Cutting forces and surface finish when machining medium hardness steel using CBN tools

International Journal of Machine Tools & Manufacture 40 (2000) 455–466 Cutting forces and surface finish when machining medium hardness steel using CBN tools Wuyi Chen Beijing University of Aeronautics and Astronautics, Beijing, PR China Received 11 November 1997 Abstract Cutting forces generated using CBN tools have been evaluated when cutting steel being hardened to 45–55 HRC Radial thrust cutting force was the largest among the three cutting force components and was most sensitive to the changes of cutting edge geometry and tool wear The surface finish produced by CBN tools was compatible with the results of grinding and was affected by cutting speed, tool wear and plastic behaviour of the workpiece material  1999 Published by Elsevier Science Ltd All rights reserved Nomenclature ap depth of cut (DOC) BUE built-up-edge f feed rate feed force Fx Fxy vector sum of Fx and Fy Fy radial thrust force tangential cutting force Fz major cutting edge angle Kr tool nose radius r⑀ v cutting speed width of flank wear VBB Introduction Machining of hardened steel using advanced tool materials, such as CBN, has certain advantages over the traditional cutting–hardening–grinding practice in terms of improved fatigue 0890-6955/00/$ - see front matter  1999 Published by Elsevier Science Ltd All rights reserved PII: S - 5 ( 9 ) 0 1 - 456 W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 Table The tool inserts used in the tests Tools Material Major cutting Orthogonal edge angle rake angle (deg.) (deg.) CBN CBN > 90 vol.% metallic binder grain size ␮m CBN > 90 vol.% ϩ Co, Fe, W grain size 50– 100 ␮m 75–90a Ϫ7 Ϫ5 1.2 0/0.5 mm ϫ 10° 75 Ϫ7 Ϫ5 0.3/1b CBN Clearance angle (deg.) Inclination Nose radius Edge angle (deg.) (mm) preparation/ chamfer a Because the depth of cut was far smaller than the nose radius, the major cutting edge angle was not practically functional b mm nose radius CBN insert was used only in cutting force tests strength of the machined parts, increased productivity and reduced energy consumption [1–3] Although CBN tools offer excellent performance on fully hardened steels, the results on steels of medium hardness have been challenged by other members of the tooling family, e.g ceramic tools or even some new carbides In this paper the performance of CBN tools is investigated when machining steel hardened to 45–55 HRC Since CBN tools are normally used in finishing operation and the cutting regime employed is likely to generate large radial thrust force which may cause chatter and deteriorate machining quality, understanding the changing patterns of cutting forces and surface finish is therefore important Experimental work 2.1 Materials The cutting tools used were high concentration CBN compacts, referred to in this paper as CBN and CBN The materials and geometric parameters of the tool inserts are detailed in Table The workpiece material used in the tests was hardened GB699-88 55 steel hardened to 45–55 HRC The compositions of the workplace material are shown in Table Table Compositions of 55 (GB699-88) steel C Cr Mn Ni P S Si Fe 0.52–0.6 0.25 0.5–0.8 0.25 0.04 0.045 0.17–0.37 Balance W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 457 2.2 Experimental procedure Multivariate tests were performed to measure cutting forces and machined surface roughness The operating parameters were v ϭ 56–182 m/min, f ϭ 0.08–0.31 mm/rev and ap ϭ 0.025– 0.1 mm In addition, the tools with chamfered/unchamfered cutting edge and with different tool nose geometry were used in certain tests All tests were conducted dry under continuous turning conditions Results and discussion 3.1 Cutting forces 3.1.1 Cutting force components Cutting forces can be divided into three components: feed force (Fx), radial thrust force (Fy) and tangential cutting force (Fz) Usually the tangential cutting force is the largest of the three components, though in finishing the radial thrust force is often larger (see Figs 1–3), while the feed force is minimal This arrangement in finishing can be explained by studying the particular cutting regime and tool geometry used in the tests From the tool geometry and the cutting conditions outlined in Section 2.2, it is clear that the depths of cut (0.025–0.10 mm) are far smaller than the nose radii of the tools (0.3–1.2 mm) Under such conditions the tool nose, i.e the curved part of the cutting edge, performs the whole cutting job, thus the acting cutting edge angle varies along the tool–work contact arc of the tool nose The largest