An Investigation of Tool Chip Contact Phenomena

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An investigation of tool chip contact phenomena in high-speed turning using coated toolsP T Mativenga*, N A Abukhshim, M A Sheikh, and B K K HonSchool of Mechanical, Aerospace, and Civil Engineering, University of Manchester, Manchester, UK

The manuscript was received on 11 April 2005 and was accepted after revision for publication on 8 November 2005.

DOI: 10.1243/09544054JEM351

Abstract: This paper focuses on the investigation of contact length and chip morphology inhigh-speed turning. In this study the performance of five different coatings: TiN, TiCN, TiAlNand CrTiAlN PVD coatings, and an advanced Dymon-IC hydrogenated amorphous carbon-based coating (a-C:H) deposited by plasma-enhanced chemical vapour deposition (PECVD)is investigated in high-speed machining (HSM) of AISI/SAE 4140 high-strength alloy steel atcutting speeds ranging between 210 and 925 m/min. Modelling of chip contact length isrevisited and the results of extensive machining tests supported by use of a high-speedcamera are reported. The effects of coatings on the chip compression ratio, back flow angle,toolchip contact length, frictional force, and tool wear are explored. The paper contributestowards a fundamental understanding of heat generation and partition in metal cutting using coated tools.

Keywords: tool coatings, high-speed turning, chip contact length, chip morphology

1 INTRODUCTION

High-speed machining has been reported to lead tohigher cutting interface temperatures [ 13 ]. There-fore, it is important that the cutting tools maintaintheir hardness and wear resistance at higher tem-peratures. The heat flowing into the tool and hencetool wear is dependent on the contact conditions(phenomena) at the toolchip interface. Largertoolchip contact lengths result in more heat dissi-pating into the tool. It is widely accepted that con-

tact length and chip form are interdependent [ 4]. A detailed understanding of contact conditions at therake face plays a role of major importance in accu-rate modelling of the cutting process.

Significant improvements in cutting tool perfor-mance can be achieved when they are coated withhard materials such as titanium-based coatings orsoft lubricant coatings. Current trends show thatcoated tools have a growing market share in excessof 65 per cent [ 5]. In terms of coating technology,the applications of chemical vapour deposition

(CVD) coatings have grown rapidly over the lasttwo-and-a-half decades and now show signs of level-ling off [ 6]. Quinto reported that rapid thermalrecovery due to compressive residual stress devel-oped in the physical vapour deposition (PVD) coat-ing compared to a tensile residual stress in the CVDcoating enables them to exhibit a better resistanceto chipping in interrupted cutting [ 7, 8 ]. This hasled to greater interest in the application of PVD coat-ings to high-speed machining.

It is well known that coatings on cutting tools play

multiple roles including reducing cutting tempera-tures and cutting forces, increasing wear resistance,and improving the surface-related properties suchas friction, wear, oxidation, and fatigue resistance[9]. It has also been suggested that coatings can actas a diffusion barrier, which reduces tool tempera-tures and consequently increases tool life. However,it is not entirely clear whether the coating affects thecutting process by an insulation effect or/and by tri-bological effect. It was suggested by Rech et al . onthe basis of heat flux studies that coatings do notinfluence heat distribution but the amount of heatgenerated in the toolchip interface [ 10 ]. The inter-esting point is how this relates to contact pheno-mena, which determine the contact mode, sticking or sliding and the magnitude of heat transferred

*Corresponding author: School of Mechanical, Aerospace, and

Civil Engineering, University of Manchester, Sackville S t ree t Build ing, Manches ter M60 1QD, UK. emai l:P Mativenga@manchester ac uk

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Nowadays, the majority of solid carbide cutting tools and almost all inserts are coated with speciallayers to enhance their performance. The machining performance of the coated tool is dependent on

the properties of both the coating and substratematerial, such as hardness, fatigue strength, fracturetoughness, internal stress, thermal expansioncoefficients, friction coefficient, microstructure, andchemical compatibility as well as the adhesion of the coating to the substrate and the depositiontechnique. As mentioned earlier, turning at high cut-ting speeds is associated with extremely hightemperatures at the interface and chemical wearbecomes more predominant. Therefore, the essen-tial requirements of a successful coating systemin HSM operations include thermal stability, highoxidation resistance, and hot hardness in additionto a high toughness and good adhesion to thesubstrate.

