Effect of chamfer angle on wear of PCBN cutting tool

Effect of chamfer angle on wear of PCBN cutting tool

International Journal of Machine Tools & Manufacture 43 (2003) 301–305 Effect of chamfer angle on wear of PCBN cutting tool J.M. Zhou ∗, H. Walter, M...

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International Journal of Machine Tools & Manufacture 43 (2003) 301–305

Effect of chamfer angle on wear of PCBN cutting tool J.M. Zhou ∗, H. Walter, M. Andersson, J.E. Stahl Division of Production and Materials Engineering, Department of Mechanical Engineering, Lund University, Lund, Sweden Received 25 July 2002; received in revised form 10 September 2002; accepted 1 October 2002

Abstract In precision hard turning, a remaining problem is to minimise tool wear to maintain the accuracy of geometry and surface finish. Tool wear not only directly reduces the part geometry accuracy but also increases cutting forces drastically. The change in the cutting forces also causes instability in the tool motion, which results in more inaccuracy. PCBN cutting tools are often used in hard turning. However, they are still relatively expensive compared to ordinary carbide cutting tools. In order to attain sufficiently high production rates at minimum cost, increase of knowledge on cutting tool geometry is necessary. This article presents a study of the effect of chamfer angle on tool wear of PCBN cutting tool in the super finishing hard turning. The correlation between cutting force, tool wear and tool life were investigated. The optimised chamfer angle for PCBN cutting tool is suggested. Finally, the distribution of stresses and maximum principal stress working on the tool edge were calculated with the use of finite element method.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Hard turning; Wear; Tool life; PCBN; Cut

1. Introduction The development of new tool materials in past ten years, such as Polycrystalline Cubic Boron Nitride (PCBN), makes precision hard turning possible, in which the hardness of workpiece is up to 58–62 HRC. Compared to grinding operations, precision hard turning enables relatively high material removal rate and flexibility, which makes it more and more attractive to industry, especially to automotive, bearing and hydraulic industries [1]. In precision hard turning, an important consideration is to minimise tool wear to maintain the accuracy of geometry and surface finish. Tool wear not only directly reduces the part geometry accuracy but also increases cutting forces drastically. The change in the cutting forces also causes instability in the tool motion and, in return, more inaccuracy. Currently, PCBN cutting tools are still relatively expensive compared to ordinary carbide cutting tools. In order to attain sufficiently high production rates at minimum cost, optimisation of cutting

Corresponding author. Tel.: +46-46-2228601; fax: +46-462224529. E-mail address: [email protected] (J.M. Zhou). ∗

tool geometry is necessary. In hard turning, often chamfered cutting tools are used due to their high wedge strength and chipping resistance. Cutting tests suggest that the chamfer angle plays an import role in tool life since the cutting action only takes places on the chamfer area in super finishing hard turning (see Fig. 1), in which very small cutting depth and feed rate are employed in most cases. Chamfered PCBN cutting tools result in higher forces and smaller flank wear than positive or zero rake angles. On the other hand, large chamfer angles will also result in more ploughing action instead of cutting, and thus degrade surface finish and produce higher friction and wear, which results in short tool life [2,3,4]. Previous research in hard turning has been mostly concerned with chip formation, tool wear, and surface integrity. Little concerned with effect of the tool geometry on the process and workpiece integrity. Early investigations on tool geometry mainly focused on tool edge geometry and the tool rake geometry, such as restricted contact length tools [5]. The cutting tool with negative chamfer angle and positive main rake face was first proposed by Hoshi [6], and he found that the chamfer traps the work material over the chamfered edge, and the formed dead metal acts like a cutting edge, which increases the tool edge strength and reduces tool wear.

0890-6955/03/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0890-6955(02)00214-6

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Nomenclature a: Bcf: gH: WL: r⑀ : VB: Fp: Fc: Ff: v: a: s: g: σ: τ: Tf: Nf: Tr: Nr:

Flank angle [deg] Chamfer angle [deg] Edge radial [mm] Chamfer length [mm] Nose radial [mm] Flank wear [mm] Passive force (or thrust force) [N] Cutting force [N] Feed force [N] Cutting speed [m/min] Depth of cut [mm] Feed rate [mm/rev.] Rake angle [deg] Normal stress [N/mm∗mm] Shear stress [N/mm∗mm] Shear force on flank face [N] Normal force on flank face [N] Shear force on rake face [N] Normal force on rake face [N]

2. Experimental procedures 2.1. Tool geometry parameters

Fig. 1.

