j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104

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Hard machining of hardened bearing steel usingcubic boron nitride tool

Mohamed Athmane Yallesea, Kamel Chaouib,∗, Nassereddine Zeghibb,Lakhdar Boulanouarb, Jean-Francois Rigal c

a Laboratoire de Mecanique et des Structures (LMS), 8 May 1945 University, P.O. Box 401, Guelma 24000, Algeriab Laboratoire de Recherche Mecanique des Materiaux et Maintenance Industrielle (LR3MI), Badji Mokhtar University,P.O. Box 12, Annaba 23000, Algeriac Laboratoire de Mecanique des Contacts et des Solides (LaMCoS) INSA of Lyon, 20 Albert Einstein Avenue,69621 Villeurbanne Cedex, France

a r t i c l e i n f o

Article history:

Received 11 July 2007

Received in revised form

7 March 2008

Accepted 15 March 2008

Keywords:

Hard turning

Hardened steel

Cubic boron nitride

Wear

Roughness

a b s t r a c t

In many cases, hard machining remains an economic alternative for bearing parts fabrica-

tion using hardened steels. The aim of this experimental investigation is to establish the

behaviour of a CBN tool during hard turning of 100Cr6-tempered steel. Initially, a series

of long-duration wear tests is planned to elucidate the cutting speed effects on the vari-

ous tool wear forms. Then, a second set of experiments is devoted to the study of surface

roughness, cutting forces and temperature changes in both the chip and the workpiece. The

results show that CBN tool offers a good wear resistance despite the aggressiveness of the

100Cr6 at 60HRC. The major part of the heat generated during machining is mainly dissi-

pated through the chip. Beyond 280 m/min, the machining system becomes unstable and

produces significant sparks and vibrations after only a few minutes of work. The optimal

productivity of machined chip was recorded at a speed of 120 m/min for an acceptable tool

flank wear below 0.4 mm. Beyond this limiting speed, roughness (Ra) is stabilized because

of a reduction in the cutting forces at high speeds leading to a stability of the machining

Cutting forces system. The controlling parameter over roughness, in such hard turning cases, remains

tool advance although ideal models do not describe this effect rationally. Surface quality

obtained with CBN tool significantly compared with that of grinding despite an increase in

the advance by a factor of 2.5. A relationship between flank wear (VB) and roughness (Ra) is

deduced from parametric analysis based on extensive experimental data.

tools is a very complicated phenomenon as contact surfaces

1. Introduction

During the process of material cutting, complex and mutualinteractions are created between tool and workpiece at the

contact surface. Consequently, in this system, significantforces and high temperatures are recorded causing wear andsometimes breakage of the tool. Usually, such conditions lead

∗ Corresponding author. Tel.: +213 38 87 11 09; fax: +213 38 87 11 09.E-mail address: chaoui@univ-annaba.org (K. Chaoui).

0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2008.03.014

© 2008 Elsevier B.V. All rights reserved.

to the damage of both contact surfaces and reduce the pre-cision on the geometrical shapes or modify the mechanicalcharacteristics. It is well known that wear process of cutting

of the system are subjected to physical and chemical changeswhich contribute to the progressive destruction of the activepart surface layers. Moreover, wear also affects geometrical

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h

Nomenclature

Nomenclatureap depth of cut (mm)C1, C2 constantsCBN cubic boron nitridee, exp exponential operatorf feed rate (mm/rev)Fa axial force (N)Fr radial force (N)Fv tangential force (N)HB Brinell hardnessHRC Rockwell hardnessK, ˛ fitting constants for roughness modelKT crater wear (mm)[KT] allowable crater wear (mm)R2 determination coefficientRa Arithmetic mean roughness (�m)Rath theoretical average roughness (�m)Rt total roughness (�m)Rtth theoretical total roughness (�m)Rz mean depth of roughness (�m)r, R� nozzle radius (mm)T tool life (min)t cutting time (min)t◦ cutting temperature (◦C)VB flank wear (mm)[VB] allowable flank wear (mm)Vc cutting speed (m/min)x1, x2, x3, constants describing cutting forcesy1, y2, y3, y4 constants describing roughness

Greek symbols˛c clearance angle (◦)�r cutting edge angle (◦)� rake angle (◦)

psls

vsutbmaaitmat(d

� cutting edge inclination angle (◦)

arameters of the tool, generated heat quantity, cutting pres-ures and induced compressive surface residual stresses, toolifetime and the micro-geometrical precision of the machinedurface.

Current technical progress helped to develop the use ofery hard materials associated with difficult machinability ineveral industrial domains of equipment and spare parts man-facture. Although the behaviour of the conventional cuttingools is limited to particular conditions, high-speed steels, car-ides and cermets tools remain largely used for turning theajority of materials. Whenever hardness is increased, such

s in the case of quenched steels or hardened cast irons as wells nickel-based refractory alloys and some metallic compos-te materials, it becomes necessary to use appropriate cuttingools, which respond to these new requirements. Therefore,

aterials having a significant abrasion resistance as well

s a raised hardness, need to be machined with super-hardools like cubic boron nitrides with polycrystalline structurePCBN). Huang et al. (2006) presented a thorough review whichiscusses CBN material tool microstructure, wear patterns

n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104 1093

involved and CBN tool wear rate modelling under hard turning.They also concluded that assessments of the cutting tool con-dition based on tool geometry, cutting regimes and the natureof wear in terms of VB and KT need better understanding.

