Effect of Tool Wear on Roughness in Hard Turning

Effect of Tool Wear on Roughness in Hard Turning

Effect of Tool Wear on Roughness in Hard Turning M. L. Penalva’, M. Arizmendi’, F. Diaz’, J. Fernandez’ Tecnun - School of Engineering, University of ...

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Effect of Tool Wear on Roughness in Hard Turning M. L. Penalva’, M. Arizmendi’, F. Diaz’, J. Fernandez’ Tecnun - School of Engineering, University of Navarra, San Sebastian, Spain Submitted by 2.Katz ( I ) , School of Engineering, Rand Afrikaans University, Johannesburg, South Africa 1

Abstract This paper attempts to make a contribution to wear estimation of CBN tools when turning hardened steels. It is well known that cutting edge geometry deteriorates with wear. Although many authors have considered tool wear process has a random nature, detailed tool examination has proved that wear has some deterministic features in these processes. Thus, plastic deformation exists in the early stages while gradual abrasion makes the cutting edge recede. On the other hand, it has also been found that there is a good replication of the tool on the roughness profile. Therefore, cutting edge state might be predicted with reasonable accuracy through roughness parameters. This strategy allows fast tool wear estimation by simple roughness measurements using a shop floor instrument.

Keywords: Hard Turning, Roughness, Tool Wear

1

INTRODUCTION

Hard turning represents an attractive alternative to grinding because of evident advantag es : more flexibility , higher machining rates, no need of coolant ... With commercial CBN tools, machining of hardened steels became feasible at economic cost. However, insufficient knowledge has reduced industrial acceptance of this process so far. Anyway, its potential fields of application have promoted research and a better understanding of the process is being achieved. Thus, pioneering works [I-31 basically provided tool life plots for a wide range of tools, workpiece materials and cutting conditions. They also pointed out benefits of cutting edge preparation, favorable geometry and main wear patterns. On the other hand, important efforts have been made to explain the mechanics involved in the process [4, 51, and even some analytical models have been proposed in recent years [6, 71. Meanwhile, other works have focused on the attainable part quality in this type of processes and how parameters such as cutting conditions, tool geometry or tool wear affect it [8-101. The aim of present work has been to study concrete causes of surface finish deterioration with tool wear, and the possibility of extracting information about tool wear level from roughness results. 2 EXPERIMENTAL WORK Finish facing tests were performed on a CNC lathe. A sole ring (150 mm outer diameter and 50 mm inner) of hardened (60 HRc) AlSl 52100 steel was used for all tests. Inserts with low CBN content (40450%) and TIC as binder with corner radius of 0.8 mm and chamfered and honed cutting edge were employed. Cutting conditions were selected according to tool manufacturer’s recommendations. Thus, feed rate was fixed at 0.1 mrdrev and depth of cut at 0.1 mm.

Only cutting speed, and some tool angles varied in each test, as it is shown in Table 1. Cutting speed (mlmin) Tool geometry

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Table 1: Specific conditions of performed tests. After each test, part surface finish was measured with a profilometer. Cut-off length was set to 0.25 mm according to I S 0 4288 and an I S 0 2CR filter was selected. Both profile and parameters (a total of 21) were stored. In order to have accurate information of roughness evolution, measurements in three diameters (150, 100 and 50 mm) were made. Besides, six measurements were made along each diameter for statistical treatment. A toolmaker’s microscope was used to measure tool wear. End of tool life criterion was set over a maximum flank wear land width of 0.3 mm. Tool was examined periodically in a scanning electron microscope (SEM). 3

ROUGHNESS PROFILE GENERATION It seems reasonable to expect an acceptable tool shape replication on the machined surface in processes involving geometrically defined cutting edges. Certainly, it has already been reported how a good correlation exists for some soft materials [ I l l . Likewise, a high correlation can be expected for hardened steels. On the one hand, BUE, which dynamically blunts the cutting edge, disturbing its replication on the workpiece seems to be unlikely, according to characteristics of this type of materials [4]. Effectively, no evidence of the phenomenon has been found when examining either the tool or the profiles.