Illustration of the cutting area in hard turning.

Experiments showed that the chamfered cutting edge is almost completely covered by a dead metal zone [7]. The chamfer has more influence on the passive force than the tangential force [8]. Moore [2] made another fundamental investigation and he claimed that chipping resistance was improved by the use of a chamfer on ceramic cutting tools. The use of chamfered tools can increase tool life during interrupted or rough cutting where the mode of failure is chipping. In the experiments [3,4] with cemented carbide tools in continuous cutting, the tool life based on flank wear was observed to drop significantly as the amount of hone or chamfer increased at a high cutting speed. In this paper, the effects of the chamfer angle on cutting force, tool life and stresses for PCBN cutting tool in super-finishing hard turning were investigated. The investigation was based on cutting tests and FEM analysis aiming at optimisation of chamfer angle designed for increased tool life and improved cutting performance.

For most applications of PCBN cutting tools, edge preparation is necessary in order to strengthen the tool edge and increase the tool life and machined surface quality. In PCBN cutting tools, several types of edge preparation can be made for hard turning operations, including sharp edge (with no additional edge processing to strengthen edge), chamfers, hones, and chamfers plus edge hones [1]. However, in most cases, chamfers with edge hones are preferred edge preparation. A typical PCBN cutting edge with chamfer and hone and the corresponding geometry parameters employed in the cutting tests is illustrated in Fig. 2. Chamfer angle bcf was between 0 and 30°; chamfer width WL was 0.1 mm; honing edge radius was 0.01 mm; flank angles were 7°. Five values of chamfer angle 0, 10, 15, 20 and 30° were used for the cutting tests. The test inserts contained

Fig. 2.

Tool geometry parameters used in the cutting tests.

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low-CBN content and composed of PCBN tips brazed to a WC substrate. The inserts were diagonal and corresponded to ANSI classification DCMW11T308S with 0.8 mm nominal nose radius. These inserts were used with a tool holder with rake angle zero deg, flank angle 7 deg and 0 deg lead angle (SDJCL1616H11). 2.2. Workpiece materials The workpiece material used was bearing steel 100Cr6 with hardness of 60–62 HRC. The workpiece was made as a tube with nominal out diameter 140 mm and inner diameter 100 mm to guarantee uniform hardness. The structure of material was martensitic. 2.3. Cutting conditions, tool wear and cutting force measurement Cutting tests were carried out on a SMT500 CNC turning centre. One objective of the super-finishing cut is to replace grinding. Therefore, fine cutting conditions were used with feed rate s and depth of cut a chosen as 0.05 mm/rev. and 0.05 mm respectively. Cutting speed v was selected as 160 m/min. Tool life was determined by measuring maximum flank wear width of the tool, VB. The flank wear was measured under a tool microscope. The cutting was continued until the tool flank wear reached 0.2 mm. The tool wear measurement was taken after every 10 min cutting. Cutting forces were measured by a Kistler force dynamometer 9212 and sampled by a InstruNet100 data acquisition system. Cutting forces were measured during the whole cutting process but only the mean values of stationary forces were employed in the analysis. Three components of cutting force were measured throughout the cut. 3. Results and discussion Compared to conventional cutting, in super finishing hard turning the depth of cut and feed rate are very small and the whole cutting area is concentrated on a relatively small area in the front tip of the cutting edge, or in the area of chamfer zone, see Fig. 3. The chamfer zone

Fig. 3.

Cutting forces under different chamfer angles.

Fig. 4.

Tool life for PCBN inserts with different chamfer angles.

forces increase with the increase of chamfer angles. Since all cutting conditions are identical except their chamfer angle, the increase in the cutting forces is mainly due to forces contributed by chamfer zone. The chamfer forces in the passive direction are higher and increase more rapidly with an increase in chamfer angle than the forces in cutting direction and feed direction. In super finishing hard turning, to guarantee the surface quality, tool wear is usually limited to a very small range. In this investigation, tool life was measured based on a 0.2 mm flank wear criterion in order to avoid excessive white layer induced on the workpiece surface due to the higher temperature under the large flank wear. Tool life results for the chamfered tool edge are shown in Fig. 4. Crater wear was also observed in the test but it was not measured. Fig. 5 shows a typical flank wear and crater wear profile on a CBN tool. The results of tool flank wear for different chamfered edges under the same cutting time and the tool life time for the same wear criteria (VB=0.2 mm) are presented in Fig. 6. As chamfer angles increase the tool life increases to a maximum value for cutting edge with 15° chamfer angle from zero chamfer angle. The tool life difference between the cutting tool with 15 and 30° of chamfer angle is 53%. The results suggest that there is an optimised chamfer angle, where the tool will have the longest tool life. On one hand, tool strength increases with an increase in chamfer angle, on the other hand, an increased chamfer angle will result in a rise of the cutting force.

