Advances in the turning process for productivity improvement – a review-2008

Advances in the turning process for productivityimprovement – a reviewV S Sharma1*, M Dogra2, and N M Suri3

1Department of Industrial Engineering, NIT Jalandhar, Punjab, India2Department of Mechanical Engineering, SSG Panjab University Regional Centre, Punjab, India3Department of Production Engineering, Punjab Engineering College, Chandigarh, India

The manuscript was received on 16 April 2008 and was accepted after revision for publication on 3 July 2008.

DOI: 10.1243/09544054JEM1199

Abstract: Increasing productivity and reducing manufacturing cost have always been keys tosuccessful business. In machining, higher values of cutting parameters offer opportunities forincreasing productivity, but this also involves greater risk of deterioration in surface quality andtool life. During the past decade significant advances have been made in the development ofcutting tool materials for machining of ‘difficult-to-cut’ materials. The cost involved in theproduction of new turning tools is very high. To overcome this cost factor researchers have triedto bring about modifications in the turning process using existing tool materials. A goodunderstanding of the cutting conditions, temperature generation, failure modes, and cuttingforces leads to efficient control of the turning process. The literature reveals thatmodifications inthe tool geometry (such as grooved/restricted contact tools (RCTs) and chamfered/honededges), applications of cooling techniques, ultrasonic assisted turning (UAT), hot machining,and cryogenic treatment of inserts have led to efficient and economic machining of modernmaterials used for aerospace, steam turbine, bearing industry, nuclear, and automotive appli-cations. This paper presents a review of the different techniques involved in turning which cansupplement the performance of cutting tools for improved economics, good surface finish, andsurface integrity during machining of the latest materials.

Keywords: ultrasonic assisted turning (UAT), cryogenic treatment, minimum quantity lubri-cation (MQL), cryogenic, restricted contact tools (RCT), hot machining

1 INTRODUCTION

While technological advancements continue to takeplace throughout the manufacturing industry, turn-ing still remains the most important process used toshape metals because in turning, the conditions ofoperations are most varied [1]. Most metals andalloys – hard or soft, cast or wrought, ductile or brit-tle, with high or lowmelting point – are machined [2].Most of the shapes used in engineering are producedby machining and, in terms of size, components fromwatch parts to aircraft wing spares (over 30m long) orship propeller shafts are machined. All over the world

over US $100 billion is spent annually on metal-part-finishing processes such as turning, milling, boring,and other cutting operations. It is also known that 10per cent of material produced by the machiningindustry goes to waste. This wastage can be reducedto a minimum by using better tooling and machiningconditions [3].

One of the primary aims inmachining is to producethe parts in the most economical way [4]. A wrongdecision causes high production cost and decreasesthe machining quality. The materials that are fre-quently used in the aerospace and nuclear industrieshave high tensile strength and wear resistance. Wear-resistant materials are difficult to machine. Duringmachining of ‘hard-to-cut’ materials, the cutting tooloften encounters extreme thermal and mechanicalstress close to the cutting edge [1]. At the presenttime, mainly heat-resisting alloys with high melting

*Corresponding author: Department of Industrial Engineering,

National Institute of Technology, Near Surranussi, Jalandhar,

Punjab, 144011, India. email: [email protected];

[email protected]

JEM1199 � IMechE 2008 Proc. IMechE Vol. 222 Part B: J. Engineering Manufacture

REVIEW PAPER 1417

temperatures are used in the manufacture of aeroengines and steam turbines, in the bearing industry,and for other automotive-related applications. Todesign the optimum, high-performance metal-cutting tool, all the principal factors such as tool wearrate, tool fracture probability, and quality of themachined products should be considered. Over theyears, many techniques have been developed forimproving tool life and surface finish for hard-to-cutmaterials, e.g. cryogenic treatment of tools, cryogeniccooling, ultrasonically assisted turning (UAT),chamfered/honed/curvilinear edge tools, minimumquantity lubrication (MQL), high-pressure coolant(HPC), self-propelled rotary tooling (SPRT), restrictedcontact tools (RCTs), solid lubricants, and hotmachining, as shown in Fig. 1. Advances in machin-ing techniques have resulted in a significant increasein material removal rate (MRR) and enhancement oftool life and surface quality.

The production of exotic and smart materials hasbecome indispensable in satisfying the robust designrequirements for the aerospace and defence sectors.The machining of these materials has posed a greatchallenge to industry, requiring cutting tools of highstrength, which are very costly and sometimesimpracticable [2]. The non-conventional machiningprocess, another viable method, is mostly restrictedto small-scale removal of material. High temperatureand catastrophic friction in the cutting zone arecharacteristic features of traditional machining (suchas turning); these conditions have motivated re-searchers to look for new cutting tool materials towithstand the high temperatures and friction. Newtool materials such as cubic boron nitride (CBN) andpolycrystalline diamond (PCD) are costly alternativesfor turning; however, higher MRR and better productqualities have been obtained by using cutting toolsmade from these newmaterials [5]. This cost factor of

new tool materials has motivated researchers to lookfor alternative arrangements in existing tool materi-als. For bulk removal of material, instead of usingnew tool materials, there has been a growing interestin approaches that help to improve the machiningperformance while using existing tool materials.

1.1 Tool wear

There are primarily three problems faced by all cut-ting tools: wear at the cutting edge, heat generatedduring the cutting process, and thermomechanicalshock [6]. The formation of chips by shearing actionat the shear plane is one aspect of metal cutting. Thepower consumed in metal cutting is largely convertedinto heat near the cutting edge of the tool. The workdone in deforming the bar to form the chip, movingthe chip, and moving the freshly cut work surfaceover the tool is nearly all converted into heat. Thevery large amount of plastic strain means that it isunlikely that more than 1 per cent of the work done isstored as elastic energy; the remaining 99 per centis expanded in heating the chip, the tool, and thework material. During cutting, the tool acts as a heatsink into which the heat flows from the flow zone,and a stable temperature gradient is built up withinthe tool [1]. The amount of heat lost from the flowzone into the tool depends on the thermal con-ductivity of the tool, the tool shape, and any coolingmethod used to lower its temperature. The tem-perature generated at the flank surface may be higherthan that on the rake surface of the same tool,because the work material moves across this surfaceat the cutting speed of the operation, while the chipspeed over the rake face may be one-half or one-thirdof this speed. This gives a strong indication thatduring continuous cutting, the flank portion requiresmore cooling when compared with the rake face.

The real solid surfaces are never completely flat ona molecular scale and therefore make contact only atthe top of the hills, while the valleys are separated bya gap [7]. The force required to move one body overthe other becomes that required to shear the weakerof the two materials across the whole area. Evidencefrom quick stop sections and from chips shows thatthe contact between tool and workpiece is so nearlycomplete over a large part of the total area of theinterface that sliding at the interface is impossibleunder most cutting conditions. Sliding only occurswhen the interfacial bond is weak, particularlywhen soft metals are cut. Conditions of seizure areencouraged by the high cutting speed and the longcutting time, where difference in hardness betweentool and work material is relatively low and the bondstrength between them is high. Seizure between tooland work material is the main cause of formation ofboth the built-up edge and the flow zone [1]. During

Hot machining

Ultrasonicassisted turning

Restricted contact/ Grooved tools

Solid coolants/Lubricants

Self propelledrotary tooling

(SPRT)High pressurecoolant (HPC)

Minimum quantity

lubrication/MQL

Cryogeniccooling

Cryogenic.treatment ofturning tools

Chamfer/Honed/Curvilinear edge

tools

Turningperformance

parameterimprovement

Fig. 1 Techniques in turning for productivity improvements

Proc. IMechE Vol. 222 Part B: J. Engineering Manufacture JEM1199 � IMechE 2008

1418 V S Sharma, M Dogra, and N M Suri

cutting, when the shear plane angle is small, theshearing force may be more than five times that atthe minimum, where the shear plane angle a¼ 45�.With the variation in a, the area of shear plane alsovaries, which affects the cutting forces. At low valuesof a, the chip is thick, the area of the shear planebecomes larger, and therefore the cutting force Fcbecomes larger and affects the tool wear drastically.The feed force Ff imposes a shearing stress on the toolover the area of contact on the rake face.

Tool wear is an important criterion in determiningthe economics of turning. Most tool wear can bedescribed by a few mechanisms, which includeabrasion, adhesion, chemical reaction, plastic defor-mation, and fracture. Theses mechanisms producewear scars that are referred to as flank wear, craterwear, notch wear, edge chipping, cracking, plasticdeformation, and breakage, as shown in Fig. 2.Among the various types of wear indicated, flankwear and crater wear are gradual in nature but theremaining types are abrupt in nature [8]. For thepurpose of study, the main focus is on gradual wear(flank and crater) of the tool, whereas abrupt changeson the tool are easy to detect. Abrasive wear resultsfrom sliding of hard abrasive inclusions in the work-piece material across the face of a cutting tool. Thismechanism can dominate while machining metalswith carbide or oxide inclusions, metal matrix com-posites, carbide, and ceramics. Adhesion results fromfriction welding of the workpiece and the tool mate-rial, which can cause a portion of the cutting tool tobe plucked out. Anisotropic properties of ceramic cancause cracking along grain boundaries, which assistgrain pluck-out [9]. Some amount of abrasive andadhesive wear is desired to allow gradual tool wearand prevent premature fracture or chipping of thecutting edge. Chemical reactions between the cuttingtool and the workpiece can also lead to accelerated

tool wear or premature tool fracture [10]. A goodexample is cutting steel with PCD tools, where diffu-sion of carbon into the steel dramatically decreasesthe tool life. Cutting fluid applied to the chip forma-tion zone improves the machining conditions byacting as a coolant and lubricant. Lubrication is moreimportant at low-speed cutting conditions, whereasthe cooling effect is more important at higher cuttingspeeds owing to the large increase in heat generationby the chip removal process. Modes by which cuttingfluids can be applied to the cutting zone are as fol-lows: manual application, flood application, jetapplication, mist application, and ‘through-the-tool’application [1, 11]. Major advances in turning ofmodern materials are now described.