1.1 Research focus

Trends towards hard machining, high-speedmachining, processing of new hard-to-machinematerials, and dry machining are key drivers in thedesign of high-performance thin layer coatings [ 11 ].In essence, this paper is concerned with the influ-ence of selected coatings upon the toolchip contactphenomena in high-speed turning of BS 970-

709M40EN19 (AISI/SAE 4140) high-strength alloy steel. The study focused on the single layer TiN,double-layer TiCN, double-layer TiAlN, and singlelayer CrTiAlN coatings deposited on cementedcarbide inserts using the closed field unbalancedmagnetron sputter ion plating (CFUBMSIP) process,and the single layer Dymon-IC, a hydrogenatedamorphous carbon-based coating (a-C:H), depositedon carbide inserts by plasma-enhanced chemicalvapour deposition (PECVD) from a hydrocarbonprecursor gas. The latter two represent new coatings, which need to be evaluated in high-speed turning.

2 CHIP CONTACT PHENOMENA

A review of studies into the nature of the toolchipinterface and contact conditions in metal cutting revealed different views of the interface contactphenomena. In finite element modelling (FEM),one commonly made assumption divides the con-tact length into two regions with full seizure occur-ring over much of the interface near the cutting edge and interfacial sliding occurring near theboundary of the contact [ 12 ]. In contrast, a secondview considers that sliding occurs over much of theinterface near the cutting edge and sticking occursnear the boundary of toolchip contact [ 13 ] A third

view is that the toolchip contact is composed of four distinct regions: a region of stagnation at thecutting edge; a region of retardation; a region of sliding, followed by one of sticking near the bound-ary of the toolchip contact [ 14 ]. In HSM, it hasbeen reported that sticking or seizure occurs overthe entire contact area owing to the high velocity of the chip at the interface and the highly localizedstresses and temperatures at the contact area [ 4].

With regard to evaluation of contact phenomena,there are different methods, which can be used tomeasure contact length. One method involves a physical examination of the rake face after chip for-mation [ 15 ]. The second method involves the identi-fication of the contact length by examining theunderside of the chip.

Another area where there is no general agreementis the mathematical modelling for contact length. Various models for the toolchip contact length pro-posed by researchers based on experimentalapproaches are presented in Table 1, where L c isthe contact length, h is the undeformed chip thick-ness, h0 is the actual chip thickness, j is the chipthickness ratio, v c is the cutting velocity, and anda are the shear and rake angles respectively. Thegeneral concept is that the chip contact length is a function of chip thickness, shear, and rake angles.Thus, the contact length increases with a reduction

in the positive rake angle value and also with anincrease in the chip thickness. While these modelsare useful in the qualitative assessments of the abovecutting variables on chip contact length, they do not yield quantitative agreements. In fact, with the exce-ption of one model reviewed here, previous researchsuggests that contact length is independent of cut-ting speed. This is the prime reason for this experi-mental investigation to elucidate the effects of cutting speeds and coating types on contact length.

3 CUTTING TESTS

The cutting tests were performed on a turning lathe where chip flow was recorded by an NAC 500

Table 1 Models for the chip contact length

Model Ref

L c 2h0 Kato [16 ]; Toropov [ 17 ]L c

h(2.05 j 0.55) Poletika [ 18 ]L c 2h[j (1 tan a )sec a ] Abuladze [ 19 ]L c h

ffiffi 2p

sin sin45 a Lee and Shaffer [ 20 ]L c 0.4850.00280 n c Stephenson [ 21]L c 1.61h0 0.28h Marinov [ 22 ]L c 8:677 :10 5h 0:515 v 0:065c 90 a

0:733 Zhang, Liu, and Hu [ 23 ]L c 1.92h0 0.09h Sutter [ 24 ]L c h sincos a sin Tay et al. [25]L c h sincos a sin f1 C n3 1214p nC g

Oxley [26 ]

658 P T Mativenga, N A Abukhshim, M A Sheikh, and B K K Hon


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high-speed video system. A Kistler cutting forcedynamometer Model 9121 (Fig. 1) was used to mea-sure the actual cutting force. The three-componentpiezoelectric force dynamometer was linkedtogether by a highly insulated resistance cable con-nected to a charge amplifier, signal-conditioning instrument, and data acquisition software installedon the computer. Chip thickness and flank wear were also studied to characterize the machining per-formance of the PVD coatings. An optical micro-scope was used to measure the chip contact lengthand flank wear on the inserts.