Chou et al. (2002) experimentally investigated the perfor-mance and wear behaviour of different CBN tools in finishturning of hardened AISI 52100 steel (DIN 100Cr6). In this study,it was established that low CBN content materials provide thebest performance in hard turning in terms of tool life andsurface finish. In the machining of hard light titanium alloys,Nabahani (2001) found that PCBN tools showed reduced toolflank wear and delivered a good surface quality compared tothe various carbide tools. The failure of these tools is the resultof plastic deformations under combined significant mechan-ical and thermal stresses in the vicinity of the cutting edgeand the low-wear rate of the PCBN is primarily attributed toa reduced chemical reactivity in contact of titanium alloys.Arunachalam et al. (2004) studied both residual stress andsurface finish generated during facing of Inconel 718 steelusing CBN and mixed ceramic cutting tools as a function ofspeed, depth of cut, coolant and tool geometry. It is found thatmixed ceramic cutting tools induce tensile residual stresseswith a much higher magnitude than CBN tools. The residualstresses and the surface roughness obtained when using CBNcutting tools are more sensitive to cutting speeds than depthof cut. The use of coolant resulted in either compressive resid-ual stresses or lowered the magnitude of the tensile residualstresses, whereas dry cutting always was associated with ten-sile residual stresses. Thiele et al. (1999) presented results ofan experimental study on the effect of cutting edge geome-try and workpiece hardness on residual stresses in finish hardturning of 100Cr6 steel. They concluded that both factors aresignificant for surface integrity of finish hard turned compo-nents.

Today, it is established that the wear resistance of the CBNis improved mainly because of the much reduced solubilityof boron. Whenever the mechanisms of dissolution–diffusionprevail, the performance of CBN tools becomes more inter-esting than that of coated carbides and mixed ceramics asshown by Yallese et al. (2005). Alternatively, Banga and Abrao(2003) found that cutting speed is the factor which most effectstool life when turning hardened 100Cr6 and PCBN cutting toolsprovide longer tool life than mixed and composite ceramics.The superiority of CBN tools for hard materials machining wasillustrated in the study performed by Lima et al. (2005) on theturning of AISI 4340 (48 HRC) steel using PCBN and coatedcarbides tools. Previously, Luo et al. (1999) studied the wearbehaviour in hard turning of the same alloy steel by CBN andceramic tools and they found that the flank wear was reducedas work material hardness increased up a critical value of 50HRC. In addition, wear mechanisms by diffusion, abrasion andadhesion were discussed by Poulachon et al. (2003) and usuallyit is concluded that these mechanisms are prevalent duringwear process of CBN tools. The major influencing factor onthe tool wear is the presence of various carbides in the steelmicrostructure. The hardness of these carbides varies signif-

icantly, causing different wear rates when turning 100Cr6,X155CrMoV5, X38CrMoV5 and 35NiCrMo16 steels. In thesecases, the flank wear on the tool has resulted in grooves causedby the major abrasive action of carbides. Chou (2003) stated

1094 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e

Table 1 – Chemical composition of 100Cr6 steel

% C % Mn % Si % P % S % Cr % Ni

1.05 0.38 0.21 0.03 0.028 1.41 0.21

that high CBN content tools with metallic binder are recom-mended for roughing, but low CBN content tools with ceramicbinder are more appropriate for finishing. Finally, Lahiff etal. (2007) concluded that different theories exist regardinginvolved tool wear mechanisms; however, there is a generalagreement among researchers that PCBN tool wear is com-plex and no single mechanism alone provides a satisfactorilyexplanation. It is also agreed that abrasion makes a significantcontribution to flank wear (VB) that is caused by hard carbideparticles and martensite in workpiece and also by CBN grainsissued from the cutting tool.

The aim of this investigation, which comprises of two parts,is to establish the machining conditions under hard turning of100Cr6 steel using a cubic boron nitride tool. In the first part,tool wear evolution is observed by optical microscopy in orderto identify the various wear patterns as a function of vari-ous cutting speeds, whereas in the second part, a roughnessstudy is conducted on the machined part according the cut-ting regime parameters. The experimental results are used todraw relationships which govern flank wear, roughness crite-rion and cutting forces under hard turning.

2. Experimental procedure

Experiments devoted to wear analysis were carried out inaccordance with long-duration wear tests as stated by stan-dard ISO 3685 in order to evaluate the lifetime of CBNtools at different cutting speeds. Alternatively, roughness andcutting forces tests were realized according to experimentplanning using both unifactorial and multifactorial meth-ods. Machining was performed on 100Cr6 (AISI 52100) steelrods. Specimen diameter and length were, respectively, 80 and400 mm, respectively, whereas for roughness experiment onlyhalf of the length was considered. Because of its high wearresistance, 100Cr6 steel is especially recommended for themanufacture of various dies, profiling rollers, balls and bearingcages. It is also employed in cold working of forming matri-ces, of profiling cylinders and for wear coating purposes. Itschemical composition is given in Table 1. The steel hardnessis increased from 285 HB to 60 HRC (the Vickers hardness from260 to 710 daN/mm2) using a quenching treatment at 850 ◦Cfollowed by tempering at 220 ◦C as suggested by the supplier’s

recommendations. A 6.6 kW Tos Trencin lathe type SN40 wasused for the turning operations. Cutting inserts are removableand offered eight squared working edges. The chosen CBN toolis commercially known as CBN7020 and it is essentially made

Table 2 – Physical properties of CBN

Material Hardness HV (daN/mm2) Tenacity (MPa m1/2) Y

CBN 2800 4.2

c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104

of 57% CBN and 35% Ti(C,N). Its ISO designation is SNGA1204 08 S01020 CB7020 and was manufactured by Sandvik. Thephysical properties of the CBN7020 tool are summarized inTable 2. Tool holders are codified as PSBNR2525K12 with a com-mon active part tool geometry described by �r = +75◦; ˛c = +6◦;� = −6◦ and � = −6◦.