2 ULTRASONICALLY ASSISTED TURNING

Ultrasonically assisted turning is a technique forimproving machining operations, where a high-frequency vibration (f� 20kHz) with amplitude a�10mm is superimposed on the continuous movementof the cutting tool. Compared with conventional turn-ing this technique allows significant improvements inmachining intractable materials such as hard metalalloys, brittle plastics, high-strength aerospace alloys,composites, and ceramics. However, differentresearchers have reported significant improvements innoise reduction, tool wear reduction, and surface finishimprovement, by applying ultrasonic vibration duringmachining operations, in particular during the turningprocess. The high accuracy in UAT is the result ofreduction in elastic deformation of both the cuttingtools and workpieces, as well as reduction in cuttingheat andworkmaterial adherence to the cutting edge ofthe tool [12]. Ultrasonic technology sets new standardsfor machining time, contour accuracy, and surfacequality. There are three independent principal direc-tions in which ultrasonic vibration can be applied dur-ing the turning process, as shown in Fig. 3: feeddirection (or horizontal vibration), direction of cuttingvelocity (or tangential direction), and radial direction[13].

Possible advantages of applying ultrasonic vibra-tion simultaneously both in the tangential and radialdirections have also been explored. However, whenthe cutting tip is vibrated ultrasonically, the followinglimitations are imposed: cutting velocity direction,V¼pnd<Vt¼ 2paf; where V is cutting speed, n isrotational speed, d is diameter of workpiece, Vt is tipvelocity, a is amplitude, f is frequency; feed direction,sn<Vt. The calculations show that for the con-temporary commercially available bolted Langevin-type transducers (a� 20mm, f� 20 kHz) the vibrationtip velocity must not exceed 150m/min [14]. More-over, reduction of tip velocity also occurs during the

Fig. 2 Types of tool wear (FW: flank wear; N: notch; CH:chipping; CR: cracking; PD: plastic deformation; BR:breakage; CW: crater wear)


Advances in the turning process for productivity improvement 1419

cutting process owing to the cutting tip interactionwith the workpiece, so the upper limit on surfacespeed is further reduced. Thus efficient ultrasoniccutting occurs when vibration is applied in thedirection of the cutting velocity. Vt>V can beachieved only for low-diameter workpieces or lowrotational speeds [15]. However, application ofultrasonic vibration along the feed direction enablesthe cutting parameters used in the manufacturingindustry for most materials to be reached indepen-dently of the workpiece diameter. The other impor-tant issue with respect to ultrasonic machining is thechoice of transducer control system. In most casesthe generator drives the transducer directly. How-ever, this simple implementation is not adequate formost industrial applications for the following rea-sons. Fitting different tool holders or cutting tipsrequires readjustment of the oscillator frequency tomatch the changes in mechanical properties of thevibrating system. In addition, the cutting tip dynamicload is changed through the cutting process, whichaffects the dynamics of the cutting process (e.g. itleads to a decrease in the vibration level) [16].

A substantial decrease in cutting forces, as well asan improvement in surface finish up to 50 per centcompared with conventional turning, was achievedduring superimposed ultrasonic vibration turning(UVT) of two modern high-grade nickel-based alloys,C263 and Inconel-718, at constant depth of cut andfeed. During UAT, by applying the vibration inthe feed direction (parallel placement of the transdu-cer to the workpiece in the horizontal plane), surfaceroughness measurements were used to identify thecharacteristics of the ultrasonic turning processcompared with conventional turning. Figure 4 showsthe spectrum analysis of surface profiles of themachined surfaces of C263 and Inconel-718 mea-sured in the axial direction at cutting velocities of 14and 17m/min. By this method, macro and micromorphological differences between ultrasonicallymachined surfaces and surfaces produced underconventional cutting can be compared. It is clear fromFig. 4 that the magnitudes of roughness profiles arereduced by nearly 50 per cent in the cases of Inconel-718 and C263 with the application of ultrasonicvibration, and the surface becomes smoother alongthe axial direction. The reason for this is that theconventional cutting process has been transformedinto a high-frequency vibro-impact process, whichin turn increases the dynamic stiffness of thelathe–tool–workpiece system as awhole and improvesthe accuracy of turning. Also, abolishing the built-upedge (BUE) through the application of ultrasonicvibration at low cutting speeds helps to reduce thesurface roughness [16].

There are various shortcomings with respect to thetransducer control system, as explained by Skelton[15]. To overcome those shortcomings an auto-resonant system for the control of vibration in thefeed direction was implemented in this work. Theautoresonant systemmaintains the resonant mode ofvibration during the dynamic changes of the load.The autoresonant system proved the possibility of

Fig. 4 Surface profiles of ultrasonically machined (left) and conventionally machined (right) workpiecesat (ap¼ 0:8mm, s¼ 0.05mm/rev): (a) C263 (v¼ 14m/min); (b) Inconel-718 (v¼ 17 m/min)

Fig. 3 Principal directions of ultrasonic vibration on cut-ting tool



keeping the controlled signal under the resonantconditions during the cutting process [15].

For turning of high-strength aerospace alloys, UAThas shown great benefits by producing a noticeabledecrease in cutting forces and a superior surfacefinish. A reduction in the cutting force can resultin the extension of the tool life, improved surfacefinish, increased MRR, and improved roundness ofmachined workpieces. During the turning of agedInconel-718 with tungsten carbide inserts usingElectrolube HDC400 as the cutting fluid, cutting for-ces were measured and dependence of the cuttingforce on the feed rate in turning was studied. Themeasured cutting force in UAT (FUAT) is invariablymuch lower than the cutting force in conventionalturning (FCT). The ratio FUAT/FCT is considerablylower at the low feed rate (0.25–0.33) and stabilizesat about 0.6 at higher feed rates. With an increasein the feed rate from 0.03 to 0.1 r/min, the FCTdoubles, whereas the FUAT grows about five-fold,but numerically still remains much lower than theFCT. The average reduction in the cutting force withapplication of the lubricant on to the surface of theworkpiece for UAT was about 30 per cent, which wasabout double the reduction observed for conven-tional turning. Thus lubrication during cutting inUAT helps to improve the cutting performance. Byincreasing the cutting speed, the cutting forces tendto increase. The growing force in UAT with speed iscaused by the increased time of contact between thecutting tool and chip, with the cutting speed growingcloser to its critical value (Vt¼ 2paf ) [17]. A three-dimensional finite element (FE) model was alsodeveloped which predicts an increase in the vibrationfrequency from 10 to 30 kHz, and results in a 47 percent drop in the level of average cutting forces, whichcould be attributed to an increased velocity of thetool vibration. Finite element analysis (FEA) calcula-tions also show that vibration in the tangentialdirection causes a lower cutting force than thatobtained with vibration in the feed direction [18].

The evaluation of cutting tool temperatures wascarried out for conventional turning and UAT interms of FEA. A steady increase in the cutting tiptemperature is observed in simulations from themoment of the initial contact between the tool andworkpiece. Figure 5 shows that the maximum tem-perature attained at the cutting tip during conven-tional turning is about 12 per cent greater than thevalue obtained in UAT. Significantly lower tool tem-peratures during UAT can be explained by the factthat the cutting tool separates from the chip withineach cycle of ultrasonic vibration, staying in contactonly for 40 per cent of the period for the chosenvibration and cutting parameters. Such an inter-mittent contact leads to a reduction in the total timeof thermal conduction between the cutting tool and

chip, and cooling owing to the convective heattransfer to the environment.

According to the results, the width of the hardenedsurface layer is half the size for the UAT (40mm)specimen when compared with conventional turning(80mm). Furthermore, the hardness of this layer forUAT (about 15GPa) is half that for conventionalturning and considerably closer to the hardness ofthe untreated material, as shown in Fig. 6. Compar-ison of simulated residual strains in the machinedlayer shows 20 per cent lower values for UAT. Thisagrees well with the results of the nano indentationtests, demonstrating twice the smaller hardenedsurface layer with half the hardness for UAT whencompared with conventional turning [18]. Tangentialvibration cutting direction (also called the perpendi-cular direction) has shown good results in ultrasonicvibration cutting (UVC) of low-alloy steel (DF2) witha CBN tool. The maximum vibrating speed of the tooltip can be calculated as V¼ 2paf¼ 107.4m/min. TheUVC process improved the surface finish and resul-ted in reduction of tool flank wear, which reduces byapproximately one-fifth under all cutting conditionsas shown in Fig. 7. Breakage of the cutting edgecaused by the BUE is one reason why flank wear isenhanced subsequently, but the BUE does not occur

Fig. 5 Tool tip temperature in UAT and conventionalturning

Fig. 6 Nano indentation analysis of UAT and conventionalturning



in UVC so tool life is greater in UVC compared withconventional turning.

As UVC is a discontinuous process, from Fig. 8 itis clear that the UVC process requires approximately50 per cent of the cutting forces at all cutting speedsfor the same feed rate of 0.1mm/rev or 0.2mm/revwhen compared with conventional turning. Anotherobservation was that the main cutting force (i.e. tan-gential force) was the highest in all cases followed byaxial (feed) force. Compared with the conventionalmethod, the UVCmethod resulted in lower roughnessvalues as shown in Fig. 9, because the BUE rarelyoccurs when using the UVC method, as reported byvarious researchers. In the analysis it was observedthat the surface roughness value was at amaximum at0.2mm/rev. Thus in UVC, increasing the feed dete-riorates the machining performance [13].

Some of the drawbacks of UAT were removed bythe development of a high-rigidity UAT tool andby providing an inclination to the vibrating directiontowards the Z-axis during cutting, as shown inFig. 10. In the turning of stainless steel using carbidetools, it was reported that during UAT of difficult-to-cut materials, owing to chipping, unusual wearoccurs at the tool edge caused by colliding or rubbingbetween the flank of the tool and the newly formedsurface of the work; when the vibrating tool leaves thecutting point of the work, reverse directional cuttingforce arises and therefore tensile stress acts on thecutting edge. The authors [19], developed a newultrasonic vibrating cutting tool as shown in Fig. 11,which has vibrating direction stability and highrigidity so as not to change the vibrating direction ofthe cutting force. A conventional ultrasonic cuttingtool system with the longitudinal vibration mode forcomparison is mounted at one nodal flange and thecutting insert is held on the tip of the tool shank bymeans of a clamping screw. In addition, the con-ventional UVC tool system with bending vibration

mode is mounted on two nodal points using tool-fixing blocks and clamping screws. The longitudinalvibration mode, in which the vibration direction isstable, is applied and the tool is strongly attached tothe tool post at four nodal points on the tool shanks.It was also reported that a stable finish cutting couldbe carried out without tool chipping by using aninclined vibrating direction, at a level of about 30�

towards the Z-axis direction [19].

