ARTICLE IN PRESS

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doi:10.1016/j.ijm

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International Journal of Machine Tools & Manufacture 48 (2008) 965–974

www.elsevier.com/locate/ijmactool

Effect of machining parameters in ultrasonic vibration cutting

Chandra Nath1, M. Rahman�

Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore–117576, Singapore

Received 7 November 2007; received in revised form 21 January 2008; accepted 25 January 2008

Available online 15 February 2008

Abstract

The ultrasonic vibration cutting (UVC) method is an efficient cutting technique for difficult-to-machine materials. It is found that the

UVC mechanism is influenced by three important parameters: tool vibration frequency, tool vibration amplitude and workpiece cutting

speed that determine the cutting force. However, the relation between the cutting force and these three parameters in the UVC is not

clearly established. This paper presents firstly the mechanism how these parameters effect the UVC. With theoretical studies, it is

established that the tool–workpiece contact ratio (TWCR) plays a key role in the UVC process where the increase in both the tool

vibration parameters and the decrease in the cutting speed reduce the TWCR, which in turn reduces both cutting force and tool wear,

improves surface quality and prolongs tool life. This paper also experimentally investigates the effect of cutting parameters on cutting

performances in the cutting of Inconel 718 by applying both the UVC and the conventional turning (CT) methods. It is observed that the

UVC method promises better surface finish and improves tool life in hard cutting at low cutting speed as compared to the CT method.

The experiments also show that the TWCR, when investigating the effect of cutting speed, has a significant effect on both the cutting

force and the tool wear in the UVC method, which substantiates the theoretical findings.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: Ultrasonic vibration cutting; Tool vibration parameters; Cutting speed; Tool–workpiece contact ratio; Cutting tool life

1. Introduction

The ultrasonic vibration cutting (UVC) method is amore effective cutting process over conventional turning(CT) in terms of cutting force, cutting instability, toolblunting, tool wear, chip generation, surface finish and soon, to machine difficult-to-cut materials such as Ni- andTi-based super alloys, hardened steels, optical glasses,ceramics, tungsten carbides, etc. [1–17]. This methodimproves productivity by saving the manufacturing timeby about 5–10% as well as the machining cost by about30% of the cost of parts as required in the deburringprocess of precision parts [18].

Along with the usual parameters for the CT method, twoadditional parameters: tool vibration frequency and vibra-tion amplitude, are considered in the UVC system that help

e front matter r 2008 Elsevier Ltd. All rights reserved.

achtools.2008.01.013

ing author. Tel.: +656516 2168; fax: +65 6779 1459.

esses: g0500294@nus.edu.sg (C. Nath),

s.edu.sg (M. Rahman).

16 4644; fax: +656779 1459.

to improve cutting quality and to increase tool life bylowering, mainly, the cutting force and improving thedynamic cutting stability.Extensive theoretical research [12–16], simulation [17]

and experimental results [8–12,19,20] for the UVC methodmention that the lower cutting force is due to aconsiderable reduction of friction between the tool andthe workpiece [17,19] and the separating or pulse cuttingcharacteristic of the tool [12–14,18,21]. Previous experi-mental reports also indicated that the UVC methodperforms better at low cutting speeds [6–8,18–22] and atboth high-tool vibration frequency and amplitude [23].Moreover, the UVC mechanism during the tool–workpieceinteraction is basically related to these above threeparameters [3,8,13,15,20]. Therefore, the cutting force isdirectly related to these three parameters. However, therelationship between the cutting force and these threeparameters has not yet been established.Furthermore, Babitsky et al. [2,6] justified and compared

the axial surface profile and roundness profile for thecutting of Inconel 718 by applying both the CT and the

ARTICLE IN PRESS

Y-axis (Radial)

X-axis (Axial)

Z-axis (Tangential)

Feed directionalvibration

Radial directionalvibration

Tangentialdirectional vibration

Tool insert

PZT

ap

ve

f, a

Fig. 1. Schematic of ultrasonic vibration cutting.

x = a sin �t

a’b’

ta tb

ta tctc

Cutting

Dis

plac

emen

t of

tool

0

T

Cut

ting

forc

e

0

R

Time

ToolTime

Freeab

vt = 0

vt = 0

(vt) max

vct

Fig. 2. Pulse cutting state in the UVC method.

C. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974966

UVC methods. However, the ability of the cubic boronnitride (CBN) tool for this excellent tough and creep-rupture resistance alloy and the effect of cutting parameterson the cutting performance by applying the UVC methodhave not yet been studied. As a hard and tough cuttinginexpensive tool material, CBN is the best choice, next onlyto the diamond tools.

