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heat in metal cutting

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Heat generation in metal cutting and various methods of measuring the cutting temperature

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heat in metal cutting

  1. 1. Theory of Metal Cutting Lecture No. (5) Faculty of Engineering Prod. & Mech. Design Engineering Department Dr. Rania Mostafa Heat in metal cutting
  2. 2. INTRODUCTION  The power consumed in metal cutting is largely converted into heat near the cutting edge of the tool, and many of the economic and technical problems of machining are caused directly or indirectly by this heating action.  The heat generated can cause temperatures to be as high as 6000C at tool chip interface.  The cost of machining is very strongly dependent on the rate of metal removal, and costs may be reduced by increasing the cutting speed and/or the feed rate, but there are limits to the speed and feed above which the life of the tool is shortened excessively.  This may not be a major constraint when machining aluminum and magnesium and certain of their alloys, in the cutting of which other problems, such as the ability to handle large quantities of fast moving chips, may limit the rate of metal removal.
  3. 3.  The bulk of cutting, however, is carried out on steel and cast iron, and it is in the cutting of these, together with the nickel based alloys, that the most serious technical and economic problems occur.  With these higher melting point metals and alloys, the tools are heated to high temperatures as metal removal rate increases and, above certain critical speeds, the tools tend to collapse after a very short cutting time under the influence of stress and temperature. “There is little doubt that when the laws of variation of the temperature of the shaving and tool with different cutting angles, sizes and shapes of cut, and of the rate of abrasion are definitely determined, it will be possible to indicate how a tool should be ground in order to meet with the best efficiency and the various conditions to be found in practice. However, determination of temperatures and temperature distribution in the important region near the cutting edge is very important and vital. INTRODUCTION
  4. 4. Cutting Temperatures Elastic deformation- Energy required for the operation is stored in the material as strain energy and no heat is generated. Plastic deformation – Most of the energy used is converted as heat. Cutting temperatures are important because high temperatures, 1. Reduce tool life. 2. Produce hot chips that pose safety hazards to the machine operator. 3. Can cause inaccuracies in work part dimensions due to thermal expansion of work piece material.
  5. 5. The effect of cutting temperature, particularly when it is high is mostly detrimental to both the tool and the job. The major portion of the heat is taken away by the chips. But it does not matter because chips are thrown out. So attempts should be made such that the chips take away more and more amount of heat leaving small amount of heat to harm the tool and the job. 1- Effect of cutting temperature on tool The possible detrimental effects of the high cutting temperature on cutting tool (edge) are  Rapid tool wear which reduces tool life  plastic deformation of the cutting edges if the tool material is not enough hot-hard and hot-strong  thermal flaking and fracturing of the cutting edges due to thermal shocks.  Built up Edge formation.
  6. 6. 2- Effect of cutting temperature on Job The possible detrimental effects of the high cutting temperature on machined job are:  Dimensional inaccuracy of the job due to thermal distortion and expansion-contraction during and after machining  surface damage by oxidation, rapid corrosion, burning etc.  induction of tensile residual stresses and micro cracks at the surface / subsurface.
  7. 7. Positive effect of Cutting Temperature (Hot machining) However, often the high cutting temperature helps in reducing the magnitude of the cutting forces and cutting power consumption to some extent by softening or reducing the shear strength, τs of the work material ahead the cutting edge. To attain or enhance such benefit the work material ahead the cutting zone is often additionally heated externally. This technique is known as Hot Machining and is beneficially applicable for the work materials which are very hard and hardenable like high manganese steel, Hadfield steel, Ni-hard, Nimonic etc.
  8. 8. Factors Affecting Temperature
  9. 9. Sources and Causes of heat generation in Machining During machining, heat is generated at the cutting point from three sources,  Primary shear zone (1) where the major part of the energy is converted into heat.  Secondary deformation zone (2) at the chip – tool interface where further heat is generated due to rubbing and / or shear.  At the worn out flanks (3) due to rubbing between the tool and the finished surfaces.
  10. 10. Thermal Aspects of Machining The heat generated is shared by the chip, cutting tool and the blank. The apportionment of sharing the heat depends upon the configuration, size and thermal conductivity of the tool – work material and the cutting condition. The following figure visualizes that maximum amount of heat is carried away by the flowing chip. From 10 to 20% of the total heat goes into the tool and some heat is absorbed in the blank. With the increase in cutting velocity, the chip shares heat increasingly.
  11. 11. Temperature distribution in Metal Cutting Fig. shows temperature distribution in work piece and chip during orthogonal cutting (obtained from an infrared photograph, for free-cutting mild steel where cutting speed is 0.38m/s, the width of cut is 6.35mm, the normal rake is 300, and work piece temperature is 6110C)