On the other hand, rubbing of flank wear land has a poor influence on the machined surface since shifting of the cutting edge with wear makes the flank area in contact with the workpiece surface remain small all tool life long, as it can be seen in Figures 1 (a) to (c).

Figure 2: Correlation between roughness profiles and cutting edge (profile lengths equal one feed). (a) 3.0 1.0 -1 .o ._ -3.0

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(dl Thus, by fitting scales and overlapping profiles to tool images it has been checked that correlation is good. As an example, see Figures 2 (a) to (d).

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Figure 3: Profile patterns (test I).

4 TOOL WEAR AND PROFILE EVOLUTION It is evident that the good replication of the tool tip profile implies that changes in its topography with wear will be transferred to the workpiece roughness. Examination of tool images together with roughness profiles has shown that tool wear process has a strong deterministic component and, consequently, the profile presents some predictable patterns.

At the very early stages, the fresh tool generates a closeto-theoretical profile, see pattern (a) from Figure 3.

point, resulting profile pattern has low amplitude and an irregular shape, as it can be seen in Figure 3 (d).

However, this pattern is soon wiped out due to the socalled "size effect". As it can be seen in Figure l (a), finishing conditions generate small chip thickness values at the trailing edge. Since the tool is unable to cut such small values, material is extruded through the flank face generating extremely high local pressures. Chamfering prevents the cutting edge under pressure from notching, but plastic deformation occurs instead, see Figure 4 (a). Evidences of this phenomenon have already been reported [10,12], and occasional traces of extruded workpiece material adhered on the clearance face support them [13]. Therefore, the resulting profile shows a bulk located at the deformed zone of the cutting edge, see Figure 3 (b).

Finally, when abrasion has generated an important increase in the leading angle, fresh trailing edge is involved in cutting, as it is shown in Figure 6 (b), so the profile reflects its shape, see Figure 3 (e). Hence, the generated profile at this stage will be highly dependent of the trailing angle, as it is seen when comparing profiles of Figures 3 (e) and 7, obtained with similar wear level but different trailing angle.

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Figure 7: Pattern (e) for test IV. As patterns (e) and (f) correspond with maximum flank wear widths close to 0.3 mm, traditionally associated with end of tool life, they can forecast soon tool replacement moment.

Figure 4: Cutting edge state for patterns (b) and (c). But most important changes in profiles are associated to abrasive tool wear. Certainly, high abrasiveness of hardened steels and scratched appearance of the worn surface, see Figure 5, suggest it as the reason for the remarkable volume wear shown in Figure 1 (b).

Figure 5: Abrasive wear (test 111). At the early stages, sharp tool topography resulting from the size effect is easily smoothed, see Figure 4 (b), generating a lower amplitude profile, as it can be seen in Figure 3 (c).

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Figure 6: Cutting edge state for patterns (d) and (e). With progressive abrasion, cutting edge starts shifting towards the fresh geometry zone, see Figure 6 (a). A this

5

TOOL STATE CHARACTERIZATION THROUGH ROUGHNESS PARAMETERS

As it has been seen, macroscopic component of tool wear has some deterministic features that are reflected on the machined profiles. Hence, they represent an interesting source of information about tool wear state. Unfortunately, analysis of profile signals is time consuming. However, observation of Figure 3 shows that changes from one pattern to another are characterized by modifications in amplitude and shape. Therefore, roughness parameters containing information about these modifications should be suitable for a correct profile representation. After checking all available parameters calculated by the profilometer, the average roughness, Ra, and the skewness of the profile, Rsk, have revealed as the most suitable ones. Effectively, average roughness is adequate for representing periodic profiles generated by a tool tip, and the skewness indicates its symmetry about the mean line [14]. Figures 8 (a) to (d) show results for all tests carried out. It can be observed that different profile patterns are discriminated by these two parameters, especially in the case of critical wear levels (maximum flank wear width close to 0.3 mm), that is, pattern (e). Patterns (a), (b) and (c) have less interest since they are associated to low wear levels. As it has been mentioned before, pattern (e) is tool geometry dependent, see Figure 3 (e) and 7. Therefore, it is not surprising that representative parameters change with geometry, as it can be seen when comparing Figures 8 (a) and (b) with (c) and (d). Different location in the map of pattern (e) in Figures 8 (c) and (d) suggests that cutting speed influences this pattern when using tool with a small trailing angle. It can also be observed, that in these cases pattern (c) is skipped.

