Amarjit Prakashrao Kene 1*, S.K. Choudhury 2. Abstract

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1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Behaviour of Cutting Forces in Hard Turning Considering Effect of Tool Wear on Principal Flank, Auxiliary Flank and Rake Faces: Individually and in Combination Amarjit Prakashrao Kene 1*, S.K. Choudhury 2 1* Ph.D. Scholar, Indian Institute of Technology Kanpur, Kanpur, , amarjitk@iitk.ac.in 2 Professor, Indian Institute of Technology Kanpur, Kanpur, , choudhry@iitk.ac.in Abstract In the present work behaviour of cutting forces was investigated considering the effect of tool wear on flank and rake face individually and also in combination. The relationship between cutting forces and wear land was studied by performing the turning operation on EN24 hardened steel of diameter 70 mm and length 400 mm using commercially available single layer PVD coated TiSiN-TiAlN nanolayer insert at constant cutting conditions. In the beginning the inserts were worn out artificially using electric discharge machining process. From the plot of cutting forces with wear land and experimental results taken individually, it has been observed that the tangential cutting force was more affected by crater wear. However axial and radial forces observed to be predominantly affected by principle flank wear. On the other hand, wear on auxiliary flank face showed no specific effect on any of the forces. Axial and radial forces were observed to be affected mostly when combination of flank wear and crater wear was considered. The average axial force F x, tangential force F z, radial force F y were found to be N, N and N respectively in case of no wear (zero wear) and N, N and N respectively in case of combination of wear. Keywords: Hard turning, cutting forces, PVD coated insert, Artificial tool wear. 1 Introduction Hard turning is a process which involves the machining of steel of above 45 HRC. In the present scenario, hard turning gained the popularity because of its noticeable advantages over conventional grinding process such as greater accuracy, higher productivity, more flexibility and above all, the economy (Hembrug Machine Tools Manual, Finish Hard Turning). Hard turning is being widely used in industries for the production of gears, hydraulic pistons and injection pump components etc. In this process, the forces generated are expected to be high, so as per convention of machining a harder tool material with low wearing capabilities is required for continuous machining operations. Hard turning is of a great interest to both the manufacturing industry and research community. Hard materials require harder tools to carry out machining process and indulge more cutting forces in the process. The cutting forces were in feed direction, in the direction normal to the workpiece axis and in vertical direction are named as axial force F x, radial force F y and tangential force F z respectively. The value of these forces is expected to be low and dependent on practical cutting conditions. Hamdi et al (2012) have claimed that the cutting force components are influenced principally by the depth of cut and workpiece hardness whereas feed is responsible for surface roughness. Chinchanikar et al. (2013) have used the optimization technique and found that cutting forces were mostly affected by depth of cut (60-70 %) and feed (25-30 %), while tool life is prominently dependent on cutting speed and then depth of cut for hard materials. Kaynak et al. (2013) proclaimed that cryogenic cooling plays a significant role on reducing notch wear which decreases the cutting force requirement. Agustina et al. (2013) have proclaimed that the magnitude of cutting forces are strongly related to the amount of heat in the cutting area, tool wear, quality of machined surface and accuracy of the workpiece. Chinchanikar et al. (2013) have concluded that coatings increases the limiting cutting speed for machining of AISI 4340 steel of 35 HRC. Dimla et al. (0) have suggested that the vertical components (z-direction) of both cutting forces and the vibration signatures were the most sensitive to tool wear

2 Behaviour of Cutting Forces in Hard Turning Individually and in Combination Considering Effect of Tool Wear on Principal Flank, Auxiliary Flank and Rake Faces: For the machining of hard materials, different tool materialslike CBN, Ceramics and carbides have been tried by many researchers. Tugrul et al. (7) have used ceramic tool insert for machining 60 HRC steel. The cutting forces were observed to be reduced but roughness remained unaffected. Suresh et al. (2012) have used multilayer coated carbide tool for machining of hardened AISI 4340 steel and proclaimed that the reduction in cutting forces were observed while turning at higher cutting speeds. Coelho et al. (7) have used PCBN coated and uncoated edges for machining of AISI 4340 steel. The surface finish was observed to be increased with PCBN compared to uncoated edge. The carbide has been chosen in present study to attain economy in the process as CBN and ceramic tools are costlier. Tool wear is also a big challenge in the field of machining as it directly affects the machining performance for a particular operation. During a continuous turning process, the online monitoring of tool condition plays a vital role which can be used to protect the process from unexpected hike in cutting forces as well as workpiece roughness. Therefore in the present work the behaviour of the cutting forces was observed with respect to the tool wear. The data was collected from the force measurement sensor (force dynamometer) which was then analysed to minimize the tool wear. The artificially worn-out insertswere used to carry out the turning operation on EN24 hardened steel of 45 HRC hardness with single layer PVD coated TiSiNconcentrated on TiAlN insert. The work was mainly the artificial tool wear generation and then machining using these worn out tools to identify performance of cutting process at constant cutting conditions. 