an experimental investigation on response of cbn super
TRANSCRIPT
An Experimental Investigation on Response of cBN SuperAbrasive’s Grinding Performance and Failure Characteristicsto the Grinding Speed
G. Zhi1 & X. Li1 & A. Luo2 & J. Yang1 & Y. Rong3
Received: 12 July 2016 /Accepted: 29 September 2016 /Published online: 8 November 2016
Abstract Cubic Boron Nitride (cBN), a kind of super abra-sive material with excellent properties, is widely applied intothe manufacturing of high end grinding tools especially forhigh speed machining fields. During the actual grinding pro-cess, abrasive failure phenomena take place, which affects thegrinding tool performance, and then the final machining qual-ity. Considering the high speed applicability of cBN grindingtools, it is of significance to investigate the failure mechanismand grinding performance of cBN abrasives at different grind-ing speeds, in particular under high speed condition.Therefore, this paper carries out an investigation of cBN abra-sive failure behavior taking grinding speed sensitivity intoconsideration. Given that the random and diversity of multi-abrasives distributed upon grinding tools contributes to thecomplexity of the study, single cBN abrasive cutting
experiments are conducted under four grinding speed levelsof 40, 60, 80 and 100 m/s, respectively. When all experimentsare completed, abrasive failure patterns can be identifiedthrough observation on the morphology of ground cBN abra-sives bonded on cutting inserts using Scanning ElectronMicroscope (SEM). Based on the analysis of experimentalresults, it is shown that for the cBN abrasives studied in thispaper, abrasive breakage occurs at low grinding speed, but asspeed increases, cBN abrasives tend to get worn. The changeof grinding forces, as well as roughness Ry values, can showthe detailed failure behavior of tested cBN abrasives with timedependent features. In addition, G ratio rises with the increaseof grinding speed, which demonstrates the high productivitypotential of cBN high speed grinding. Finally, an in-depthanalysis concerning on cBN material’s crystal characteristicsand energy threshold (binding energy) is discussed, whichprovides an intrinsic explanation on the influence of grindingspeed upon cBN abrasive failure mechanism.
Keywords Grinding . cBN . Grinding speed . Grindingperformance . Failuremechanism
Introduction
High speed grinding offers high performance machining com-bined with high productivity compared to conventionalfinishing processes. Appropriate grinding machines andgrinding tools make it possible to expand the scope of ma-chined materials that can be applied for high performancegrinding technology [1]. Based on the theoretical analysis ofhigh speed grinding principle, it can be found that an increasein the cutting speed will result in a decrease of the undeformedchip thickness, thus leading to a reduction in grinding forces,grinding wheel wear and workpiece surface roughness [2, 3].
* X. [email protected]
1 Department of Mechanical Engineering, Tsinghua University,Beijing 100084, China
2 Superabrasives Department, Saint-Gobain Research (Shanghai) Co.,Ltd., Shanghai 200245, China
3 Department of Mechanical and Energy Engineering, SouthernUniversity of Science and Technology, Shenzhen 518055, China
Exp Tech (2017) 41:117–130DOI 10.1007/s40799-016-0159-9
# The Society for Experimental Mechanics, Inc. 2016
Moreover, the increase of cutting speed raises the specificmaterial removal rate which is defined as the material removalvolume per unit cutting length per unit cutting time.Consequently, a development of the machining quality andproductivity can be achieved for high speed grinding. On theother hand, as the cutting speed increases, the increasing totalthermal energy introduced into the workpiece not only influ-ences the surface integrity, but also requires grinding powerfurther. Reducing the contact time of abrasives with the work-piece can reduce the generated machining heat, but unfortu-nately affects material removal rate. These considerations in-dicates that high performance grinding at high speed conditionhas to take the inevitable and undesirable thermal effect intoaccount. As one of super abrasives, cubic boron nitride (cBN)has been intensively accepted in high end grinding toolmanufacturing especially for high speed industrial machiningfields due to its superior characteristics such as high hardness,high wear resistance, good thermal stability, and low chemicalreactivity with iron group metals compared to diamond [4, 5].In consequence, cBN grinding tools that cBN abrasives areattached to the tool substrate with adhesive bonding materialscan meet the special requirements of high speed machiningregarding resistance to fracture and wear.