ISCAR triangular cutting inserts whose geometry was specified to ISO standard as TPUN 160308 were used in the turning tests. The cutting edge was specified as class E, i.e. honed preparation. Theinserts had a clearance angle of 11 and the edgeradii were found to vary between 55 and 60 mm forthe uncoated tool. The inserts were mounted on a tool holder effecting a 60 approach angle. The depthof cut was such that cutting was done on the noseradius. The inserts were coated using the PVD andPVD/ PECVD method to the specifications as shownin Table 2. Inserts were mounted on a left-hand styletool-holder with a rigid clamping system. Each of thecutting inserts was used at cutting lengths of 35 mm.The workpiece used was BS 970-709M40EN19(AISI/SAE 4140) high strength alloy steel in the

form of a round solid bar of 220 mm diameter and200 mm length. The cutting speeds used in theexperiments ranged between 210 and 925 m/min.The feedrate and depth of cut were 0.15 mm/rev and 0.1 mm respectively and were kept constant forall experiments.

4 RESULTS

To assess the surface quality of coated tools, the sur-face roughness was measured on the rake of theinserts. This would influence the heat generation inthe burn-in period, as the tool surface would needto be smoothed out. Figure 2 shows the surface fin-ish on the rake face for the different coatings. TiCNhad the most consistent surface finish, followed by the TiAlN. While the TiN had the best average sur-face finish, it had a wide variation in surface rough-ness similar to CrTiAlN and Dymon-IC.

The use of the high-speed camera enabled thechip back-flow angle and some contact phenomena to be captured. In general, the chip compressionratio reduced with cutting speed for TiN and TiAlN(Fig. 3). This trend is consistent with the formationof thinner chips as reported for HSM [ 28 ]. However,the chip thickness for TiCN could almost beassumed to be independent of cutting speed The

Table 2 Properties of PVD coatings [ 27 ]

TiN TiCN TiAlN CrTiAlN Dymon-IC

Coefficient of friction against steel (dry) 0.4 0.4 0.4 0.4 0.030.06Deposition method PVD PVD PVD PVD PVD/PACVD Adhesion layer ( mm) 0.1Ti 0.1Ti 0.1Ti 0.2Cr 0.43Cr1st layer thickness ( mm) 2.7 1.8 0.85 3.11 0.37CrC2nd layer thickness ( mm) 1.6 2.1 0.6DLCTotal layer thickness ( mm) 2.80 3.50 3.05 3.31 1.40

Fig. 1 Experimental set-up

An investigation of tool chip contact phenomena 659


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CrTiAlN and Dymon-IC generate thinner chips withincreasing cutting speeds within the conventionalmachining range. However, the chip thickness thenincreases for cutting speeds in the HSM range.

Thus, the variation of chip thickness with cutting velocity actually depends on the tool coating.

The toolchip contact length L C in this experimen-tal work was measured from the images of the worninserts obtained from the optical microscope. Theevolution of the chip contact length for the differentcoated tools is presented in Fig. 4. Based on existing models and Fig. 3, the chip contact length would beexpected to reduce with cutting speed for TiAlNand TiN, be independent of cutting speed for TiCNand increase with cutting speeds for CrTiAlN andDymon-IC. Figure 4 shows that the coatings yielddifferent chip contact length over the range of cut-ting speeds tested. The chip contact lengths forCrTiAlN and Dymon-IC are almost double that of the other coatings TiN, TiCN, and TiAlN. It is inter-esting to note that the coatings with the best

consistency in surface finish, i.e. TiCN and TiAlN,gave a chip contact length that was almostunchanged for higher cutting speeds. It could beinferred that these coatings have the capacity not

to increase the secondary heat generation forincreased cutting speeds. On the other hand, thecontact length for TiN, CrTiAlN, and Dymon-ICincreased for higher speeds above 800 m/min and600 m/min for the latter two.