value of the angle appears at the position where the cutting edge meets the original work part surface as in Fig The maximum cutting edge angle can be obtained from: Kr ϭ arccos r⑀ Ϫ ap r⑀ (1) where Kr is the cutting edge angle, r⑀ is the tool nose radius and ap is the depth of cut If r⑀ ϭ mm, ap ϭ 0.025 mm, then Kr ϭ 12°8Ј Such a small cutting edge angle is seldom used in metal cutting, moreover, if considering the average value along the tool–work contact arc the angle is even smaller As the cutting edge angle decreases the horizontal component of the cutting force Fxy will alter direction clockwise; see Fig As a result, Fy will increase whereas Fx will decrease Fz will also increase but to much less extent [4] Eventually Fy will surpass Fz, and Fx will reduce to a negligible quantity The increase in Fy can lead to instability through vibration From this point of view the tool nose radius should be kept as small as possible This is not ideal however in respect of good surface finish In addition, this may also cause temperature concentration at the tool nose and increase the likelihood of spalling, resulting in a short tool life 3.1.2 Cutting regime vs cutting forces With an increase in cutting speed, both the radial thrust force and the tangential cutting force showed a decrease (Fig 1) This is a standard effect when cutting most metals with carbide tools 458 W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 Fig Cutting forces vs cutting regime and edge chamfer using CBN (a) f ϭ 0.15 mm/rev, ap ϭ 0.05 mm; (b) v ϭ 95 m/min, ap ϭ 0.05 mm; (c) v ϭ 95 m/min, f ϭ 0.15 mm/rev W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 459 Fig Cutting forces vs feed rate and tool nose radius using CBN v ϭ 85 m/min, ap ϭ 0.1 mm Fig Cutting forces vs tool wear and edge chamfer with CBN v ϭ 95 m/min, f ϭ 0.15 mm/rev, ap ϭ 0.05 mm Trent [5] attributed this phenomenon in part to the softening of the workpiece material at high temperature and in part to a decrease in tool–chip contact area owing to a thinner chip When the feed rate was increased the forces also increased, but the radial thrust forces generated by CBN tools (Fig 1) appeared to be not as sensitive to the change as those produced by CBN tools (Fig 2) Depth of cut seemed to influence cutting forces more significantly than cutting speed and feed rate In fact, the feed force (Fx) showed visible changes only when increasing DOC On substituting the tool nose radius used, r⑀ ϭ 1.2 mm, in the cutting force tests into Eq (1), it can be seen that when the depth of cut increases from 0.025 to 0.1 mm, as in Fig 1, the maximum cutting edge 460 W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 Fig The maximum cutting edge angle with a large tool nose radius and small depth of cut Fig The influence of cutting edge angle on the direction of Fxy angle increases from 11°3Ј to 23°3Ј This is a major reason for the increase in Fx, as illustrated in Fig 3.1.3 Edge geometry and cutting forces Chamfered and unchamfered CBN inserts were used in the cutting force tests It can be seen from Figs 1–3 that all three force components generated by the chamfered tools were greater than those recorded when using the unchamfered ones The radial thrust force was affected the most On the chamfered tools, Fy was doubled or even tripled, yet the increase in Fz was only about 10–50% There are other observations that may also be related to geometric parameters In the cutting force tests, CBN inserts were ground to different nose radii yet were tested under otherwise identical cutting conditions It can be seen from Fig that as the nose radius increases from 0.3 to mm, Fy increases by about 30%, whereas the changes in Fx and Fz are negligible This phenomena can be explained using Eq (1) When the nose radius changes from 0.3 to mm, for a depth of cut of 0.1 mm, the maximum cutting edge angle decreases from 48 to 26° Such a change may turn the horizontal component of the cutting force (Fxy in Fig 5) clockwise, then the radial component of the cutting force increases W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 461 3.1.4 Influences of tool wear on the cutting forces It can be seen in Fig that tool wear had a negligible influence on feed force and tangential cutting force, however, the radial thrust force showed a 90–150% increase when the wear land VBB had an increment of about 0.18 mm Because the tool nose radius is much larger than the depth of cut, the flank wear land may almost be parallel to the feed direction, thus the force normal to the flank wear land will be approximately in the direction