Wear follow-up was achieved using an optical Hund(W-AD) microscope equipped with a digital display and acolour charge-coupled device camera, enabling a precision of0.001 mm to be obtained. The cutting forces in X, Y and Zdirections were recorded using a standard quartz dynamome-ter (Kistler 9257B) allowing measurements from −5 to 5 kN.Instantaneous roughness criteria measurements (Ra, Rt, andRz) for each cutting condition were obtained from a Surftest301 Mitutoyo roughness meter coupled with a profile printer.It consists of a diamond point (probe) with a 5 �m radius andmoves linearly on the working surface. The length examinedis 4.0 mm with a basic span of 0.8 mm. The measured values ofRa are within the range 0.05 to 40 �m while for Rt and Rz, theylay between 0.3 and 160 �m. Roughness measurements weredirectly obtained on the same lathe without disassembling theturned part in order to reduce uncertainties due to resump-tion operations. The measurements were repeated 3 times outof 3 generatrices equally positioned at 120◦ and the result isan average of these values for a given machining pass. Errormagnitude is globally estimated around 10% for the obtaineddata and between 15 and 20% for very low roughness valueswhich are in agreement with the instrument characteristicsand the experimental conditions. A Rayner 3I infrared pyrom-eter was used to determine the temperatures of the chip, part,and cutting tool from a remote data acquisition system. Thispyrometer is designed to measure temperatures within therange from −30 to 1200 ◦C and covers emissivities range from0.10 to 1.00. The cutting conditions adopted for the wear androughness experiments are illustrated in Table 3. The weartests were carried out without lubrication at a feed rate (f) of0.08 mm/tr, a depth of cut (ap) of 0.5 mm and a varying cut-ting speed (Vc) from 90 to 350 m/min. On the other hand, thefollowing intervals were selected for both roughness and cut-ting forces experiments: 0.08 < f < 0.24 mm/tr; 0.1 < ap < 1.0 mmand 60 < Vc < 350 m/min. The wear behaviour of the CBNwas assessed on the basis of allowable flank wear limits of[VB] = 0.30 mm and [KT] = 0.15 mm. Some wear and roughnessvalues are obtained beyond these limits in order to observethe complete behaviour of the tool under extreme conditions.

3. Results and discussion

The various combinations of the cutting regime parameters(Vc, f and ap) are used for specific correlations as shownin Table 3. Such correlations have already been tested forroughness measurements and the simulated predictions were

oung’s modulus (GPa) Density (g/cm3) Grain size (�m)

570 4.3 2.5

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104 1095

Table 3 – Roughness and cutting force as a function of the experimental plan

Parameters Criteria/values

Codified values Actual values Roughness Cutting force

Test X1 X2 X3 f (mm/rev) ap (mm) Vc (m/min) Ra (�m) Rz (�m) Rt (�m) Fr (N) Fa (N) Fv (N)

1 −1 −1 −1 0.08 0.2 90 0.23 1.4 1.9 132.2 71.1 106.32 +1 −1 −1 0.2 0.2 90 1.08 4.9 5.2 166.5 87 1573 −1 +1 −1 0.08 0.6 90 0.27 1.6 2.1 216.3 222.3 230.54 +1 +1 −1 0.2 0.6 90 1.05 5.2 5.8 302.1 274 3855 −1 −1 +1 0.08 0.2 180 0.18 1.1 1.4 111.5 66.1 90.5

000

stI(sa

F

R

wteiu

3

FwIw5fp

F4

6 +1 −1 +1 0.2 0.2 187 −1 +1 +1 0.08 0.6 188 +1 +1 +1 0.2 0.6 18

atisfactory. In this case, an extension of the model is madeo describe the cutting forces data using a similar procedure.n other words, these correlations concern the cutting forceF) represented by (Fr, Fa and Fv) and the roughness (R) repre-ented by (Ra, Rt and Rz) in the general following forms whichre used in the work of Yallese (2005):

= C1 f x1 ax2p Vx3

c (1)

= C2 f y1 ay2p Vy3

c (2)

here C1, C2, x1, x2, x3, y1, y2 and y3 are constants experimen-ally assessed. Depending on the sign and the value of eachxponent (xi or yi), the importance of each cutting parameters deduced which helps understanding the basic effect thatnderlines the optimum machining conditions.

.1. Tool wear

ig. 1a–d shows the morphology of flank (VB) and crater (KT)ears as a function of time for a cutting speed of 90 m/min.

nitially, flank wear develops according to a regular band

hich widens with cutting time and becomes irregular beyond

0 min. Wear according to the surface attack begins in theorm of a small crater which extends until it reaches thereceding flank wear. At this time and in the vicinity of the

ig. 1 – Flank and crater wear micrographs at Vc = 90 m/min, f = 00 min, (c) 56 min and (d) 80 min.