Fig. 7 Tool flank wear at 50, 70, and 90m/min in UVC and conventional turning, at 1mm/rev feed (a, b, care cutting speeds 50, 70, 90m/min respectively, and subscript u indicates UVC, while subscript cindicates conventional turning)

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100120

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30 50 70 90 110

Cutting speed (m/min)

)N( ecr

of laitne

gna

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UVC .1mm/rev CT .1mm/rev

UVC .2mm/rev CT .2mm/rev

Fig. 8 Comparison of main cutting force at different speedand feed rates

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Cutting Speed(m/min)

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Fig. 9 Analysis of surface roughness at a feed rate of2mm/rev



2.1 Benefits of UAT

The major benefits of UAT are listed below.

1. The UVC method results in low tool flank wearowing to lower abrasive wear effects when com-pared with the conventional turning process withCBN tools, while turning at low feed and vibrat-ing speeds below the critical value.

2. It is found that tool wear acceleration in UVC isvery low compared with that in conventionalturning. In addition, in UVC the wear rate in-creases very slowly when compared with theconventional turning method. This is mainly as aresult of the intermittent cutting mechanism ofthe UVC method.

3. Ultrasonic vibration cutting always requires lowercutting forces compared with conventional turn-ing; applying lubricants during machining canfurther decrease these forces.

4. The surface finish with UAT improves dramati-cally, as the BUE rarely occurs. Good surfacefinish can be achieved with vibration cutting atlower feed rates; even nano-finished surfacequalities (Ra< 1mm) can be generated with UAT.

5. An increase in either vibration frequency oramplitude leads to a decrease in cutting forces inthe UAT process, which is beneficial for increas-ing the accuracy of the cutting.

6. By increasing the cutting speed, the forcesincrease owing to the increased time of contactbetween the cutting tool and chip; also the cut-ting speed approaches its critical value. Thusvibrating tip velocity should not exceed the cri-tical value.

2.2 Limitations of UAT

There are a few drawbacks of the UAT methodology.The initial cost of set-up is high. In addition, fittingdifferent tool holders or cutting tips requires readjust-ment of the oscillator frequency to match the changesin mechanical properties of the vibrating system. Thiscan affect machine utilization time. A highly rigidmachine tool/tool post is required for UAT. At highrotational speeds, ultrasonic cutting is not effectiveowing to the restriction on maximum tip velocity(150m/min) with Langevin-type transducers.

3 CRYOGENIC TREATMENT OF TOOLMATERIAL

Cryogenic treatment refers to subjecting materials tovery low temperatures. This process is not limited inapplication to metals, but can also be used for a widerange of materials, with differing results. It is believedthat the life of cutting tools extends substantially withcryogenic treatment. It is a one-time permanenttreatment affecting the entire section of a component,unlike coatings [20]. The literature reveals that thebasic reason behind enhanced performance of cryo-genically treated carbide tools may be the fact thatcryogenic treatment not only facilitates the carbideformation (carbide refinement) but also makes thecarbide distribution more homogeneous. High wearresistance of cryogenically treated tool steel is attri-butable to reduction of the retained austenite, whichis achieved by transformation of the retained auste-nite into martensite during cryogenic treatment [21].

Researchers have tested several cryogenic pro-cesses. These include a combination of deep freezingand tempering cycles. Generally, these can bedescribed as a controlled lowering of temperaturefrom room temperature to the boiling point of liquidnitrogen (�196 �C), maintenance of this temperaturefor about 24 h, followed by a controlled raising of the

Fig. 10 Illustrations of inclined directions of vibrations forultrasonically vibrated cutting tool (F is angle ofinclination to feed direction (Z-axis), C is angle ofinclination of depth of cut (Y-axis))

Fig. 11 Illustration of newly fabricated ultrasonic vibrationcutting tool system with high rigidity (1: boltedLangevin-type transducer, 2: ultrasonic vibrationgenerator, 3: tool shank, 4: cutting insert, 5: toolholder, 6: tool fixing blocks, 7: clamping bolts,8: tool post)



temperature back to room temperature. Subse-quently tempering processes may follow [22].

The general procedure of cryogenic treatment forturning tools follows.

1. Inserts are placed in a chamber.2. Temperature is gradually lowered over a period

of 6 h from room temperature to about �184 �C.3. Temperature is held steady for about 18h.4. Temperature is gradually raised over a period of

6 h to room temperature.5. Inserts are tempered at the end.

A proprietary cryogenic treatment cycle with tem-perature reduced to very low levels as mentioned inthe literature is shown in Fig. 12. As coatings arecurrently sought after to improve tribological prop-erties and wear resistance, it is considered reasonableto compare the effect of cryogenic treatment with theeffects of coatings. Most of the research on cryogenictreatment in the area of machining tools and cuttingtool materials has concentrated mainly on toolsteels, especially high-speed steel. The mechanismsresponsible for the improvement in properties of toolsteel have also been well documented. However, littleresearch has been done on other cutting tool mate-rials. So far, few researchers have proposed othermechanisms that explain the effect of cryogenictreatment on tungsten carbide [22]. Bryson [23]attributes the wear resistance, and hence the increasein tool life, of carbide tools to the improvement in theholding strength of the binder after cryogenic treat-ment. He believes that cryogenic treatment also actsto relieve the stresses introduced during the sinteringprocess under which carbide tools are produced.However, Bryson also warned that under certainconditions, cryogenic treatment would have little orno effect on carbide tools, such as when reprocessedcarbides are used. In the heat treatment of toolsteel the problem of retained austenite occurs; it isunstable and likely to transform into martensiteunder certain conductive conditions. Freshly formedmartensite is also brittle, and only tempered mar-tensite is acceptable. To further aggravate this pro-blem the transformation of austenite to martensite

yields a 4 per cent volume expansion, causing dis-tortion; thus retained austenite should be alleviatedto the maximum extent possible before any tool isput into service [24].

Cryogenically treated tungsten carbide tools havemuch greater resistance to chipping compared withuntreated ones. Also, the cryo-treated tools per-formed better than the untreated versions at highercutting speeds. Finally, it was concluded that increasein wear resistance was attributable to an increase inthe number of Z-phase particles after cryogenictreatment, and this was confirmed on the basis ofphotographs taken by scanning electron microscope(SEM) [25].

Flank wear was measured during orthogonal turn-ing of medium-carbon steel (ASSAB 760) using tung-sten carbide square inserts with chip breakers(Sumitomo SNGG 230408RUM) at various cuttingspeeds. In addition, turning tests were performed forcontinuous and repeated cuts. When metal cuttingwas continuous, without any breaks, the depth of cutand feed were kept constant, while cutting speedwas varied between 150m/min and 300m/min.Figures 13(a) and (b) show the comparison betweenthe untreated and cryogenically treated tools for flankwear analysis. The cryogenically treated insertsexperience less flank wear than the untreated insertsduring the earlier part of the cutting operation, unlessthe cutting speed is very high (300m/min). As theduration of cut increases, the treated insert graduallyloses its wear resistance and at some point has almostthe same wear resistance as the untreated insert [22].

This phenomenon of continuous cutting showsthat prolonged heating of the cutting tool interfacehas a detrimental effect on the wear resistance of thecryogenically treated tool. So it is possible to con-clude that cryogenically treated tools perform betterif used for short periods of time or with repeated cuts,with breaks in between, both in terms of decreasedtool wear and increased resistance to chipping. Inlight of the fact that cryogenically treated tools per-form best when the tool temperature is kept low,their effectiveness can be extended if coolants orsuitable methods of cooling are used to keep the tooltemperature low.

Different heat/cryogenic treatments on samples ofM2, T1, and D3 steel tool were performed for turningof mild steel material under dry conditions. The toolsamples were heat treated as prescribed in AmericanSociety for Metals (ASM) standards; after conven-tional quenching some samples were tempered andsome were directly subjected to cryogenic treatment.After tempering, some of the samples were cryo-genically treated; of these some were left withoutfurther treatment and some were coated with TiNusing the physical vapour deposition technique.Some samples were coated with TiN and then

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Time (Hrs)

)K( er

utarep

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Fig. 12 General cryogenic treatment cycle for tool inserts



cryogenically treated. The objective was to study theeffect of cryogenic treatment on tool steel when therewas no retained austenite. The absence of retainedaustenite above the 0.1 per cent level after conven-tional heat treatment was ascertained through X-raydiffraction pattern studies [26].

It can be seen from Figs 14 and 15 that TCT(93/24)-M2 and -D3 steel has imparted a 69 per centand 33.9 per cent improvement respectively, whileCT (93/24)-M2/D3 steel has imparted a 86.6 per centand 48 per cent improvement; hence temperingbefore cryogenic treatment is not desirable. This canbe attributed to the stabilization of carbides andmicrostructural phases during the tempering pro-cess, which inhibits further transformation duringthe cryogenic treatment. The tempered M2 samplessubjected to CT (133/24) have only 13.3 per centimprovement whereas those tempered and subjected

to CT (93/24) have 69 per cent improvement, whichmay improve further if cooled below 93K. Thus it isinferred that it is desirable to avoid tempering beforecryogenic treatment; also low-temperature temper-ing at 423K after cryogenic treatment is carried out torelieve any brittleness remaining.

It is clear from Fig. 16 that CT (93/24) T1 samplesyield 110.2 per cent improvement when comparedwith standard heat treated tools. In T1, M2, and D3tools CTþTiN yields a life improvement of 205.3, 153,and 146 per cent respectively, while TiN coating fol-lowed by CT yielded improvement of only 104 and 109per cent on M2 and D3 steels. Also TiN coating aloneimparts 48.4, 42, and 41 per cent improvement, whilecryogenic alone imparts 110.2, 86.6, and 48 per centimprovement in T1, M2, and D3 steels. So from theseresults it is inferred that cryogenic treatment on TiNcoating is not favourable, as it has resulted in shorter

Fig. 13 (a) Flank wear at cutting speed 150m/min; (b) flank wear at cutting speed 300m/min

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CT SHT CT(133/24) TCT(133/24)

CT(163/24) CT(93/6) TCT(93/24) CT(93/6) +LN2

Fig. 14 Tool life up to 0.4mm flank wear of different cryogenic/heat treated D3 samples (SHT: standardheat treated; CT: cryogenic treated; CT(163/24): cryo-treated at 163K for 24 h; CT(133/24):cryotreated at 133K for 24 h; CT(93/6): cryo-treated at 93K for 6 h; CT(93/6þ LN2): cryo-treatedat 93K for 6 h and quenched in LN2 for 2 h; TCT (133/24): tempered and cryo-treated at 133K for24h; TCT(93/24): tempered and cryo-treated at 93K for 24h)



tool life. The uneven contraction of the coated mate-rial and the substrate during cryogenic treatmentresults in incipient cracks appearing at the interface.The cryogenic treatment imparts better wear resis-tance throughout the section and therefore it issuperior to any coatings. Tempered samples if treatedat lower temperatures may yield better results [26].

3.1 Benefits of cryogenic treatment of toolmaterial

The cryogenic treatment of tool material provides thefollowing benefits.

1. Cryogenically treated tools perform better interms of tool wear resistance when the tool tem-perature is kept low. Their effectiveness can be

extended if coolants or suitable methods of cool-ing are used to keep the tool temperature low.

2. The samples cryogenically treated showed afraction very close to 0 per cent of retained aus-tenite. This means that practically 25 per cent involume of the retained austenite observed in theuntreated sample was transformed into marten-site by the cryogenic treatment.

3. Cryogenic treatment not only facilitates carbideformation (carbide refinement) but also can makethe carbide distribution more homogeneous, thusincreasing the wear resistance of carbide tools.

4. Cryogenic treatment has shown good results withdie steel, high-speed steel, and carbide tools interms of improved wear resistance of the toolmaterials. Coating on cryogenically treated basematerial is favourable for improved performance.

It is further suggested that tempering before cryo-genic treatment is not desirable as the stabilization ofcarbides and microstructural phases during thetempering process inhibits further transformationduring cryogenic treatment.

3.2 Limitations of cryogenic treatmentof tool material

There are a few drawbacks of the cryogenic treatmentmethodology; for example, in continuous cutting theperformance of the cryogenically treated tool dete-riorates and comes down to the performance level ofthe untreated version. At higher depth of cut or inrough turning operations, the performance of thecryo-treated tool can be maintained by supportingthe turning process with certain cooling methods,

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000 2500 3000

Machining time in sec

mm ni rae

w k na lF

CT TCT93/24 CT(93/6+LN2) CT(133/24)

CT(163/24) SHT TCT(133/24)

Fig. 15 Tool life up to 0.4mm flank wear of different cryogenic/heat treated M2 samples (SHT: standardheat treated; CT: cryogenic treated; CT(163/24): cryo-treated at 163K for 24 h; CT(133/24):cryotreated at 133K for 24 h; CT(93/6þLN2): cryo-treated at 93K for 6 h and quenched in LN2 for2 h; TCT(133/24): tempered and cryo-treated at 133K for 24h; TCT(93/24): tempered and cryo-treated at 93K for 24 h)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 1000 2000 3000 4000 5000 6000 7000

Machining Time in Seconds

m ni raeW knalF

m

SHT TiN CT(93/24) CT+TiN

Fig. 16 Tool life comparison of cryogenically treated toolscompared with TiN coated tools (CT(93/24): cryo-genic treated at 93K for 24h; CTþTiN: cryogenictreated then coated; TiNþCT: titanium nitridecoated then cryogenic treated)



which involves extra cost. Treatment is not effectiveon coated tools, as this results in shorter tool life.The uneven contraction of the coated material andthe substrate during cryogenic treatment can causeincipient cracks to appear at the interface.

4 HOT MACHINING

Tigham first innovated the process of hot machiningin 1889, with the idea of softening the workpiecematerial whereby part or all of the workpiece isheated. Heating is performed before or during themachining. Researchers utilize various methods forheating the workpiece, such as flame heating, plasmaheating, electrical resistance heating [27] and laser-assisted heating. The hot machining process preventscold work-hardening by heating the workpiece abovethe recrystallization temperature, thereby reducingthe resistance to cutting and favouring machining.The general set-up for flame heating of a workpiece isgiven in Fig. 17 [28].

In the machining of high-manganese steel (56 HRCin the Rockwell scale) with a carbide tool by con-tinuously heating the workpiece using flame heating,tool flank wear was measured. The torch burned amixture of liquid petroleum gas and oxygen. Thevariation of mean chip reduction coefficient withrespect to temperature of the workpiece is shown inFig. 18. It is observed that the chip reduction coeffi-cient reduces with increase in temperature. Hencethe machinability of the material improves withincrease in temperature. The chip produced at hightemperature is of the continuous type, whereas it is ofdiscontinuous type at room temperature. FromFig. 19 it quite evident that a flank wear value of0.4mm in low-temperature heating is reached in9min of cutting time, while this value in the case ofhigh-temperature heating is reached at around

20min of cutting time, with the same feed and depthof cut. Thus the effect of the temperature of theworkpiece is clearly found to be the most significanton tool life [28].

Plasma enhanced machining (PEM) has improvedthe machining performance of Inconel when turningwith silicon carbide whisker-reinforced aluminium-oxide inserts. During experimentation with a fixedvalue of plasma gas flowrate, shield gas flowrate, andcathode set-back, the flank wear was measured; theexperiments also considered the effects of (a) plasmacurrent I, (b) initial bulk temperature of the work-piece T0, (c) workpiece diameter D, (d) cutting speedV, and (e) feed rate F based on the empirical equation

Ts ¼ 80:3275I0:584T 0:06

0

V 2:206D0:405F0:2026ð1Þ

where Ts is the surface temperature (K).The influence of the nozzle height above the

workpiece was also considered. There was no pro-tection applicable to avoid the heat on the tool cut-ting edge. Furthermore, the tool notching wear wasanother problem associated with PEM. Thus, in

Fig. 17 Flame heating system: ((a) lathe head stock; (b)chuck; (c) workpiece; (d) torch; (e) oxygen flowvalve; (f) oxygen cylinder; (g) liquid petroleum gas(LPG) flow valve; (h) LPG cylinder; (i) oxygen pipe;(j) LPG pipe; (k) temperature indicator; (l) tailstock; (m) thermocouple; (n) wire; (o) distanceadjustment handle; (p) cutting tool)

Fig. 18 Analysis of chip reduction coefficient withtemperature

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40

Time in minutes

m ni raew knal

Fm

Vc43m/min,S=.7mm/rev,200CVc21m/min,S.5mm/rev,600CVc43m/min,S=.5mm/rev,200CVc43m/min,S=.05mm/rev,600C

Fig. 19 Analysis of flank wear with time at d¼ 1.5mm



addition to plasma heating of the workpiece, a liquidnitrogen cooling system was added to cool the tool,i.e. hybrid machining [29].

From Fig. 20 it is clear that, compared with con-ventional machining, hybrid machining increasestool life by 156 per cent. Compared with PEM, the toollife in hybrid machining was prolonged by about 170per cent. The experimental data suggest that PEMalone did not extend tool life. It is reasonable to con-clude that during plasma heating the heat flowinevitably increases the temperature in the cuttingzone and leads to increase in tool temperature as well.These experiments indicate that the tool wear wasconsistently reduced in the hybrid machining. Thesurface roughness value was less in PEM comparedwith conventional machining, owing to a decrease inworkpiece hardness by heating. The tool life hasincreased greatly in hot machining of manganesesteel specimens compared with that of room tem-perature machining. The longest tool life has beenobtained at 600 �C machining. The tool life obtainedat 600 �C machining is approximately the same as thetool life obtained at 400 �C machining. Consequently,400 �C machining is the optimum heating tempera-ture if the microstructure of the workpiece and thecost of heating are considered. In hot machining,therefore, the effect of heating on microstructure ofthe workpiece and the economics of heating must betaken into consideration also [30].

During turning of Inconel-718 with ceramic insertsunder laser-assisted machining (LAM), better cuttingperformance has been achieved by using the materi-al’s high absorptivity of CO2 laser energy. This highabsorptivity of CO2 is achieved by choosing a suitablecoating type, the right coating condition, and opti-mum processing parameters. Also, with changingworkpiece diameters, the laser power and r/min havebeen varied as appropriate to obtain the requiredthermal fields. The specific cutting energy is reducedto 25 per cent by increasing the temperature from 0 �C(conventional machining) to 620 �C. The surfaceroughness decreases as the material removal tem-perature is increased, from 1.7mm in conventionalmachining to 0.9mmduring LAM at 540 �C. Increasing

the velocity from 1 to 3m/s is beneficial during LAM,in that notch wear is decreased by one-half; also, withincrease in temperature the amount of notch weardecreases from 0.58mm in conventional machiningto 0.36mm using LAM at 540 �C. It is known thatturning processes usually yield tensile residual stres-ses; the compressive residual stresses in the axialdirection indicate that LAMdoes not yield any adverseeffect on the resultant subsurface. The average flankwear during LAM is significantly lower than conven-tionalmachining. There are large economic benefits ofLAM, as the cost of machining a 1m length of Inconel-718 with carbide tools under LAM decreases by 66 percent compared with conventional machining, and byalmost 50 per cent with ceramic tools compared withconventional machining at 3.0m/s [31].

4.1 Benefits of hot machining

1. Effectiveness of the LAM process can be achievedby having a high absorptivity of CO2 laser energyin the metals, which has been accomplished bychoosing a suitable coating type (for exampleaerodag, aquadag, and graphite adhesive, as wellas oxide and black paint coating were reportedgiving absorptivity of 1 in the case of Inconel718), the right coating condition, and optimumprocessing parameters. With the use of LAM,specific cutting energy, surface roughness, andtool notch wear all are reduced. Also, there is noadverse effect on the resultant subsurface.

2. In hot machining, the limiting highest tempera-ture will be the recrystallization temperature ofthe workpiece, as higher heating temperaturemay induce unwanted structural changes in theworkpiece material and even increase the cost ofheating.

3. The effect of temperature of the workpiece isfound to be the most significant with respect totool life. The chip-reduction coefficient decrea-ses with increase in temperature, thus enhancingmachinability. However, the recrystallizationtemperature of the workpiece limits the max-imum value of temperature in hot machining.

4. If, with hot machining, tool cooling is applied itwill improve the overall machining performanceas in the case of hybrid PEM.

4.2 Limitations of hot machining

There are a few drawbacks of the hot machiningmethodology; for example, during heating of theworkpiece, it is very difficult to orient the direction ofthe heating process exactly on the work material,because sometimes tool heating also takes place,which leads to deteriorated performance of the tool.This external heating action also affects micro-Fig. 20 Comparison of flank wear with cutting length



structure of the newly generated workpiece surfaceif the intensity of heat applied is not properlycontrolled.

5 CRYOGENIC COOLING

The heat generation becomes more intense inmachining of hard materials because the machiningprocess requires more energy than for cutting a low-strength material [1,11]. In addition, the thermalconductivity of advanced materials such as siliconnitride (about 13W/m �C), titanium alloy (about15W/m �C), and Inconel (about 11W/m �C) is muchlower than that of commonly used alloy steels. As aresult, the cutting temperature in the tool and theworkpiece rises significantly during machining ofthese advanced materials. The most practical andeffective way to enhance machining performance incutting of difficult-to-cut materials is therefore toreduce the temperature generated during cutting[32]. Cryogenic cooling is the efficient way of main-taining the temperature well below the softeningtemperature of the cutting tool material. Cryogeniccooling is an environmentally safe alternative toconventional emulsion cooling. In the past, commoncryogenic cooling approaches included pre-coolingthe workpiece, indirect cooling, general flooding, andenclosed bath. The liquid nitrogen absorbs the heat,evaporates quickly, and forms a fluid gas cushionbetween the chip and the tool face that functions as alubricant [33]. Reduction in tool temperature duringcryogenic cooling is by lubrication and cooling thehottest spot, which in turn reduces the crater andflank wear. Cryogenic cooling, if properly employed,can provide (besides environmental friendliness) sig-nificant improvement in both productivity and pro-duct quality, and hence overall machining economyeven after covering the additional cost of the cryo-genic cooling system and cryogen [34]. The beneficialeffect of cryogenic cooling by liquid nitrogen may beattributed to effective cooling, retention of tool hard-ness, and favourable interactions of the cryogenicfluid with the chip–tool and work–tool interfaces [35].

A specially designed tool holder as shown in Fig. 21was used, in which liquid nitrogen is converted intothe gaseous state before coming in contact with thetool. Nitrogen is made to flow just beneath the insertthrough a small hole. During turning of stainless steelwith coated carbide tools, tool life with a cuttingspeed of 100–300m/min was plotted, as shown inFig. 22. The results show that with conventionalcoolant the tool life was 13.45min at 100m/mincutting speed and 0.5mm depth of cut, whereasunder the same cutting conditions in cryogeniccooling the tool life was 57.45min [32]. Maximumflank wear reduces by 3.4 times in cryogenic cooling

when compared with dry turning and two times whencompared with wet turning (soluble oil) during themachining of Ti-6Al-4V alloy using uncoated micro-crystalline K20 tungsten carbide inserts at 70m/mincutting speed, 0.2mm/rev feed rate, and 2mm depthof cut. Such reduction in wear is seemingly due toreduction of temperature-sensitive wear phenomenasuch as diffusion and adhesion, enabled by direct andindirect cooling with the liquid nitrogen jet. It wasobserved that such benefits decreased under highvelocities of 100 and 117m/min, possibly because ofimproper penetration of the liquid nitrogen into thechip–tool interface. Flaking of the rake surface just atthe end of the crater wear region was observed,especially under the cryogenic machining condition.This is attributed to higher thermal gradient at theend of the crater contact [36].

During the machining of reaction-bonded siliconnitride (RBSN) with polycrystalline boron nitride(PCBN) tools under dry cutting and cryogenic coolingconditions, the maximum temperature experiencedwith cryogenic cooling is only 829 �C as opposed to1153 �C in dry cutting. A special cup-type arrange-ment of tool holder is used and there is little vapor-ization and no frozen ice blocking in the coolant

Fig. 21 Tool for cryo-cooling. 1: screw head, 2: insert, 3:liquid nitrogen passage, 4: inlet of liquid nitrogen,5: tool holder, 6: hole for nitrogen exit, 7: expand-ing chamber, 8: length of screw

0

20

40

60

80

0 100 200 300Cutting speed in m/min.

etunim ni efil loo

Ts

Cryogenic,d=.5mm

Conventional,d=.5mm

Cryogenic,d=1mm

Conventional,d=1mm

Fig. 22 Tool life at different cutting speeds (f¼0.1mm/rev)



circulation system, which frequently occurs with theexternal spray method. Also, in this method liquidnitrogen does not come directly into contact with theworkpiece, so there is no extra size change or pooraccuracy caused by serious temperature changes onthe machined surface. There is an increase of rough-ness value by 16mm with dry cutting when comparedwith cryogenic cooling; this variation of surfaceroughness is attributable to an increase in tool wearunder dry conditions [37]. The cutting force withcryogenic cooling is less than that with dry cutting.This is because application of cryogenic fluid reducesthe coefficient of friction at the interface of thetool–chip over the rake face [38].

It is observed that during machining of steel rods(AISI 1040 and E4340C steel) with carbide insertshaving grooves along the cutting edges and hills onthe tool rake face, as shown in Fig. 23, cryogeniccooling reduces the average cutting temperature,because the aforementioned geometry has helpedthe cryogenic jet to come closer to the chip–toolinterface, thus effectively cooling the interface [39].

Two different designs for cryogenic cooling with amodified tool holder were proposed. In design I, thegas is directed towards the tool cutting edge, to coolthe newly generated chips. This will enhance the chipbrittleness for easy chip breaking. The cryogenic fluidexit is formed closer to the tip of the cutting tool. Indesign II, the fluid exit is formed away from the tip ofthe cutting tool; as a result discharging gas is directedaway from the workpiece. This design is useful for

those materials in which excessive cooling has anegative effect on their ductility.

Analysing Fig. 24, it can be clearly seen that designII exhibited better wear resistance compared withdesign I. This is owing to the fact that, in the case ofcryogenic tool design II, the workpiece is cooled bythe nitrogen outflow, which discharges the evapo-rated gas away from the cutting edge and the chip.This will maintain the workpiece ductility, whilekeeping the tool insert itself at a very low tempera-ture, thus achieving better wear resistance. Thisdesign is recommended for cutting materials thathave strong temperature–ductility relations [34].Cryogenic cooling reduced main flank and auxiliaryflank wear as expected because the liquid nitrogen jetimpinged particularly along the auxiliary cuttingedge. During the turning of AISI-4037 steel undercryogenic conditions, the liquid nitrogen jet has beenused mainly to target the rake surface and flank sur-face along the auxiliary cutting edge [40].

5.1 Benefits of cryogenic cooling

Cryogenic cooling provides the following benefits.

1. Optimization of the flowrate and applicationpressure of liquid nitrogen is important to obtaincontinuous flow of liquid nitrogen without over-cooling the workpiece. If overcooling of theworkpiece takes place, a greater cutting force isrequired. Further, this may lead to embrittlementof the work material. Proper positioning of thenozzle is also required to protect the flank, rake,and auxiliary flank face.

2. The cutting force required in cryogenic cooling isless than that required for dry cutting. This isbecause application of cryogenic fluid reducesthe coefficient of friction at the interface of thetool–chip over the rake face.

3. The cryogenic cooling effect decreases slightly athigher cutting speed; this may be attributed tothe fact that with the increase in cutting speed,the chip–tool contact tends to become fullyplastic, obstructing penetration of the cryogeninto the hot chip–tool interface. Grooves alongthe cutting edge of the tool may be advantageousin high-speed cryogenic cutting.

4. At a higher feed rate chip thickness is higher;plastic deformation at the shear zone takes placeat a faster rate, generating more heat. Therefore,cryogenic cooling is more effective at higher feedrate.

5. By selectively applying liquid nitrogen to the chipand tool rake face through using a well-controlledjet, tool life can be enhanced. Microtemperaturemanipulation with cryogenic cooling is thebest means of chip control in the machining ofdifficult-to-cut materials.

Fig. 23 Carbide insert with grooves along the cutting edge

Fig. 24 Tool wear for different designs



6. During turning with carbide tools under cryo-genic cooling, notching, abrasion, adhesion, anddiffusion-type wear can be retarded effectively,leading to remarkable improvement in tool wear.

5.2 Limitations of cryogenic cooling

There are a few drawbacks of the cryogenic coolingmethodology. The cost of the set-up is high. Some-times frozen action at the nozzle obstructs the flowof the fluid and overcooling of workpiece can eventake place, which can affect the performance of theprocess.

6 CHAMFERED/HONED/CURVILINEAREDGE TOOLS

The design of cutting edge geometry and its influenceon machining performance has been a research topicin metal cutting for some time. Emerging machiningtechniques such as hard turning, hard milling, andmicromechanical machining, where the uncut chipthickness and the tool edge dimension are of thesame order of magnitude, require cutting edgeswhich can withstand high mechanical and thermalstresses, hence wear resistance, for a prolongedmachining time [41]. Research on metal cuttingmainly focuses on machining with sharp edgeinserts/tools. The investigation of tool geometryfocuses on categories such as (a) the tool edge geo-metry and (b) the tool rake geometry. Tools with achamfered edge are used for machining hard mate-rials owing to their edge strength. A chamfered cut-ting tool traps the work material over the chamferededge and the formed dead metal acts like a cuttingedge, which increases the tool edge strength andreduces tool wear [42]. In PCBN cutting tools, severaltypes of edge preparation can be made for hardturning operations, including sharp edge (with noadditional edge processing to strengthen the edge),chamfers, hones, and chamfers plus edge hones, asshown in Fig. 25. However, in most cases, chamferwith edge hones is the preferred edge preparation inhard turning [43]. The cutting tool edge geometry,which means the chamfer angle, chamfer width, andedge hone, has a significant influence on tool life and

to a large extent determines the surface finish andsurface integrity of the machined part. Tool wear, onboth flank and rake face, constitutes a change in edgegeometry. The feeds and depths of cut used in finishhard turning are relatively small (0.2mm), and gen-erally of the same magnitude as that of tool edgegeometry. As a result, cutting is confined to a smallarea on the nose radius and edge. Owing to theextreme hardness of the workpiece in hard turning, anegative rake angle with strong edge geometry with achamfer and hone is employed to withstand the highcutting forces, stress, and temperature that are gen-erated during turning [44].

As the material is harder, specific cutting forces arelarger than in conventional turning and thus theengagement between cutting tools and the workpiecemust be limited. The small cutting depth requiredmeans that cutting takes place on the nose radius ofcutting tools, and the tools are prepared with cham-fered or honed edges to provide a stronger edgegeometry that is less prone to premature fracture[45]. Cutting with a chamfered or honed edge equatesto a large negative effective rake angle, while neutralor positive rake angles are typical in conventionalmachining. The large negative rake angle yieldsincreased cutting forces compared with machiningusing positive rake tools, and also induces largercompressive loads on the machined surface. Duringthe finish turning of AISI 52100 bearing steel (hard-ness 60HRC with PCBN inserts having chamferedgeometry 0, 10, 20, 30�, honed edge radius 0.01mm,and chamfered width 0.1mm), a very small feed rateand depth of cut are used. Cutting area is confined toa small area in the front tip of the cutting edge, or inthe area of the chamfer zone. As shown in Fig. 26,cutting forces increase with the increase of thechamfer angle. The passive forces (Fp) in the passivedirection are higher than the primary cutting forces(Fc) in the cutting direction and increase morerapidly with the increase in chamfer angle [6].

There is an optimum value of chamfer angle wherethe tool life has maximum value. As shown in Fig. 27tool life reaches its maximum up to a 15� chamferangle and after that it reduces drastically. The tool lifewas measured up to the value of 0.2mm flank wear toavoid the excessive white layer induced on theworkpiece surface owing to the higher temperatureunder large flank wear. As determined by FEA thecutting edge with a 15� chamfer angle has the smal-lest value of flank wear when compared with othercutting tools [46].

The benefits of honed/chamfered edge tools seemmore obvious when turning commercially availableInconel-718 using PCBN tools. Different cutting edgegeometries are used during turning. The cutting edgepreparations employed include large chamfer(100mm· 30�) CWI, small chamfer (100mm · 20�)

Fig. 25 Chamfered/honed edge tools and their functionduring cutting



CWII, and large chamfer plus honed edge (100mm ·30�) CH to eliminate edge-related problems duringmachining such as edge chipping, cracks, andbreakage. It is observed that the cutting force com-ponent is significantly higher (2–3 times) in magni-tude than the other force components. The cuttingforces reduce considerably at 475m/min cuttingspeed, whereas the magnitude of forces is higher at125m/min cutting speed. This can be attributed tothe increased thermal softening of the work materialat higher cutting speeds [47].

As shown in Fig. 28, it is observed that the overallmagnitude of radial force is not affected because itacts more or less parallel to the chamfered cross-section on the cutting edge. As the tool wearprogresses it affects the rake angle negatively andthus increases cutting force; even by increasing thecutting speed the force component does not reduce.Increasing tool tip radius causes the main cuttingforce to increase [48].

The geometry of the cutting edge and its pre-paration can play a significant role in insert perfor-mance, directly affecting tool life, surface finish, andsurface integrity. Turning tests were performed onnickel-based alloy Inconel-718�, with Al2O3-based,

Al2O3-basedþ SiC, and PCBN inserts having modifiededges. The round edge inserts were best in termsof tool wear and surface roughness. The round cera-mic inserts produced a compressive layer on themachined surface, which is beneficial for fatigueresistance. Notching was a common phenomenonobserved on all the inserts. It was possible to producesurface roughness Ra below 0.5mm when turningInconel-718� at 500m/min. Such results stronglyencourage the use of modified edges for work mate-rials where better fatigue resistance is required [49].

The influence of hone radius and chamfer angle issuch that when the values of both the variablesincrease, burnishing at the cutting edge becomes adominant factor in chip formation. The burnishingprocess is prone to producematerial side flow, leadingto the deterioration of the final surface quality. It wasalso observed that hone radius had more of an influ-ence on the cutting edge temperature than chamfer.When hone radius increases from 20 to 100mm (at achamfer angle of 20�), the maximum edge tempera-ture increases to more than 90 �C. Therefore, usingchamfer plus hone for cutting edge preparation isbetter from the tool wear point of view, if the requiredresidual stress level can be reached [50].

The chamfered tool helps to increase compressiveresidual stress but its effect is less than that ofincreasing the hone radius. It is recommended thatchamfer plus hone radius be used to obtain the bestresidual stress profile. The best approach is to uselarge-hone-radius cutting tools [51]. The finishingcondition of Ra 0.2mm is attained only for feed ratessmaller than 0.06mm/rev while machining with thechamfered tools. Increase in productivity togetherwith good finish can be achieved with these tools, ifonly the last pass of the cutting is made using theseconditions. The cutting forces developed in hardmetal turning with chemical vapour deposited (CVD)diamond tools increase with the bluntness of the

Fig. 26 Correlation of cutting force and chamfer angle

Fig. 27 Analysis of tool life with chamfer angle

Fig. 28 Undeformed chips and cutting forces with thechamfered tool



cutting edge in the following order: sharp< chamfer<hone [52].

During the orthogonal turning of AISI 4340 steel,waterfall-hone (oval-like edge) tools yielded lowerforces than honed and chamfered tools. Waterfallhone with 20:40mm edge dimension yielded thelowest thrust forces. It was clear from the experi-mental results that, as edge radius increases, thrustforces increase. A large edge radius is not suitable formachining low uncut chip thickness. The purpose ofusing cutting tools with curvilinear edges is to protectthe cutting edge from chipping, to improve its impactresistance, and to increase surface area of heattransfer on the cutting zone [41].

Chamfered tools are usually used in rough andinterrupted turning. The stable trapped material(dead metal zone (DMZ) or cap) in front of thechamfered cutting edge increases the strength of thetool tip; however, it also increases cutting forces [53].Honed tools are employed in finish-turning opera-tions since the application of hone to the tool tipincreases the impact resistance. Waterfall hone edgegeometry combines the appropriate characteristics ofchamfered and honed tools such as increased tool tipstrength and increased rake angle; the oval-like geo-metry eases the flow of work material in front of thetool [54].

6.1 Benefits of chamfered/honed/curvilinearedge tools

The following benefits are provided by chamfered/honed/curvilinear edge tools.

1. The chamfer angle has a great influence on thecutting force and tool stress. All cutting forcecomponents increase with an increase in thechamfer angle, especially the level of passiveforce.

2. An increase of chamfer angle will increase toollife up to a certain value; after that the tool lifedecreases. This increase of tool life is attribu-table to the increase in wedge strength of thePCBN tool.

3. The magnitude of all the cutting forces is lowerat higher cutting speed than at lower speed; thismeans the honed and chamfered cutting edgegeometry influences the cutting forces onlywhen the MRR is low.

4. A higher value of chamfer angle produces lowsurface roughness at higher cutting speed.

5. During turning with ceramic tools, increasingthe tool-tip radius causes the main cutting forceto increase.

6. Edge radius must be selected according to cut-ting conditions. Large edge radius is not sui-table for machining low uncut chip thickness.The ratio of uncut chip thickness to edge radius,

which is approximately equal to 3, seems to bean appropriate ratio for edge preparations usedin the cutting tests.

7. Curvilinear edges protect the cutting edge fromchipping, improve its impact resistance, andincrease surface area for heat transfer fromthe cutting zone. Curvilinear edge preparationaffects the chip formation mechanism owing toincreased cyclical plastic deformations alongthe face of these edges.

8. Round inserts made of alumina-based ceramicC50 were the best, in terms of tool wear andsurface roughness. All edges presented notchwear and flank wear. Wear was less severe onround inserts with modified (M) edge. Theround ceramic inserts produced a compressivelayer on the machined surface, which is bene-ficial for fatigue resistance.

9. If the required residual stress profile can beassured, use of medium hone radius pluschamfer in cutting edge preparation is a goodoption to keep tool temperature and cuttingforce low. As the effect of chamfer is equivalentto the increasing hone radius, medium honeradius (0.02–0.05mm) plus chamfer angle of 20�

is recommended.10. A chamfered tool helps to increase compressive

residual stress but its effect is less than that ofincreasing the hone radius. Therefore, it isrecommended that chamfer plus hone radius beused to obtain the best residual stress profile forturning thorough hardened AISI-52100 bearinggrade steel.

11. The cutting forces developed in hard metalturning with CVD diamond tools increase withthe bluntness of the cutting edge in the follow-ing order: sharp< chamfer<hone.

6.2 Limitations of chamfered/honed/curvilinear edge tools

There are a few drawbacks to the methodology; forexample, the amount of force involved in cuttingincreases with these configurations of tools, which inturn demands high-rigidity machine tools.

7 RESTRICTED CONTACT/GROOVED TOOLS

With the development of advanced manufacturingtechnology, metal machining operations are nowbeing carried out at high speed to secure maximumproductivity. The disposal of long continuous chipsproduced at high cutting speeds has posed a problemfor industry in the age of automation. For easy dis-posal of chips, the volume of chips relative to thevolume of the same material in bulk should be as low



as possible. It is reported that well-broken chips havea volume ratio of between 3 and 10, whereas entirelyunbroken chips have a volume ratio of 50. Long chipscurl around the tool and can pose serious hazardsto the workpiece surface, the operator, and themachine-tool operations. The situation becomesmore critical in the environment of automatedmachine loading, unloading, and in-process inspec-tion of the machined parts [55]. To overcome thisdifficulty, a number of researchers have investigatedeffective control of chip flow and chip breaking.Using an obstacle across the chip flow direction,commonly known as a chip breaker or chip former,chip curl can be controlled. Broadly speaking thereare two types of chip breakers: obstruction type andgroove type. The obstruction type is further dividedinto two categories: the step-type chip breaker andthe ram-type chip breaker. An experimental investi-gation of metal turning with a ram-type chip breakerwas carried out by Nakayama [56]. The effect ofincreasing feed was an increase in length of contactbetween chip and tool, which resulted in extension ofthe heated area further from the edge to the rake face,accompanied by an increase in the maximum tem-perature. The artificial reduction of chip–tool contactlength substantially reduces the power consumed bythe cut and as a result, heat generated is reduced [57].

Restricted contact tools are also called artificiallycontrolled contact tools. Their land length is less thanthe natural tool–chip contact length for a given set ofcutting conditions [58]. As seen in Fig. 29 the area ofcontact is less; thus RCTs offer several advantages formachining processes, such as substantial reductionin power consumption, lower tool–chip interfacetemperatures, and improved surface integrity of themachined parts [57]. On the basis of turning testsunder a wide range of cutting conditions withcommercially available grooved tools, the resultsachieved are quite satisfactory. Many groove-typechip breaker tools, which have a primary rake landbetween the cutting edge and the chip-groove, havebeen widely applied in modern automatic machiningoperations [59]. These tools are also referred to asrestricted contact grooved tools. The restricted con-tact effect in machining with grooved tools forms a

basis for the generated chip to back-flow into thechip-groove, and hence it provides favourable con-ditions for the chip to be broken. In this study,restricted contact grooved tools produced from sin-tered carbides P20 were used to cut medium-carbonsteel under dry conditions. The results of experi-mentation are validated through the developedmodel. The chip back-flow angle is defined first.Considering the contact between the chip and thechip-groove back wall, the predicted chip back-flowangles are compared with their experimental resultsfor a given set of machining conditions. The impor-tance of chip back-flow angle is that it directly gov-erns the tool–chip contact pattern. Experimentally,chip back-flow angle is measured by using a high-speed filming technique. If the chip back-flow angleis less than the tool secondary rake angle, the gener-ated chip will be in contact with the primary rake faceonly; otherwise, the chip will be in contact with boththe primary and secondary rake faces, which adds tothe complexity of the machining processes. Studyshows that the state of stresses in the plastic defor-mation region is one of the most important factorsgoverning chip back-flow. Chip form/chip break-ability and tool wear/tool life are two of the majormachining performance measures that have been thesubject of extensive study over several decades.Understanding the chip flow and curl mechanisms isessential for predictive assessment of the chipbreaking process. In practical machining operationssuch as turning, grooved tools are used rather thanflat-faced tools, effectively to curl and break the chipsinto small sizes and shapes for handling and disposalpurposes. It has been shown that the nature of chipcurl and breaking contributes to the decrease in toollife and reduction in cutting forces [60].

The beneficial effect of controlled contact cuttingwith respect to energy reduction became more pro-nounced with an increase in feed rate and depth ofcut. During the turning of C45 carbon steel, chro-mium alloy steel, and austenitic steel with coatedindexable inserts of triangle negative molded chip-breaker geometry (TNMG) having four different chipgroove geometries, results reveal that specific cuttingenergy seems to be governed (remarkably) by thecontact length. Chip breaking increases steadily athigher feed rates, and savings in cutting power con-sumption can be achieved by controlling the contactlength at higher feed rate and depth of cut [61].

To obtain an optimum tool performance for pro-viding good chip breakability and tool life, it appearsthat the contact length should be in the range of55–65 per cent of the natural contact length. Theassociated benefits resulting from the effects ofrestricted tool contact are a tendency towards mini-mum power consumption, lower cutting forces, andreduced cutting temperature [62]. Figure 30 shows

Fig. 29 Restricted contact cut-away and grooved-typechip-breaker tools



actual temperature distribution at various points inthe groove, during the machining of AISI 1045 steelwith coated (TiN/TiCN/TiC) grooved tools. It wasobserved that after 50 s of machining, the maximumtemperature was located at the secondary face ofthe tool and after an additional 190 s the back wallarea temperature increased by approximately 13 percent and the location of the maximum temperaturewould be an area of high tool wear rate. Thus tem-perature distribution clearly indicates the advantageof grooved tools [63].

Inappropriate design of the chip grooves orobstructions can cause ‘unfavourable’ flow of thechip, resulting in wear of the obstruction and con-sequent tool failure. At low feed conditions, i.e. forcases where the tool face land is large with respect tothe feed, chip groove utilization is very low and thechip strikes the back wall directly and leads to abra-sive wear, when the feed is very high compared withthe tool face land. Smallness of the land not onlymakes the tool edge vulnerable to chipping, but alsothe high feed conditions force the chip to have amuch larger back-flow. The chip in this case is forcedto remain in contact with a larger section of the rakeand groove-face (inner wall) and therefore curlsnaturally before coming in contact with the backwall. The uneven wear pattern results in failure of thetool edge without significant wear of the back wall. Insuch cases, extremely high contact pressure at thetool nose results in accelerated nose wear.

As shown in Fig. 31(c) there is an optimum utili-zation of the chip groove, in which a major part of thechip groove is utilized. The uniform pressure on thetool edge and the back wall normally results in acombination of wear of the nose, rake face, and theback wall. This combination wear generally results inmore predictable tool wear and longer tool life. Forconditions involving lesser side-curl, the chip defor-mation on the inside surface is less. This results inlower formation of burrs and, consequently, lesserdepth of notch wear [60].

The target for a tool designer in using restrictedcontact length is to produce acceptable chip formand to reduce cutting forces. The common factoraffecting both tool life and chip form is the contactlength. In general, tool–chip contact length decreasesif the cutting speed is increased; tool–chip contactlength increases if the feed increases; and tool–chipcontact length increases with increasing depth of cut.During turning of carbon steel with carbide tools withvarious restricted contact lengths, reduction of thecontact from the natural length leads to reduction inflank wear. If the contact length is too heavilyrestricted, tool wear will increase rapidly. Extremereduction in contact length also leads to a con-centration of high specific compressive stress on thissmall area. The optimum contact length wouldappear to be in the range 55–65 per cent of the nat-ural contact length. For a low combination of cuttingspeed and feed, the tools with restricted contact

Fig. 30 Tool tip temperature distribution after 50 s and 240 s of cutting

Fig. 31 Different tool wear mechanisms in grooved tools



length show a low too1 temperature. For a highcombination of these cutting data, tools give betterperformance for less time and, after that, perfor-mance deteriorates [64]. Deshayes reported that thetool with chip-breaker geometry has the smallestfriction angle. For restricted contact length tools, theminimum and maximum speed ranges were decidedon the basis of stability in the specific cutting energyas shown in Fig. 32 [65].

7.1 Benefits of restricted contact/grooved tools

The following benefits have been derived by use ofrestricted contact/grooved tools.

1. The optimum performance of RCTs mainlydepends on selection of chip-breaker groovegeometry and combination of different cuttingparameters. Using a restricted contact length inthe low range of the feed produces a naturalcontact length which is smaller than the restric-ted contact length, and the chip does not reachthe chip-breaker groove. This means that thefunction of the chip-breaker groove is suspendedand the tool with the groove behaves like a toolwith a natural contact length. Also, using arestricted contact length in the high range of thefeed produces a natural contact length, which isseveral times larger than the restricted contactlength. In this case the chip breaks into a shortsegment of dark blue colour combined withvibration and high cutting forces. Thus the use ofrestricted contact length in the correct range ofcutting data leads to a controlled chip form and afavourable process in the sense of low cuttingforces, low tool temperature, and long tool life.

2. The beneficial effect of controlled contact cuttingwith respect to energy reduction becomes morepronounced with an increase in feed rate anddepth of cut.

3. While machining with RCTs, the cutting tem-peratures corresponding with efficient chip con-trol are in general higher than for chips, which isassumed to be unacceptable. This is due to thefact that chip breaking is usually related to anincrease in feed rate, which in turn results in anincrease in the chip–tool interface temperature.

4. The failure of grooved tools is mainly attributableto improper groove utilization by the chip andthis has resulted from either poor chip groovedesign or inappropriate application of the cuttingconditions for a particular chip groove.

Further it is suggested that the secondary edge formof wear can be averted by causing the chip-flow angleto be larger (e.g. by reducing depth of cut, and so on),so that the chip strikes the back wall instead of theminor edge.

7.2 Limitations of restricted contact/groovedtools

There are a few drawbacks of this methodology; forexample the technique is only feasible in tool insertsproduced through powder metallurgy. Also, the useof restricted contact length in a high range of cuttingdata leads to the cutting edge being exposed to a highconcentration of specific compressive stress andtemperature, which in turn leads to shorter tool lifecaused by the rapid plastic deformation of the cuttingedge.

8 MINIMUM QUANTITY LUBRICATION

Metal cutting fluids change the performance ofmachining operations because of their lubrication,cooling, and chip-flushing functions, but the use ofcutting fluid has become more problematic in termsof both employee health and environmental pollu-tion. The minimization of cutting fluid also leads toeconomic benefits by way of saving lubricant costsand workpiece/tool/machine cleaning cycle time [1].Nearly a decade ago the concept of MQL was sug-gested as a means of addressing the issues of envir-onmental intrusiveness and occupational hazardsassociated with airborne cutting fluid particles onfactory shop floors.

As far as lubrication is concerned, the load appliedand the working conditions, which characterize thecut, suggest that it is impossible to lubricate thecutting area continuously by fluid film lubrication.So, to guarantee lubrication, it is necessary to uselubricant with additives that react chemically withthe workpiece and tool material to generate chemicalcompounds that allow lubrication of the cuttingsurface. Moreover, workpiece cooling is necessary toremove the heat generated during the chip formation

Fig. 32 Analysis of cutting speed with specific cuttingenergy (where Kcc is the specific force for Fc, Kcf isthe specific cutting force for Ff, Kcp is the specificcutting force for Fp)



and by the friction between tool and workpiece. Toreach the cutting surface is not easy; in fact the highcutting pressure in the contact area and the smallspace between chip and tool do not allow the cuttingfluid to access this zone. To obtain a good coolingaction, the cutting area is generally flooded withlubricant. Finally, the cutting fluid flow can be usedto prevent the chip remaining in the cutting zone,thereby reducing the possibility of damaging theworkpiece. Many application methods can be usedfor this and each method is selected depending onthe advantages that it provides [66].

The main types of application are listed as follows.

1. Hand application: this type of application is usedonly in small batch production, because whenusing this lubrication method it is not easy toapply the cutting fluid continuously and suffi-ciently to cool the workpiece. A low level oflubrication, cooling, and chip removal are guar-anteed.

2. Flooding: this application is the most common. Avery good level of lubrication, cooling, and chipremoval are guaranteed. Applying this method oflubrication, it is also possible to orient the nozzleto the clearance tool surface, thereby reducingthe flank wear, especially when the cutting speedis slow.

3. Minimal quantity lubrication: in MQL a verysmall lubricant flow (ml/h instead of l/min) isused. In this case, the lubricant is directlysprayed on the cutting area. A good level oflubrication is guaranteed, but the cooling actionis very small and the chip removal mechanism isobtained by the airflow used to spread thelubricant.

Two different mixing methods can be used forMQL: mixing inside the nozzle and mixing outsidethe nozzle. Using equipment with mixing inside thenozzle, pressurized air and lubricant are mixed intothe nozzle by a mixing device, as shown in Fig. 33(a).The lubricant performs the lubrication action, whilethe pressurized air that reaches the cutting surfaceperforms the cooling action. This method has severaladvantages. Mist and dangerous vapours are reducedand the mixture setting is very easy to control. In themethod of mixing outside the nozzle, as shown inFig. 33(b), the mixture is obtained in a mixing devicepositioned in a specific tank. Also, in this case,lubrication between workpiece and tool can beachieved.

In MQLmachining, a small amount of vegetable oilor biodegradable synthetic ester is sprayed on to thetool tip with compressed air. The consumption of oilin industrial applications is in the range of approxi-mately 10–100ml/h. Machining using MQL is nearly

equal to or often better than traditional wetmachining in terms of tool life [67].

During finish turning of Inconel-718 with coatedcarbide tools (with chip breakers) under MQL (bio-degradable synthetic ester as lubricant), the cuttingfluid was supplied to the cutting point with com-pressed air through oil holes on both the flank andrake faces of the tool, as shown in Fig. 34. Compara-tive analysis of oxygen and argon as carrier gasreveals that the poor heat capacity, poor thermalconductivity, and poor lubrication characteristics ofargon gas have increased cutting temperature andtool wear. Extending the quantity of lubricant canonly help in improving the surface finish.

In turning normalized 100Cr6 steel using com-mercial triple-coated carbide tips with a negativerake angle, tool flank wear was studied under MQL(inside nozzle mixing device) and dry cutting as perguidelines of the ISO 3685 standard. Comparison ofdry, rake MQL, and flank MQL for surface roughnessand tool wear was made as mentioned in Figs 35(a)and (b). Under similar cutting conditions, thevolumes removed with flank MQL are equal to orgreater than those achieved with the other condi-tions. For fixed cutting length, a rise in feed ratealways causes a reduction in tool lifetime, but withflank MQL, tool life lies consistently above the others.

Fig. 33 Different methods of mixing lubricant in MQL

Fig. 34 Holes in the tool holder for MQL



Also, it is observed that dry cutting and rake faceMQL generally have the same behaviour. It isassumed that when MQL is applied on the rake sur-face, the lubricant does not reach the cutting surface.Furthermore, energy dispersive X-ray microanalysisof tips used in rake MQL conditions does not showany presence of chemical compounds on the wornsurfaces. This means that, when MQL is applied onthe rake surface, lubricant does not reach the cuttingarea. Under this condition it is impossible to reducethe tool wear. As far as the influence of cutting lengthis concerned, the mean tool life in flank MQL condi-tions increases with cut length, whereas in dry andrake MQL conditions, cutting length does not influ-ence life. Traces of lubricant compounds have beenfound on the worn surfaces only when MQL has beenapplied on the flank surface [68].

The MQL jet has been used mainly to target therake and flank surface and to protect the auxiliaryflank to enable better dimensional accuracy. The rateof average principal flank wear is decreased by MQL.This is attributable to reduction in the flank tem-perature by MQL, which helps in reducing abrasionwear by retaining tool hardness, and also adhesionand diffusion types of wear, which are highly sensi-tive to temperature. Also temperature control byMQL reduces the growth of notch and groove wearon the main cutting edge. Surface roughness duringMQL reduces mainly through controlling the dete-rioration of the auxiliary cutting edge by abrasion,chipping, and BUE formation [69].

8.1 Benefits of MQL

The benefits of MQL are listed as follows.

1. During turning with coated tools under MQL,optimization of air pressure is needed if oxygen isused as the carrier gas for applying MQL to finishturning, because by increasing the air pressurethe oxidation of the coating is accelerated byabundant oxygen.

2. The cooling efficiency depends on the specificheat of the coolant gas. A coolant of higher

specific heat can receive more heat from the tooland workpiece. Thus air acts as a better carriergas in comparison with argon.

3. With the MQL technique, a remarkable reductionof machining costs can be obtained because thequantity of lubricant used is small.

4. Lubricating the rake surface of a tip by theMQL technique does not produce evident wearreduction.

5. Lubricating the flank surface of a tip by the MQLtechnique reduces the tool wear and increasesthe tool life.

8.2 Limitations of MQL

There are a few drawbacks of the MQL methodology,for example, if the mixture is not properly controlledit may lead to the formation of mist or dangerousvapours, and thus contamination of the workingenvironment.

9 HIGH-PRESSURE COOLANT

High-pressure coolant delivery is an emerging tech-nology that delivers a high-pressure fluid to the tooland machined material. The high fluid pressureallows a better penetration of the fluid into thetool–workpiece and tool–chip contact regions, thusproviding a better cooling effect and decrease in toolwear through lubrication of the contact areas.

In finish turning of AISI 1045 steel using coatedcarbide tools under high-pressure fluid (with highand low flowrates), dry cutting, and conventionalfluid application (low pressure, high flowrate), thetool wear was studied. For HPC three directions ofhigh-pressure fluid were used: (a) towards thechip–tool interface (tool rake face); (b) towards theworkpiece–tool interface (flank face); and (c) towardsboth flank and rake face.

The longest tool lives were obtained when fluid wasapplied either simultaneously on the rake and flankfaces with high pressure and high flowrate, or when itwas applied solely on the flank face with high pres-sure and low flowrate. When fluid was injected on the

Fig. 35 Tool lives and surface finishes of the three coated tools in MQL, wet, and dry cutting: (a) tool life;(b) surface finish at tool life



rake face, the adhesion between chip and tool wasstrong, causing the removal of tool particles. Whenthe adhered chip material was removed from the toolby the chip flow, this resulted in a large crater wear.The fluid was not able to penetrate between chip andtool to perform lubrication, since no fluid elementswere found on the crater wear region in the energydispersive X-ray analysis [70].

Tool life generally increases with increase in cool-ant supply pressure. This can be attributed to theability of the high-pressure coolant to lift the chipand gain access closer to the cutting interface. Thisaction leads to a reduction of the seizure region, thuslowering the friction coefficient, which in turn resultsin reduction in cutting temperature and cutting for-ces. Compared with conventional coolant supplies,tool life improved as much as 740 per cent, whilemachining at 203 bar coolant pressure at a speed of50m/min. Chip segmentation depends on the cut-ting conditions employed and further, to a greaterextent, on the coolant pressure [71]. Lower cuttingforces were generated while machining Inconel-718with a whisker-reinforced ceramic tool at highercoolant supply pressure, owing to improved coolingand lubrication at the cutting interface. Thisimprovement may be the result of chip segmentationcaused by the high-pressure coolant jet. Tool lifedecreased with 20.3MPa coolant supply pressureowing to accelerated notching of the tool [72].

9.1 Benefits of HPC

The following are the major benefits of HPC.

1. Tool life increases with increasing coolant pres-sure supply; once a critical value of pressure hasbeen reached, any further increase in coolantpressure will only result in marginal increase intool life.

2. During machining of aerospace alloys at highcoolant pressure, well-segmented C-shapedchips are generated. Thus it is clear that chipsegmentation depends to a greater extent on thecoolant pressure employed.

3. Low cutting forces are generated owing toimproved cooling and lubrication with HPC.Surface finish is acceptable and free from physi-cal damage such as tears, laps, or cracks inalmost all the cutting conditions.

4. During turning of hard metals with CBN tools,low-CBN-content tools give better performanceunder HPC in terms of tool life and reducednotch wear.

9.2 Limitations of HPC

Drawbacks of this methodology include the fact thatpressure generated by the fluid in the nozzle may

produce certain subsurface defects on the newlygenerated surface.

10 CONCLUSIONS

Conclusions with respect to the various technologiesused in turning which have been reviewed in thispaper are listed below.

1. Many researchers have reported their work onsupplementary techniques in turning, which canhelp in improving the efficiency of commonlyused turning tool materials such as HSS, carbides(coated/uncoated), ceramics, and even for CBN.It has been clearly indicated in the literature thatthe use of these supplementary techniques suchas cryogenic treatment, UAT, hot machining,grooved tools, MQL, HPC, cryogenic cooling,chamfered/honed/curvilinear edges, and use ofsolid lubricants has improved the machiningproductivity of difficult-to-cut materials andsuper alloys.

2. The application of cryogenic cooling for turningof difficult-to-cut materials has resulted in aseveral-fold increase in tool life without com-promising the environmental conditions. Toollife improves dramatically owing to the fact thatcryogen is able to penetrate the chip–tool inter-face and perform both lubrication and coolingfunctions satisfactorily. Productivity is also high,as cryogenic cooling shows better results athigher feed rates.

3. Low tool flank wear and cutting forces in UAThave helped in improving cutting efficiency. As aresult of the non-occurrence of BUE in UAT,surface finish is improved compared with con-ventional turning.

4. Cryogenic treatment has enhanced the wearresistance of HSS and carbide tools by bringingabout metallurgical changes in the micro-structure of these tool materials. If these tools aresupplemented with additional cooling, tool lifeimproves dramatically.

5. In hot machining the effect of the temperature ofthe workpiece is found to be the most significanton tool life. However, the recrystallization tem-perature of the workpiece limits the maximumvalue of the temperature of hot machining. If thecutting tool is simultaneously cooled during hotmachining, the effectiveness of the turning pro-cess improves. This process, commonly knownas hybrid machining, has shown good potentialin turning of super alloys.

6. During the turning of hard alloy steels with cer-amic/CBN tools, with chamfer plus honed, cur-vilinear edges, the tool geometry protects thecutting edge from chipping, to improve its



impact resistance. It also helps in increasing thesurface area for heat transfer from the cuttingzone.

7. Machining with RCTs having proper chipgroove design and optimum selection of cuttingparameters in a particular moderate feed ratewill lead to efficient chip control, energy re-duction, less heat generation, and lower cuttingforces.

8. With the MQL technique, a remarkable reductionof machining cost, quantity of lubricant used,and surface roughness can be achieved by prop-erly orienting the nozzle on the flank face of thetool.

9. Turning with the HPC technique gives rise toformation of segmented chips and better pene-tration at the interface, and thus lower cuttingforces, better tool life, and acceptable surfacefinish.

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Advances in the turning process for productivity improvement – a review-2008