of the y-axis Meanwhile the friction force on the flank wear land is always in the direction of the z-axis The fact that Fz changes only slightly while Fy increases dramatically seems to indicate that either the increase in the normal force on the flank wear land does not lead to the increase in friction force, or the friction force on the flank face is too small to have significant influence on the total force in the z-direction 3.2 Surface roughness 3.2.1 Hardness vs roughness The majority of Ra data collected during the tests were summarised by using histograms; see Fig The horizontal axis of the graphs represents the observed roughness readings and the vertical axis gives the frequency of the readings The graphs are able to show the variations of surface roughness with the changing workpiece hardness From Fig 6, it is evident that the harder the workpiece material, the lower is the surface roughness obtained for a given set of operating parameters This phenomenon may be explained by a finding presented by Usui [6] In orthogonal cutting, the material flow is mainly two-dimensional, on a plane normal to the cutting edge The deformation in the third direction, i.e the direction parallel to the cutting edge, is usually disregarded However, this deformation does exist and causes slight lateral plastic flow of the workpiece material in the region adjacent to the two free surfaces, e.g the internal and external surfaces if a thin wall tube is used as the workpiece when conducting orthogonal cutting on a lathe When there is only one free surface, as in turning a solid bar, the lateral flow on the constrained side may increase the peak-to-valley height of the machined surface profile as in Fig By increasing the workpiece hardness, the plasticity of the workpiece material is reduced and so is the level of the lateral plastic flow As a result the surface roughness becomes lower 3.2.2 Influence of cutting regime Surface finish was shown to be improved by increasing cutting speed (Fig 8), though the improvement was very limited Producing a better surface finish at higher cutting speed is not something unusual in metal cutting, but the conventional explanations are usually related to BUE [4] That is, the formation of a built-up-edge is favoured in a certain range of cutting speed By increasing cutting speed beyond this region, BUE will be eliminated and as a result the surface finish will improve When hardened steel was machined under present cutting conditions, the cutting speeds adopted were higher than those favouring BUE formation [5] Indeed BUE was not apparently observed even at the lowest speed of 56.5 m/min Therefore, the phenomenon needs further explanation Two possible reasons are given below According to Liu [7], the properties of metals are influenced by the deformation velocity The higher the velocity, the less significant the plastic behaviour will be Based on the reasoning in 462 W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 Fig Surface roughness vs workpiece hardness (a) By CBN tools; (b) by CBN tools Fig Additional increase in surface roughness caused by lateral plastic flow W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 463 Fig Surface roughness vs cutting speed and tool geometry with CBN f ϭ 0.1 mm/rev, ap ϭ 0.1 mm Section 3.2.1, the lateral plastic flow of the workpiece material along the cutting edge direction may increase the peak-to-valley height of the surface irregularity If the material presents less plasticity by increasing cutting speed and hence deformation velocity, the surface finish can be improved as a result of less significant lateral plastic flow and thus less additional increase in the peak-to-valley height of the machined surface roughness The second possible reason is based on SEM observations At low cutting speed, grooves developed on the flank wear land (Fig 9) When such a cutting edge is engaged with a workpiece, the defects will in part be copied on to the newly generated surface In any event it is likely that the surface will be rough With an increase in cutting speed the grooves will gradually be reduced, Fig Wear scar of CBN tool: (a) ϫ 60; (b) ϫ 1250 v ϭ 82.5 m/min, f ϭ 0.2 mm/rev, ap ϭ 0.025 mm 464 W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 Fig 10 Wear scar of CBN tool: (a) ϫ 60; (b) ϫ 1250 v ϭ 145 m/min, f ϭ 0.2 mm/rev, ap ϭ 0.025 mm thus the cutting edge and wear land will become smoother (see Fig 10), as will the workpiece surface The influence of wear land grooves on surface roughness was also observed by Solaja [8], and Ansell and Taylor [9] They