In the actual grinding process, the cBN abrasives acting as thecutting medium interact with the workpiece, withstanding thegrinding forces and completing thematerial removal process [6].On account of the interaction complexity caused by the appliedload condition in the grinding zone and the diversity of abrasiveson geometry and orientation, cBN abrasives tend to fail mainlyin two forms namely breakage and wear, which is revealed fromthe experimentally observed results in the current work [7].These two abrasive failure patterns present different grindingperformances with a significant influence on the machining pro-ductivity and tool life. Abrasive wear takes place on theabrasive-workpiece contact interface with an abrasive flatteningand dulling phenomena, which is affected significantly by plas-tic flow crumbling and chemical reaction. It reduces the grindingtool wear rate, but contributes to the contact friction with work-piece surface. As for abrasive breakage, it is caused by the partialremoval from the abrasive substrate. In this case, new cuttingedges emerge enhancing the sharpness of a grinding tool, yetwheel life descends sharply if this failure pattern occurs domi-nantly [8]. Moreover, the abrasive failure patterns probablytransform into each other when the working condition changes.If one cBN abrasive cannot bear the external load and fails inbreakage and wear form, the subsequent abrasive will be sub-jected to much high loads particularly at high speed grindingcondition. It means that this abrasive will either immediatelyfracture or get worn quickly until it breaks [9]. Since that cBNgrinding tools can be considered as a collection of numbers ofindividual cBN abrasives, the cutting characteristics and wearbehavior of the employed abrasives dominate the overall grind-ing performance and service life of the grinding tools [10].
The failure characteristics of cBN abrasives depend onmany parameters, e.g. the properties of abrasives on geometry,hardness and yield stress, working conditions such as cuttingspeed and depth of cut, and bonding strength of adhesivesystem. Considering the high speed applicability of cBNgrinding tools, it is of importance to evaluate the cBN abrasivegrinding performance considering speed sensitivity in order tosatisfy the practical demands of grinding speed. Althoughmany studies have been carried out on the optimization ofgrinding operation, development of grinding tools and im-provement of abrasive synthesis, the reason why grinding per-formance at different grinding speeds varies is not elucidatedsufficiently, and the abrasive failure mechanisms influencedby the speed needs to be revealed as well [11]. In addition, interms of the experimental Installation, despite the fact that theconventional single abrasive scratching test can be used toinvestigate the failure mechanism and cutting behavior ofthe tested abrasives, the major drawback of low cutting speedlimit the test scope, that is to say, it cannot be suitable for thehigh speed performance analysis [9]. Therefore, an experi-ment equipment which is able to meet the high speed testrequirement ought to be designed and set up.
This paper presents the grinding performance and failurecharacteristics of cBN abrasives during grinding under differ-ent cutting speeds. Single cBN abrasive cutting experimentsdesigned similar to single point helix turning are carried out atfour cutting speeds (40 m/s, 60 m/s, 80 m/s, 100 m/s) rangingfrom low speed to high one. After all the experiments, failurepatterns of breakage or wear are discriminated by observingthe morphology of cBN abrasive bonded upon the cuttinginsert head using SEM. The specific tangential cutting forcescaptured by ultra-precision dynamometer throughout the ex-periment are compared as the change of cutting speed, and theabrasive failure behavior can also be reflected in the forcechange curves. Through calculating the abrasive volume lossand material removal volume, G ratio values can be obtainedto evaluate the grinding performance evolution consideringthe speed sensitivity. With an assistance of grinding energytheory, the difference on the grinding behavior is analyzed toestablish the relationship between the cutting speed and theabrasive failure mechanism in the grinding process. This re-search not only provides an available approach to study thecBN abrasive high speed performance, but also figure out theappropriate applications of the employed cBN abrasives forhigh end grinding tool manufacturing.
Experiment Setup and Measurement Approaches
Cutting Insert and Workpiece Preparation
To investigate the grinding performance of cBN super abra-sives considering cutting speed sensitivity, single cBN
118 Exp Tech (2017) 41:117–130
abrasive cutting inserts are employed in this research. Thenormalized #45 steel is chosen as the insert substrate, andparticularly the truncated head with flatness and parallel re-quirement guarantees the jointing of cBN abrasives to theinsert holder and the insert-workpiece alignment process inthe cutting test, which can be seen in Fig. 1. After beingselected through FEI Quanta 200 FEG EnvironmentalScanning Electron Microscope (ESEM) with a nano-level res-olution, one individual cBN abrasive with typical morphologyis mounted upon the insert head by electroplating. The com-mercial cBN abrasives with grit size of 120/140 (ISO stan-dard) are available, and this abrasive size has a mean equiva-lent diameter of 135.23 μm at a standard deviation of 8.92 μmaccording to the dimension measurement. Two countersunkholes on the insert holder, whose upper side and back side aremarked in green plus sign and yellow minus sign respectivelyin Fig. 1, helps to enable the location of cBN abrasive underthe ESEM observation view be corresponding to the one inactual grinding process, so that the orientation of each cBNabrasive electroplated on the insert head with an indication ofcutting edge shape can be determined. Besides, by comparingthe pre-test cBN abrasive morphology with the post-test one,failure patterns of abrasive breakage or abrasive wear can bediscriminated. Herein, the material of the electroplated layer isNickel-Cobalt alloy, and the bonding layer thickness is ap-proximately 60% of the average dimension of cBN abrasives,which is equal to around 81 μm. Through electroplating pa-rameter control, the layer thickness can be guaranteed, andfurther verified by measuring the cBN abrasive protrusionheight, whose values range from 45 to 62 μm. This protrusionheight satisfies the demand of effective material removal pro-cess during grinding, as well as efficient bonding force.