The authors in this paper investigate firstly the mechan-ism of the effect of these three parameters in the UVCprocess. It is theoretically understood that the amount ofcutting force is directly related with these three parametersthat establish two key factors: tool–workpiece contact ratioand tool–workpiece relative speed (hereafter called‘‘TWCR’’ and ‘‘TWRS’’, respectively), for this intermittentcutting technique. This paper focuses mainly on controllingthe first key factor, TWCR. It is found that the increase inboth the tool vibration frequency and the vibrationamplitude, and the decrease in the workpiece cutting speedreduce the TWCR. As the TWCR decreases, the non-cutting time of the tool increases, which decreases thecutting force and enhances both increased tool life andimproved cutting quality.

This paper also investigates the effect of cutting para-meters such as cutting speed and feed rate in cuttingInconel 718. The cutting quality is evaluated in terms ofthree force components, tool wear, chip formation andsurface roughness for both the UVC and the CT methods.Additionally, the study observes and discusses the behaviorof tool failure and the formation of chips at differentcutting conditions by means of scanning electron micro-scopy (SEM). Finally, a comparison is made to show theadvantages of the UVC method over the CT method. It isconcluded that the UVC method performs better than theCT method up to a certain cutting speed range. However,beyond this range, the CBN tools catastrophically failed inmachining of Inconel 718 in the UVC method. The authorsconsider that this type of failure during ultrasonic cutting isdue to a number of consecutive high impacts between thetool and the workpiece for long durations at a compara-tively high-cutting speed. The higher the cutting speed, thelarger is the TWCR. Therefore, the cutting speed has asignificant effect on cutting force and tool wear in the UVCmethod, which substantiates the theoretical findings.Finally, it is remarked that the UVC method offers bettercutting performance in hard and tough cutting materials atlow cutting speed and at high-tool vibration frequency andamplitude.

2. Theory

2.1. Study of the UVC mechanism

Fig. 1 shows an illustration of a UVC system wherea piezoelectric transducer (PZT) is configured into thetool shank to excite the tool in any desired directioncorresponding to three conventional axes. In this study,a tangential or cutting directional type UVC system is

considered as implemented by most of the previousresearchers [3,8–16,20]. The rest of cutting system in thistechnique is the same as the CT set-up.The cutting principle of the UVC method was explained

by previous researchers [8,12,13,15,20]. The tool oscillatesat an ultrasonic frequency f (i.e. vibration period, T=1/f)with a very small vibration amplitude a where theworkpiece rotates with a constant cutting speed vc.Accordingly in Fig. 2, the cutting edge starts to vibratefrom the origin O and then cuts the workpiece materialduring the interaction periods ta�b, ta0�b0.The displacement of vibrating tool is described by

x ¼ a sin ot ¼ a sin 2pft (1)

where x and o are the displacement and the angularvelocity of the tool, respectively.Thus the tool vibration speed is vt ¼ _x ¼ ao cos ot,

which varies from a minimum of (vt)min ¼ 0 at any peak orvalley to a maximum of (vt)max ¼ ao ¼ 2paf at midpoint ofits either upward or downward motion, except for theinitial tool speed at the origin O. It was realized[3,8,12,13,15,20] that ultrasonic cutting is satisfied if

ARTICLE IN PRESSC. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974 967

ao4vc, otherwise it becomes a conventional cuttingprocess. Therefore, the critical cutting speed in the UVCmethod is defined as (vc)cr=ao=2paf=(vt)max.

Fig. 2 also illustrates the three following basic equationsthat govern the UVC system:

vc þ ao cosotb ¼ 0 ðat t ¼ tbÞ (2)

a sin oðT þ taÞ � a sin otb ¼ vcðT þ ta � tbÞ (3)

r ¼ tc=T (4)

where r is the tool–workpiece contact ratio (TWCR).Eqs. (2)–(4) formulate a final equation as obtained

by [13]

vcð1� rÞ ¼ 2af sin pr cos½cos�1ð�vc=ð2paf ÞÞ � pr�

ðfor 2paf4vcÞ (5)

Therefore, it is clear from the final equation (5) that theterm TWCR during ultrasonic cutting is dependent onthree vital parameters, namely f, a and vc. Since thistechnique cuts the workpiece for a certain period in eachvibration cycle, the cutting force in this method also shouldbe TWCR ( ¼ tc/T) times of that for the continuous cutting[11,14]. That means a lower value of TWCR can decreasethe cutting force in the UVC system. Equation (5) directlyindicates that the TWCR can be lowered by controllingthese three important parameters. In Sections 2.2–2.4, theeffect of these factors is studied theoretically first. Then theeffect of the third factor, workpiece cutting speed, isjustified in the experiment in Section 4.1.

2.2. The effect of tool vibration frequency

Suppose two UVC systems vibrate at two differentfrequencies f1 ¼ 20 kHz and f2 ¼ 35 kHz with the sameamplitude a ¼ 15 mm. Hence the tool vibration period T1 ishigher than T2. Fig. 3(a) combines both the UVC systemsincluding the tool displacement curves and their corre-sponding pulse cutting states as a function of time.

0

0

0

0

TW

CR

Too

l dis

plac

emen

tC

uttin

g st

ate

1

0

-1

0 1 2 3 4 5 6 7

time, t x 10-5

x 10-5

A J

B K D F M H

C L G

E N I

a2a1

b1b2 a'2

a'1 a''

b'2

b'1

tc1

tc2tc2 tc2

tc1T2

T1

2b''

2

Fig. 3. UVC process: (a) tool displacement and resultant cutting force for tw

35 kHz, respectively), (b) relation between TWCR and tool vibration frequenc

Now let a workpiece, rotating with a constant cuttingspeed vc, engages at a1 and disengages at b1 during theupward and downward motion, respectively, of the tool forthe low-frequency UVC system. Thus the system for thiscase follows the same contact period tc1 as tb1�a1 ; tb01�a0

1;

tb001�a001; . . . in the consecutive vibration cycles. Similarly, the

high-frequency tool for the same workpiece cutting speedfollows the contact period tc2 as tb2�a2 ; tb02�a0

2; tb002�a00

2; . . .. It is

clear from the figure that tc14tc2 . But since T14T2, itcannot directly be determined whether r14r2 or vice versa.Fig. 3(b) plots the TWCR against the tool vibration

frequency using Eq. (5) where a ¼ 15 mm and vc ¼ 15m/min. Two different values of TWCR, r1 ¼ 0.2162 andr2 ¼ 0.1600, can be found for f1 ¼ 20 kHz and f2 ¼ 35 kHz,respectively. Thus, it is clear that the TWCR for a low-frequency tool is higher than for a high-frequency tool inthe UVC system. Therefore, the tool cutting areaexperiences for a short duration of pulsating cutting forcewhen using a high-frequency tool.Though the number of contact between the tool and the

workpiece will always be higher for a relatively high-frequency cutting tool, it was experimentally reported [23]that the increase of tool vibration frequency in the UVCsystem improves cutting quality and prolongs tool life thatagrees with the theoretical studies.

2.3. The effect of tool vibration amplitude

Again suppose other two tangential UVC systemsoperate two different tool vibration amplitudes a1 ¼

10 mm and a2 ¼ 25 mm where the frequency f ¼ 20 kHz isfixed. Thus the tool vibration period T is same for both thesystems. Fig. 4(a) illustrates a combined diagram of twodifferent displacement curves and the corresponding pulsecutting states against the time cycle for these two systems.If a workpiece rotates with a constant cutting speed vc

for both the conditions, then the tool of small vibrationamplitude interacts with the workpiece earlier at c1 and

0 5 10 15 20 25 30 35 400

.2

.4

.6

.8

11

Tool frequency, f (kHz)

35, 0.160020, 0.2162

o different tool vibration frequencies (subscripts: 1 and 2 are for 20 and

y, f.

ARTICLE IN PRESS

0 5 10 15 20 25 30 35 400

0.2

0.4

0.6

0.8

1

Vibration amplitude, a (micron)

TW

CR

10, 0.2710

25, 0.1644

Too

l dis

plac

emen

t,C

uttin

g st

ate

2

1

0

-1

-2

0 2 4 6

time, t x 10-5

x 10-5O Q

NJ

K

L

M

T

P

c2

c1d1

d2

tc1

tc2

tc1

tc2

c'2

d'2

d '1c'

1

Fig. 4. UVC process: (a) tool displacement and resultant cutting force for two different tool vibration amplitudes (subscripts: 1 and 2 are for 10 and

25mm, respectively), (b) relation between TWCR and tool vibration amplitude, a.

0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

1

0

-1

0 2 4 6 8

Cutting speed, v (m/min)

TWC

R

20, 0.2536

40, 0.3803

time, t x 10-5

Cut

ting

stat

e

x 10-5

p2

p1 p'1

q'1q'

2p'2

T

q1q2

S

R

T

U W

V

tc2tc1

tc2tc1

Too

l dis

plac

emen

t

Fig. 5. UVC process: (a) tool displacement and pulsating cutting force against time at f ¼ 20 kHz and a ¼ 15 mm (subscripts: 1 and 2 are for low and high-

cutting speed, respectively), (b) relation between TWCR and workpiece cutting speed, vc.

C. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974968

separates from the workpiece later at d1 as compared withthe tool of large vibration amplitude. In this manner,let the tool for the former case follows the tool–workpiececontact time tc1 as td1�c1 ; td 01�c0

1; . . . in the successive passes

where the tool for the later case follows tc2 as td1�c1 ;td 01�c0

1; . . .. It is clearly seen from Fig. 4(a) that tc14tc2 . Thus

r14r2 because T1 ¼ T2.Fig. 4(b) shows the TWCR against the tool vibration

amplitude using the final equation (5) where f ¼ 20 kHzand vc ¼ 15m/min. Two different values of TWCR, r1 ¼

0.2710 and r2 ¼ 0.1644, can be picked out at a1 ¼ 10 mmand a2 ¼ 25 mm, respectively, where r14r2. Therefore, it isobvious that the TWCR for a tool of a high-vibrationamplitude is lower than for a tool of a small-vibrationamplitude. As the number of contacts between the tooland the workpiece is same for both the tool conditions [seeFig. 4(a)], the tool for the second condition favors bettercutting performance in the UVC system.

Zhang [23] experimentally investigated that the increaseof tool vibration amplitude in the UVC system improves

cutting quality and saves tool life, which substantiates thetheoretical findings.

2.4. The effect of workpiece cutting speed

Fig. 5(a) shows a tool displacement diagram andcorresponding pulse cutting state against time for a singletool following f ¼ 20 kHz and a ¼ 15 mm. Let two cuttingspeeds vc1 and vc2 for the workpiece be considered where,vc1ovc2 . Since vc1 is the smallest, the workpiece for this caseholds with the tool later at p1 and then separates earlier atq1 during the upward and downward motion of the toolrespectively. Accordingly, the tool–workpiece in thiscondition follows the contact period tc1 as tq1�p1 ; tq0

1�p0

1; . . .

in the consecutive vibration cycles. On the other hand, theworkpiece of cutting speed vc2 engages with the tool earlierat p2 and disengages later at q2 following the tool–work-piece contact period tc2 as tq2�p2 ; tq0

2�p0

2; . . .. From Fig. 5(a),

it is clear that tc1otc2 and hence r1or2, because the samefrequency was applied to the cutting tool.

ARTICLE IN PRESS

Table 1

Experimental conditions

Workpiece Material Inconel 718

Diameter 175mm

Length 600mm

Tool Material CBN (BN250)

Rake angle +101

Relief angle 111

Approach angle 301

Nose radius 0.4mm

Cutting conditions Depth of cut 0.10mm

Feed rate 0.025–0.1mm/rev

Cutting speed 5–20m/min

Vibration conditions Frequency 1971.5 kHz

Amplitude 15 mm

C. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974 969

The curve for TWCR vs. the workpiece cutting speed isplotted in Fig. 5(b) again using the final equation (5). If twodifferent cutting speeds, 20 and 40m/min, are consideredthen the TWCR is found to be about 0.2536 for 20m/minas compared to about 0.3803 for 40m/min. Therefore, theTWCR for a low-workpiece cutting speed in the UVCsystem is lower than that for a high-workpiece cuttingspeed. This means the tool experiences a short durationof the pulsating cutting force when applying low cuttingspeed.

Moreover, the TWRS in the UVC method increases withthe increase in cutting speed that definitely effects cuttingquality in machining. Previous experimental works[6–8,18–22] indicated that the UVC method improvescutting quality and saves tool life at low cutting speedvalues. Therefore, low values of cutting speed are suggestedto be used for the UVC technique.

Table 2

Properties (at RT) of workpiece Inconel 718

Density 8.19 g/cm3

Melting temp. range 1260–1336 1C

Avg. thermal exp. coeff. 13.0mm/mK

Specific heat 435 J/kgK

Thermal conductivity 11.4W/mK

Ultimate tensile strength 1240MPa

Yield strength (0.2% off) 1036MPa

Elongation in 50mm 12%

Elastic modulus (tension) 211GPa

Hardness 36 HRC

Table 3

Properties of CBN tool inserts used

CBN contents 85–90 (vol%)

CBN grain size 3–5 (mm)

Binder Co, etc.

Poission’s ratio 0.22

Thermal conductivity 100–130 (W/m K)

Thermal stability 1270 (K in air)

Hardness (GPa) 35–40 (at room temp.)

12 (at 1273K)

3. Experimental set-up and procedure

All the cutting tests were conducted with a modern CNClathe machine Okuma LH35-N. One end of the workpiecewas held tightly in the three-jaw chuck and the other endwas supported by the lathe center. A Sonic impulse SB-150device containing a PZT (see Fig. 6) was used to vibrate thetool tip in the tangential direction. The available powersource supplied for the device was AC 100V with 50–0Hzfrequency that consumes 260VA of electric power. Finally,a fresh CBN tool insert (601 type) was mounted on thecutter head in each test.

Table 1 presents the experimental conditions used forboth the CT and the UVC methods where Tables 2 and 3show the physical and mechanical properties of both theworkpiece Inconel 718 and the CBN tool materials,respectively. When switching off the generator of thevibration device, the cutting becomes CT. On switching iton, the device provides the frequency of about 19 kHz andthe amplitude of about 15 mm. Therefore, the maximumvibrating speed of the tool tip can be calculated as(vt)max=2paf=107.4m/min.

In order to maintain separating-type vibration cutting,the cutting speeds in all the operations were chosen to beless than this critical speed.

Fig. 6. Photograph of a tool holder containing a PZT and tool insert in

the UVC method.

In regular intervals (about 2min) of one-pass, machiningwas stopped in order to observe and measure outputparameters such as tool flank wear width, chip formationand surface roughness. A KISTLER 3-Component Tool

Dynamometer was used to measure the cutting forcecomponents for tangential, radial and axial directions.For analysis, the force signals were measured viaa Graphtec chart recorder. Also, the width of the tool wearalong the flank, VB, was measured with a Toolmaker’s

Microscope while the topography of the tool wear and theformation of chips were examined with a Scanning Electron

Microscope (SEM). Moreover, a surface analyzer, Surtro-nic 10, was used to measure the average surface roughness,Ra, throughout the experiments.

ARTICLE IN PRESSC. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974970

4. Results and discussions

4.1. The effect of cutting speed on cutting force and on tool

wear

Fig. 7(a) shows the effect of cutting speeds on the cuttingforce components for both the cutting techniques at a feedrate of 0.1mm/rev. It is observed that all the forcecomponents for the UVC method are reduced up to15–25% that is required for the CT method at all thecutting speeds. Another finding is that, for both the cuttingmethods, the thrust force component is the highest amongall the components followed by the tangential componentand then the axial component.

Fig. 7(b) plots the tool flank wear width, VB against thecutting speeds after 10min of cutting and Fig. 8 illustratesthe CBN tool wear characteristics with the following SEMphotographs for various cutting conditions by applyingboth the cutting techniques. Though highest tool wear ratewas seen, the cutting operation was continued throughoutall the cutting speeds in the CT method. In contrast, thewear rate in the UVC method was negligible at a cuttingspeed up to 10m/min. However, beyond this speed such as15 and 20m/min, the tool nose as well as the cutting edgeexperienced high-wear rate and eventually failed after4min of machining. Since the tools were worn out within ashort time at 15 and 20m/min by applying the UVC

0

10

20

30

40

50

60

70

80

2.5

Cutting speed, m/min

Cut

ting

forc

es, N Fr in CT

Ft in CTFa in CTFr in UVCFt in UVCFa in UVC

0

0.1

0.2

0.3

0.4

0.5

5

Cutting speeds, m/min

Flan

k w

ear

wid

th, m

m

CTUVC

7.5 10 12.5 15 20

5 7.5 10 12.5 15 17.5 20

Fig. 7. Effect of cutting speed in both the cutting methods: (a) cutting

force components, and (b) tool flank wear width, VB after 10min of

cutting.

