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Int J Adv Manuf Technol (2002) 19:157162 2002 Springer-Verlag London Limited

On the Dry Machining of Steel Surfaces Using Superhard ToolsA. G. Mamalis 1 , J. Kundrak 2 and K. Gyani 21 Manufacturing Technology Division, Department of Mechanical Engineering, National Technical University of Athens, Greece; and2 Department of Production Engineering, University of Miskolc, Hungary

Coolant for cutting and grinding not only increases the pro-duction costs, but also damages the environment and health of the employees. Therefore, attention should be directed towardsmachining processes, where the use of coolant can be reduced signicantly or even stopped. Analysis of the cutting processesand the tool materials throws light on the area where wet,moist, or dry machining are applicable.

Dry machining with CBN tools, so called hard turning, performed on hardened steels has produced very favourableresults, which are applicable in industry. The characteristicsof the surface integrity with dry machining are more favourablethan with grinding and the operation costs are reduced. Inhard turning, the compressive residual stress eld developed,in contrast to the tensile stresses developed in grinding,increases the fatigue life of the machined components.

Keywords: Dry machining; Fatigue life; Hard turning; Pre-cision grinding; Residual stress elds; Surface integrity

1. Introduction

Application of coolants in cutting is widespread. Their coolingand lubricating effect ensures the economical tool life of certaintool materials, e.g. tool steels (TS) and high-speed steels (HSS),since their washing effect results in the cleanliness of themachined workpieces, and higher production and good surfacequality are obtained. However, besides these favourable charac-teristics, the cutting coolant both evaporates and is washedaway; this lost coolant has a very harmful effect on theenvironment. In addition, the purchase, handling, and properdisposal of the coolant constitutes a signicant cost. These twofactors, i.e. the environmental pollution and the cost, havebecome important and, thus, work is directed towards drymachining. In this eld, new tool materials provide manyadvantages because cutting is performed under dry conditions,

Correspondence and offprint requests to : Professor A. G. Mamalis,Manufacturing Technology Division, Department of Mechanical Engin-eering, National University of Athens, 42, 28th October Avenue, 10682 Athens, Greece. E-mail: mamalis central.ntua.gr

and their productivity is also high. Grinding hardened steels,which requires a large amount of coolant, can be replaced bydry machining.

Environmental protection has attracted much attention inmanufacturing. In metal removal processes, environmentallyfriendly methods of applying coolant can be classied as fol-lows:

1. Modifying the composition of the coolant by:Application of composition without oil.Using synthetic materials.Preference for natural materials.Application of biologically breaking down materials.Application of materials with signicantly longer tool life.

2. Reducing the amount of coolant by:Programmed feeding of coolant.Optimised dosing.

Supervised conduction.3. Minimising the amount of liquid (minimal cooling) by:

Using less then 50 ml h 1 of liquids.Inner conduction through the tool.Externally controlled conduction with special dosing equip-ment.Application of liquid mixed with air.

4. Application of coolant not in liquid state by:Cooling with compressed air or with cold gas.Cooling with solid coolant.Using impregnated tools.

5. Dry machining

This modied environmentally friendly cooling process is appli-cable in the case of superhard tools and of ceramic tools.These tool materials do not require any cooling because theypossess outstanding wear resistance and heat strength. Fordiamonds, heating is avoided because of their excellent heatconductivity. However, with the traditional, but still widelyused, tool materials (HSS, TS), the possibility of environmen-tally friendly cooling has to be examined in each case.

Grinding requires a larger amount of coolant than all theother kinds of metal removal processing because of the very

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high contact temperature (1000 C) and the necessity for wash-ing. Two possibilities emerge:

Replacing grinding with hard turning.Application of new or dry grinding.

The cooling conditions in metal removal processing, dependingon the machine tools used, are given in Fig. 1.

In the present paper, these new tendencies in metal removalprocessing, with minimal cooling and dry machining, are inves-tigated. The surface integrity of hardened steel surfaces sub- jected to hard turning and grinding and the residual stresselds developed are reported and discussed.