In terms of the workpiece preparation, cast iron QT500-7 isemployed as the workpiece material, which is commonly usedin cBN machining applications especially for camshaft orcrankshaft of an engine under grinding. Its main chemicalcomposition and mechanical properties are presented inTables 1 and 2, respectively [12–15]. The cast iron ingot witha dimension of Φ300mm(D) × 1000 mm(L) is sliced into
several disks ofΦ210mm(D) × 20mm(L). In order to increasethe strength and wear resistance, and ensure the uniform onmechanical properties of the cast iron, high temperature nor-malizing treatment is the following, heating to 900 °C andinsulation for 3 h, then air cooling to room temperature.Additionally, tempering process, which is for the reductionof stress inside the workpiece, is conducted with heating to550 °C and insulation for 3.5 h, and finally air cooling to roomtemperature [12–14]. The fine grinding processing after roughcutting is carried out on attachment hole, both sidesand circumferential faces. The final dimension isΦ200mm(D) × 15 mm(L) considering the magnetic bearingability of the spindle, which can be seen in Fig. 2(a). To meetthe high speed requirement in the test, the dynamic balancingadjustment under high speed condition is conducted on thedisc-shaped workpiece assembled with work holder attach-ment by means of drilling balancing holes on high speed bal-ancer (Haimer TD2009) in Fig. 2(b). Moreover, under thesame platform setup after the workpiece is installed into thespindle, a wiper insert is used for pre-test fine polishing todecrease the workpiece roughness to less than 2 μm, furtherto ensure the accuracy of tool alignment and cutting depth. InFig. 2, two little flat surfaces distributed symmetrically aroundthe circumferential surface of workpiece, which are markedwithin red dotted line frames, are the remained surfaces ofworkpiece sampling for cutting track observation and rough-ness measurement after experiments.
Experiment Setup and Test Procedure
The grinding can be regarded as an integration of multi-abrasive cutting process. Consequently, the single abra-sive cutting test is one commonly used methodology for
Top view of the cutting insert Detailed cBN abrasive Single cBN grain
cutting insert
Single cBN abrasive
Electroplated layer
Insert holder
Countersunk holes
Upper side of the cutting insert
Back side of the cutting insert
Fig. 1 Single cBN abrasivecutting insert
Table 1 Chemical composition of cast iron QT500-7
C Si Mn S P Mg RE
3.55 ~ 3.85 2.34 ~ 2.86 <0.6 <0.025 <0.08 0.02 ~ 0.04 0.03 ~ 0.05
Exp Tech (2017) 41:117–130 119
grinding research, which allows a focus on an individualabrasive to estimate the overall performance of grindingtools [16, 17]. For this test, the experimental platform isbuilt based on CNC milling machine (DMG DMU 80MonoBlock) with a spindle speed up to 18,000 rpm andposition accuracy of 1 μm, where dry grinding is conduct-ed. In Fig. 3, the ultra-precision dynamometer (Kistler9725B) with a resolution of 1 μN and a sampling rateof 5 kHz is installed upon the working table of the ma-chining tool. The single cBN abrasive cutting insert isfixed on the dynamometer. The work tool holder assem-bled with workpiece cast iron QT500-7 is installed intothe spindle. An industry camera (MER-500-7M/UC) with500 M pixels and 7 frame rates is used to provide thevisual assistance for tool-workpiece alignment. The de-tailed test parameters are summarized in Table 3. Thesingle abrasive cutting inserts are tested under the speedof 40 m/s, 60 m/s, 80 m/s and 100 m/s, respectively. Toimprove the precision and creditability of experimentalresults, 15 cutting inserts with random selection of cBNabrasives are employed for each speed level condition(40 m/s, 60 m/s, 80 m/s, 100 m/s), thus totally 60 samplesare completed throughout all the experiments. The ran-domness on cBN abrasive selection suing SEM is a guar-antee of experimental results to be an effective estimationfor grinding behavior and failure features of abrasives onpractical grinding tools. The error bar, which is defined asthe difference between the maximum value and the min-imum value around the average value, is used to indicatehow results vary derived from multiple experiments. The
depth of cut (DOC), which is along X direction, is kept asa constant of 8 μm in all experiments given the averagecutting depth of an individual abrasive bonded upon thesurface of one grinding wheel, and the feed rate along Zdirection is 50 μm/rev to ensure separate tracks on theworkpiece surface left after the cutting procedure, furtherto reduce the influence of overlapping tracks on the abra-sive failure formation.