Figure 5 shows another important aspect of contact phenomena. While it is often assumed thatthe contact width is uniform, the images show thatcontact width in HSM reduces away from the cutting edge and this could be influenced by chip curling.Furthermore, Figs 6 and 7 show that the chip back-flow angle increases with an increase in cutting speed. The rake face images and increase of chipback-flow angle with cutting speed supportsdominant sticking contact for HSM.

0.10

0.15

0.20

0.25

0.30

0.35

0.40

TiN TiCN TiAlN CrTiAlN Dy mon-IC

S u r f a c e r o u g

h n e s s

R a

( m

)

Fig. 2 Measured surface roughness on the rake face of a new tool

a p = 0.1 mm, f = 0.15 mm/rev

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

200 400 600 800 1000Cutting Speed (m/min)

C h i p

C o m p r e s s i o n R a t i o


Fig 3 Chip compression ratio

a p = 0.1 mm, f = 0.15 mm/rev

0.0

0.1

0.2

0.3

0.4

0.5

0.6


C o n t a c t l e n g t h ( m m )


Fig. 4 Chip contact length

Fig. 5 Tool chip contact areas at 924 m/min for CrTiAlN,Dymon-IC and TiAlN respectively

(a)

Tool

Chip

W o r k p i e c e

Chip

Tool W o r k p i e c e

(c)

Tool

Chip

W o r k p i e c e

(b)

Fig. 6 Photographs of chip flow for TiN coated tools.(a) V c 210 m/min; (b) V c 604 m/min; (c) V c 925 m/min



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Figure 7 shows that at the cutting speed of 210 m/min, all coatings showed a negative valuefor the chip back-flow angle. The chip velocity atthe separation point, where the chip departs from

the tool rake face, is almost parallel to the rake face(Fig. 6). This is in good agreement with the tradi-tional assumption as reported by Rao and Shin[29 ]. In general, TiN, TiCN, and the Dymon-ICcoatings showed a slight linear increase in the chipback-flow angle with cutting speed. This has beenattributed to the reduction in coefficient of friction[24 ]. At cutting speeds of 415 to 604 m/min, slightdecreases of the chip back-flow angle for the TiAlNand CrTiAlN coatings were recorded. This correlates well to the reduced chip thickness given in Fig. 3.

It is worth comparing the experimental measuredcontact lengths in this study with the resultsobtained by models reported in the literature. Accu-rate models are very useful to support FEM analysis.Results of the comparison for different coatings arepresented in Figs 812. It should be noted that the

model proposed by Oxley et al . is not included inthis comparison as it requires additional data includ-ing work material properties, n, and process con-stants, C , which can only be known experimentally

following numerous cutting tests. Oxleys modelcan be viewed as a representation of Tay et al .smodel with the inclusion of a correction factorexpression. Stephenson et al .s model, whichexpresses the contact length as a function of cutting speed showed an overestimation of the trend andis not included in the comparison. The modeldoes not show the well-established dependenceof contact length on chip compression ratio.Stephensons model implies that the contact lengthhas a minimum value of 0.485 mm, which is unrea-listic in high-speed machining where the unde-formed chip thickness is very small.

In this paper the well-known model for shearangle by Merchant (equation (1)) was used to esti-mate the shear angle [ 30 ]. The other required angle was calculated from knowledge of the shear angleand the direction of the resultant force as derivedfrom measured cutting forces

tan cos a

l sin a 1

Referring to Fig. 8, it can be seen that in the case of TiCN all models produce an almost constantcontact length over the range of cutting speeds

considered in this study. Indeed, the measured data also show that the contact length for TiCN-coatedinserts could almost be assumed to be independentof cutting speed. In particular, the model proposedby Tay and Marinov represents the closestestimates of the chip contact length while othersolutions show an overestimation of contact length.Tays model produced an average 17 per cent

a p = 0.1 mm , f = 0.15 mm/rev

-6

0

6

12

18

200 400 600 800 1000

Cutting Speed (m/min)

C h i p b a c k f l o w a n g l e ( d e g . )


Fig. 7 Chip back flow angle

TiCN, a p = 0.1 mm, f = 0.15 mm/rev

0

0.1

0.2

0.3

0.4

0.5

0.6

200 400 600 800 1000


C o n t a c t L e n g t h

, m m

Kato et al,Toropov Poletika

Abuladze

Lee and Saffer Marinov

Sutter

Experimental results Zhang et al

Tay

Fig. 8 Benchmarking of chip contact length models to HSM data for TiCN coated tool



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underestimation in the contact length (Fig. 9). TheTiCN coating (the smoothest of all the toolcoatings) showed a chip thickness that was almostindependent of cutting speed. This would imply that the shear angle would also remain almostconstant with cutting speed. The frictional forcealso did not change significantly as cutting speed was increased.

In Fig. 10, it is clear that all models showed a similar trend for the contact length for the TiN overthe range of cutting conditions considered. Theexperimental results are in qualitative agreementwith the model predictions up to a cutting speed of

800 m/min. However, all models except Tays yieldhigher values for L c in comparison with theexperimental results. Tays model has an averageprediction accuracy of 10 per cent at speeds up to830 m/min. The large increase in the measuredcontact length at the highest cutting speed could beattributed to the highest tool wear as reportedearlier. Severe nose radius wear reduces effectiverake angle and increases contact length.

Figure 11 shows the contact length for TiAlN-coated inserts benchmarked against the variousmodels. The overall trend of reducing contact lengthfor increased cutting speeds predicted by the models

-60

-40

-20

02040

60

80

100

T i C N -

T a y

T i N - T a

y

T i A l N -

S u t t e

r

C r T i A

l N - A b

u l a d z

e

D y m o

n - I C -

K a t o

D y m o

n - I C -

A b u l a

d z e

% d e v i a t i o n i n L c

Fig. 9 Estimation of maximum, minimum, and average model performance for L c

TiN, a p = 0.1 mm, f = 0.15 mm/rev

0

0.1

0.2

0.3

0.4

0.5


C o n t a c t L e n g t h ( m m )


Abuladze

Lee and Shaffer Marinov

Sutter

Experiment al results Zhang et al

Tay

Fig. 10 Benchmarking of chip contact length models to HSM data for TiN coated tool



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is not evident in the experimental results. Theexperimental results seem to show an overallopposite trend. The model used by Kato as well asToropov is found to be closest for TiAlN-coatedtools. The results on TiAlN suggest a reduced depen-dence of contact length on the chip thickness; hence,a departure from the fundamental assumption takenin developing existing models.

In Fig. 12, for the Dymon-IC coating, the cutting speed effect on the contact length is shown as

less significant for cutting speeds from 210 to604 m/min. The experimental curve for the contactlength then rises quite steeply after the cutting speedof 604 m/min. All other plotted curves in the figureshow a decrease in contact length when the cutting speed is increased from 210 to 415 m/min, a con-stant contact length when the cutting speed isincreased to 604 m/min. All other models, except Abuladzes and Zhang et al .s models, which showedan overestimation of the trend for the present test

TiAlN, ap = 0.1 mm, f = 0.15 mm/rev

0

0.1

0.2

0.3

0.4

0.5

0.6

200 400 600 800 1000




Abuladze


Sutter


Tay

Fig. 11 Benchmarking of chip contact length models to HSM data for TiAlN coated tool

Dymon-IC, a p = 0.1 mm, f = 0.15 mm/rev

0

0.1

0.2

0.3

0.4

0.5

0.6




Abuladze


Sutter


Tay

Fig. 12 Benchmarking of chip contact length models to HSM data for Dymon-IC coated tool



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material, showed a slight increase in contact length when the cutting speed exceeded 604 m/min. Thus,the DLC coating seems to be associated with

enlarged contact lengths in the high-speed cutting regime (>604 m/min). This trend seems to make Abuladzes model more relevant in the high-speedcutting zone for the Dymon-IC coating. However, Abuladzes model is still inaccurate with a 23 percent average overestimation error. The broadening of the contact length with increased cutting speedmakes conventional machining contact lengthmodels less relevant.

Figure 13 shows the contact length benchmark for the CrTiAlN-coated insert. For this coating themodels seem to show a similar trend to that of the Dymon-IC. However, the experimental data forcontact length show a large underestimation of thecontact length by all models except that by Abuladzeand Zhang et al Dymon IC and CrTiAlN seem to

follow a close trend to the uncoated tool for whichthe results were reported earlier [ 31 ]. Thus, high-speed machining using uncoated, CrTiAlN, andDymon-IC coatings did not show the potential formaintaining the chip contact length, but in fact ledto its expansion.

As shown in Fig. 9, existing models for the predic-tion of contact length give rise to significant errors inthe quantitative prediction of the contact length.Tays model appears to yield the closest results forTiCN and TiN. While Sutters model is the best esti-mate in the case of TiAlN, the spread of the resultsis too large and unacceptable. Abuladze and Katosmodels seem relevant for Dymon-IC and CrTiAlN;however, the spread of the error still makes theminaccurate for predicting contact length. Thus, new investigations are needed in the case of TiAlN,CrTiAlN, and Dymon-IC better to understand factors which influence and can be used to model contactlength.

Figure 14 shows the flank wear at the end of a cut-

ting pass for the various coatings. The data show thatTiAlN displayed consistently the lowest flank wearfor all cutting speeds, followed by CrTiAlN, Dymon-IC, TiCN, and, lastly, TiN. Flank wear occurs at a location remote to the chip contact area, but in cases where ploughing occurs, the heat generation on therake face will have an increased effect on flank wear. There was no obvious reduction in the contactlength observed for the TiAlN for increased cutting speeds. Thus, a reduction in the frictional force would be a factor in reducing heat generation inthe secondary deformation zone. The results imply that contact length cannot fully account for flank wear evolution. It is plausible to postulate that thesuperior wear performance of TiAlN to TiCN couldbe attributable to the hard insulating layer of Al 2 O3

CrTiAlN, a p = 0.1 mm, f = 0.15 mm/rev

0

0.1

0.2

0.3

0.4

0.5

0.6

200 400 600 800 1000


C o n t a c t L e n g t h ( m m

)


Abuladze


Sutter


Tay

Fig. 13 Benchmarking of chip contact length models to HSM data for CrTiAlN coated tool

a p = 0.1 mm, f = 0.15 mm/rev

0

0.2

0.4

0.6

0.8


M a x F l a n k W e a r

( m m )


Fig. 14 Maximum flank wear (linear cutting length 400 mm)



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as reported in the literature. Such an insulating layercould alter the heat partition balance and directmore heat into the chip.

Figure 15 shows the variation of frictional force with cutting speed. It is noted that TiN shows anodd trend compared to the other coatings. This coat-ing was also observed to be associated with built-upedge formation and this could account for the occa-sional increase in the force components. In general,the frictional force could be assumed to follow simi-lar trends for all the other coatings, a generaldecrease in the frictional force followed by an ulti-mate increase at the highest speed. Indeed, the high-est speed was associated with a significant degree of melting and that would alter the frictional contactforce. The superiority of the TiAlN coating is againdisplayed by the lowest frictional force in the highcutting speed regime (604925 m/min). The TiAlNcoating, with the lowest frictional force in the high-speed machining zone exhibited the best wear per-formance (Fig. 14). Thus, the superiority of the TiAlNcoating can be recognized for both the rake face (low frictional force on secondary deformation zone) andthe flank face (better wear resistance). The additionof the Cr in the CrTiAlN coating seems to have led

to a very good wear performance, but does notappear to yield a significant reduction in the fric-tional force and contact length (Fig. 14). Thus, thereis a possibility that the addition of Cr compromisesthe lubricant properties of the TiAlN coating. Withregard to the Dymon-IC, there seems to be no clearfrictional force reduction advantage recorded inthis study. Its frictional force performance was com-parable to TiCN.

The correlation data in Table 3 shows thatthe variation of contact length with cutting speedis weakly correlated to the frictional force in thecase of TiN, TiCN, and TiAlN. On the otherhand, strong correlations exist in the case of theCrTiAlN, 0.97, and the lubricant Dymon-IC coating,0 77 Thus it could be inferred that the use of

effective coatings such as TiN, TiCN, and TiAlNreduces the dependence of contact length onthe frictional force. Thus, the effective coatingsreduce the expansion of the contact length (andhence heat flux area) that seems to be experiencedin the case of the soft Dymon-IC coating forincreased cutting speeds. Indeed, the chip thicknessdata and chip compression ratio for TiCN wasfound to be almost independent of cutting speed,as reported earlier.

5 CONCLUSIONS

This paper presents new findings on contact

phenomena in high-speed turning with coatedtools. This study found that coatings influencethe contact area and hence heat flux in metal cut-ting. In general, existing models of chip contactlength are empirical in nature and are useful for qua-litative inference of the effect of chip thickness andrake angle. The models by Poletika, Abuladze, Leeand Shaffer, Marinov, Tay, and Sutter have beenfound qualitatively to predict the contact length forthe TiN, TiCN, and TiAlN coatings for increased cut-ting speeds. For these coatings the contact lengthseems to be independent of the frictional force.However, the models do not yield agreeable resultsin quantitative estimation of contact length. Thebest quantitative estimates of contact length witherrors of 17 per cent and 10 per cent were foundusing Tays model for TiCN- and TiN-coated toolsrespectively. This most accurate model assumesthat the contact length is influenced by the unde-formed chip thickness, shear angle, rake angle, andthe inclination of the resultant cutting force to theshear plane. While most modelling assumes a regu-lar chip contact area, there is new evidence in thisinvestigation, which suggests that the shape of thearea in HSM is in fact parabolic, which reduces in width from the cutting edge. This fact should betaken into account in the input distribution of heatflux in FEM

a p = 0.1 mm, s = 0.15 mm/rev

0.0

10.0

20.030.0

40.0

50.0


F r i c t i o n a l F o r c e ( N )


Fig. 15 Frictional force

Table 3 Correlation between contact length andfrictional force

Speedm/min


Fr L c Fr L c Fr L c Fr L c Fr L c

210 31.9 0.138 24.7 0.096 22.8 0.156 27.9 0.396 29.8 0.306415 45.0 0.090 23.9 0.156 26.2 0.240 28.5 0.408 26.2 0.336604 31.1 0.100 23.0 0.120 20.4 0.210 24.3 0.330 22.2 0.318830 28.7 0.132 27.6 0.138 18.7 0.245 33.8 0.420 30.3 0.468925 35.8 0.490 38.2 0.110 31.1 0.280 42.6 0.552 34.0 0.540R [0.03] [ 0.29] [0.49] [0.97] [0.77]



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The TiCN coating was found to display the lowestvariation in rake face roughness, chip compressionratio, and contact length for both conventionaland high-speed cutting. In general, the chip back-

flow angle was found to increase slightly with cutting speed owing to a reduced coefficient of friction.However, TiAlN and CrTiAlN showed a reducedchip thickness and back-flow angle in the conven-tional cutting speed zone. The Dymon-IC coating did not lead to thinner chips and reduced contactlength compared to other coatings. The TiAlN coat-ing showed superior wear performance as well asdisplaying the lowest frictional force and hencelowest heat generation in the secondary defor-mation zone.

Existing models are inadequate for the quantita-tive prediction of chip contact length in high-speedmachining using coated tools. New models arerequired, especially for the use of TiAlN, CrTiAlN,and Dymon-IC coatings and uncoated carbides toolsin high-speed cutting. These models should embracethe frictional force as another factor influencing contact length, as revealed by the correlationstudies.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the coopera-tion of Teer Coatings Ltd in this investigation.

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An investigation of the tool chip contact length and

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APPENDIX

Notation

a p depth of cut (mm)C constantDLC diamond like carbon f feedrate (mm/rev)h undeformed chip thicknessh0 actual chip thickness (mm)L c contact lengthn constantR correlation factor L c and FrV c cutting velocity (m/min)

a rake angle in degrees inclination of resultant cutting force to shear

planej chip compression ratio shear angle



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An Investigation of Tool Chip Contact Phenomena

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