2. The Effect of Minimal Cooling and DryMachining on the Cutting Parameters

Minimal cooling, and dry machining combined with therequirements of environmental protection, as outlined above,have turned work on cutting in a new direction. At the moment,the initial results are very promising and further breakthrough

results are expected, in spite of the fact that changing thecooling method requires changing all the cutting parameters(ank wear, tool life, cutting strength, speci c energy, contacttemperature, etc.) which leads to a change of the surfaceintegrity (surface roughness, residual stresses, material micro-structure of the surface layer, etc.). The user is interestedmainly in the tool life and the reliability of the tool, expressedby the fatigue limit, which is mainly related to the residualstress elds developed.

Surface integrity of machined surfaces has been extensivelyinvestigated in the past. In particular, a high correlationbetween the developed subsurface residual stresses and thefatigue limit was shown [1,2]. It was observed, during testing,that surface roughness is not the critical factor that it hastraditionally been assumed to be. It appears that the effects of roughness are overshadowed by the effects of residual stressesand the other factors of surface integrity. These experimentsindicate that there are alloys where surface roughness has verylittle or no effect on the fatigue strength.

When analysing the effect of surface roughness, the effectof tool marks, sharp corners, scratches and other geometricalconsiderations has to be taken into account, since these factorsconstitute sources of stress concentrations and reasons for

Fig. 1. Cooling depending on the tool materials.

premature failure due to fatigue. The relationship betweenresidual stresses, roughness, and fatigue limit is shown inFig. 2; it depends on the tool and workpiece materials, on theprocess combinations, on machining data, and on many otherfactors [3,4].

3. Experimental

Turning with CBN tools, often called hard turning, is moreand more frequently applied for the nishing of hardened orsuperhard steels and cast iron. This cutting process can replaceabrasive machining which has been applied previously, if it ismore economical and also meets the quality and accuracyrequirements of the parts.

Applications of hard turning have been widely used in theautomotive industry in recent years and, in particular, forprecision machining. The main advantages of hard turning arethe cost reduction of these operations and the increase of thefatigue limit of hard turned surfaces. Hard turning results incompressive residual stresses in the surface layer, which

increases the fatigue life. For rolling contact surfaces, it isvery important to generate compressive residual stresses, toincrease their fatigue strength limit [5].

Our hard turning experiments were performed under thefollowing conditions:

Workpiece material: hardened ball-bearing steel (100Cr6;HRC60 2) cylinders with a diameter d = 45100 mm.Cutting tools: superhard tools based on boron nitride for cuttinghardened steels. In this case, the chip removal is characteristi-cally in uenced by the physical-mechanical features of thepolycrystals (high hardness, high temperature conductivity, highresistance to wear and temperature) and their polycrystallinecharacter. The superhard tools used for the experiments were

Composite 01 (C01) and Composite 10 (C10) tool materialswith tool geometry o = 5 ; o = o = 15 ; s = 0 ; r = 45 ;r = 2 ; r = 15 ; b = 0.3 mm.

Machine tool: E400 1000 universal latheCutting parameters: cutting speed, vc = 10400 m min

1 ; depthof cut, a p = 0.050.4 mm; feedrate, f = 0.025 0.4 mm rev

1 .

Fig. 2. Relationship between surface roughness, residual stresses andfatigue limit at room temperature in cutting Inconel alloy [3].

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Dry Machining of Steel Surfaces 159

Our grinding experiments were performed under the follow-ing conditions:

Workpiece material: hardened ball bearing steel (100Cr6; HRC60 2) rings, with outer diameter, d w = 120 mm, inner diameter,d = 108 mm, and width, b = 20 mm. After grinding, a 100 mmarclength was cut off with Erosimat for residual stress measure-ments.