The overall experiment procedure mainly consists offour steps, namely tool alignment, workpiece removing,parameter setup and cutting completion, as are illustratedin Fig. 4. As for the insert alignment, considering its effecton the accuracy of the DOC setup, two coupling ap-proaches are applied to provide a relatively precise indica-tion. One is the camera-observation of contact when therotating workpiece moves slowly and smoothly towardsthe single cBN abrasive at the top of cutting insert, andthe other one is the dynamometer-detection of instanta-neous peaks occurring in the recorded curves of three com-ponent cutting forces, which can be seen in Fig. 5.Afterwards, the workpiece is removed by the spindle alongZ direction away from the fixed cutting insert, and thenkeeps moving along X direction until the DOC valuereaches 8 μm. Finally, the workpiece rotates at thepredetermined cutting speed (40 m/s, 60 m/s, 80 m/s and100 m/s), and moves along Z direction at the feed rate of50 μm/rev to complete the workpiece material removalscratched by the single cBN abrasive cutting insert, so asthat a spiral trajectory will be reserved upon the workpiecesurface. Besides, to reduce the temperature influence, ablower is used beside the cutting insert although the totalcutting time is short and the chip space is large enough forheat dissipation under this experiment condition.
When all groups of the experiments are completed, themorphology of cBN abrasives electroplated upon the headof the cutting insert is observed by ESEM in order todetermine different abrasive failure patterns of wear or
Table 2 Mechanical properties of cast iron QT500-7
Tensile strength, σb Yield strength, σ0.2 Elongation, δ Hardness
500 MPa 320 MPa ≥7 % 250HB
Fig. 2 Workpiece cast ironQT500-7 and its balancing hole(a) upper side to show theassembly of workpiece with workholder (b) back side to show thedetail and position of balancinghole
120 Exp Tech (2017) 41:117–130
breakage. The evolution of cutting force as the increase ofcutting speed, especially the tangential one (Ft ), is com-
pared by specific force (Ft0). This force, which is defined
as the cutting force per unit cross section area (A ) of thecBN abrasive along the cutting direction so as to eliminatethe impact of the cBN abrasive shape diversity on theforce value, is given as
Ft0 ¼ Ft
Að1Þ
During grinding, it is difficult to conduct a real-timerecord of the cBN abrasive shape change, but the initialand final geometrical information of cBN abrasives canbe obtained through microscope. Due to the approximatelylinear wear rate which will be discussed later, A can beequivalent to the average of initial (A0) and final (A1)cross-sectional areas of cBN abrasive cutting part embed-ded into the workpiece in Fig. 6(a). If the cBN abrasivebreaks during grinding, the value of A is just the half of theinitial cross-sectional area (A0) because A1 is zero, as alsocan be seen in Fig. 6(a). Thus the value A implies theinformation of cBN abrasive shape and its change, as wellas the failure mode. In this way, the correlation of grindingspeed and grinding force, and the abrasive failure patternscan be established to reveal the abrasive failure mecha-nism. In addition, the protrusion volume of cBN abrasiveson cutting inserts is measured through 3D profile scanningby utilizing digital microscope (Keyence VHX-2000) bothbefore and after the test. Herein, the protrusion volume isdefined as the volume part of employed cBN abrasivewhich is not embedded into the electroplating layer [18].This part actually undertakes the cutting work, and tend tofail in the form of wear or breakage during grinding. TheVHX-2000 microscope supplies an effective ability tomeasure the volume beyond a reference surface on the3D image with an accuracy of 0.1 μm3, and through com-paring the average protrusion volume of cBN abrasivesderiving from multiple measurements before and after thegrinding, the volume loss due to abrasive breakage or wearcan be acquired, which is shown in Fig. 6(b). Based on the
calculation of the abrasive volume loss (VG) and the work-piece material removal volume (VW), single abrasive grind-ing ratio (G) can be figured out to compare the grindingperformance under different cutting speed conditions. Theexpressions of VG, VW and G can be formulated as
VG ¼ V0−V1 ð2Þ
VW ¼X n
k¼1Ak ⋅π⋅d ð3Þ
G ¼ VW
VGð4Þ
Where, V0 and V1 are the cBN abrasive protrusion volumebefore and after the test, respectively. Given that the amount ofchips removed fromworkpiece are small to collect completelyand weigh accurately, and some chips possibly are welded onthe rake face of the cBN abrasive cutting edge, a methoddirectly through volume calculation for Vw is adopted, as isshown in equation (3). Ak, which is different from the defini-tion of A for the deformation of workpiece material, is illus-trated in Fig. 6(a) with a meaning of the cross-sectional area ofkth cutting track distributed upon the workpiece surface be-cause of the scratch of cBN abrasive, and d is the diameter ofworkpiece cast iron. From equation (3), it is obvious that Vw
depends largely on the value of Ak, which varies as cBN abra-sive fails in the grinding process. Therefore, a 3D laserprofilemeter with a high measuring resolution is applied torecord the topography of cutting tracks by scanning the work-piece surface, so as that relatively accurate Ak values of eachcutting track can be obtained with an assistance of an areameasuring software.
Results and Discussion
With SEM observation, cBN abrasive morphology shows thefailure pattern of breakage or wear under four grinding speeds.Taking the speed influence into consideration, the differenceof grinding performance and failure behavior is analyzed interms of cutting tracks remained upon the workpiece surface,grinding forces and G ratio values. The nature of the
Industrycamera
High speedspindle
Workpiece
cBN insert
Dynamometer
Working table
ZX
Y
Fig. 3 Experimental platform ofthe single cBN abrasive cuttingtest
Exp Tech (2017) 41:117–130 121
difference, as well as the failure mechanism under low andhigh grinding speeds, can be explained from an energy per-spective at an atomic scale.