0.88 4.1 4.8 144.5 83.1 126.40.21 1.3 1.5 206.4 202.2 215.40.90 4.5 5.2 280.6 247.7 356.4

cutting edge, a significant wear in the form of cavity appearsas shown in Fig. 2 at 120 m/min. This state of damage weak-ens the tool nozzle leading progressively to its final collapseand this observation is common and foreseeable for CBN toolsfor subsequent speeds. At a much higher speed of 280 m/min,the degradation of the CBN insert according to its both con-tact surfaces becomes intense and catastrophic as shown inFig. 3 and tool nozzle deterioration is definitely visible afteronly 7 min of machining (Fig. 3b). At 15.5 min chipping tookplace in both wear directions and extended in the diagonalpart of the insert forming a rather deep crucible at the noz-zle tool (Fig. 3d). On the nozzle contour, the fracture processdeteriorated the chamfer reinforcement of the cutting edge,whereas on the opposite side, it reached the limit of the CBNinsert.

Under extreme conditions of cutting speed, i.e. at350 m/min, the effect on wear morphology of is more pro-nounced as indicated in Fig. 4. The collapse of the cuttingedge following the junction of the two wear forms (VB) and(KT) appeared as of the first working minute (Fig. 4a). As cut-ting time elapsed, general wear accompanied by a series ofchippings on the different tool faces developed very quickly

and lead to total collapse of the CBN insert within few min-utes. As a result, working at high cutting speeds, in particularbeyond 280 m/min, the wear phenomenon turn out to be veryfast reducing drastically tool life and negatively influencing

.08 mm/rev and ap = 0.5 mm. Machining times: (a) 8 min, (b)

1096 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104

Fig. 2 – Final aspect of crater wear: (a) at beginning of machining, (b) after 80 min of machining at Vc = 120 m/min,f = 0.08 mm/rev, ap = 0.5 mm.

, f =

Fig. 3 – Flank and crater wear micrographs at Vc = 280 m/min(b) 7 min, (c) 9 min and (d) 15.5 min.

workpiece surface quality. Under such conditions, tool wearevolves according to several mechanisms. As an indication,

both abrasion and diffusion wear mechanisms are generallyobserved as illustrated for the results obtained by Thiele andMelkote (1999). The first is due to the friction phenomenonat the interfaces associated to high specific pressures which

Fig. 4 – Flank and crater wear micrographs at Vc = 350 m/min, f =(b) 2.5 min, (c) 4.5 min and (d) 7.5 min.

0.08 mm/rev and ap = 0.5 mm. Machining times: (a) 1.5 min,

are the result of the interaction between machined materialhardened phases and the components of the tool. When cut-

ting speeds are high, the interface temperature rises quicklyand allows a softening of the tool material, thus engagingthe diffusion mechanism that favours chemical reactions. Thecombination of these two mechanisms accelerates surface

0.08 mm/rev and ap = 0.5 mm. Machining times: (a) 1.5 min,

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104 1097

Fc

cc

Ct11tm(3

Fc

ig. 5 – CBN flank wear (VB) evolution as a function ofutting time and speed.

hipping process of the active tool part and degradation of theutting edge.

Figs. 5 and 6 illustrate the evolution of VB and KT of theBN as a function of cutting time for speeds ranging from 90

o 350 m/min. VB and KT are rather regular and slow at 90 and20 m/min. In both cases, wear increased in a notable way after20 m/min but KT increase rate is relatively low compared to

hat of VB, comforting the fact that VB is considered as the

ain criterion for tool life assessment. The allowable tool wear[VB] = 0.3 mm) is reached in 35 min at 180 m/min whereas at50 m/min the lifespan is only 5 min.

ig. 6 – CBN crater wear (KT) evolution as a function ofutting time and speed.

Fig. 7 – Tool life of CBN at various cutting speeds.

3.2. Tool life

Generally, tool behaviour is influenced by several factors mostimportantly the cutting speed. Speed impact on tool lifespanfor both allowable wear criteria (i.e. [VB] = 0.3 or 0.4 mm) canbe deduced from Fig. 5. The idea of extending the limit valueof [VB] to 0.4 mm is supported by microscopic observations ofan acceptable surface quality even beyond such a limit whichcould be economical if the requirements towards dimensionalaccuracy and surface finish would allow it. For these 2 limitingwear criteria, tool life is extended from 25% to 61% between90 and 220 m/min whereas, for Vc ≥ 220 m/min, it increased bya factor of 1.5 (Fig. 7). For the criterion [VB] = 0.3 mm, speedeffect can be interpreted by comparing tool life ratios. Theratio T90/T120 is only 1.15 whereas T120/T180 is the double.For speeds above 220 m/min, such ratio reaches an importantvalue of 19.16 indicating a significant wear which is accen-tuated by higher temperatures. Consequently, between 280and 350 m/min, machining becomes unstable after just fewminutes as vibrations and continuous sparks make machin-ing thereafter dangerous and even impossible. Globally, it isdeduced that cutting speeds higher than 280 m/min shouldnot be recommended for CBN tools in this case because of thevery fast wear rate.