demonstrated that with the development of the grooves the surface finish deteriorated The roughness increases with increases in feed rate (see Fig 11), but the trend is less significant for the tools with large nose radius A recommendation is therefore made to the tool users that if the inserts of mm nose radii are used, feed rates as large as 0.3 mm/rev may be used in order to promote productivity when finishing without significant deterioration in surface roughness However, low DOC should be used in order to reduce the tendency to chatter The DOC has little direct influence on the surface roughness, however, with increases in DOC, chatter may result causing degradation of the workpiece surface Therefore, if the tool–work Fig 11 Surface roughness vs feed rate and tool nose radius v ϭ 85 m/min, ap ϭ 0.1 mm W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 465 system is not very rigid, such as in cutting slender parts, very fine DOC should be employed to avoid chatter In this way very good surface finishes can be obtained For example, when a DOC of 0.025 mm and a feed rate of 0.2 mm/rev were employed, a roughness of Ra ϭ 0.22 ␮m was achieved using CBN inserts, which is compatible with grinding 3.2.3 Influences of tool wear As mentioned in the last section, large surface roughness values produced at low cutting speed probably resulted in part from the grooves on the wear scars of the tools It can be seen from Fig 12 that the roughness is also associated with the width of the flank wear land The relationship may be explained as follows When a new insert starts to work, the machined surface is determined, for a given feed rate, by the geometry of the fresh tool edge If the DOC is far smaller than the nose radius, then the principal geometric parameter is only the nose radius As the tool wears, however, the round corner becomes flatter, in other words the nose radius increases substantially As a result the machined surface finish improves With the development of excessive flank wear, however, increased cutting force and temperature may destabilise the machining process and the surface quality is degraded Conclusions When finish cutting of hardened steel, the radial thrust force (Fy) became the largest among the three cutting force components and was the most sensitive to the changes of cutting edge chamfer, tool nose radius and flank wear Although an unchamfered tool with small nose radius generated low Fy and hence reduced the tendency to chatter, such geometry decreased tool life Fig 12 Surface roughness vs tool wear (1) v ϭ 82.5 m/min, f ϭ 0.2 mm/rev, ap ϭ 0.025 mm; (2) v ϭ 121 m/min, f ϭ 0.1 mm/rev, ap ϭ 0.1 mm 466 W Chen / International Journal of Machine Tools & Manufacture 40 (2000) 455–466 Lateral plastic flow of the workpiece material in front of a cutting edge increased roughness of machined surfaces Therefore, the harder, and hence less plastic, the workpiece material, the better the surface finish Surface roughness could be improved by increasing cutting speed Two possible reasons are: (i) workpiece material presents less plastic behaviour at higher deformation velocity and (ii) the flank wear scar becomes smoother at higher cutting speed A better surface finish could be produced using the tool with a certain degree of tool wear, which has increased the tool nose radius Excessive tool wear, however, resulted in a rough surface Acknowledgements The author would like to thank General Electric CO, USA, Shanxi Natural Science Foundation, PRC and Taiyuan Heavy Machinery Plant, PRC for funding the work The author is also grateful to Mr D.K Aspinwall, School of Manufacturing and Mechanical Engineering, The University of Birmingham, for helpful discussions References [1] Y Matsumoto, Effect of machining process on the fatigue strength of hardened AISI3040 steel, Trans ASME Journal of Engineering for Industry 113 (1991) 154–159 [2] A.A Panov, Intensifying components machining by means of tools provided with synthetic superhard materials and ceramics, Soviet Engineering Research (11) (1989) 45–49 [3] N.G Boim, I.N Sokolov, The use of super-hard material and ceramic cutting tools in machine tool construction, Soviet Engineering Research (7) (1984) 55–56 [4] South China Institute of Technology (Ed.), Principles of Metal Cutting and Design of Cutting Tools, vol 1, Shanghai Science and Technology Press, 1979 (in Chinese) [5] E.M Trent, Metal Cutting, 3rd ed., Butterworth-Heinemann, 1991 [6] E Usui, The Principles of Cutting and Grinding, Machinery Industry Press, 1982 (Chinese version translated from Japanese by X Gao and D Liu) [7] H Liu, Mechanics of Materials, The People’s Education Press, 1979 (in Chinese) [8] V Solaja, Wear of carbide tools and surface finish generated in finish turning of steel, Wear (1958/59) 40–58 [9] C.T Ansell, J Taylor, The surface finishing properties of carbide and ceramic cutting tools, in: Proc 3rd Int MTDR Conf., 1962 [...]