Abrasive Morphology Comparison Via SEM
When the experiments are completed, the morphologies of allground cBN abrasives electroplated upon cutting inserts areobserved through SEM, and compared with correspondingpre-test ones, so that the failure patterns under different grind-ing speeds can be determined. Herein, four typical SEMphotos which are able to show the detailed failure conditions
of samples at 40, 60, 80 and 100 m/s are selected respectivelyin Fig. 7. Under the magnification of 1000, a significant sen-sitivity of grinding speed on cBN abrasive failure mechanismis revealed. At a relatively low speed level of 40, 60 or 80 m/s,cBN abrasive breakage occurs for all 15 samples. It can befound from Fig. 7(a), (b) and (c) that a large piece falls offfrom the cBN abrasive and an irregular fracture surface isremained. As for the 80 m/s samples not showing large frac-ture area as 40 and 60 m/s samples do, more cleavage planeswith river patterns can be seen in Fig 7(c). When the grindingspeed increases up to 100 m/s, the observing results of 15samples indicate that the cBN abrasives tend to get worn grad-ually as cutting process continues. In consequence, a clearflattened area left upon the cBN abrasive surface, which ispresented in Fig. 7(d), is a typical feature of abrasive wearfailure pattern. Energy Dispersive X-Ray Spectroscopy(EDX) can be used to distinguish the wear flat area from theclogging of workpiece on the cBN abrasive. Additionally,some cutting scratches on the wear area are the indication ofcutting direction, as well as the intensity of interaction be-tween the employed cBN abrasive and the workpiece. Basedon the microscopic observation, a speculation could be putforward that between 80 and 100 m/s, there possibly exists atransition where abrasive breakage and abrasive wear takeplace at the same time, which is potentially useful for theindustrial application of this type of cBN abrasives.
ω
Spindle
WorkpiececBN insert
Working table
Dynamometer
X
Z
ω
Spindle
Workpiece
cBN insert
Working table
Dynamometer
X
Z
ωn
Spindle
Workpiece
cBN insert
Working table
Dynamometer
X
Z
DOC=8μm
Spindle
Workpiece
cBN insert
Working table
Dynamometer
X
Z
DOC=8μm
Tool alignment Workpieceremoving
Parameter setup(DOC/rotation speed/feed rate)
Cutting completetion
ωn
Fig. 4 Schematic diagram of testprocedure
Table 3 Detailed test parameters
Parameters Values
Abrasives Cubic Boron Nitride (cBN)
Cutting speed levels 40 m/s, 60 m/s, 80 m/s, 100 m/s
Depth of cut (DOC) 8 μm, X direction
Feed rate 50 μm/rev, Z direction
Workpiece material Cast iron QT500-7
Workpiece dimension Φ200mm(D) × 15 mm(L)
Grinding mode Dry grinding
Experiment amount 15 × 4 = 60(15 cutting inserts per each speed level)
122 Exp Tech (2017) 41:117–130
When all experiments are completed, cutting tracksremained upon the workpiece surface are observed underVHX-2000 microscope, and the distance between adjacent
two tracks, which is perpendicular to the cutting direction, isthe value of feed rate (50 μm/rev), as is shown in Fig. 8(a). Forall machined workpiece surfaces, the roughness value Ry is
(a) Cross-sectional area of cBN abrasive cutting part and kth cutting track
(b) Protrusion volume of cBN abrasive
Worn or broken cBN abrasive
Electroplated layer
Cutting insert holder
Protrusion volume of cBN abrasive
PRE-TEST single cBN abrasive cutting insert
Initial cBN abrasive
V0 V1
VG = V0 - V1
Protrusion volume loss of cBN abrasive
after cutting
POST-TEST single cBN abrasive cutting insert
Fig. 6 Indication for thecalculation of A, Ak and VG. (a)Cross-sectional area of cBNabrasive cutting part and kthcutting track. (b) Protrusionvolume of cBN abrasive
Fig. 5 Camera observation anddynamometer detection in thealignment process
Exp Tech (2017) 41:117–130 123
measured by using surface roughmeter with a 0.01 μm accu-racy, in particular at initial, intermediate and finishing posi-tions of the grinding process. Ry is defined as the distancefrom the peak to the valley of the surface contour line withina certain area, and if the original workpiece surface is set as themeasuring reference, this parameter can be used to representthe evolution of actual cutting depth due to the failure of aspecific cBN abrasive in the form of breakage or wear duringthe single abrasive cutting process. The Ry results of four-speed-level samples with fitting trend lines are illustrated inFig. 8(b), and the error bars show the spread of data resultingfrom multiple measurements. Considering that the numberdiffers in cutting revolutions caused by various failure condi-tions, data of 40 and 60 m/s samples after about 150 revolu-tions, and 80 m/s samples after about 250 revolutions arevacant. From Fig. 8(b), it indicates that there is no strict cor-responding relation between Ry values and grinding speeds,because single abrasive cutting process is different from prac-tical grinding or cutting process. However, no matter underwhich speed level, the values of Ry generally declines linearlyas the cutting process continues, which hints that the depth ofcutting track becomes smaller due to that the protrusion part ofcBN abrasives as effective cutting medium gets worn gradu-ally, even broken during grinding. This finding can beregarded to reflect the failure behavior of cBN abrasives withtime dependent features in the experiment. In addition to thetime dependent failure condition reflected by the evolution ofRy values, the comparison of slopes of four linear fitting linesshows that the higher the grinding speed is, the smaller thedecreasing slope will be, which means a slower wear rate ofthe specific cBN abrasive. The reason of relatively larger re-ducing slopes at low speeds might rely on that the cBN abra-sive fails partial volume by partial volume because of abrasivebreakage. This measuring result is also a potential evidence ofthe transition of cBN abrasive failure pattern from breakage towear.