The xturing of the workpieces was on the inner diameterusing a exibly expanding precision xing device.Grinding wheels: wheel for traverse external grinding400 40 127 89A 60K 9V (Tyrolit); pre-strained wheel forinternal grinding 400 20 300 EK60 46L VX (special),grinding with its inner diameter, d w = 300 mm.Coolant: Syntilo-4 (synthetic, Castrol); the quantity used was30 l min 1 .Machine tools: KE-250 04 Cylindrical Grinding Machine andSI-4 Internal Grinding Machine.A pre-strained wheel is mounted in place of the spindle of theinternal grinding machine (Fig. 3). Note that, pre-straining of the wheel which was put in a frame was necessary, in orderto avoid bursting because of the high r.p.m.Grinding parameters: wheel speed, vs = 3060 m s

1 ; workpiecespeed, vw = 1530 m min

1 ; depth of cut, ae = 0.010.05 mm/two stroke; feedrate, v fL = 10 m/work rev, feedrate(for the pre-strained wheel), v fR = 16 m s

1 , spark out time,05 s.Dressing: single-point diamond 1.5 carat; depth of cut of diamond, a pD = 0.02 mm, three times; feed of diamond, f D = 0.1 mm/wheel rev, with coolant.Measurements of the residual stresses were made after grindingwith a medium worn wheel, a material volume, vw = 300 mm

3 .

Residual stresses, developed during the hard turning and theprecision grinding were measured on ring-shape samples, usingthe method of continuous stress release made by acid solution.Tangential residual tensions depending on the distance fromthe surface [6] were measured and the deformation was evalu-ated by means of an appropriate computer program.

Fig. 3. High-speed grinding machine equiped with special pre-strainedwheel.

4. Results and Discussion

Using the elaborated hard turning process and replacing thegrinding process with hard turning, many turning operationsand operations performed previously with grinding can beperformed in dry conditions. During our work, we investigatedthe attainable accuracy, the achievable surface roughness, thechange of the microstructure in the surface layer and thedevelopment of the residual stresses. In this paper, only theresults of the investigation of the residual stresses are reportedbecause this has the most signi cant effect on fatigue life of the parts.

4.1 Surface Integrity

4.1.1 Hard Turning

The distribution of residual stresses, R R, with depth below thesurface, h, for the workpiece turned with a superhard tool forvarious cutting conditions are presented in Figs 4 and 5. Themain characteristics of the distribution of residual stresses are

their maximum value and their distance from the surface.Examination of the surface and subsurface layers reveals anincrease in microhardness and the existence of compressiveresidual stresses. Note that, the distribution of microhardnessand its magnitude are in accordance with the distribution of residual stresses. In hard turning, performed under the sameconditions, the maximum microhardness corresponds with themaximum compressive residual stress.

The increase of the cutting speed results in a slight decreaseof the maximum of the compressive residual stress pro le, seeFig. 4( a ), whilst the feed has no effect on it, see Fig. 4( b). Aslight increase of the maximum residual stress is indicatedwith increasing depth of cut, see Fig. 4( c). Correspondingly,the depth of the layer, where the maximum compressiveresidual stress occurs, increases with increasing feed and depthof cut, while it decreases in the case of increasing cuttingspeed, see Figs 4( a ) to 4(c).

The ank tool wear in uences the depth of the compressiveresidual stresses, see Figs 5( a ) to 5(c). Cutting speed, depth of cut and feedrate, adjusted in cutting, have hardly any effecton the characteristics of the residual stresses in the surfacelayer and in the tangential direction (compare Figs 5( a ), 5(b)and 5( c)).

4.1.2 Precision Grinding

Residual stress distribution below the ground surface is shownin Figs 6 and 7. The speed of the wheel is the grindingparameter with the most in uence on the residual stresses forthe steel examined. The contact temperature, which increaseswith increasing wheel speed, results in microstructural changesand, subsequently, in volumetric changes of the structuralelements. Measurements show that the depth of cut doesnot increase, or increases much less than the residual tensilestresses [7].