2 Experimental details 2.1 Cutting conditions Turning operations were carried out on the HMT center lathe with artificially worn out inserts. Wear patterns on principle flank face, auxiliary flank face and rake face, were considered as varying parameters in present experimentation. Therefore experiments were carried out at constant values of cutting parameters viz. cutting speed(v), feed( (f) and depth of cut(d). According to capability of machine, recommendation from insert manufacturer, and literature review, the values of cutting parameters used in the present work are as given in Table 1. Table 1 Cutting parameters Parameters Cutting Speed, V (m/min) Feed, f (mm/rev) Depth of Cut, d (mm) Values Workpiece material and cutting insert In this study, 45 HRC EN24 hardened steel of diameter 70 mm was turned. The hardness was assumed to be constant (± 1 HRC) throughout its cross section because of uniform hardening and tempering process. The workpiece material was of 400 mm in length. The chemical composition of EN24 workpiece material is given in Table 2. The turning operation has been carried out using commercially available single layer PVD coated TiSiN-TiAlN nanolayer, graded as SECO TH0, CNMG (80 o diamond shape with 0.8 mm nose radius) insert with MF2 chip breakerr geometry. The fractograph of insert has been described in Figure 1. A right handside tool holder designated by ISO as PCBNR 2020 K12 was used for mounting the insert. Table 2 Chemical composition of EN24 Steel by weight percentage C Mn Si S P Cr Mo Ni Figure 1 Fractograph of PVD coated insert [2] 2.3 Experimental procedure Experiments were carried out with varying flank wear and crater wear. The wear lands were created on insert artificially using Micro-Electro-Discharge Machining (µ-edm) process. The MIKROTOOLS DT-110 system was used for creating wears on different faces of insert as shown in Figure 2. Digital microscope having maximum amplification of 230X was used to identify the amount of wear produced on inserts. Average values of the cutting force components were measured using a three-component piezo-electric dynamometer (KISTLER- Type 9257B). 3 Result and discussion In this section, the experimental observations are summarized.graphs showing relationship between cutting force components with respect to varying tool wear are presented keeping cutting parameters and workpiece hardness constant. The behaviour of cutting force components with change in tool wear on principle flank, auxiliary flank and rake faces was considered individually and in combination

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India 3.1 Preparation of worn out inserts The inserts were worn out artificially before performing the actual turning operation on lathe. As discussed earlier the µ-edm process was used for wearing the insert. Three wear patterns were generated on auxiliary flank, principle flank and rake faces separately by varying wear land, VB on flank faces assuming the rectangular wear geometry as shown in Figure 2. The crater wear has been considered as circular in shape and is varied by varying diameter of crater keeping depth of crater as constant at 0.03 mm. Table 3 gives the actual dimension of the wear generated on different faces along with its designation. The titanium wire was used as a cathodeand insert was used as anode in µ- EDM process. The other parameters like voltage, current, discharge gap, dielectric etc. were selected accordingly as per the literature. Table 3 Tool wear dimensions and their designations Insert Face Dimensions Designation VB (mm) Fresh Tool 0 T T Auxiliary flank T face T 3 Principle flank face Rake face (Crater Wear) Combination of wears 3.2 Fresh tool inserts 0.2 P P P 6 Φ C 7 Φ C 8 Φ C Φ0.5 R 10 The coating on inserts used is comparatively much harder than work material. Therefore it provides more abrasion resistance and high temperature resistance to the insert so that most of the heat could get away along with the chips (Chinchanikar et al., 2013). In this section, the behaviour of cutting forces using fresh insert (T 0 ) has been presentedwhich was used as reference data for further comparison of cutting forces using inserts T 1, T 2, T 3, P 4, P 5, P 6, C 7, C 8, C 9 and R 10. The dynamic force signal plot shown in Figure 3 illustrates that the feed force (F x ) is lower as compared to tangential force (F z ) and radial force (F y ). In machining, the behaviour of feed force and radial force is mostly dependent on feed rate and depth of cut respectively. Also, chip breaker geometry controls the size of chips flowing out of the machining zone and it will affect the tangential