Results obtained from Fig. 5 allow tool life models to beestablished based on least square data analysis for the 0.3 and0.4 mm limit wear criteria. The latter as set according to thecutting speed as the main variable for industrial production.For the cutting speed interval considered in this study, tool lifecan be described by the following equations:

T〈[VB]=0.3 mm〉 = e14.65 V−2.18c (R2 = 0.997) (3)

T〈[VB]=0.4 mm〉 = e14.14 V−2.02c (R2 = 0.996) (4)

Eqs. (3) and (4) express quantitative relationship betweenthe life and cutting speed; on one hand, they make it possibleto calculate the cutting speed necessary for a preset tool life

and on the other hand, to optimize the machining conditionsin combination with the relevant cutting regime parameters.The determination coefficients (R2) are high enough implyinggood agreement with experimental results. On the industrial

1098 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104

Fig. 8 – Machined chip volume at different cutting speeds.

level, the influence of the cutting speed on the productivity,expressed in total machined chip volume is shown in Fig. 8.This volume is calculated according to the following equation:

Volume = Vc f ap T (5)

It can be concluded that the optimal cutting speed is forthe system CBN-100Cr6 (60HRC) is 120 m/min for the two wearlimit criteria. At 0.3 and 0.4 mm values of [VB], the tool cuttingedge could produce 336 and 504 cm3 of chip during 80 and105 min, respectively (Fig. 8).

3.3. Cutting forces

Tools wear in particular during the machining of hardenedmaterials, takes place under very severe conditions of frictionindicating appreciable local cutting forces. It is known thatthe basis for the determination of the cutting forces is theproduct of the material removal rate by the cutting specificenergy. Indeed, in Kitagawa et al. (1997), the force magni-tude in contact surfaces for machine components is very low(in the order of few MPa) with a surrounding temperaturebelow 100 ◦C, whereas in machining contacts it may attainabout 1000–2000 MPa with a temperature ranging from 100 ◦Cto above 1000 ◦C.

For the given machining system (CBN-100Cr6), the evo-lution of the cutting forces (F) according to the cuttingparameters (i.e. Vc, f and ap) is presented in Fig. 9a–c. Anincrease in the cutting speed leads to a gradual decrease forthe 3 components of the cutting forces (Fr, Fa and Fv). This isdue to temperature rise (more heat is generated) in the cuttingarea, which softens the metal and thus requires less frictionforces (Fig. 9a). It should be noted that the cutting conditionsemployed did not favour any material adherence on tool cut-ting edge. It is observed that the forces (N) decreased quickly inthe interval 60–180 m/min, Fr, Fa and Fv are reduced by 18.40%,22.31% and 23.72%, respectively; however, these drops are only11.63%, 9.47% and 7.18% between 180 and 280 m/min. When

turning 100Cr6-tempered steel with a cut of depth of 0.2 mm,the radial force (Fr) becomes dominant when compared to theother components of F. This can be explained by the importantstresses exclusively applied on the rounded part of the tool

Fig. 9 – (a) Speed effect on cutting forces at ap = 0.2 mm;f = 0.08 mm/rev. (b) Feed rate effect on cutting forces atVc = 120 m/min; ap = 0.2 mm. (c) Depth of cut effect oncutting forces at Vc = 120 m/min; f = 0.08 mm/rev.

t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104 1099

nTftapi5iFFdabbtff

3

ItipeltsfciOdaotcatfiod

3

HbdOgemoFtatt

j o u r n a l o f m a t e r i a l s p r o c e s s i n g

ozzle which has a radius (R�) of 0.8 mm (i.e. ap is only 1/4 R�).he influence of the feed rate (f) is given in Fig. 9b as the cutting

orces increase with f since the sheared chip section is propor-ional feed rate. In all feed-rate experiments, the radial force islways dominating but closely followed by the tangential com-onent. Practically, the increase in (f) from 0.08 to 0.16 mm/rev

ncreased the components Fr, Fa and Fv, respectively, by 33%,1% and 58%. On the other hand, from 0.08 to 0.24 mm/rev, thencrease is, respectively, of 50%, 65% and 77% (Fig. 9b). Finally,ig. 9c shows a quasi-linear increase of the components ofwith growing depth of cut (ap) due to expanded volume ofeformed metal during the cutting process. At lower values

p (up to 0.3 mm) Fr is dominating but at 0.4 mm and above,oth tangential and axial forces exceed the radial one. It cane understood that at low ap values, cutting is mainly done byhe rounded tool nozzle part and at higher ap, metal removal isulfilled outside nozzle limit radius, which produces resistingorces in both tangential and axial directions.

.4. Roughness evolution

n order to study machined surface quality, roughness charac-erization is limited to the 3 criteria (Ra, Rz and Rt). Fig. 10a–cllustrates effects of the cutting regime (Vc, ap and f) on work-iece roughness. Basically, Vc improves the surface qualityspecially for speeds up to 120 m/min (Fig. 10a). Above thisimit, roughness is stabilized because of the drop in the cut-ing forces that translate a relative stability of the machiningystem. Alternatively, an increase of f or ap deteriorates sur-ace quality with f noted as a determinative factor (Fig. 10b and). The generated surface comprises helicoid furrows result-ng from the tool shape and the form of tool-part movements.bviously, the printed grooves in the hardened material areeeper and broader as the feed rate is higher implying to workt the lower values of f for surface finish. Importantly, in spitef varying f with a factor of 2.5, the resulting surface quali-ies obtained with CBN remain always acceptable and can beompared with those of grinding as also found by Remadnand Rigal (2006). Consequently, the cutting regime effects onhe measured absolute roughness (Ra) indicated that changingfrom 0.08 to 0.2 mm/rev and ap from 0.2 to 1.0 mm resulted

n roughness increase by factors of 5.25 and 1.45, respectively,therwise, an augmenting Vc from 60 to 180 m/min caused arop by a factor of 1.6 times.