During the grinding process the tensile residual stressesdeveloped are a direct consequence of the high temperature,which is developed in the wheel workpiece contact zone [5].

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Fig. 4. Distribution of residual stresses with depth below surface inhard turning of hardened ball bearing steel (100Cr6). ( a ) Effect of cutting speed, vc for constant depth of cut ( a p = 0.2 mm) and feedrate( f = 0.1 mm rev 1 ). 1, vc = 56 m min

1 ; 2, vc = 81 m min1 ; 3,

vc = 160 m min1 . (b) Effect of feedrate, f for constant cutting speed

(vc = 56 m min1 ) and depth of cut ( a p = 0.2 mm). 1, f = 0.1 mm rev

1 ;2, f = 0.15 mm rev. ( c) Effect of depth of cut, a p for constant cuttingspeed ( vc = 56 m min

1 ) and feedrate ( f = 0.1 mm rev 1 .) 1, a p = 0.1 mm;2, a p = 0.2 mm; 3, a p = 0.3 mm.

It is important to reduce the high tensile residual stresses, forexample by the increasing peripheral workpiece speed anddecreasing the depth of cut, see Fig. 6( a ). Note, also, thatanother way to reduce the high tensile residual stresses is theemployment of the spark-out process and the reduction of thewheel speed. From Fig. 6( b) it can be seen that, by reducingthe wheel speed from 60 m s 1 to 30 m s 1 and with 5 s spark-out time the tensile residual stresses decrease by 50%.

To reduce the tensile residual stresses and increase theproductivity, high-speed grinding was employed using thegrinding equipment shown in Fig. 3 [8]. The experimentalresults and the grinding conditions are shown in Fig. 7. The

Fig. 5. Distribution of residual stresses with depth below surface inhard turning of hardened ball bearing steel (100Cr6). Effect of ank wear, VB for constant depth of cut ( a p = 0.2 mm) and feedrate( f = 0.1 mm rev 1 ) and for varying cutting speed (1, vc = 56 m min

1 ;2, vc = 81 m min

1 ; 3, vc = 112 m min1 ; 4, vc = 160 m min

1 ). (a )VB = 0 mm (sharp tool). ( b) VB = 0.2 mm. ( c) VB = 0.4 mm (worntool).

employment of this new grinding method with a suitable spark-out time resulted in a signi cant reduction of the residualtensile stresses, but their distribution remained unchanged.

When comparing the parameters of surfaces, which wereprecision ground or machined with cutting tools made of different tool materials, the advantage of hard cutting withCBN tool was seen to be considerable. In the latter case,advantageous compressive residual stresses and hardened layerswere formed over a wide range of technological parameters.

4.2 Effect of Residual Stress Fields on Fatigue Life

From these results, the residual stresses developed in precisionturning and grinding are signi cantly different (compare Figs 4

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Dry Machining of Steel Surfaces 161

Fig. 6. Distribution of residual stresses with depth below surface inexternal precission grinding of hardened ball bearing steel (100Cr6)cylinder of diameter, d w = 120 mm (grinding wheel A99 60K-9V; cool-ant: Syntilo 4). ( a) Effect of workpiece speed, vw and depth of cut, ae for constant wheel speed vs = 32 m s

1 . 1, vw = 15 m min1 ,

a e = 0.05 mm/two stroke; 2, vw = 60 m min1 , ae = 0.01 mm/two stroke.

(b) Effect of workpiece speed, vs for constant workpiece speed,vw = 30 m min

1 , depth of cut, ae = 0.01 mm/two stroke, spark-out 5 sand v fL = 10 m/workpiece rev. 1, vs = 60 m s

1 ; 2, vs = 30 m s1 .