G Ratio Calculation and Grinding Force Characteristics
G ratio is commonly used as an important grinding technicalparameter to evaluate the grinding ability and efficiency.Through measuring the protrusion volume loss of cBN abra-sive and the material removal volume of workpiece underdigital microscope, the value of G ratio for one individualcutting experiment can be obtained, so that the average G ratiovalues at four grinding speed levels of 40, 60, 80 and 100 m/sare presented in Fig. 9 with the indication of data floatingranges. The exponentially fitting trend line of all data pointscan also be seen in the green dotted line of Fig. 9. Because ofsmall amount of volume loss of around 104–105 μm3 for anindividual cBN abrasive, relatively higher G ratio values aregenerated compared to industrially practical ones. It is appar-ent that Fig. 9 displays a different grinding performance of
(a) Cutting tracks on the workpiece
Fade rate direction
Cutting tracks
Workpiece sample @ v=40m/s
(b) Ry evolutions at four speed levels
Fig. 8 Ry measurement to indicate time dependent failure behavior ofcBN abrasive (a) Cutting tracks on the workpiece (b) Ry evolutions atfour speed levels
(b) 60m/s sample
Abrasive Breakage Abrasive Breakage
(a) 40m/s sample
(c) 80m/s sample (d) 100m/s sample
Abrasive Breakage Abrasive Wear
Fig. 7 The morphology of ground cBN abrasives under differentgrinding speeds to indicate failure patterns (a) 40 m/s sample (b) 60 m/s sample (c) 80 m/s sample (d) 100 m/s sample
124 Exp Tech (2017) 41:117–130
studied cBN abrasives at different cutting speeds, and thevalues of G ratio approximatively performs an exponentialgrowth as the grinding speed increases. This calculated resultconfirms that cBN abrasives have superiority in high speedapplications concerning on productivity. In particular, a rapidgrowth rate of over three times occurs from 80 to 100 m/s ascan be seen in the trend line, which is higher than the approx-imately two times growth rate from 40 m/s to 60 m/s and from60 m/s to 80 m/s, respectively. This increasing tendency isassociated with the change of cBN abrasive failure patternsfrom breakage to wear as a function of grinding speed. Incomparison to abrasive breakage with less workpiece materialremoval volume at low cutting speed, abrasive wear takingplace at high speed makes it possible to complete the wholecutting process and remove more workpiece material, whichresults in a fast development of G ratio from 80 to 100 m/s.Despite of the fact that abrasive breakage might generate morecutting edges, the reduction of the effective cutting part ofcBN abrasives leads to low G ratio values under a constantDOC test condition. Furthermore, there might be a speedrange where abrasive breakage and abrasive wear coexist be-tween 80 and 100 m/s, and the existence of abrasive wearfailure pattern within this range could significantly improvethe grinding efficiency.
In terms of the grinding forces recorded by Kistler ultra-precision dynamometer, it can be considered as the indicationof loading condition of one cBN abrasive under a specificgrinding speed, so as to provide some explanations on thechange of cBN abrasive failure pattern ranging from breakageto wear, as well as the increasing tendency of G ratio values.
The specific tangential grinding force (Ft0), which can stand
for the applied load bearing status of the tested cBN abrasiveregardless of the abrasive geometrical influence, is calculatedby means of averaging grinding force values in the stablecutting process, as is shown in Fig. 10. Apparently, in thegrinding force curve, it displays a decreasing change as thegrinding speed increases, which is in agreement with the
cutting force test result of single point diamond cutting study.A gradually reducing drop rate of that curve can also be foundin Fig. 10. Moreover, it can be generally concluded that at lowgrinding speeds of 40, 60 or 80 m/s, relatively large forcevalues are captured. In this case, cBN abrasives cannot with-stand such severe load, so the failure pattern is abrasive break-age, which potentially damages the rake face of cBN abra-sives. Afterwards, the cutting process deteriorates, which inturn makes the grinding force become larger. As the grindingspeed increases up to a higher level of 100 m/s, practicalcutting depth becomes thinner, meanwhile the loading condi-tion get improved with an indication of a smaller tangentialgrinding force, so the cBN abrasives are able to bear the load,further leading to the occurrence of cBN abrasive wear [19].Besides, on account of that cBN abrasives get worn graduallyat this case, the rake face can be retained, which allows asmoother workpiece material removal process, and the forcecould keep a relatively lower value as well. As for 80 m/ssamples, more cleavage surfaces than the ones of 40 and60 m/s samples might be the reason for the improvement ofgrinding force and loading condition.
