CHAPTER # 2 Thermal Aspects in Machining

2. Topics to be covered: Sources of heat generation in machining and its effects Temperature Measurement techniques in machining Types of cutting fluids Functions of cutting fluid Characteristics of cutting fluid Application of cutting fluids Economics of Metal Cutting Operations

3. Heat generation: Temperature is the main limitation in the selection of process parameters. Heat generation depends on the rate of metal cutting, cutting speed, specific heat and thermal conductivity of the work piece and tool material. Distributed between work piece, tool, chip and surrounding. The amount of dissipated by surroundings is very small and can be neglected. The heat distributed in metal cutting operation is approximately 70:15:15 between chips, tool and work piece.

4. Heat distribution :

5. Source of heat generation in machining: In the elastic deformation , the energy required for the operation is stored in the material as strain energy and no heat is generated. However, in case of plastic deformation , most of the energy used is converted into heat. There are three sources/regions for heat generation in metal cutting:- Primary deformation zone Tool-chip interface zone or Secondary deformation zone Tool work piece interface zone

6. Source of heat generation in machining: contd…

7. Source of heat generation in machining: contd… Heat generated in primary zone is mainly due to plastic deformation and viscous dissipation. In secondary zone , heat is generated due to chip distortion. Metal layer inside the bend and adjacent to tool tip is stretched while layer away from the tip is contracted. Hence, there is distortion of grains in the chip which leads to internal friction between heated chip and tool face. In Work-tool interface zone , heat is generated due to friction by rubbing action between portion of tool flank and work piece.

8. Effect of various factors: Material of tool and work: Higher hardness and tensile strength of material required more cutting force and hence higher heat generated. Tool Geometry : The rake angle has complex influence on the cutting temperature. If rack angle increase in positive direction both cutting force and amount of heat generated are decreased. Negative rack angle causes deformations. As the nose radius increase, volume of tool point is increases and it promotes more heat conduction. Large cross section area of tool shank leads more heat conduction and it reduces cutting temperature.

9. Effect of various factors: contd… Cutting Parameter: As cutting speed increases, friction will increases and it raise the temperature of cutting zone. Cutting temperature is also increase with increase in feed and depth of cut. Cutting Fluid: Cutting fluid is not only reduces friction but also absorb and carry away heat from work, chips and tool.

10. Effect on cutting tool: Plastic deformation of cutting edges if tool material is not enough hard Thermal fracturing of cutting edges Formation of built up edge Rapid tool wear

11. Effect on workpiece: Dimensional inaccuracy Rapid corrosion due to surface damage by oxidation Burning Micro-cracks at the surface

12. Measurement of chip-tool interface temperature: There are number of methods for measuring the chip tool interface temperature as… Tool-work thermocouple, R adiation pyrometers, E mbedded thermocouples, T emperature sensitive paints etc .

13. Measurement of chip-tool interface temp. contd… Tool and work material as two elements of thermocouple . The hot junction is contact area at the cutting edge. Cold part of tool forms cold junction . [Tool-work thermocouple]

14. Measurement of chip-tool interface temp. contd… The thermoelectric emf [electromotive force] generated between the tool and work piece during metal cutting is measured using sensitive milli -voltmeter . These emf readings are converted in terms of temperature.

15. Cutting fluid: Cutting fluid are those liquid and gases which are applied to cutting zone to facilitate cutting operation by removing heat .

16. Functions of cutting fluid: To keep tool cool and prevent it from being heated . To reduce friction and power consumption, hence increase the tool life . To decrease adhesion between chip and tool . To cool workpiece to prevent expansion . To wash away chips and keep cutting region free mainly during deep hole drilling, Hacksawing, milling and grinding . To protect the finished surface from corrosion . To Improve Machinability.

17. Characteristics of cutting fluid: contd… High specific heat and thermal conductivity. Ability of spreading and wetting. High lubricity without gumming and foaming. Chemically stable and non corrosive. Non toxic, odorless and colorless . It should have high film boiling point and high flash point. High resistance to bacterial growth. Easily available with low cost.

18. Method of application of cutting fluid: The effectiveness and cutting fluid significantly depends on how it is applied in respect of flow rate and direction of application . Cutting fluid generally employed by following ways: 1. Flood Cooling : It is most common method. Flow rate is 10 to 20 liter /min for single point cutting tool. U p to 230 liter /min for multiple tooth cutters . Multiple nozzles should be used.

19. Method of application of cutting fluid: contd … 2. Mist Cooling : Small jet equipment is used to disperse water base fluid as fine droplets in carrier. As carrier generally air at pressure 70 to 550 kPa . Mist cooling provides high velocity fluids and better visibility. L imitation due to requirement of venting .

20. Method of application of cutting fluid: contd … 3. High Pressure refrigerated Cooling : It is very effective in removing heat at high rates. Generally used in CNC . Fluid directed at relief angle at pressure of 35,000 kPa . In this system continuous filtering of fluid is essential to eliminate damage of workpiece due to impact of any contaminants that may be present in cooling system.

21. Recommended cutting fluids: Workpiece Material Cutting Fluid Steels For HSS tools: Soluble oil for (1:20) For Carbide tools: Thinner solution oil Grey C I Air blast for cooling and flushing chips Soluble oil for high speed machining Al and Al alloy Preferably dry Light and oily soluble oil Kerosene for stringent cuts Copper and its alloy Water based fluids Oil for High grade copper S S High performance soluble oil Ceramics Dry condition mostly Light neat oil in case of fine finishing High Speed grinding Soluble oil ( 1:50 to 1:100) Low viscosity neat oil

22. Effect of cutting speed on various cost of machining:

23. Effect of cutting speed on various time and production rate: