A Comparative Study of Tool Life Between Ceramic and CBN Cutting Tools when Machining Steel and Optimization of Cutting Parameters

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1 International Journal of Manufacturing Science and Technology 5(2) December 2011; pp Serials Publications A Comparative Study of Tool Life Between Ceramic and CBN Cutting Tools when Machining Steel and Optimization of Cutting Parameters P. V. Rangarao *, K. Subramanyam ** and C. Eswar Reddy *** Abstract: This paper describes a comparison of tool life between ceramics and cubic boron nitride (CBN) cutting tools when machining hardened steels using the Taguchi method. An orthogonal design, signal-to-noise ratio (S/N) and analysis of variance (ANOVA) were employed to determine the effective cutting parameters on the tool life. The results indicated that the V was found to be a dominant factor on the tool life, followed by the TH, lastly the f. The CBN cutting tool showed the best performance than that of ceramic based cutting tool. In addition, optimal testing parameter for cutting times was determined. The confirmation of Experiment was conducted to verify the optimal testing parameter. Improvements of the S/N ratio from initial testing parameters to optimal cutting parameters or prediction capability depended on the S/N ratio and ANOVA results. Moreover, the ANOVA indicated that the cutting speed was higher significant but other parameters were also significant effects on the tool lives at 90% confidence level. The percentage contributions of the cutting speed, tool s hardness, and feed rate were about 42.88, 32.44, and on the tool life, respectively. Keywords: ANOVA, Flank wear, Hard Turning, optimization, Signal to noise ratio, Taguchi method. 1. INTRODUCTION Engineers want materials with long service lives, and processes for shaping them into finished products with tight geometric tolerances and excellent surface finish. Hardened steel is one such material, used particularly in the automotive industry for components such as bearings, gears, shafts, and cams. Soft steel must be hardened to increase the strength and wear resistance of parts made from this material. Hardened steels are machined by grinding process in general, but grinding operations are time consuming and are limited to the range of geometries to be produced. The hardened steel surfaces have an abrasive effect on the tool material, and the high temperature on the cutting edge causes diffusion between tool and chip. Therefore, improved technological processes, optimum tool selection, determination of optimum cutting parameters or tool geometry should be considered. The developments of new cutting tools have led to the use of higher cutting speeds compare with conventional machining. High speed cutting reduces machining costs by increasing production rate. However, high speed cutting leads to the rapid wear of cutting tools, which is caused by the high temperatures generated at the cutting zone and as a result tool life decreases. The ability of polycrystalline cubic boron nitride (CBN) cutting tools to maintain a workable cutting edge at elevated temperature is, to same extent, * M. Tech., Department of Mechanical Engineering S.V.University, ( niseguyranga@gmail.com) ** Department of Mechanical Engineering S.V. University, ( kalthireddysubramanyam@gmail.com) *** Prof., Department of Mechanical Engineering S. V. University.