... machined surface finish improves With the development of excessive flank wear, however, increased cutting force and temperature may destabilise the machining process and the surface quality is degraded 4 Conclusions 1 When finish cutting of hardened steel, the radial thrust force (Fy) became the largest among the three cutting force components and was the most sensitive to the changes of cutting edge... (Chinese version translated from Japanese by X Gao and D Liu) [7] H Liu, Mechanics of Materials, The People’s Education Press, 1979 (in Chinese) [8] V Solaja, Wear of carbide tools and surface finish generated in finish turning of steel, Wear (1958/59) 40–58 [9] C.T Ansell, J Taylor, The surface finishing properties of carbide and ceramic cutting tools, in: Proc 3rd Int MTDR Conf., 1962 ... super-hard material and ceramic cutting tools in machine tool construction, Soviet Engineering Research 4 (7) (1984) 55–56 [4] South China Institute of Technology (Ed.), Principles of Metal Cutting and Design of Cutting Tools, vol 1, Shanghai Science and Technology Press, 1979 (in Chinese) [5] E.M Trent, Metal Cutting, 3rd ed., Butterworth-Heinemann, 1991 [6] E Usui, The Principles of Cutting and Grinding,... front of a cutting edge increased roughness of machined surfaces Therefore, the harder, and hence less plastic, the workpiece material, the better the surface finish 3 Surface roughness could be improved by increasing cutting speed Two possible reasons are: (i) workpiece material presents less plastic behaviour at higher deformation velocity and (ii) the flank wear scar becomes smoother at higher cutting. .. International Journal of Machine Tools & Manufacture 40 (2000) 455–466 465 system is not very rigid, such as in cutting slender parts, very fine DOC should be employed to avoid chatter In this way very good surface finishes can be obtained For example, when a DOC of 0.025 mm and a feed rate of 0.2 mm/rev were employed, a roughness of Ra ϭ 0.22 ␮m was achieved using CBN 1 inserts, which is compatible... As mentioned in the last section, large surface roughness values produced at low cutting speed probably resulted in part from the grooves on the wear scars of the tools It can be seen from Fig 12 that the roughness is also associated with the width of the flank wear land The relationship may be explained as follows When a new insert starts to work, the machined surface is determined, for a given feed... of Manufacturing and Mechanical Engineering, The University of Birmingham, for helpful discussions References [1] Y Matsumoto, Effect of machining process on the fatigue strength of hardened AISI3040 steel, Trans ASME Journal of Engineering for Industry 113 (1991) 154–159 [2] A.A Panov, Intensifying components machining by means of tools provided with synthetic superhard materials and ceramics, Soviet... becomes smoother at higher cutting speed 4 A better surface finish could be produced using the tool with a certain degree of tool wear, which has increased the tool nose radius Excessive tool wear, however, resulted in a rough surface Acknowledgements The author would like to thank General Electric CO, USA, Shanxi Natural Science Foundation, PRC and Taiyuan Heavy Machinery Plant, PRC for funding the... edge chamfer, tool nose radius and flank wear Although an unchamfered tool with small nose radius generated low Fy and hence reduced the tendency to chatter, such geometry decreased tool life Fig 12 Surface roughness vs tool wear (1) v ϭ 82.5 m/min, f ϭ 0.2 mm/rev, ap ϭ 0.025 mm; (2) v ϭ 121 m/min, f ϭ 0.1 mm/rev, ap ϭ 0.1 mm 466 W Chen / International Journal of Machine Tools & Manufacture 40 (2000) ... this paper the performance of CBN tools is investigated when machining steel hardened to 45–55 HRC Since CBN tools are normally used in finishing operation and the cutting regime employed is likely... chatter and deteriorate machining quality, understanding the changing patterns of cutting forces and surface finish is therefore important Experimental work 2.1 Materials The cutting tools used... Machine Tools & Manufacture 40 (2000) 455–466 459 Fig Cutting forces vs feed rate and tool nose radius using CBN v ϭ 85 m/min, ap ϭ 0.1 mm Fig Cutting forces vs tool wear and edge chamfer with CBN