Chapter 6: Thermal aspects of machining

Transcription

1 Chapter 6: Thermal aspects of machining LEARNING OBJECTIVES Heat Generation in metal cutting Temperature distribution in work and chip in orthogonal cutting Measurement of temperature in metal cutting INTRODUCTION: According to the first law of thermodynamics, when work is transformed into heat, the quantity of heat produced is equivalent to the quantity of work. Heat generated, through conversion of mechanical energy. The three distinct sources of heat in metal cutting are given below: The shear zone, 1,where the primary plastic or shear deformation takes place The chip-tool interface, 2, where secondary plastic deformation due to friction between the heated chip and tool takes place. The work- tool interface, 3, at flanks where frictional rubbing occurs. Fig.1: Sources of heat in metal cutting For example, in a typical study of machining mild steel at 30 m/min at about 750 deg of cutting temperature at tool-chip interface, the distribution of total energy developed at the shear zone is as follows Energy at chip 60 percent Energy to workpiece 30 percent Energy to tool - 10 percent Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 1

2 The rate of energy consumption during orthogonal cutting is given by W c = F c V c Where F C = Cutting force, N Vc = cutting speed, m/min When a material is deformed clastically, the energy used is stored in the material as strain energy and no heat is generated. However, when a material is deformed plastically almost all the energy used is converted into heat. In metal cutting, the material is subjected to extremely high strains and elastic deformation forms a very small proportion of the total deformation, hence all the energy is assumed to be converted into heat. Thus Q = F c V c / J where J is mechanical equivalent of heat The cutting energy is converted into heat in two principal regions of plastic deformation The shear zone or primary deformation zone AB Secondary deformation zone BC If, as is common in most practical situation, the cutting tool is not perfectly sharp, a third heat source BD would be present due to friction between the tool and the newly machined surface. However, unless the tool is severely worn, the heat generated at this source will be small and hence could be neglected. The temperature distribution in the workpiece, in this instance the chip zone, as seen in typical experimental study, is given in Figure 2. As point X in the material moves towards the cutting tool, it approaches and passes through the primary deformation zone, and is heated till it leaves the zone, being carried away within the chip, However point Y passes through both deformation zones and continues to get heated till leaves the region of secondary deformation. It is then cooled as the heat is conducted into the Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 2

3 body of chip, and eventually the chip achieves a uniform temperature throughout. The maximum temperature thus occurs along the tool face some distance from the cutting edge. The point Z, that remains in the workpiece, is heated as it passes below the tool cutting edge, by conduction of heat from the primary deformation zone. Some heat is removed from the secondary deformation zone by conduction into the body of the tool. Fig.2: Temperature distribution in work and chip during orthogonal cutting. EFFECTS OF THE HIGH CUTTING TEMPERATURE ON TOOL AND WORK High cutting temperatures are 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. The possible detrimental effects of the high cutting temperature on cutting are: On tool Rapid tool wear, which reduces tool life Cutting edges plastically deform and tool may loose its hot hardness Thermal flaking and fracturing of cutting edges may take place due to thermal shock Built up edge formation Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 3

4 On work Dimension inaccuracy of work duet to thermal distortion and expansion and contraction during and after machining Surface damage by oxidation, rapid corrosion, burning etc. Tensile residual stresses and microcracks at the surface and sub surfaces. 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 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. DETERMINATION OF CUTTING TEMPERATURE Cutting temperature can be determined by two ways Analytically using mathematical models (equations) if available or can be developed. This method is simple, quick and inexpensive but less accurate and precise. Experimentally this method is more accurate, precise and reliable. The temperatures which are of major interests are : Average shear zone temperature Average /Maximum temperature at the chip tool interface Temperature at the work tool interface ( tool flanks) Average cutting temperature Experimental methods of determination of cutting temperature The feasible experimental methods are Calorimetric method quite simple and low cost but inaccurate and gives only grand average value Decolourising agent some paint or tape, which change in colour with variation of temperature, is pasted on the tool or job near the cutting point; the as such colour of the chip (steels) may also often indicate cutting temperature Tool-work thermocouple simple and inexpensive but gives only average or maximum value Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 4

5 Moving thermocouple technique Embedded thermocouple technique Photo-cell technique Infra ray detection method Tool work thermocouple technique Fig.3: Tool-work thermocouple technique of measuring cutting temperature 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 as shown in Figure 3. Then the average cutting temperature is evaluated from the mv after thorough calibration for establishing the exact relation between mv and the cutting temperature. Moving thermocouple technique This simple method, schematically shown in Fig. 4. enables measure the gradual variation in the temperature of the flowing chip before, during and immediately after its formation. A bead of standard thermocouple like chrome-alumel is brazed on the side surface of the layer to be removed from the work surface and the temperature is attained in terms of mv Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 5

6 . Fig.4: Moving thermocouple technique of measuring cutting temperature 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. 5. 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. 5 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. Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 6