.5. Cutting temperature changes

ard machining of 100Cr6 steel by CBN is characterizedy flow of chips at very high temperature and extremelyeformed signs of intensive shearing at cutting edge (Fig. 11).bservations and measurements confirmed that most of theenerated heat is dissipated by the intermediate chips as theyxperienced intensive frictions forces and high plastic defor-ations for a volume relatively small compared to the tool

r the workpiece as noted by O’Sullivan and Cottrell (2001).ig. 12 presents recorded temperature evolution according to

he machining time for various cutting speeds. At 90 m/minnd for a duration of 8 min, the maximum temperatures forhe chip, the tool and the part are 574, 95 and 46 ◦C, respec-ively (Fig. 12a). Chip temperature started stabilizing around

Fig. 10 – (a) Cutting speed effect on roughness atap = 0.2 mm and f = 0.08 mm/rev. (b) Depth of cut effect onroughness at Vc = 90 mm and f = 0.08 mm/rev. (c) Feed rateeffect on roughness at Vc = 90 mm and ap = 0.2 mm.

1100 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104

chip

possible to calculate the theoretical maximum furrow depthdrawn on the machined surface. In turning conditions and foran ideal geometrical tool profile, total surface roughness Rt

for f too small compared to nozzle radius (r), the following

Table 4 – Models for roughness criteria as separatefunctions of feed rate, cutting speed and depth of cut

Variable Model R2

Feed rate (mm/rev)Ra = 16.161. f1.667 0.976Rt = 53.333. f1.353 0.971Rz = 52.712. f1.423 0.967

Cutting speed (m/min)Ra = 2.018 V−0.469

c 0.949Rt = 10.825 V−0.408

c 0.979−0.407

Fig. 11 – Heat dissipation through

3 min of machining and similar trends are observed for othercutting speeds (Fig. 12b and c). Indeed, at 180 and 350 m/minand for shorter times (4 and 2 min), the maximum chip tem-perature reached 620 and 870 ◦C, respectively, while tool andworkpiece temperatures varied little (Fig. 12b and c). The ratioof chip to workpiece temperatures (t◦chip/t◦wp) may be as highas 16 times. Consequently, even under hard-turning condi-tions, heat dissipation through the workpiece remains toosmall. It should be emphasized that Vc, VB, T (tool life) and t◦

(temperature) are closely interdependent during hard turningowing to the fact that a change in Vc involves a change in tem-peratures (chip and tool) and a heat diffusion within the toolprobably causing variations of its mechanical characteristicsvis-a-vis wear processes. Cutting-temperature evolutions canprovide valuable information on thermal effects in relation tofinal workpiece characteristics.

3.6. Correlations according to wear

In this section, it is intended to correlate both cutting forcesand roughness with resulting wear behaviour. Fig. 13 illus-trates the evolution of the cutting forces according to VB. Itis noted that as wear evolves with cutting times, the corre-sponding forces show a continuous increase because of largercontact surfaces between the part and the tool. Globally, cut-ting forces describe 3 zones: (i) the first 10 min, (ii) between10 and 22 min and (iii) from 22 up to 32 min. In zone (i), VBreached 0.175 mm and the corresponding % increases in termsof F components (Fr, Fa, Fv) are 33.3%, 25.1% and 9.5%, respec-tively. Subsequently in zone (ii), VB attained the allowablewear (i.e. [VB]) of 0.3 mm with a somewhat lower rate andcaused Fr, Fa, Fv to increase, respectively, by 11.1%, 16.2%, and16.0%. Finally, when VB increases from 0.3 to 0.41 mm, Fr, Fa

and Fv reached 50%, 26.3% and 10.3%, respectively. During the32 min of machining at Vc = 180 m/min, f = 0.08 mm/rev andap = 0.5 mm, the corresponding changes in Fr, Fa, and Fv aresuccessively 150%, 135% and 52%.

Using the multifactorial method, the 8 constants of theparametric Eqs. (1) and (2) are obtained from statistical leastsquare analysis:

Fv = e7.97f 0.48a0.81p V−0.19

c (R2 = 0.990) (6)

Fr = e7.18f 0.31a0.54p V−0.16

c (R2 = 0.992) (7)

while machining, Vc = 220 m/min.

Fa = e7.01f 0.23a1.02p V−0.12

c (R2 = 0.980) (8)

Ra = e4.15f 1.62a0.07p V−0.32

c (R2 = 0.990) (9)

Rt = e5.17f 1.23a0.08p V−0.30

c (R2 = 0.980) (10)

Rz = e5.18f 1.36a0.10p V−0.28

c (R2 = 0.990) (11)

where ‘e’ designates the exponential operator.These equations may be employed to predict, for machin-

ing conditions within the limits of the constructed models,both roughness and cutting forces. As expected, tool feed rateremains the preponderant factor for roughness criteria assess-ment as the associated exponent; i.e. y1 > 1 in Eq. (2). It isinteresting to rationalize Eq. (2) by the following relation:

R = C3 f y4 (12)

with C3 and y4 specific constants and R indifferently represent-ing Ra, Rt or Rz. This approach was applied to the experimentaldata and a computer-processing program allowed writing thecorrelations in the form of power functions as indicated inTable 4 for different variables.