Fig. 9 The change of G ratiovalues as the increase of cuttingspeed
Fig. 10 Change of specific tangential cutting force under differentgrinding speed conditions
Exp Tech (2017) 41:117–130 125
Moreover, according to the feed rate value (50 μm/rev)along Z direction and the workpiece thick (15 mm), the totalnumber of cutting passes (N) should be 300 for one completesingle abrasive cutting process. In consequence, the overallcutting length (l) can be figured out as
l ¼ πdN ¼ π� 200� 10−3 � 300 ¼ 188:4 mð Þ ð5Þ
The actual cutting pass number (N 0 ) for each experimentcan be determined by counting the amount of cutting trackremained upon the workpiece surface under optical micro-scope, as can be seen in Fig. 11. The distribution of maximumvalues and minimum values around average values is alsopresented to indicate how data vary. The floating of resultsat one speed level largely rely on the failure condition of eachcBN abrasive, which probably results from the DOC deviationof insert-workpiece alignment from 8 μm and the diversity oftested cBN abrasives on shape, dimension and orientation. Itcan be found from Fig. 11 that the number of cutting tracksdiffers in grinding speeds because of the different failure rateof effective cutting part of cBN abrasives, which leads todifferent cutting distances under four speed levels. At 40 and60 m/s, the cutting passes stay in an approximate number ofabout 100 to 150 due to the existence of abrasive breakage. Asfor cutting speed of 100 m/s, the failure of all tested cBNabrasives turns to be in the form of abrasive wear, so the totalcutting passes of 300 can be achieved. Although abrasivebreakage occurs at 80 m/s, more cutting passes of about 200to 250 are observed probably because of the decreasing grind-ing force, which implies that cBN abrasive failure phenome-non improves compared to the condition at relatively lowcutting speed. In addition, by using the measured cutting passnumber (N 0 ), the actual cutting length (l0 ) can be calculated as
l0 ¼ πdN 0 ¼ π� 200� 10−3 � N 0 ¼ 0:63N 0 mð Þ ð6Þ
The value of l0 should be consistent with the informationrevealed from the grinding force curve captured in each singlecBN abrasive cutting test. Beside, through observing the
cutting track with revolution counting, the failure conditionof cBN abrasives can be estimated.
Two typical grinding force curves are selected to presentthe characteristics of grinding performance in the cutting pro-cess, one is for cBN abrasive breakage in red curve at grindingspeed of 40 m/s, and another one is for cBN abrasive wear inblue curve at grinding speed of 100 m/s, as are illustrated inFig. 12. In the red curve as a typical grinding force change ofabrasive breakage, a sharp peak occurs at the beginning of thecutting process due to the initial contact between theemployed cBN abrasive and the cast iron workpiece. Thenthe cBN abrasive keeps interacting with the workpiece alonga helical-shaped trajectory upon the surface at a constant cut-ting depth. At around cutting distance of 80 m, another peakcan be seen, which demonstrates that abrasive breakage takesplace. From then on, no any cutting track can be observedupon the workpiece surface, which means that the cuttingprocess ends. For cutting speed of 80 m/s with better cBNabrasive failure condition, the cutting distance should be lon-ger and the grinding force should be lower. As for the bluecurve to represent the abrasive wear failure pattern, a peakappears as the same with the force change condition of abra-sive breakage, and several continuous cutting tracks at thebeginning position of workpiece imply that no breakage hap-pens at the initial contact. However, after that peak the cBNabrasive removes the workpiece material with a smaller grind-ing force until it finishes the complete cutting process of about188.4 m. Finally, the flat wear morphology is remained on thecBN abrasive surface and 300 revolutions of cutting tracks arecompleted. The change of grinding forces can present thedetailed failure condition during the cutting process, and alsohelps to estimate the failure pattern of the specific cBN abra-sive by force recording.