Fig. 5.

Observed flank wear and crater wear profile in hard turning.

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Fig. 6.

Flank wear and tool life of PCBN inserts with different chamfer angles.

4. FEM calculation The stress distribution in the tool during the cutting was analysed by the finite element method. In the analysis, a cross section of the tool model was divided into quadrilateral elements. The number of elements and nodes used were 2483. Measured values of the elastic modulus and Poisson’s ratio for the sintered body of the PCBN tool were used in the calculations. The other input data for the calculations are the measured stresses developed in the tool rake face when the tool-workpiece contact is completed during practical cutting. The stresses on the chamfer area flank face are resolved into normal stress and shearing stresses. The load condition in the area of cutting edge is illustrated in Fig. 7. The contact normal and shear stresses are calculated based on the model presented by Zhou [9]. The contact stress models are illustrated in Fig. 8. The distribution of the stresses in the cutting tool close to the cutting edge was presented in Fig. 9, which shows the maximum stresses was induced just on the cutting edge during the cutting. The correlation between the stresses and chamfer angles is plotted in the Fig. 10 and it exhibits the same correlation pattern with tool wear and chamfer angles.

Fig. 8. tip.

Distribution of normal stresses and shearing stresses on tool

Fig. 9.

Distribution of the stresses in the cutting edge.

5. Conclusions

Fig. 7.

Normal forces and friction forces in chamfer area.

Cutting experiments and FE calculations have been carried out in this study in order to investigate the effects of chamfer angle on the wear of PCBN cutting tools in super finishing hard turning. The chamfer angle has a great influence on the cutting force and tool life. All three forces’ components increase with an increase of the chamfer angles, especially in the level of passive force. Although an increase of chamfer angle will increase the wedge strength of the PCBN tool, tool life, however, it does not follow the same trend. Tool life, with measured flank wear 0.2 mm as the criterion, reaches maximum

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like to thank European Community for supporting this project.

References

Fig. 10. Correlation of maximum principal stresses and chamfer angles.

value for the tool with 15° chamfer angle and minimum value for 30° chamfer angle. The difference in the life time is up to 53%. The results from FE calculation indicate that the principal stresses acting on the cutting edge with 15° chamfer angle has the smallest value compared to other cutting tools, which suggests that there is an optimal chamfer angle around 15° where the cutting tool has maximum tool life. Acknowledgements Co-operation from all partners in the MicroHard project is gratefully acknowledged. The authors would also

[1] D. Jeffrey, N.M. Shreyes, Effect of cutting edge geometry and workpiece hardness on surface residual stresses in finish hard turning of AISI52100 steel, MED-Vol. 10, Manufacturing Science and Engineering, ASME 10 (1999) 805–979. [2] H.D. Moore, D.R. Kibbey, Ceramic tool geometry and preparation, ASTE paper, No. 19, 1957. [3] J.E. Mayer, Jr., S. Cowell, Cemented titanium carbide cutting tools—performance of finishing, semifinishing and roughing grades, SME Paper No. MR 71-934. [4] J.E. Mayer Jr., D.J. Stauffer, Effects of tool edge hone and chamfer on wear life, Manufacturing Engineering Transaction 3 (1974) 5–15. [5] H. Ren, Y. Altntas, Mechanics of machining with chamfered tools, Journal of Manufacturing Science, Transaction of ASME 122 (2000) 650–659. [6] M. Hoshi, Fundamental machinability research in Japan, Journal of Engineering Industry 83 (1961) 531–544. [7] M. Hiraro, J. Tlustly, R. Sowerby, G. Chandra, Chip formation with chamfered tools, Journal of Engineering Industrial 104 (1982) 339–342. [8] D.J. Waldorf, R.E. DeVor, S.G. Kapoor, A slip-line field for ploughing during orthogonal machining, Wear, 143, 29-43. [9] J.M. Zhou, M. Andersson, J.E. Stahl, Cutting tool fracture prediction and strength evaluation by stress identification, Part I: stress model, International Journal of Machine Tools and Manufacture 37 (12) (1997) 1691–1714.