For roughness study, knowing f and tool geometry, it is

Rz = 9.043 Vc 0.949

Depth of cut (mm)Ra = 0.333a0.158

p 0.974Rt = 2.451a0.205

p 0.942Rz = 2.109a0.203

p 0.985

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104 1101

Fig. 12 – Temperature evolution as a function of time andcutting speed at (a) Vc = 90 m/min, (b) Vc = 180 m/min and (c)Vc = 350 m/min.

Fig. 13 – Cutting force evolutions as a function of flankwear; Vc = 180 m/min; f = 0.08 mm/rev; ap = 0.5 mm.

equation is derived:

Rt(Theoretical) = f 2

8r(13)

Eqs. (10), (12) and (13) are used to calculate total rough-

ness values and to compare them with experimental data.Fig. 14 summarizes the results of the various approachesof total roughness according to feed rate. The parametricmodels represented by Eqs. (10) and (12) give satisfactory cor-

Fig. 14 – Comparison of predicted, experimental andtheoretical values of Rt as function f at Vc = 90 m/min andap = 0.2 mm.

g t e

1102 j o u r n a l o f m a t e r i a l s p r o c e s s i n

relations based on experimental results for 90 m/min andap = 0.2 mm. The one parameter model exhibits a good agree-ment with both measured experimental data and analyticalEq. (13). On the other side, the multi-parameter model inte-grating cutting regime conditions remains of unquestionableutility as it considers at the same time the effect of Vc, ap

and f.During the wear tests, the corresponding surface quality is

recorded in order to establish VB effect on roughness. Fig. 15illustrates this evolution for 4 cutting speeds (90; 120; 180 and220 m/min). The analysis concludes that any increase in VBimplies some deterioration of the surface quality; however, itshould be specified that, as long as wear is regular and does notexceed [VB] = 0.3 mm, roughness (in particular the criterion Ra)

evolves very slowly since Ra does not exceed the 0.5 �m at 90and 120 m/min (Fig. 15a and b). For higher speeds, roughnessis subjected to a relative increase, but it remains acceptable(Ra < 1.2 �m) as indicated in Fig. 15c and d. Again, the choice

Fig. 15 – Cutting speed effect on roughness evolution as a functiVc = 90 m/min, (b) Vc = 120 m/min, Vc = 180 m/min and (d) Vc = 220

c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104

of the standard criterion for wear limit ([VB] = 0.3 mm) is welladapted to describe in a suitable way the CBN tool lifespan.

In order to explain the changes in roughness behaviouraccording to CBN wear, optical micrographs of the tool noz-zle were taken at wear levels of 0.11 mm (time t1), 0.31 mm(time t2) and 0.47 mm (time t3) and for 220 m/min cuttingspeed (Fig. 15d). At t1 = 7 min, the tool wear bandwidth istoo small which leads to an almost constant roughness(Ra = 0.29 �m). This state is associated with the beginningof regular wear characterized with an abrasion mechanism.However, at t2 = 25 min, the allowable wear limit is reachedresulting in an abrupt roughness increase (Ra = 0.54 �m). Atthis stage, tool wear is experiencing a commencement of cut-ting edge deterioration and announcing relatively significant

effects of the generated heat. Finally, for important wear lev-els (t3 = 38 min), tool collapse is catastrophic which increasescontact surface between the tool and the workpiece and thus,intensifying heating as well as cutting forces. Dominating dif-

on of flank wear for f = 0.08mm/rev; ap = 0.5 mm: (a)m/min.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h

Table 5 – Roughness (Ra) models as a function of flankwear [VB] = 0.3 mm

Vc (m/min) Model R2

90 Ra = 0.241 e2.80VB 0.973120 Ra = 0.204 e2.11VB 0.953

1.67VB

fm

acftDc

R

TiVaItt

4

Tc

(

(

(

(

r

180 Ra = 0.258 e 0.969220 Ra = 0.229 e2.9VB 0.865

usion phenomenon results in a significant degradation of theachined surface as Ra equals 1.18 �m.To understand the interaction, which governs tool wear

nd workpiece roughness, it is judicious to establish a relationonnecting Ra to VB. Although VB and Ra are associated to dif-erent parts, it is accepted that the one mutually influenceshe other and then, must obey an independent correlation.ata analysis from Fig. 15a–d permitted to develop the generalorrelation:

a = K exp[˛(VB)] (14)

he numerical values of the constants K and ˛ are summarizedn Table 5. This approach offers the possibility of controllingB using measured values of roughness from the workpiecend at the same time computing the remaining tool lifetime.n other words, for a given machining time, correlation (14)ranslates into a simpler way the tool behaviour data (difficulto obtain) via 2 constants (K and ˛).

. Conclusions

his experimental study allows to draw the main followingonclusions:

1) Hard turning of 100Cr6 steel with CBN inserts showeda satisfactory wear resistance for relatively high-cuttingspeeds. Speeds in the interval 280–350 m/min should beless recommended in industrial production because ofimportant wear rates. On the other hand, speeds between90 and 220 m/min can be taken as the most interest-ing cutting conditions for the system CBN7020-100Cr6.The optimal productivity was recorded at the speed of120 m/min for the both limit wear criteria.

2) The radial force component is dominating especially whenmachining is within the limit of tool nozzle radius. Suchfinding is in contradiction with what is known from con-ventional turning as Fr = (0.3–0.5) Fv. Consequently, theradial force cannot be neglected in characterizing staticand dynamic behaviours of such machining system.