force. Figure 2 Optical microscope images of the wornout inserts before machining The machining was carried out at higher cutting velocity with low feed rate. The average value of F x was found to be N whereas F z and F y were found to be N and N respectively. Cutting Force (N) T 0 Zero Wear Figure 3 Dynamic cutting forces at insert T Worn out inserts Sample point Series 1 -Feed force Series 2- Tangential force Series 3 -Radial force As discussed in the section 3.1, the inserts were worn out on respective faces by removing the nanolayer of TiSiN-TiAlN coating. Machining operation has been carried out using these inserts individually at constant cutting conditions as given in Table 1. The average cutting force value has been found out from the data obtained from force dynamometer as given in Table 4. Machining was carried out at room temperature without cutting fluid. The relationship between wear on auxiliary flank, principle flank and rake face and cutting forces have been plotted in Figure 4.The variation of cutting forces with respect to increasing wear has been plotted in Figure 4 (a). As the Auxiliary flank wear (AFW) increases, the feed force increases whereas radial force decreases initially and then a rapid increase was observed. This is because, at VB = 0.2 mm the temperature of the workpiece increases rapidly beyond the temperature of recrystallization causingthermalsoftening. This 516-3

4 Behaviour of Cutting Forces in Hard Turning Considering Effect of Tool Wear on Principal Flank, Auxiliary Flank and Rake Faces: Individually and in Combination softening causes the work material to become softer reducing the forces required for machining. But as wear land increases further, the work tool contact length increases which causes forces to increase. Later as wear land increased beyond 0.6 mm radial force did not vary and showed a constant trend line. Auxiliary flank wear has very less effect on tangential cutting force as there was no formation of crater on the rake face. Table 4 Average values of cutting force at different wear patterns Insert F x -Axial F z - Tangential F y -Radial T T T T P P P C C C R The behaviour of cutting forces with the increase in principle flank wear are described in Figure 4(b). Feed force was observed to be increased as wear land increases from 0 to 1.0 mm whereas radial force showed the same nature as in case of auxiliary flank wear. Initially radial force decreases because of thermal softening and then increases as VB increases. Compared to feed and radial forces, tangential forces have shown negligible change but overall range of tangential force increases compared with the range in case of auxiliary flank wear. In Figure 4(c), the behaviour of cutting forces with varying crater wear has been presented. The attempthas been made to wear out the chip-breaker geometry on the rake face. The crater wear was assumed to be circular in shape.the plot shows that, as the crater wear diameter increases, forces decrease down rapidly. Tangential force was observed to be reduced compared to the case of auxiliary flank wear and principle flank wear. The crater wear alone, specifically does not limit the tool life, instead the crater increases the effective rake angle and thus reduces the cuttingforces. In Figure 4 (c), the region from wear 0.2mm to 1.0 mm, the forces were observed to be constant. This happened because the diameter of the crater was varied by keeping the depth of the crater constant i.e mm. The optical images of cutting inserts after machining have been shown in Figure Figure 4 Behaviour of cutting forces (a) Auxiliary flank wear Vs Cutting force (b) Principle flank wear Vs Cutting force (c) CraterWear Vs Cutting force. 3.4 Combination of wear Auxiliary Flank Wear (mm) Fx - Feed Force Fz - Tangential Force Fy - Radial Force (a) Principle Flank Wear (mm) Crater Wear (mm) Fx - Feed Force Fz - Tangential Force Fy - Radial Force (b) Fx - Feed Force Fz - Tangential Force Fy - Radial Force In the previous section, the inserts were worn out individually on different faces and bahaviour of cutting forces was observed. In this section, the combinationof wear on the three facesi.e. auxiliary (c) 516-4

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India AFW T 10 Series 1 -Feed force Series 2- Tangential force Series 3 -Radial force CW PFW Cutting force (N) Sample points Figure 7 Dynamic cutting forces at insert R 10 Figure 5 Optical microscope images of the wornout insertsafter machining flank, principle flank and rake faces,simultaneously has been presented.the flank and crater wear were generated on the insert as described in section 3.1. The middle values from the Table 3 were selected for wears i.e. 0.6 mm on auxiliary flank and principle flank wear, and Ф 0.5 mm for crater wear. The turning experiment has been performed using insertr 10 on AISI 4340 work material. Figure 6 shows the optical microscope images of fresh as well as combined worn-out insert. In Figure 6 (b), it can be observed that edge of the insert has become weak because of the wearing and therefore the cutting forces increase as compared to fresh tool insert. The plot shown in Figure 7 explains that the cutting forces are comparatively higher for the insert R 10 (combination of wear) than the cutting forces in case of fresh inserts. The average axial force F