Fig. 8. The SEM photographs of tool wear characteristics at different

cutting conditions. CT method: (a) 10m/min, 0.05mm/rev; (b) 10m/min,

0.1mm/rev; (c) 15m/min, 0.05mm/rev; (d) 15m/min, 0.1mm/rev; and

UVC method: (e) 10m/min, 0.05mm/rev; (f) 10m/min, 0.1mm/rev;

(g) 15m/min, 0.05mm/rev; and (h) 15m/min, 0.1mm/rev.

method, the wear values for those two cutting speeds arenot shown in Fig. 7(b). Figs. 8(a)–(f) were captured after10min of machining while Figs. 8(g) and (h) were takenafter 4min of operation.Fig. 8 also shows that the CT method continuously

generated built-up-edge (BUE) that left the pits and debrison the tool rake and flank faces. These pits and debrissticking on the tool cutting area always increase the cuttingforce and hence increase the tool wear. In contrast, a verysmall amount of BUE and pits was produced in the UVCmethod. Therefore, Figs. 7 and 8 reveal that the removal ofBUE, the separating cutting characteristic, the consequentreduction of the surface tearing during UVC [19],aerodynamic lubrication [1,19] and the generation of

ARTICLE IN PRESS

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.025

Feed rate, mm/rev

Flan

k w

ear

wid

th, m

m

CT

UVC

00

10

20

30

40

50

60

70

Feed rates, mm/rev

Cut

ting

forc

es, N

Fr in CTFr in UVCFt in CTFt in UVCFa in CTFa in UVC

0.05 0.075 0.1

0.025 0.05 0.075 0.1

Fig. 9. Effect of feed rate in both the cutting methods at a cutting speed of

10m/min: (a) cutting force components and (b) tool flank wear width VB

after 10min of cutting.

C. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974 971

comparatively thinner and even chips (Fig. 11) could be themain reasons of producing both the lowest cutting forceand the tool wear at less than 15m/min in the UVCmethod.

However, in the UVC method, a sudden increase of forcecomponents was observed when the cutting speed shiftsfrom 10 to 15m/min. This is because both the TWCR andthe TWRS increase with the increase of the cutting speedsas discussed in Figs. 5(a) and (b). Ignoring the TWRSfactor in this study, the TWCRs for 10 and 15m/min arefound to be about 0.1739 and 0.2163, respectively. Hencethe tool–workpiece contact time is reduced by more than4% in each vibration cycle if the cutting speed reduces from15 to 10m/min. This means the tool engaged with theworkpiece for a relatively long duration at high-cuttingspeed and also was attacked by a number of consecutivehigh-mutual radial and tangential impacts during theupward motion of the tool. As Inconel 718 has excellenttoughness, ultimate transverse strength, yield strength andcreep-rupture resistance (see Table 2), and a relatively high-tool relief angle (111) was used in the tests, it is assumedthat the CBN tool material, which is next to diamond inhardness, could not sustain these high impacts for a longduration in this method. It is seen in Fig. 10(b) that theCBN tool started to wear off at the beginning of cutting ata comparatively high-cutting speed of 12.5m/min orbeyond. Though the failure is insignificant at a cuttingspeed of 10m/min (see Figs. 8(e) and (f)) even after 10minof cutting, it is seen in Figs. 8(g) and (h) that the failurefirstly started with fracture (or abrasion) at the tool nose-flank area and it increased with the increase in cuttingspeed. Because of the worsening tool condition, themeasured cutting force components were found to behigher immediately when the cutting speeds shift toward12.5m/min. Due to the long duration of tool–workpieceinteraction at these speeds (i.e. high TWCR) as comparedto 10m/min, the high number of impacts in vibrationcutting led to fatigue failure of the tool just after 4min ofcutting at either 15 or 20m/min. The tool failed along withthe cutting edge, starting from the tool nose area as seen inFig. 8. Since the failure was very severe, it was consideredto be a catastrophic failure. Therefore, the UVC methodresults in long tool life at low cutting speed.

4.2. The effect of feed rate on cutting force and on tool wear

Fig. 9(a) reveals that the cutting force increases with thefeed rate in both the processes, which means that the UVCmethod accords with the same rule in the CT method.However, all the force components, at almost all the feedrates, of the UVC process are reduced approximately toabout 12–20% of that of the CT process. Moreover, thecutting force increase rate with the feed rate is insignificantin the UVC process unlike in the CT process.

Fig. 9(b) shows that the tool flank wear width in the CTmethod increases suddenly when the feed rate is increasedfrom 0.025 to 0.05mm/rev and then it maintains a steady

increase rate. In contrast, in the UVC method, the wearincreases linearly with a very small slope throughout thewhole range of the feed rates. Thus it is clear that the toolwear rate in the CT method is significantly higher than inthe UVC method.Figs. 9(a) and (b) also demonstrate that the lower cutting

force components in the UVC method reduced the toolwear rate that lengthens the tool life. At all the feed rates, itis observed that the tool wear in the UVC method isreduced to about 12–14% of that in the CT method.According to these results, it is predicted that the tool lifefor the UVC method is almost 7–8 times higher than thatfor the CT method.

4.3. Tool wear vs cutting time

Figs. 10(a) and (b) illustrate that the tool wear rate in theUVC method is significantly lower than in the CT methodup to the cutting speed of 10m/min. According to thefollowing experimental results, it is observed that the toollife in UVC of Inconel 718 is almost 4–8 times higher thanthat in CT up to that cutting speed limit. This is becauseboth the TWCR and the TWRS are low at low cuttingspeeds in the UVC method. Thus a small value of both theTWCR and the TWRS results in low tool wear rate.

ARTICLE IN PRESSC. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974972

However, as discussed earlier, due to the high value ofboth the TWCR and the TWRS beyond the cutting speedof 10m/min, the tools eventually failed just after 4min ofcutting in the UVC method. On the other hand, though

0

0.1

0.2

0.3

0.4

0.5

0

Cutting time, min

Too

l wea

r, m

m 5 m/min

7.5 m/min

10 m/min12.5 m/min15 m/min

20 m/min

2 4 6 8 10

0

0.2

0.4

0.6

0.8

1

0

Cutting time, min

Too

l wea

r, m

m 5 m/min

7.5 m/min10 m/min12.5 m/min15 m/min20 m/min

2 4 6 8 10

Fig. 10. Tool flank wear width, VB against cutting time at a feed rate of

0.1mm/rev for (a) CT method and (b) the UVC method.

Fig. 11. SEM photographs of the chips produced at different cutting conditio

(b) 10m/min, 0.1mm/rev and UVC method: (c) 10m/min, 0.05mm/rev; (d) 10

100% TWCR causes a rise in temperature and differentwear mechanisms at the tool–workpiece contact areas andelastic and plastic deformations occur to cause fast toolwear; cutting still could be continued in the CT method athigh speed because no impact is faced by the tool edge inthis method.The above studies thus reveal that the CT method limits

the repeatability of the tool, which in turn increases theproduction costs. On the other hand, the UVC methodlimits the use of high-cutting speeds with hard and toughcutting like Inconel 718.

4.4. Analysis of chip formation

Fig. 11 shows the SEM photographs of chips at differentfeed rates for both the cutting methods. It can be easilyobserved that the CT method generated thick, uneven,short and cracked chips whereas the UVC methodproduced comparatively thin, smooth and long chips.The generation of thicker, uneven and cracked chips is

not favorable for high-quality machining because thesetypes of chips negatively affect the tool cutting arearesulting in non-uniform friction and generating hightemperatures, high-cutting forces, high-regenerative chatterand rapid tool wear, which shorten the tool life. Incontrast, non-continuous interaction between the tool andthe workpiece in the UVC method generated thin, smoothand long chips that did not affect the tool life significantly.

4.5. The effect of cutting speed and feed rate on surface

roughness

Fig. 12(a) shows the roughness values Ra against thecutting speeds that were taken after 10min of machining

ns by both the cutting methods. CT method: (a) 10m/min, 0.05mm/rev;

m/min, 0.1mm/rev.

ARTICLE IN PRESSC. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974 973

with both the cutting methods. Since the tools were wornout in the UVC method after 4min of cutting at 15 and20m/min, the Ra values for those conditions were notconsidered. It is observed that the Ra values increase withthe increase in cutting speeds where the increase rate for theUVC method is very insignificant, unlike that for the CTmethod.

It is also seen from Fig. 12(b) that a minimum surfaceroughness Ra value in the CT method was 2.4 mm for thecutting of Inconel 718, whereas it was 0.6 mm in the UVCmethod. Furthermore, the Ra values with the UVCtechnique did not cross 0.8 mm at the maximum feed rateof 0.1mm/rev. Thus the Ra values in the UVC process donot increase markedly with the feed rates, as it does in theCT process.

Figs. 12(a) and (b) also reveal that the surface finish withthe UVC method is improved by about 75–85% over theCT method. Therefore, a high-quality surface finish fortough cutting could be achieved with the UVC method.