3) For the 100Cr6 steel roughness, the machining surface is afunction of the local damage form and the wear profile ofCBN tool. When augmenting Vc, tool wear increases andleads directly to the degradation of the surface quality. Inspite of the evolution of wear up to the allowable limit

[VB] = 0.3 mm, Ra did not exceed 0.55 �m.

4) The major part of the generated heat at the cutting inter-face is mainly dissipated through the chip and thus,reducing substantially workpiece and tool temperatures.

n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104 1103

At Vc = 360 m/min, the ratio chip to workpiece tempera-tures is 16.

(5) In view of roughness measurements, hard turning maybe presented as a real alternative to substitute grindingoperations. The established advantages for such substitu-tion are materialized by a shorter production cycle and byworking on the same workstation.

(6) Roughness is largely influenced by the feed rate underhard-turning conditions although the theoretical modeldoes not describe rationally this effect. Therefore, theuse of parametric models may allow better descrip-tions of roughness phenomena as a function of variousfactors.

(7) A relation between VB and Ra in the form Ra = K e˛(VB)isproposed. Coefficients K and ˛ vary within the ranges0.204–0.258 and 1.67–2.90, respectively. It permits thefollow-up of tool wear from easily accessible work-piece roughness data. This is a very significant issuefor automated monitoring of industrial machiningprocesses.

Acknowledgements

This work was completed in the laboratories LMS (GuelmaUniversity, Algeria) and LR3MI (Annaba University, Algeria)in collaboration with LaMCos (CNRS, INSA-Lyon, France). Theauthors would like to thank the Algerian Ministry of HigherEducation and Scientific Research (MESRS) and the Dele-gated Ministry for Scientific Research (MDRS) for grantingfinancial support for two CNEPRU Research Projects–LMS: J-2401/03/80/06 (Guelma University) and LR3MI: J-2301/03/04/04(Annaba University). The authors are thankful to the MDRSwhose financial assistance made possible the acquisitionof the piezoelectric dynamometer and the microscopyequipment.

e f e r e n c e s

Arunachalam, R.M., Mannan, M.A., Spowage, A.C., 2004. Residualstress and surface roughness when facing age hardenedInconel 718 with CBN and ceramic cutting tools. Int. J. Mach.Tool. Manuf. 44, 879–887.

Banga, G.C., Abrao, A.M., 2003. Turning of hardened 100Cr6bearing steel with ceramic and PCBN cutting tools. J. Mater.Process. Technol. 143–144, 237–241.

Chou, Y.K., Chris, C.J., Barash, M.M., 2002. Experimentalinvestigation on CBN turning of hardened AISI 52100 steel. J.Mater. Process. Technol. 124, 274–283.

Chou, Y.K., 2003. Hard turning of M50 steel with differentmicrostructures in continuous and intermittent cutting. Wear225, 1388–1394.

Huang, Y., Chou, Y.K., Liang, Y.S., 2006. CBN tool wear in hardturning: a survey on research progresses. Int. J. Adv. Manuf.Technol. 35, 443–453.

Kitagawa, T., Kubo, A., Maekawa, K., 1997. Temperature and wear

of cutting tools in high speed machining of Inconel 718 andTi–6Al–6V–2Sn. Wear 202, 142–148.

Lahiff, C., Gordon, S., Phelan, P., 2007. PCBN tool wear modes andmechanisms in finish hard turning. Robot. Comput. Integr.Manuf. 23, 638–644.

g t e

1104 j o u r n a l o f m a t e r i a l s p r o c e s s i n

Lima, J.G., Avila, R.F., Abrao, A.M., Faustino, M., Paulo Davim, J.,2005. Hard turning: AISI 4340 high strength low alloy steel andAISI D2 cold work tool steel. J. Mater. Process. Technol. 169,388–395.

Luo, S.Y., Liao, Y.S., Tsai, Y.Y., 1999. Wear characteristics inturning high hardness alloy steel by ceramic and CBN tools. J.Mater. Process. Technol. 88, 114–121.

Nabahani, F., 2001. Wear mechanisms of ultra-hardware cuttingtools materials. J. Mater. Process. Technol. 115, 402–412.

O’Sullivan, D., Cottrell, M., 2001. Temperature measurement in

single point turning. J. Mater. Process. Technol. 118, 301–308.

Poulachon, G., Bandyopadhyay, B.P., Jawahir, I.S., Pheulpin, S.,Seguin, E., 2003. The influence of microstructure of hardenedtool steel workpiece on the wear of PCBN cutting tools. Int. J.Mach. Tool. Manuf. 43, 139–144.

c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1092–1104

Remadna, M., Rigal, J.-F., 2006. Evolution during time of tool wearand cutting force in the case of hard turning with CBN inserts.J. Mater. Process. Technol. 178, 67–75.

Thiele, J.D., Melkote, S.N., 1999. Effect of cutting edge geometryand workpiece hardness on surface generation in the finishhard turning of AISI 52100 steel. J. Mater. Process. Technol. 94,216–226.

Yallese, M.A., Rigal, J.-F., Chaoui, K., Boulanouar, L., 2005. Theeffects of cutting conditions on mixed ceramic and cubicboron nitride tool and on surface roughness during machining

of X200Cr12 steel (60 HRC). Proc. I Mech. E 219 (Part B),35–55.

Yallese, M.A., 2005. Wear behaviour of modern cutting toolmaterials under hard turning conditions. PhD Thesis. BadjiMokhtar University, Annaba, Algeria.