  12. 12. Experimental methods of determination of cutting temperature  Amongst θS, θi, and θf , θi is obviously the highest one and its value is maximum almost at the middle of the chip – tool contact length. Experimental methods generally provide the average or maximum value of θi.  Some techniques also enable get even distribution of temperature in the chip, tool and job at the cutting zone. The temperatures which are of major interests are: θs : average shear zone temperature θi : average (and maximum) temperature at the chip-tool interface θf : temperature at the work-tool interface (tool flanks) θavg : average cutting temperature Cutting temperature can be determined by two ways : •Experimentally – this method is more accurate, precise and reliable. •analytically – using mathematical models (equations) if available or can be developed. This method is simple, quick and inexpensive but less accurate and precise.
  13. 13. • Pioneering work in this area was done by Benjamin Thompson, who in 1798 investigated that the heat generated in the boring of a cannon and developed the concept of mechanical equivalent of heat. • The total heat was measured by performing the drilling operation with the work piece, the chips, and the tool submerged in water. • Three different calorimetric setups were used for determining (i) the total heat generated in drilling, (ii) heat in the tool after the cut, (iii) heat in the chips. Calorimetric Method
  14. 14. • The heat in the tool was determined by cutting an sample test bar dry and dropping the tool into the calorimeter immediately upon the completion of cutting. Heat in the chips was obtained by noting the temperature rise of the calorimeter and water into which only chips were permitted to fall.
  15. 15. Major points: Quite simple Low cost Inaccurate Only grand average value  Much of the heat generated in cutting was carried out by the chips ( 70–80%) with 10% entering the work piece, and the remainder into the tool.
  16. 16. Decolorizing Agent • Paints or Tape are placed on the tool or job near cutting point. • Variation of temperature causes change in color. • In steels color of chips may also indicate temperature.
  17. 17. Tool work Thermocouple Technique In a thermocouple two dissimilar but electrically conductive metals are connected at two junctions. Whenever one of the junctions is heated, the difference in temperature at the hot and cold junctions produce a proportional current which is detected and measured by a milli-voltmeter. In machining like turning, the tool and the job constitute the two dissimilar metals and the cutting zone functions as the hot junction. Then the average cutting temperature is evaluated from the mV after thorough calibration for establishing the exact relation between mV and the cutting temperature.
  18. 18. • Advantages of Thermocouples include the following:  simple in construction,  ease of remote measurement,  flexibility in construction,  simplicity in operation and signal processing,  low cost.
  19. 19. 2-Moving thermocouple technique
  20. 20. 3- Embedded thermocouple technique • In operations like milling, grinding etc. • The standard thermocouple monitors the job temperature at a certain depth, hi from the cutting zone. The temperature recorded in oscilloscope or strip chart recorder becomes maximum when the thermocouple bead comes nearest (slightly offset) to the grinding zone. • With the progress of grinding the depth, hi gradually decreases after each grinding pass and the value of temperature, θm also rises as has been indicated in Fig. • For getting the temperature exactly at the surface i.e., grinding zone, hi has to be zero, which is not possible. So the θm vs hi curve has to be extrapolated up to hi = 0 to get the actual grinding zone temperature. Log – log plot helps such extrapolation more easily and accurately.
  21. 21. • The limitations of the embedded thermocouples include the following:  difficulty in placing thermocouple close to tool-chip contact & further causes interference in flow of heat.  the technique is difficult to implement as it involves the use of fine holes.  thermocouples have limited transient response due to their mass and distance from the points of intimate contact. plotting of the temperature isotherms using embedded thermocouples in the tool can be difficult.
  22. 22. 4- Chip-tool interface temperature by compound tool In this method a conducting tool piece (carbide) is embedded in a non-conducting tool (ceramic). The conducting piece and the job form the tool-work thermocouple as shown in Fig. 8 which detects temperature θi at the location (Li) of the carbide strip. Thus θi can be measured along the entire chip-tool contact length by gradually reducing Li by grinding the tool flank. Before that calibration has to be done as usual.
  23. 23. 5- Photo-cell technique • This unique technique enables accurate measurement of the temperature along the shear zone and tool flank as can be seen in Fig. The electrical resistance of the cell, like PbS cell, changes when it is exposed to any heat radiation. • The amount of change in the resistance depends upon the temperature of the heat radiating source and is measured in terms of voltage, which is calibrated with the source temperature. • It is evident from Fig. that the cell starts receiving radiation through the small hole only when it enters the shear zone where the hole at the upper end faces a hot surface. Receiving radiation and measurement of temperature continues until the hole passes through the entire shear zone and then the tool flank.
  24. 24. 6- Infrared photographic technique • This modern and powerful method is based on taking infra-red photograph of the hot surfaces of the tool, chip, and/or job and get temperature distribution at those surfaces. • Proper calibration is to be done before that. This way the temperature profiles can be recorded as indicated in Fig. • The fringe pattern readily changes with the change in any machining parameter which affect cutting temperature.
  25. 25. Infra-red photographic technique

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