The above studies demonstrate that, since the TWCR is100% in the CT method, the generation of thick, unevenand severe cracked chips, BUE, high-cutting force compo-nents, frictional heat, and high-cutting instability, etc.deteriorated the machined surface and finally produced arough and coarse surface. On the contrary, only a fractionof TWCR and consequent reduction of the surface tearingin the UVC method reduced the cutting force, frictional

0.0

1.0

2.0

3.0

4.0

5.0

6.0

5

Cutting speeds, m/min

Surf

ace

roug

hnes

s, m

icro

n

CTUVC

0

1

2

3

4

5

6

0.025

Feed rate, mm/rev

Surf

ace

roug

hnes

s, m

icro

n

CTUVC

7.5 10 12.5 15 20

0.05 0.075 0.1

Fig. 12. Average surface roughness values, Ra in both the cutting methods

after 10min of cutting: (a) against cutting speeds at a feed rate of 0.1mm/

rev and (b) against feed rates at a cutting speed of 10m/min.

force and frictional heat and produced comparativelysharper fine chips that have less influence on the machinedsurface. Thus the surface finish obtained in the UVCmethod is regular and smooth, which is much better thanthat in the CT method.It is also observed that both the TWCR and the TWRS

in the UVC method influence the cutting quality for toughcutting superalloy Inconel 718. For example, since theTWCR and the TWRS are the lowest for 5m/min ascompared to other cutting speeds (see Section 2 for theTWCR values), the surface finish is better for this cuttingspeed. Thus, as low as the values of these key parameters,the cutting quality distinctly improves.

4.6. Comparative analysis between the CT and UVC

methods

Lastly, Fig. 13 presents a brief comparison betweenthe CT and the UVC methods based on the aboveexperimental findings. Four different output parametersnamely the tangential and radial cutting force components,the flank wear width and the Ra values at a feed rate of0.1mm/rev, were taken into account for this comparison.Since the UVC method performs remarkably better up to acutting speed of 10m/min in cutting of Inconel 718, thisspeed was selected as best suited for this analysis.The chart in Fig. 13 shows that the UVC method, in all

the cases, promises better cutting performance than the CTprocess. Therefore, it is concluded that the UVC methodnot only attains high-quality cutting of difficult-to-machinematerials but also raises the tool life distinctly and savesmachining cost.

5. Conclusions

The effects of tool vibration frequency, tool vibrationamplitude and workpiece cutting speed in the UVC methodwere studied theoretically. Also the effect of cuttingparameters on a superalloy Inconel 718 with CBNtools was investigated thoroughly by applying the UVCmethods. In order to compose a comparative analysis,

0

10

20

30

40

50

60

70

1

1) T. force, N; 2) R. force, N; 3) T. wear, mm X 100; and 3) Roughness Ra, micron X 10

CTUVC

2 3 4

Fig. 13. Comparative analysis of cutting performances between the CT

and the UVC methods at a selected cutting speed of 10m/min.

ARTICLE IN PRESSC. Nath, M. Rahman / International Journal of Machine Tools & Manufacture 48 (2008) 965–974974

the cutting conditions applied in the UVC method werealso considered for the CT method. The cutting forcecomponents, the tool flank wear width, the chip formationand the surface roughness were justified as the outputparameters. Based on the theoretical studies and experi-mental results achieved, the following conclusions can becompiled:

1.

In the UVC technique, the cutting quality dependsmainly on two important factors: TWCR and TWRS.The cutting mechanism shows that the TWCR relies onthree independent key parameters: the tool vibrationfrequency, the tool vibration amplitude and the work-piece cutting speed.

2.

To achieve high-quality cutting, the TWCR should bekept as low as possible. The value of TWCR can belowered by increasing both the tool vibration frequencyand amplitude, as well as by decreasing the workpiececutting speed.

3.

The test results show that the cutting force for the UVCmethod was required to the about 12–25% of that forthe CT method in cutting of Inconel 718.

4.

The tool flank wear in the UVC method was found to beabout 12–25% of that in theCT method when cutting upto 10m/min. Accordingly, the tool life with the UVCmethod is increased by at least 4–8 times over the CTmethod.

5.

A minimum Ra value of 0.6 mm was achieved with theUVC method whereas 2.4 mm was achieved with the CTmethod for the same cutting condition. Hence, thecutting quality with the UVC method was improved byabout 75–85% over the CT method.

6.

However, beyond the cutting speed 10m/min, the CBNtools catastrophically failed after 4min of machiningapplying the UVC method. This type of failure may bedue to high TWCR when a high-cutting speed was used.A number of consecutive high impacts between the tooland the workpiece with high-TWCR induced to causefast tool wear that agrees with the theoretical study onthe effect of the third factor, i.e. the effect of workpiececutting speed.

7.

To conclude, the UVC method has been found to be asuitable technique to achieve high-quality finish surfacesfor Inconel 718; though low cutting speed range shouldbe maintained in this method.

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