x, radial force F y and tangential force F z are found to be N, N and N respectively. Figure 6 Optical microscope images of (a) fresh insert (b) combination of wear 4 Conclusion The following conclusions were made on the basis of experimentation performed. For the fresh PVD coated TiSiN-TiAlN nanolayer Carbide insert, the feed force (F x ) is lower as compared to tangential force (F z ) and radial force (F y ) because for fresh insert the chip breaker geometry increased the tangential force and the high depth of cut increased the radial force. For worn out tools, initially, higher values of cutting forces were recorded as compared to the subsequent valuessince thermal softening plays a vital role. With increasein auxiliary flank wear (AFW), the feed force increases whereas radial force decreases initially and then a rapid increase were observed. In case of principle flank wear (PFW), Feed force was increased as wear land increases from 0 to 1.0 mm, whereas radial force showed the same nature as in case of AFW because the temperature of the workpiece increases rapidly beyond the temperature of recrystallization causing thermal softening. This softening causes the work material to become softer reducing the forces required for machining. As the crater wear diameter increases, forces lowers down rapidly. Overall range of values of cutting forces has also been reduced to a great extend i.e. axial forces by 48.43%, radial forces by 20.94% and tangential forces by 48.05%. In case of combination of wears, feed and tangential forces are comparatively higher than the cutting forces in case of fresh inserts as sharp edge of the insert was used for machining. The average axial force F x, radial force F y and tangential force F z were found to be N, N and N respectively.the tangential cutting force was more affected by crater wear as chip flows on the rake face and the 516-5

6 Behaviour of Cutting Forces in Hard Turning Considering Effect of Tool Wear on Principal Flank, Auxiliary Flank and Rake Faces: Individually and in Combination tangential i.e. vertical force was reduced as continuous chips were generated. However feed and radial forces observed to be predominantly affected by principle flank wear. On the other hand, wear on auxiliary flank face showed no specific variation in any of the forces. Feed and radial forces were observed to be affected mostly when combination of flank wear and crater wear was considered. Nomenclature EN : Euronorms (European standards) PVD : Physical Vapour Deposition ISO : International organization for standardization TiAlN : Titanium Aluminum Nitride TiSiN : Titanium Silicon Nitride AFW : Artificial Flank Wear PFW : Principle Flank Wear CW : Crater Wear References Agustina B., Bernal C., Camacho A.M. and Rubio E.M. (2013), Experimental Analysis of the Cutting Forces Obtained in Dry Turning Processes of UNS A97075 Aluminium Alloys, Procedia Engineering, Vol. 63, Chinchanikar S.and Choudhury S.K. (2013), Effect of Work Material Hardness and Cutting Parameters on Performance of Coated Carbide Tool When Turning Hardened Steel: An Optimization Approach, Measurement, Vol. 46, Chinchanikar S.and Choudhury S.K.(2013), Experimental Investigations To Optimise And Compare The Machining Performance Of Different Coated Carbide Inserts During Turning Hardened Steel, Proceedings of the Institution of Mechanical Engineers, Part B:Journal of Engineering Manufacture. Chinchanikar S.and Choudhury S.K. (2013), Investigations On Machinability Aspects Of Hardened AISI 4340 Steel At Different Levels Of Hardness Using Coated Carbide Tools, International Journal of Refractory Metals and Hard Materials, Vol. 38, Chinchanikar S.and Choudhury S.K. (2013), Wear Behaviors Of Single-Layer And Multi-Layer Coated Carbide Inserts In High Speed Machining Of Hardened AISI 4340 Steel, Journal of Mechanical Science and Technology, Vol. 27 No. 5, Dimla D.E. and Lister P.M. (0), On-line metal cutting tool condition monitoring. I: force and vibration analyses, International Journal of Machine Tools & Manufacture, Vol. 40, Hamdi A., Mohamed A. Y., Kamel C., Tarek M. and Rigal J. (2012), Analysis of Surface Roughness and Cutting Force Components in Hard Turning With CBN Tool: Prediction Model and Cutting Conditions Optimization, Measurement, Vol. 45, "Hembrug Machine Tools Manual", Finish Hard Turning. Kaynak Y., Karaca H.E., Noebe R.D. and Jawahir I.S. (2013), Analysis of Tool-wear and Cutting Force Components in Dry, Preheated, and Cryogenic Machining of NiTi Shape Memory Alloys, Procedia CIRP, Vol. 8, Reginaldo T. and Ngb E. (7), Tool Wear When Turning Hardened AISI 4340 With Coated PCBN Tools Using Finishing Cutting Conditions, International Journal of Machine Tools & Manufacture, Vol. 47, Suresh R., Basavarajappa S. and Samuel G.L.(2012), Some Studies On Hard Turning Of AISI 4340 Steel Using Multilayer Coated Carbide Tool, Measurement, Vol. 45, Suresh R., Basavarajappa S., Gaitonde V.N. and Samuel G.L. (2012), Machinability investigations on hardened AISI 4340 steel using coated carbide insert, International Journal of Refractory Metals and Hard Materials, Vol. 33, Tugrul O., Yigit K., Lus F. and Paulo D. (7), Modelling of Surface Finish and Tool Flank Wear in Turning of AISI D2 Steel With Ceramic Wiper Inserts, Journal of Materials Processing Technology, Vol. 189, Web link: Access Date: