This paper presents the experimental results of the turning of spheroidal iron (EN-GJS-500-7 grade) using L-CBN tools. The cutting process can be classified as a High Performance Cutting (HPC) due to a relatively high material removal rate of about 190 cm3/min. The investigations performed include fundamental process quantities and machined surface characteristics, i.e. componential cutting forces, specific cutting energy, average and maximum values of cutting temperature as well as temperature distribution in the cutting zone, tool wear progress visualized by appropriate wear curves and 2D/3D surface roughness parameters

D. Kowalczyk

J. Rech

K. Żak

P. Kiszka

W. Grzesik

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33METALURGIJA 53 (2014) 1, 33-36W. GRZESIK, P. KISZKA, D. KOWALCZYK, K. ŻAK, J. RECHINVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC BORON NITRIDE (CBN) TOOLSReceived – Prispjelo: 2013-04-22Accepted – Prihvaćeno: 2013-06-20Original scientifi c paper –Izvorni znanstveni radISSN 0543-5846METABK 53(1) 33-36 (2014)UDC – UDK 621.74.04:669.781:669.789:621.96=111W. Grzesik, P. Kiszka,D. Kowalczyk, K. Żak, Opole University of Technology, Department of Manufacturing Engineering and Automa-tion, Opole, Poland, J.Rech, Université de Lyon, ENISE, LTDS, Saint-Etienne, FranceThis paper presents the experimental results of the turning of spheroidal iron (EN-GJS-500-7 grade) using L-CBN tools. The cutting process can be classifi ed as a High Performance Cutting (HPC) due to a relatively high material removal rate of about 190 cm3/min. The investigations performed include fundamental process quantities and ma-chined surface characteristics, i.e. componential cutting forces, specifi c cutting energy, average and maximum val-ues of cutting temperature as well as temperature distribution in the cutting zone, tool wear progress visualized by appropriate wear curves and 2D/3D surface roughness parameters.Key words: spheroidal cast iron, CBN tool, cutting force, cutting temperature, surface roughnessINTRODUCTIONRecently, advanced manufacturing has continued to apply materials with speciﬁ c service properties. In par-ticular, machine building, automotive and energy/wind power industries consume more ductile irons including nodular cast iron/spheroidal cast iron (NCI/SCI), com-pacted graphite iron (CGI) and austempered ductile iron (ADI) grades. This trend can be explained in terms of their higher mechanical strengths keeping good yield (plasticity) and fatigue strengths [1]. According to the machinability rating, these irons are classiﬁ ed as difﬁ -cult-to-machine materials. Their machinability rates are comparable to high-strength alloy steels, for example P-F SCI with AISI 4140 steel [2]. As a result, they need special technological routines to be machined efﬁ cient-ly. On the other hand, cutting tool manufactures do not yet recommend appropriate tooling and cutting condi-tions [3]. The machining of SCI needs, due to character-istic two-phase microstructure with globular graphite inclusions and high ductility, more wear resistant cut-ting tool materials such as multilayer coated carbides, uncoated and coated silicon nitride ceramics Si3N4, (coated with Al2O3/TiN), SIALON and L-CBN depend-ing on iron grade and machining conditions [4].Because previous cutting experiments when machin-ing P-F SCI material with coated and uncoated silicon nitride cutting tools indicated rather low tool lives of maximum several minutes [3-5] this study was performed using L-CBN tools. It is obviously known that CBN tools are capable of machining EN-GJS-500-7 iron with dis-tinctly higher cutting speed up to about 500 m/min. More-over, the material removal rate (MRR) can be increased considerably [6]. This experimental study is focused on the cutting performance and surface quality in terms of componential cutting forces, cutting temperature distribu-tion in both primary and secondary zones and obtainable surface roughness. The cutting tests were performed un-der orthogonal (O) and semi-orthogonal (S-O) conditions with variable cutting speed between 100 and 400 (500) m/min, feed rate varying between 0,04 and 0,24 mm/rev and depth of cut equal to 3,3/0,8 (O/S-O) mm.EXPERIMENTAL METHODOLOGYWorkpiecematerialsThe workpiece material was the pearlitic-ferritic spheroidal (nodular) iron (PF-SCI), EN-GJS-500-7 grade.Table 1  Chemical composition of EN-GJS-500-7 iron / wt. % [4]C Si Cu Mg Mn P S3,78 2,46 0,01 0,05 0,32 0,038 0,065Its microstructure contains approximately 50 % pearlite, 40 % ferrite and 10 % graphite. The basic me-chanical properties are: Rm = 500 MPa and hardness of 175 HB. Chemical composition of the SCI machined is speciﬁ ed in Table 1.Cutting tool and cutting conditionsThe machining of SCI workpieces was performed us-ing cutting inserts of N123H1040004S01025 type 34  METALURGIJA 53 (2014) 1, 33-36W. GRZESIK et al.: INVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC...mounted in LF 123H25–2020BM type holder and trian-gular TNGA160408S01030 inserts clamped in PTGNL 2020K16 holder for orthogonal and semi-orthogonal op-erations respectively. CBN cutting inserts with low con-tent of CBN (L-CBN), grade CB 7015 according to Sandvik designation, were used [7]. The measured radius of the cutting edge was equal to 7 mm. The chamfer an-gle was γch=- 25 ° and its length was lch= 100 mm.Cutting experiments were divided into the two series: Series I under orthogonal conditions: a) with varia-ble cutting speed of 100, 160, 240, 280, 320, 400 m/min, constant feed rate of 0,12 mm/rev and constant depth of cut of 3,3 mm, and b) with constant cutting speed of 240 m/min, variable feed rate of 0,04, 0,08, 0,12, 0,16, 0,2 0,24 mm/rev and constant depth of cut of 3,3 mm.Series II under semi-orthogonal with variable cut-ting speeds of 100, 160, 240, 320, 400, 480 m/min, two feed rates of 0,08 and 0,12 mm/rev and constant depth of cut of 0,8 mm. During experiments all trials were repeated three times.Measurement techniquesAll machining tests were carried out on a CNC turn-ing centre - Transmab 450TD. A thin disk was orthogo-nally machined with radial tool engagement as shown in Figure 1. In the ﬁ rst part of experiments the cutting Fc and feed Ff forces were measured by means of the Kis-tler’s commercial piezoelectric dynamometer, model 9257A included in the measuring circuit shown in Fig-ure 1. Force signals were transmitted in on-line mode to the Kistler 5070 ampliﬁ er and a PC with National In-struments LabView 6i software to acquire and analyze. The generated signals were recorded with the frequency of 1 000 Hz.Parallel to force measurements, the temperature dis-tribution in the cutting zone was recorded using the in-frared technique. In this case, IR-CCD set-up (Figure 1) consists of an infrared camera ThermaCAMTM Phoe-nix, Flir System, special lens and protective window. The CCD-IR camera used can register thermal images with the resolution of 320 × 255 pixels, which is enough to obtain high quality thermal maps. The cutting tem-peratures were determined using the emissivity curve (shown in Figure 1) obtained previously for the nodular cast iron (NCI) machined in the temperature range of 50 °C ÷ 600 °C. The calibration procedure is described in details in Ref. [5].The ﬂ ank and rake surfaces of CBN inserts were successively examined during tool wear period using a LOM Leica equipped with a CCD camera. The images obtained were digitized using special graphical pro-gram. After turning the surface proﬁ les/ topographies were recorded using a proﬁ lometer and the 2D and 3D roughness parameters were determined.EXPERIMENTAL RESULTSComponents of the resultant cutting forceFigure 2 shows the inﬂ uence of the cutting speed and feed rate on the values of the cutting Fc and feed Ff forces acting on the CBN tools. The predicted values of both forces are maximum 5 % lower or higher than ex-perimental results.Figure 2 shows that Fc and Ff forces decrease slowly when the cutting speed increases. For instance, the maximum value of Fc force of about 850 N was record-ed for the minimum cutting speed of vc = 100 m/min. In turn, the minimum value of Fc force of about 750 N cor-responds to the cutting speed of vc = 400 m/min. The values of the feed force (course #2) are visibly lower. For example, the maximum Ff force of about 620 N was also recorded for the cutting speed of vc = 100 m/min. It can be observed in Figure 2 that the difference between values of Fc and Ff forces is about 200 N.It can also be seen from Figure 2 that both forces increase when feed increases up to 0,24 mm/rev. For example, the effect is that force Fc increases from about  100 200 300 400Cutting speed / m/min4006008001 0001 2001 400Force F cand F f/ N0,04 0,08 0,12 0,16 0,2 0,24Feed / mm/revFc-vc (1)Ff-vc (2)Fc-f (3)Ff-f (4)1234Figure 1 Scheme of IR thermal mapping techniqueFigure 2  Infl uence of cutting speed and feed rate on Fc and Ff forces35METALURGIJA 53 (2014) 1, 33-36W. GRZESIK et al.: INVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC...550 N to 1245 N. In the case of Ff local minimum value appears at the feed of 0,12 mm/rev. Another character-istic observation is that for the feed of 0,04 mm/rev the values of Fc and Ff forces are comparable, probably due to intensive friction.Figure 3 shows the energetic characteristic of the ma-chining process performed in terms of the relationship between the speciﬁ c cutting energy (SCE) and volumet-ric machining rate (Qv). It is evident in Figure 3 that the obtainable machining rate is about 190 cm3/min.Cutting temperatureFigure 4 presents exemplarily colored thermal maps recorded by means of thermographic technique. In or-der to localize the thermal ﬁ elds, the curled chip and the CBN insert are also captured in Figure 4. It can be noted in Figure 4a-c that the localizations of both average and maximum temperatures within the cutting zone were distinguished for different cutting speeds.The areas with higher temperatures in the CBN in-serts are probably due to the fact that thermal conduc-tivity of CBN is relatively high [1]. In previous investi-gations with coated nitride ceramic tools this phenom-enon was not observed [5].One particular ﬁ nding from the thermal analysis shown in Figure 4 is that the differences between maxi-mum and average values of temperatures measured in the cutting zone are not distinct. As a result, in this study the average contact temperature changes in the range of t = 412 – 512 °C, (see Figure 5) whereas its maximum value varies between tmax = 426 – 541 °C. The maximum difference between maximum and average temperatures of about 40 °C was recorded for the maximum cutting speed of vc = 400 m/min. Surprisingly, low temperatures recorded in the cutting zone result from the speciﬁ c chip formation of the two-phase material in which graphite inclusions facilitate its decohesion. In fact, a high-ve-locity stream of small chips was formed during the turn-ing of SCI material. Moreover, this speciﬁ c effect can result from substantial decreasing of the tool-chip con-tact length/area and concentration of the heat within very small area in comparison to steel machining [5].As shown in Figure 5, the increase of cutting speed from 100 to 400 m/min causes the average temperature to increase linearly of about 50 °C. When considering the inﬂ uence of feed, the temperature plot tends to display  0 40 80 120 160 200Volumetric MRR / cm3/min12345Specific cutting energy / GJ/m312Figure 3  Specifi c cutting energy vs. volumetric machining rate: 1-infl uence of cutting speed, 2- infl uence of feed rateFigure 4  Thermal maps for orthogonal cutting for diff erent cutting speeds: (a) vc = 280 m/min, (b) vc = 400 m/min keeping constant f = 0,12mm/rev and ap = 3,3 mma)b)Figure 5  Infl uence of cutting speed on the cutting temperature36  METALURGIJA 53 (2014) 1, 33-36W. GRZESIK et al.: INVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC...the minimum value of 430 °C at the feed of 0,12 mm/rev. This fact can be explained in terms of the corresponding values of the speciﬁ c cutting energy, shown in Figure 3. In fact, for the feed of about 0,12 mm/rev the value of ec approaches 1,9 GJ/m3. It can also be noticed that the computation accuracy depends on the errors in the deter-mination of SCI emissivity versus temperature.Surface roughness and surface topographyFigure 6 presents the inﬂ uence of the cutting speed on the surface roughness average Ra. For the cutting speeds of 240 - 480 m/min and feed rate of f = 0,08 mm/rev the Ra value decreases from 0,6 to 0,38 μm. The maximum value of Ra = 1,34 μm was measured for feed of 0,08 mm/rev and cutting sped of vc = 100 m/min, which is typically observed in machining practice [1]. At a higher feed rate of f = 0,12 mm/rev, the Ra param-eter varies in the range of Ra = 1,1 - 0,83 μm. In this case, the trend showing lower surface roughness with higher cutting speed is less pronounced.In addition, Figure 7 shows surface topography ob-tained at the maximum cutting speed of 400 m/min and low feed rate of 0,08 mm/rev. In general, surfaces with regular asperities and good bearing properties (negative Ssk values) are generated. At higher feed rate of 0,12 mm/rev small craters resulting from the removal of graphite inclusions are observed.CONCLUSIONS1)  The cutting and feed forces decrease when ma-chining with higher cutting speeds and increase non-linearly when feed rate increases. The local minimum of the speciﬁ c cutting energy corre-sponds to the minimum value of feed force.2)  The variations of the average tool-chip contact temperature under machining conditions em-ployed are not so distinctive as in the steel ma-chining. Maximum temperature in the cutting zone of 510 °C was recorded for f = 0,24 mm/rev and vc = 240 m/min. On the other hand, the mini-mum temperature of 410 °C corresponds to cut-ting speed of vc = 100 m/min.3)  SCI machining with high cutting speeds visibly improves the surface ﬁ nish. In this study, the min-imum value of Ra roughness parameter of about 0,4 μm was recorded for vc = 400 min and f=0,08 mm/rev. Under these machining conditions de-fect-free surfaces with regularly distributed feed marks are produced. REFERENCES[1] Grzesik W.: Advanced machining processes of metallic materials. Elsevier, Amsterdam 2008.[2] Graham D.: Machining cast iron. Manufacturing Enginee-ring, 136(2006) 2, www.sme.org.[3] Klocke F., Klöpper C., Lung D., Essig C.: Fundamental Wear Mechanisms when Machining Austempered Ductile Iron (ADI). Annals of the CIRP, 56(2007) 1, 73-76.[4] Małecka J., Grzesik W., Kowalczyk D., Kiszka P.: Image analysis of wear effects when machining nodular cast iron with silicon nitride tools. Tribologia, 42(2011) 2, 71-84[5] Schneider J., Richter G.: Eine Keramik für viele Fälle. Werkstatt und Betrieb, 139(2006) 6, 51-55.[6] Byrne G., Dornfeld D., Denkena B.: Advancing cutting technology, CIRP Annals-Manufacturing Technology, 52(2003) 2, 483-507.[7] Machining Handbook, Sandvik Coromant, 2010.Note: The responsible for English language is the lecturer from Opole University 100 200 300 400 500Cutting speed / m/min00,40,81,21,6Surface roughness Ra / µmf = 0,08 mm/revf = 0,12 mm/revFigure 6  Infl uence of cutting speed on the roughness average RaFigure 7  Surface topography at diff erent magnifi cations generated with vc = 400 m/min, f = 0,08 mm/rev and rε = 0,8 mm. Sa = 0,49 µm, Sz = 13,2 µm, Sp = 5,43 µm, Sku = 7,82, Ssk = -0,67

Investigation of the machining process of spheroidal cast iron using cubic boron nitride (CBN) tools

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33 METALURGIJA 53 (2014) 1, 33-36W. GRZESIK, P. KISZKA, D. KOWALCZYK, K. ŻAK, J. RECHINVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC BORON NITRIDE (CBN) TOOLSReceived – Prispjelo: 2013-04-22Accepted – Prihvaćeno: 2013-06-20Original scientifi  c paper –Izvorni znanstveni radISSN 0543-5846METABK 53(1) 33-36 (2014)UDC – UDK 621.74.04:669.781:669.789:621.96=111W .  G r z e s i k ,  P .  K i s z k a , D .  K o w a l c z y k ,  K .  Ż a k ,  O p o l e  U n i v e r s i t y  o f  Technology, Department of Manufacturing Engineering and Automa-tion, Opole, Poland, J.Rech, Université de Lyon, ENISE, LTDS, Saint-Etienne, FranceThis paper presents the experimental results of the turning of spheroidal iron (EN-GJS-500-7 grade) using L-CBN tools. The cutting process can be classifi  ed as a High Performance Cutting (HPC) due to a relatively high material removal rate of about 190 cm3/min. The investigations performed include fundamental process quantities and ma-chined surface characteristics, i.e. componential cutting forces, specifi  c cutting energy, average and maximum val-ues of cutting temperature as well as temperature distribution in the cutting zone, tool wear progress visualized by appropriate wear curves and 2D/3D surface roughness parameters.Key words: spheroidal cast iron, CBN tool, cutting force, cutting temperature, surface roughnessINTRODUCTIONRecently, advanced manufacturing has continued to apply materials with speciﬁ  c service properties. In par-ticular, machine building, automotive and energy/wind power industries consume more ductile irons including nodular cast iron/spheroidal cast iron (NCI/SCI), com-pacted graphite iron (CGI) and austempered ductile iron (ADI) grades. This trend can be explained in terms of their higher mechanical strengths keeping good yield (plasticity) and fatigue strengths [1]. According to the machinability rating, these irons are classiﬁ  ed as difﬁ  -cult-to-machine materials. Their machinability rates are comparable to high-strength alloy steels, for example P-F SCI with AISI 4140 steel [2]. As a result, they need special technological routines to be machined efﬁ  cient-ly. On the other hand, cutting tool manufactures do not yet recommend appropriate tooling and cutting condi-tions [3]. The machining of SCI needs, due to character-istic two-phase microstructure with globular graphite inclusions and high ductility, more wear resistant cut-ting tool materials such as multilayer coated carbides, uncoated and coated silicon nitride ceramics Si3N4, (coated with Al2O3/TiN), SIALON and L-CBN depend-ing on iron grade and machining conditions [4].Because previous cutting experiments when machin-ing P-F SCI material with coated and uncoated silicon nitride cutting tools indicated rather low tool lives of maximum several minutes [3-5] this study was performed using L-CBN tools. It is obviously known that CBN tools are capable of machining EN-GJS-500-7 iron with dis-tinctly higher cutting speed up to about 500 m/min. More-over, the material removal rate (MRR) can be increased considerably [6]. This experimental study is focused on the cutting performance and surface quality in terms of componential cutting forces, cutting temperature distribu-tion in both primary and secondary zones and obtainable surface roughness. The cutting tests were performed un-der orthogonal (O) and semi-orthogonal (S-O) conditions with variable cutting speed between 100 and 400 (500) m/min, feed rate varying between 0,04 and 0,24 mm/rev and depth of cut equal to 3,3/0,8 (O/S-O) mm.EXPERIMENTAL METHODOLOGYWorkpiecematerialsThe workpiece material was the pearlitic-ferritic spheroidal (nodular) iron (PF-SCI), EN-GJS-500-7 grade.Table 1   Chemical composition of EN-GJS-500-7 iron / wt. % [4]CS i C u M g M n PS3,78 2,46 0,01 0,05 0,32 0,038 0,065Its microstructure contains approximately 50 % pearlite, 40 % ferrite and 10 % graphite. The basic me-chanical properties are: Rm = 500 MPa and hardness of 175 HB. Chemical composition of the SCI machined is speciﬁ  ed in Table 1.Cutting tool and cutting conditionsThe machining of SCI workpieces was performed us-ing cutting inserts of N123H1040004S01025 type 34   METALURGIJA 53 (2014) 1, 33-36W. GRZESIK et al.: INVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC...mounted in LF 123H25–2020BM type holder and trian-gular TNGA160408S01030 inserts clamped in PTGNL 2020K16 holder for orthogonal and semi-orthogonal op-erations respectively. CBN cutting inserts with low con-tent of CBN (L-CBN), grade CB 7015 according to Sandvik designation, were used [7]. The measured radius of the cutting edge was equal to 7 mm. The chamfer an-gle was γch=- 25 ° and its length was lch= 100 mm.Cutting experiments were divided into the two series: Series I under orthogonal conditions: a) with varia-ble cutting speed of 100, 160, 240, 280, 320, 400 m/min, constant feed rate of 0,12 mm/rev and constant depth of cut of 3,3 mm, and b) with constant cutting speed of 240 m/min, variable feed rate of 0,04, 0,08, 0,12, 0,16, 0,2 0,24 mm/rev and constant depth of cut of 3,3 mm.Series II under semi-orthogonal with variable cut-ting speeds of 100, 160, 240, 320, 400, 480 m/min, two feed rates of 0,08 and 0,12 mm/rev and constant depth of cut of 0,8 mm. During experiments all trials were repeated three times.Measurement techniquesAll machining tests were carried out on a CNC turn-ing centre - Transmab 450TD. A thin disk was orthogo-nally machined with radial tool engagement as shown in Figure 1. In the ﬁ  rst part of experiments the cutting Fc and feed Ff forces were measured by means of the Kis-tler’s commercial piezoelectric dynamometer, model 9257A included in the measuring circuit shown in Fig-ure 1. Force signals were transmitted in on-line mode to the Kistler 5070 ampliﬁ  er and a PC with National In-struments LabView 6i software to acquire and analyze. The generated signals were recorded with the frequency of 1 000 Hz.Parallel to force measurements, the temperature dis-tribution in the cutting zone was recorded using the in-frared technique. In this case, IR-CCD set-up (Figure 1) consists of an infrared camera ThermaCAMTM Phoe-nix, Flir System, special lens and protective window. The CCD-IR camera used can register thermal images with the resolution of 320 × 255 pixels, which is enough to obtain high quality thermal maps. The cutting tem-peratures were determined using the emissivity curve (shown in Figure 1) obtained previously for the nodular cast iron (NCI) machined in the temperature range of 50 °C ÷ 600 °C. The calibration procedure is described in details in Ref. [5].The ﬂ  ank and rake surfaces of CBN inserts were successively examined during tool wear period using a LOM Leica equipped with a CCD camera. The images obtained were digitized using special graphical pro-gram. After turning the surface proﬁ  les/ topographies were recorded using a proﬁ  lometer and the 2D and 3D roughness parameters were determined.EXPERIMENTAL RESULTSComponents of the resultant cutting forceFigure 2 shows the inﬂ  uence of the cutting speed and feed rate on the values of the cutting Fc and feed Ff forces acting on the CBN tools. The predicted values of both forces are maximum 5 % lower or higher than ex-perimental results.Figure 2 shows that Fc and Ff forces decrease slowly when the cutting speed increases. For instance, the maximum value of Fc force of about 850 N was record-ed for the minimum cutting speed of vc = 100 m/min. In turn, the minimum value of Fc force of about 750 N cor-responds to the cutting speed of vc = 400 m/min. The values of the feed force (course #2) are visibly lower. For example, the maximum Ff force of about 620 N was also recorded for the cutting speed of vc = 100 m/min. It can be observed in Figure 2 that the difference between values of Fc and Ff forces is about 200 N.It can also be seen from Figure 2 that both forces increase when feed increases up to 0,24 mm/rev. For example, the effect is that force Fc increases from about  100 200 300 400Cutting speed / m/min4006008001 0001 2001 400Force Fcand Ff/ N0,04 0,08 0,12 0,16 0,2 0,24Feed / mm/revFc-vc (1)Ff-vc (2)Fc-f (3)Ff-f (4)1234Figure 1 Scheme of IR thermal mapping techniqueFigure 2   Infl  uence of cutting speed and feed rate on Fc and Ff forces35 METALURGIJA 53 (2014) 1, 33-36W. GRZESIK et al.: INVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC...550 N to 1245 N. In the case of Ff local minimum value appears at the feed of 0,12 mm/rev. Another character-istic observation is that for the feed of 0,04 mm/rev the values of Fc and Ff forces are comparable, probably due to intensive friction.Figure 3 shows the energetic characteristic of the ma-chining process performed in terms of the relationship between the speciﬁ  c cutting energy (SCE) and volumet-ric machining rate (Qv). It is evident in Figure 3 that the obtainable machining rate is about 190 cm3/min.Cutting temperatureFigure 4 presents exemplarily colored thermal maps recorded by means of thermographic technique. In or-der to localize the thermal ﬁ  elds, the curled chip and the CBN insert are also captured in Figure 4. It can be noted in Figure 4a-c that the localizations of both average and maximum temperatures within the cutting zone were distinguished for different cutting speeds.The areas with higher temperatures in the CBN in-serts are probably due to the fact that thermal conduc-tivity of CBN is relatively high [1]. In previous investi-gations with coated nitride ceramic tools this phenom-enon was not observed [5].One particular ﬁ   nding from the thermal analysis shown in Figure 4 is that the differences between maxi-mum and average values of temperatures measured in the cutting zone are not distinct. As a result, in this study the average contact temperature changes in the range of t = 412 – 512 °C, (see Figure 5) whereas its maximum value varies between tmax = 426 – 541 °C. The maximum difference between maximum and average temperatures of about 40 °C was recorded for the maximum cutting speed of vc = 400 m/min. Surprisingly, low temperatures recorded in the cutting zone result from the speciﬁ  c chip formation of the two-phase material in which graphite inclusions facilitate its decohesion. In fact, a high-ve-locity stream of small chips was formed during the turn-ing of SCI material. Moreover, this speciﬁ  c effect can result from substantial decreasing of the tool-chip con-tact length/area and concentration of the heat within very small area in comparison to steel machining [5].As shown in Figure 5, the increase of cutting speed from 100 to 400 m/min causes the average temperature to increase linearly of about 50 °C. When considering the inﬂ  uence of feed, the temperature plot tends to display  0 40 80 120 160 200Volumetric MRR / cm3/min12345Specific cutting energy / GJ/m312Figure 3   Specifi  c cutting energy vs. volumetric machining rate: 1-infl  uence of cutting speed, 2- infl  uence of feed rateFigure 4   Thermal maps for orthogonal cutting for diff  erent cutting speeds: (a) vc = 280 m/min, (b) vc = 400 m/min keeping constant f = 0,12mm/rev and ap = 3,3 mma)b)Figure 5   Infl  uence of cutting speed on the cutting temperature36   METALURGIJA 53 (2014) 1, 33-36W. GRZESIK et al.: INVESTIGATION OF THE MACHINING PROCESS OF SPHEROIDAL CAST IRON USING CUBIC...the minimum value of 430 °C at the feed of 0,12 mm/rev. This fact can be explained in terms of the corresponding values of the speciﬁ  c cutting energy, shown in Figure 3. In fact, for the feed of about 0,12 mm/rev the value of ec approaches 1,9 GJ/m3. It can also be noticed that the computation accuracy depends on the errors in the deter-mination of SCI emissivity versus temperature.Surface roughness and surface topographyFigure 6 presents the inﬂ  uence of the cutting speed on the surface roughness average Ra. For the cutting speeds of 240 - 480 m/min and feed rate of f = 0,08 mm/rev the Ra value decreases from 0,6 to 0,38 μm. The maximum value of Ra = 1,34 μm was measured for feed of 0,08 mm/rev and cutting sped of vc = 100 m/min, which is typically observed in machining practice [1]. At a higher feed rate of f = 0,12 mm/rev, the Ra param-eter varies in the range of Ra = 1,1 - 0,83 μm. In this case, the trend showing lower surface roughness with higher cutting speed is less pronounced.In addition, Figure 7 shows surface topography ob-tained at the maximum cutting speed of 400 m/min and low feed rate of 0,08 mm/rev. In general, surfaces with regular asperities and good bearing properties (negative Ssk values) are generated. At higher feed rate of 0,12 mm/rev small craters resulting from the removal of graphite inclusions are observed.CONCLUSIONS1)   The cutting and feed forces decrease when ma-chining with higher cutting speeds and increase non-linearly when feed rate increases. The local minimum of the speciﬁ  c cutting energy corre-sponds to the minimum value of feed force.2)   The variations of the average tool-chip contact temperature under machining conditions em-ployed are not so distinctive as in the steel ma-chining. Maximum temperature in the cutting zone of 510 °C was recorded for f = 0,24 mm/rev and vc = 240 m/min. On the other hand, the mini-mum temperature of 410 °C corresponds to cut-ting speed of vc = 100 m/min.3)   SCI machining with high cutting speeds visibly improves the surface ﬁ  nish. In this study, the min-imum value of Ra roughness parameter of about 0,4 μm was recorded for vc = 400 min and f=0,08 mm/rev. Under these machining conditions de-fect-free surfaces with regularly distributed feed marks are produced. REFERENCES[1]  Grzesik W.: Advanced machining processes of metallic materials. Elsevier, Amsterdam 2008.[2]  Graham D.: Machining cast iron. Manufacturing Enginee-ring, 136(2006) 2, www.sme.org.[3]  Klocke F., Klöpper C., Lung D., Essig C.: Fundamental Wear Mechanisms when Machining Austempered Ductile Iron (ADI). Annals of the CIRP, 56(2007) 1, 73-76.[4]  Małecka J., Grzesik W., Kowalczyk D., Kiszka P.: Image analysis of wear effects when machining nodular cast iron with silicon nitride tools. Tribologia, 42(2011) 2, 71-84[5]  Schneider J., Richter G.: Eine Keramik für viele Fälle. Werkstatt und Betrieb, 139(2006) 6, 51-55.[6]  Byrne G., Dornfeld D., Denkena B.: Advancing cutting technology, CIRP Annals-Manufacturing Technology, 52(2003) 2, 483-507.[7]  Machining Handbook, Sandvik Coromant, 2010.Note: The responsible for English language is the lecturer from Opole University 100 200 300 400 500Cutting speed / m/min00,40,81,21,6Surface roughness Ra / µmf = 0,08 mm/revf = 0,12 mm/revFigure 6   Infl  uence of cutting speed on the roughness average RaFigure 7   Surface topography at diff  erent magnifi  cations generated with vc = 400 m/min, f = 0,08 mm/rev and rε = 0,8 mm. Sa = 0,49 µm, Sz = 13,2 µm, Sp = 5,43 µm, Sku = 7,82, Ssk = -0,67

Investigation of the machining process of spheroidal cast iron using cubic boron nitride (CBN) tools

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