Fig. 7. Distribution of residual stresses with depth below surface inexternal high-speed grinding of hardened ball bearing steel (100Cr6)cylinder of diameter, d w = 120 mm with strained wheel EK60-LX-VXand coolant Syntilo 4. Effect of spark out for constant grindingconditions vs = 60 m s

1 , vw = 30 m min1 , v fR = 16 m s

1 . 1, spark-out:0 s; 2, spark-out: 5 s.

and 5 with Figs 6 and 7). In hard turning compressive residualstresses are developed, whereas in grinding tensile stresses aredeveloped. The difference between the residual stresses in hardturned and ground surfaces has an effect on the fatigue life of the workpieces.

Fatigue tests under similar conditions on hard turned andground ball-bearing rings were carried out by the Timken ballbearing company [2]. Before the fatigue test, the specimenswere super nished, so that the surface roughness, Ra was the

same, and less than 0.1 m. The results of the fatigue test forthe hard turned and precision ground components are shownin Figs 8( a ) and 8( b), respectively. Examination showed thatthe hard turned bearings have longer fatigue life than theground and super nished bearings.

5. Conclusions

Summarising the main features of the results reported, formetal removal processing in dry conditions, the followingconclusions may be drawn:

1. The experimental results prove that, in the case of hardenedsteels, cutting with CBN tools performed in dry conditionsis a suitable substitute for precision grinding and, therefore,the number of materials which can be hard turned withtools with de nite edges can be increased.

2. In many situations, residual stresses play an important rolein the quality of the surface integrity and durability. Theaim of the advanced machining processes is to assurepredictable, optimal residual stress distribution in the sur-face layer.

3. In dry hard turning performed on hardened steels, thepresence of compressive residual stresses in the surfacelayer results in an increased fatigue life of the hard turnedcomponents, which is higher than that for the ground steels.

4. In precision grinding, owing to the high contact temperatureand the small depth of cut, the surface integrity changes,and residual tensile stresses develop. Therefore, as men-tioned above, the fatigue life of the ground workpieces islower than that for the hard turned workpieces.

5. In hard turning, the surface roughness, the magnitude of the allowance and the material removal rate are the sameas in grinding. Thus, in many cases, if a suitable machinetool is available, dry hard turning is cheaper than grinding

Fig. 8. Effect of nishing process on the fatigue life for ( a ) smallbearing [2], ( b) large bearing [2].

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and can replace grinding, which requires a large amountof coolant.

References

1. H. Popke and T. Emmer, Minimalschmiertechnik und Trocken-bearbeitung erreichter stand und perspektivische Entwicklungen ,Rezanie i instrument, 55, pp. 190 194, 1999.

2. K. Matsumoto, F. Hashimoto and G. Lahoti, Surface integritygenerated by precision hard turning , Annals CIRP, 48(1), pp. 59 62, 1999.

3. V. P. Koster et al. Manufacturing methods for surface integrityof machined structural components , US Air Force LaboratoryReport AFMC-TR-71 258, 1974.

4. Metcut: Machining Data Handbook, Cincinnati, Ohio, 1980.5. W. Ko nig, A. Berktold and K. Koch, Turning versus grinding:

a comparison of surface integrity aspects and attainable accuracy ,Annals CIRP, 42(1), pp. 39 43, 1993.

6. L. Gribovszki, Method for determination of residual stresses ,Metallurgy Journals, 10, pp. 458 462, 1962.

7. J. Peters, R. Snoeys and K. Maris, Ober achenspannungen beimEinstechschleifen , Feinbearbeitung Kolloquium, Braunschweig,pp. 12 16, 1986.

8. L. Gribovszki et al. Modernization of some operations of therolling-contact bearing s production , Research Report, TechnicalUniversity, Miskolc, Hungary, 1973.

Notation

a e depth of cut in grindinga p depth of cut in turninga pD depth of cut of the dressing diamondb width of edge breaking off d w diameter of the workpiece f feed f D feed of the dressing diamondh depth below surfaces timevc cutting speedv fL axial feed per minutev fR radial feed per minutevs circumferential speed of wheelvw circumferential speed of workpiecev B ank wear

0 tool orthogonal clearance0 tool minor edge angle0 tool orthogonal raker tool cutting edge angles tool cutting edge inclination