(a) Test I sensitive enough to discriminate different tool wear states, as well as to indicate when the tool should be repIaced. Nevertheless, further work will be required for a better understanding of influence of the process conditions on results.

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REFERENCES Narutaki, N., Yamane, Y., 1979, Tool wear Cutting Temperature of CBN Tools in Machining of Hardened Steels, Annals of the CIRP, 28/1: 23-28. Hodgson, T., Trendler, P. H. H., 1981, Turning Hardened Tool Steels with Cubic Boron Nitride Inserts, Annals of the CIRP, 30/1: 63-66.

(b) Test I1

Konig, W., Komanduri, R., Tonshoff, H. K., Ackershott, G. 1984, Machining of Hard Materials, Annals of the CIRP, 33/2: 417-427.

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Figure 8: Pattern representation. 6

CONCLUSIONS

It has been proved that, in the studied process, replication of tool tip on the workpiece is acceptable. Therefore, the machined profile reveals tool wear state. It has also been seen that, for fixed tool and cutting conditions, information provided by roughness profiles can be helpful to estimate tool wear. Concretely, the average roughness and the skweness of the profile are

Nakayama, K., Arai, M., Kanda, T., 1988, Machining Characteristics of Hard Materials, Annals of the CIRP, 37/1: 89-92. Vyas, A,, Shaw, M. C., 1999, Mechanics of SawTooth Chip Formation in Metal Cutting, Journal of Manufacturing Science and Engineering, 121: 163172. Wang, J. Y., Liu, C. R., Wang, K. K., 1999, The Effect of Tool Flank Wear on the Heat Transfer, Thermal Damage and Cutting Mechanics in Finish Hard Turning, Annals of the CIRP, 48/1: 53-58. Elbestawi, M. A,, Srivastava, A. K., El-Wardany, T. I., 1996, A model for Chip Formation During Machining of Hardened Steels, Annals of the CIRP, 45/1171-76. Klocke, F., Lung, D., Liermann, J., Rottger, K., 1998, Part Quality in Hard Turning, Seminar on Improving Machine Tool Performance, San Sebastian, Spain, 389-400. Konig, W., Berktold, A,, Koch, K. F., 1993, Turning versus Grinding - A Comparison of Surface Integrity Aspects and Attainable Accuracies, Annals of the CIRP, 42/1: 39-43. [ l o ] Kishawy, H. A,, Elbestawi, M. A,, 1999, Effects of process parameters on material side flow during hard turning, International Journal of Machine Tools 8 Manufacture, 39: 1017-1030. [ I l l Mori, M., Kumehara, H., Suda, H., Takahasi K., 1985, Variation of Machined Surface Roughness with Progressing of Tool Wear, Bull. Japan SOC.of Prec. Engg., vol 19, No 3: 216-217. [I21 Kukino, S., Harada, T., Fukaya, T., Shiraishi, J., Nakai, T., 1997, High Speed and Precision Cutting Technology of Hardened Steel with PCBN Tool, 2nd International Conference on High Speed Machining, Darmstadt, Germany, 217-221. [I31 Penalva, M. L., Fernandez, J., 2000, Tool wear characterization in finish hard turning processes through acoustic emission signal (in Spanish), 13th Conference on Machine Tools 8 Manufacturing Technologies, San Sebastian, Spain, 383-396. [I41 Whitehouse, D. J., 1978, Beta Functions for Surface Typologie?, Annals of the CIRP, 27/1: 491-497. 8 ACKNOWELDGMENTS The authors would like to thank the Asociacion de Amigos de la Universidad de Navarra and the Diputacion Foral de Gipuzkoa for supporting this work.