2 92 / International Journal of Manufacturing Science and Technology shared with several conventional ceramic tools. These tools are characterized by high hot hardness, wear resistance and good chemical stability and low fracture toughness. CBN and ceramic tools are used in the manufacturing industry for hard turning because of its inertness with ferrous materials and its high hardness. Though CBN particles and binder phases such as TiN are harder than carbides in steels, it is still possible that the tool will encounter soft abrasive wear. The machining of hardened bearing steel represents grooving proportion of applications involving hard cutting tools such as CBN and ceramics. Various studies have been conducted to investigate the performance of CBN and ceramic tools when machining hard steels or hardened steels. Eras Aslant and Kneecap Caucus [1] have optimized the cutting parameters when turning of hardened AISI 4140 steel (63 HRC) with a ceramic tool. Combined effects of three cutting parameters, namely cutting speed, feed rate, and depth of cut on two performance measures, flank wear (VB) and surface roughness (Ra), were investigated employing an orthogonal array and analysis of variance. They found that the cutting speed is the only statistically significant factor influencing the tool wear. Axial depth of cut has also physical influence on tool wear. Also the relationship between the parameters and the performance measures were determined using multiple linear regressions. M.L. Penal, M. Arisen [2] investigated experimentally the effect of tool wear on surface roughness in hard turning. It has been found that there is a good replication of tool on the roughness profile. Concretely, the average roughness and skewness of the profile are sensitive enough to discriminate different tool wear states, as well as to indicate when the tool should be replaced. Therefore, cutting edge state might be predicted with reasonable accuracy through roughness parameters. Their strategy allows tool wear estimation by simple roughness measurements using shop floor instrument. Cora Lahiff [3], J. P. Costes [4], and, S. Y. Luo conducted studies on tool wear modes and mechanisms in CBN turning of hard materials. They identified the primary wear modes and discussed the many theories proposed to explain the mechanisms contributing CBN tool wear and failure. They considered the critical factors that influence the behavior of CBN tools in continuous hard turning and how this knowledge can be applied to optimize tool performance. They showed that the dominant wear mechanisms are abrasion, adhesion and diffusion due to chemical affinity between elements from workpiece and insert. S.Y. Luo [5] conducted experiments to study wear characteristics in turning high hardness alloy steel by ceramic and CBN tools. The experimental results showed that the main wear mechanism for the CBN tools was the abrasion of the binder material by the hard carbide particles of the workpiece. Variations of tool wear with the cutting speed and the hardness of the work material are discussed accordingly. Tool life for CBN and ceramic tools is increased with cutting speed until it reaches a maximum value, thereafter the tool life starts to decrease. They found that flank wear increases gradually with the cutting time. Radu Pavel [6] studied experimentally the effect of tool wear on surface finish for a case of continuous and interrupted hard turning. They presented new findings concerning the evolution of common surface roughness parameters as well as the evolution of surface topography. They found a good correlation between flank wear aspect and machined surface. As cutting time progresses surface roughness increases due to the increase in tool wear. In case of continuous cutting Ra, Rz, Rpk tend to increase significantly with the tool wear.

3 A Comparative Study of Tool Life Between Ceramic and CBN Cutting Tools / EXPERIMENTAL DETAILS 2.1. Machine and Cutting Tool Specifications Experiments were conducted on a Standard precision lathe with main motor capacity of 15 kw and spindle speeds ranging from rpm. Three types of cutting tools were used for the present work. These are mixed alumina ceramic tools, coated ceramic cutting tools and CBN cutting tools. One of the tools was a mixed alumina ceramic with an Al2O 3 (70%): TiC (30%) matrix, which is designated by KY1615. The other insert was coated using a physical vapor deposition (PVD) method. Coating substance takes place on the mixed ceramic substrate and PVD-TiN coated mixed ceramic with a matrix of Al2O 3 (70%):TiC (30%) + TiN, which is called as KY4400 grade. The inserts are from Sandvik, reference numbers S-TNGA KY1615 and S-TNGA KY4400. A tool holder PTGNR1616H11 with an approach angle of 91 º was used for the experiments. The cutting tool geometry for ceramics as follows Nominal rake angle 6 º, Back rake angle 6 º, Clearance angle 6 º, Approach angle 75 º, Major tool cutting angle 60 º,Cutting edge length 11mm, Nose radius 0.8 mm, Insert thickness 3.18 mm. The inserts were rigidly attached to a tool holder of ISO designation of PTGNR1616H11. The last one was a CBN with an Al2O 3 + TiC matrix, which is designated by CBN/TiC. The CBN/ TiC tools contained CBN (50%), TiC (40%), WC (6%), AlN, and AlB 2 (4%). However, the CBN/TiC insert type was CNGA S-L0. The cutting tool geometry as follows Negative rake angle -20 º, Side rake angle-6 º, Clearance angle 6 º, Approach angle 75 º, Major tool cutting angle 80 º. Hardness s of these cutting tools are about 83, 87 and 140HRC, respectively. Details of cutting tools are given in Table 1. Table 1 Cutting Tool Details Type of Cutting Tool Chemical Composition of Material Hardness (HRC) (KY 1615) Al2O3 (70%) + TiC (30%) 83 (KY 4400) Al2O3 (70%) + TiC (30%) + TiN 87 (CBN/TiC) CBN (50%) + TiC (40%) + WC (6%) + AlN, AlB2 (4%) Work Piece Preparation and Heat Treatment The material used throughout this work was an AISI (commercially known as EN 31 steel) alloy steel with 60 mm diameter and 450 mm length with hardness of 45 HRC. The chemical analysis of the material is indicated in Table 2. Table 2 Chemical Analysis of Material (% wt) C Si Mn Cr Co Required hardness is obtained through heat treatment after skin turning. Before hardening annealing was carried out at a temperature of 800 C and followed by furnace cooling. Hardening

4 T h e d e p t h o f c u t w a s f i x e d a s 0. 2 m m i n a l l t e s t c o n d i t i o n s. T h r e e f e e d r a t e s ( f) 94 / International Journal of Manufacturing Science and Technology is carried out by heating uniformly to C until heated through. Allow 30 minutes per inch of ruling section and quench immediately in oil. And tempering was carried out at a temperature of 300 C for getting hardness of 45 HRC by heating uniformly and thoroughly at the selected tempering temperature and hold for at least one hour per inch of total thickness Tool Wear Measurement Tool flank wear (V B ) is measured by using an optical microscope of 0.5 microns accuracy after properly placing the tool below the lens of the microscope to get good view of the tool flank surface. Accurate measurements are carried out by proper focusing and by providing proper illumination of the tool flank surface. After each test, the worn cutting tool was measured with the optical tool microscope to determine the degree of flank wear. The width of the flank wear criteria was taken as 0.3 mm. In general, experiment was stopped to measure the width of the wear land at each 5, 10, 15 and 30 min Experimental Design The Taguchi design was selected to find out the relationships between independent variables and cutting time. The independent variables were cutting speed, feed rate, depth of cut and tool s hardness. The experiments were carried out to analyze the influence of cutting parameters on tool life for machining hardened AISI steels. Cutting parameters were selected keeping in mind that the hard turning operation was generally used as a finishing operation as an alternative to grinding. 0.06, 0.08, and 0.11 mm/rev were selected. Three cutting speeds ( V) were chosen: 100, 140, and 196 m/min. Details of experimental design, control factors and their levels, and results for tool lives are shown in Table 3. This table showed that the experimental plan had three levels. Table 3 Experimental Design and Results for Tool Lives and their S/N Ratios Control Factors Experimental Values Trail No V(rpm) f (mm/rev) TH (HRC) Measured tool S/N ratio life, T (min) (db) Mean S/N ratio (db)

5 A Comparative Study of Tool Life Between Ceramic and CBN Cutting Tools / 95 A standard Taguchi experimental plan with notation L9 (3 4 ) was chosen. The rows in the L9 orthogonal array used in the experiment corresponded to each trial and the columns contained the factors to be studied. The first column consisted of cutting speed, the second contained the feed rate and the consecutive column consisted of the cutting tool s hardness. The experiments were conducted twice for each row of the orthogonal array to circumvent the possible errors in the experimental study. In the Taguchi method, the experimental results are transformed into a signal-to-noise (S/N) ratio. This method recommends the use of S/N ratio to measure the quality characteristics deviating from the desired values. To obtain optimal testing parameters, the-higher-the-better quality characteristic for machining the steels was taken due to measurement of the tool life. The S/N ratio for each level of testing parameters was computed based on the S/N analysis. This design was sufficient to investigate the three main effects. With S/N ratio analysis, the optimal combination of the testing parameters could be determined Analysis of Control Factors 3. RESULTS AND DISCUSSION Analysis of the influence of each control factor (V, f, and TH) on the tool life was performed with a so-called signal-to-noise (S/N) response table, using a Minitab 15 computer package. The experimental design, results for tool lives and S/N ratios are shown in Table 3. The control factors and tool lives were included in this table. Table 4 shows the S/N response table of tool lives for machining the hardened steels. It indicated the S/N ratio at each level of control factor and how it was changed when settings of each control factor were changed from level 1 to level 2. The influence of interactions between control factors was neglected here. The control factor with the strongest influence was determined by differences value. The higher the difference, the more influential was the control factor. The control factors were sorted in relation to the difference values. It could be seen in Table 4 that the strongest influence was exerted by cutting speed, followed by hardness of cutting tool, lastly feed rate, respectively. Since the first level of the cutting speed was about db while the third level of the cutting speed was about db the difference being the most highest of db. It is followed by the hardness of cutting tool. The difference between the first level of the tool hardness and third level of the tool hardness was found to be about db, which is significant level again. The feed rate showed the least effect on the tool life since the difference between the first level and third level were about db. Table 4 S/N Response Table of Tool Lives in Machining Hardened Steels Average S/N Ratio (Db) Symbol Control Factors Level 1 Level 2 Level 3 Max-min V Cutting speed (rpm) f Feed rate (mm/rev) TH Cutting tool s hardness (HRC)

6 96 / International Journal of Manufacturing Science and Technology 3.2. Main Effect on the Tool Life Figure 1: Main Effect Plots for Tool Lives in Machining Steels: (a) S/N ratio (db); (b) mean (min).

7 f o r t h e t e s t e d s a m p l e s ( V1, A Comparative Study of Tool Life Between Ceramic and CBN Cutting Tools / 97 Figure 1 a and b shows the main effect plots for tool lives of the cutting tools for S/N ratios and mean values, respectively. The greater is the S/N ratio, the smaller is the variance of the tool life around the desired value. Optimal testing conditions of these control factors could be very easily determined from the response graph. The best tool life value was at the higher S/N value in the response graph. For main control factors, Figure 1a indicates the optimum condition f1, and TH3). Thus, it could be concluded that the best tool life of cutting tools can be achieved and their optimal setting of control factors for tested samples are shown in Table 5. Table 5 Optimum Level of Control Factors Main control factors Symbol Optimum level Optimum value Cutting speed V Feed rate f Hardness of cutting tool TH 3 CBN/TiC From the results of control factors, higher tool life was obtained under cutting conditions of V = 580 rpm, f = 0.06 mm/rev when machining AISI workpiece by CBN/TiC cutting tool. The experimental work was carried out on the same steel using the determined optimal control factors. The tool life was found to be about min. Then this value was transferred to the S/N ratio (db), average value of S/N ratio was calculated and it was about db. Moreover, the mean tool life of cutting tool is shown in Figure 5.1b. It was evident that the cutting speed had the greatest effect on the optimal testing conditions. It might be that higher cutting speed led to higher flank wear width because of easy for removal of particles from the place. It is followed by the cutting tool s hardness. It was observed that the tool life obviously increased highly as cutting tool changed from level 1 to level 3 due to the having considerable hardness s between these two cutting tools. The feed rate was also effective on the tool life of the cutting tool (see Table 1). Effects, however, were lower compared to those of cutting speeds. The effects of cutting parameters and their interaction effects on the tool life are shown in Fig. 2 as a three-dimensional surface contour graph. As shown in Figure 2a, longer tool life was observed when reducing the cutting speed. Increasing the feed rate was not so effective on the tool life when machining at lower cutting speed and feed rate. However, the tool life decreased with increasing the feed rate when machining medium cutting speed. It was obvious that performance of all tools were good at lower cutting speeds. The tool life for the KY1615 and KY4400 insert decreased considerably with increasing the cutting speed, but the CBN/TiC cutting tool showed the best performance because of retaining their hardness at elevated temperature and fracture toughness (Figure 2b). In addition, performances of all cutting tools were good when machining at lower feed rate (Figure 2c). The cutting time reduced significantly when tested with the KY1615 cutting tool. Moreover, the tool life decreased with increasing the feed rate, especially for the KY1615 and KY4400 cutting tools because of not having high enough chemical stability of these tools. In the current study of machining hardened steel, an orthogonal design, S/N ratio and ANOVA were employed to determine the effective cutting parameters such as V, f and tool s hardness on the tool life. It was concluded that the cutting speed was found to be the most important parameters on the tool life among control parameters.

8 w h e n m a c h i n i n g h a r d e n e d b e a r i n g s t e e l s. T h e L 9 ( Analysis of Variance 98 / International Journal of Manufacturing Science and Technology The ANOVA was used to investigate which design parameters significantly affect the quality characteristics of the tool life for the turning process and to check the adequacy of the models under development. Examination of the calculated value of variance ratio ( F), which is the variance of the factor divided by the error variance for all control factors. The results of the ANOVA of tool lives in machining hardened steels are shown in Table 6. In addition to degree of freedom, mean of squares (MS), sum of squares (SS), F-ratio and P-values associated with each factor level were presented. This analysis was performed for a confidence level of 90%. The F value for each design parameters was calculated. The calculated value of the F showed a high influence of the cutting speed (V) on the tool life since F-calculation was equal to while F-table was about 9.0, but the feed rate ( f), and hardness of the tool (TH) had also significant effects on the tool life since F-test was equal to 53.42, 71.54, respectively. The last column of the Table 6 indicated the percentage of each factor contribution ( P) on the total variation, thus exhibiting the degree of influence on the result. It was important to observe the P-values in the table. From the analysis of Table 6, the factor A (P 42.88%) showed a high significant effect. It was followed by cutting tool s hardness ( P 32.44%), and feed rate (P 24.22%) as well. Table 6 Results of ANOVA for Tool Lives in Machining Hardened Steels Symbol Degree of Sum of Mean of F- F- Contribution, freedom (d.f.) squares (SS) squares (MS) Calculation Table P (%) v f TH Error Total CONCLUSIONS The following conclusions could be drawn from results of tool lives of different cutting tools 4 ) orthogonal arrays were adopted to investigate the effects of cutting speed, feed rate and hardness of cutting tools on the tool life. The results showed that the cutting speed exerted the greatest effect on the tool wear, followed by the hardness of cutting tool, lastly the feed rate. Furthermore, CBN/TiC cutting tools showed the best performance than those of other tools. Moreover, the ANOVA indicated that the cutting speed was high significant but other parameters were significant effects on the tool life at 90% confidence level. The percentage contributions of cutting speed, tool s hardness, and feed rate were about 42.88, 32.44, and on the tool life, respectively. 5. FUTURE RECOMMENDATIONS The present work considers only 3 input parameters for optimization of hard turning process. More number of input parameters in hard turning such as work piece hardness, coolant condition can be taken in to study.

9 A Comparative Study of Tool Life Between Ceramic and CBN Cutting Tools / 99 Acknowledgements I wish to place on record my deep sense of gratitude to Asst. Prof. K. Subramanyam for his involvement and guidance throughout this work. Many people helped me throughout this work. I thank Mr. Vijay of CAD lab, Mr. Eswar Reddy and Mr. Madan of Machine Tool Laboratory, S.V. University for their major help in conducting experiments. I am thankful to Mr. Eswar Reddy for his cooperation in carrying tool wear measurements. References [1] Ersan Aslan, Necip Camuscu, Burak Birgoren (1993), Design Optimization of Cutting Parameters When Turning Hardened AISI40 Steel (63 HRC) with Al2O3 + TiCN Ceramic Tool, Materials & Design, 28, [2] Penevala M. L., Arizmendi M., Diaz F., Fernandez J. (2007), Effect of Tool Wear on Roughness in Hard Turning, Annals of CIRP, 51, [3] Cora Lahiff, Seamus Gordon; Pat Phelan (2007), PCBN Tool Wear Modes and Mechanisms in Finish Hard Turning, Robotics and Computer-Integrated Manufacturing, 23, [4] J. P. Costes, Y. Guillet, G. Poulachon, M. Dessoly (2007), Tool Life and Wear Mechanisms of CBN Tools in Machining of Inconel 718, International Journal of Machine Tools and Manufacture, 47, [5] Luo, S.Y., Liao Y. S., Tsai Y.Y. (1999), Wear Characteristics in Turning High Hardness Alloy Steel by Ceramic and CBN Tools, Journal of Materials Processing Technology, 88, [6] Radu Pavel, Ioan Marinescu, Mick Deis, Jim Pillar (2005), Effect of Tool Wear on Surface Finish for a Case of Continuous and Interrupted Hard Turning, Journal of Materials Processing Technology, 170,