Further Analysis on Abrasive Failure Mechanismwith Grinding Energy
In order to obtain an in-depth understanding on the failurebehavior and cutting characteristics of cBN abrasive underdifferent grinding speeds, crystal structure of cBN materialin combination with energy theory is analyzed at an atomiclevel. Some previous studies have revealed that cBN, which isisoelectronic with diamond, exhibits a zinc-blende crystalstructure with sp3 orbital hybridization covalent bond betweenboron atoms and nitrogen atoms [20]. cBN crystals can besynthesized from hexagonal boron nitride (hBN) or other bo-ron nitride’s allotrope under high pressure and high tempera-ture (HPHT) either directly or with the aid of catalysts selectedfrom the nitrides of alkali metals or alkaline earth metals [21,22]. The nature of phase transformation during this synthesisis the completion of old chemical bond breaking and new oneforming which are both related to the binding energy of B-B,N-N or B-N [22]. Herein, the binding energy can be
Fig. 11 Measurement on grinding passes distributed upon the workpiecesurface
126 Exp Tech (2017) 41:117–130
considered as an energy threshold that separating B atoms orN atoms ought to cross, and can be derived based on first-principle [23]. In theory, given that the value of atomic kineticenergy Ek at equilibrium state is too small to be ignored, in-teratomic binding energy Eb is approximately equal to inter-action potential energy Ep, and can be written as
Eb ¼ Ek þ Ep≈Ep Ek≪Ep� � ð7Þ
As for Ep, it is an accumulation of attraction energy EA andrepulse energyER existing between any two atoms as is shownin Fig. 13, and can be represented as
Eb rð Þ ¼ EA rð Þ þ ER rð Þ ¼ −Αrα
þ Brβ
; α < βð Þ ð8Þ
where, A,α, B, β are parameters depending on the cBN crystalstructure. The lowest binding energy Eb,min at equilibrium
position r = r0 under a stable crystalline state can be obtainedby calculating the extreme point of Eb(r) as
dEb rð Þdr
����r¼r0
¼ αAr0αþ1
−βBr0βþ1
¼ 0 ð9Þ
r0 ¼ βBαA
� � 1β−α
ð10Þ
Eb;min ¼ Eb r0ð Þ ¼ −A 1−αβ
� �⋅1
r0αð11Þ
As for the ith atom (B or N) and the jth atom (B or N), theirinteratomic distance at equilibrium position r0,ij can be relatedto the distance between two any adjacent atoms r0 with adistance conversion coefficient αk, and expressed as
r0;i j ¼ akr0; k ¼ 1; 2; 3;⋯;N ð12Þ
Therefore, the total binding energy Eb,total of cBN materialbased on the derived consequence of Eb,min can be expressedas
Eb;total ¼ 1
2
X N
i
X N
jEb r0;i j
� � ¼ N2
Xi≠ jEb r0;i j
� �; j≠1; j ¼ 2; 3;⋯;Nð
ð13Þ
Eb;total ¼ N2
Aαβ−1
� �Xi≠ j
1
akr0ð Þα ¼ N2
Aαβ−1
� �X N
k
1
akα⋅1
r0α
ð14Þwhere, N stands for the number of total atoms.
Based on the derivation of energy threshold of cBN abra-sives indicated by binding energy, the failure mechanism ofcBN abrasive at low and high speeds can be explained better.
Fig. 12 Typical curves ofgrinding force change for cBNabrasive breakage and wear
r0
Eb
r
Attraction energy EA(r)
Eb,min
Repulsive energy ER(r)
Binding energy Eb(r)
Br β
ER(r)=
ArEA(r)=
Fig. 13 The accumulation of attraction energy EA and repulse energy ERfor binding energy Eb
Exp Tech (2017) 41:117–130 127
For the interaction of cBN abrasive with the workpiece duringgrinding, energy is produced continuously, and some of it willbe conducted into the cBN abrasive. It is generally thoughtthat the higher the grinding speed, the more severe the inter-action as an integration of grinding forces and heat, especiallyfor the impact between the cBN abrasive and the non-machined surface of workpiece, thus leading to a higher ex-ternal energy conduction. This external energy enables Batoms or N atoms distributed upon the cBN abrasive surfaceto tend to be activated, and the activation strength reducesgradually from the surface to the inside. The activationstrength herein means the intensity of B atoms or N atomsvibrate at equilibrium positions and the breaking tendency ofB-B, N-N or B-N bonds. Obviously, higher activation strengthsuggests a larger trend of bond breaking. If this strength
reaches the energy threshold cBN abrasives possess, the bond-ing strength between atoms (B or N) becomes weaker, thusresulting in a macro potential failure phenomenon [24–27].
For the cBN abrasives investigated in the single abrasivecutting experiments, it can be deduced that as the grindingspeed goes up to 100 m/s, relatively higher grinding energyis generated and applied upon the cBN abrasive surface.Whenthe applied external energy exceeds the energy threshold ofcBN abrasives, the surface atoms will be activated. At the tipof potential failure positon, the first few bonds break and thefollowing bonds allow a local reconstruction due to high en-ergy conduction [28–30]. Meanwhile, lower grinding forcesare recorded indicating a better load condition, so the cBNabrasives will be peeled off layer by layer at a macro scaleaccumulated from several atoms by several atoms at a micro
(a) cBN abrasive wear mode
(b) cBN abrasive breakage mode
Fig. 14 Schematic diagram ofcBN abrasive failure mechanismconsidering grinding energy (a)cBN abrasive wear mode (b) cBNabrasive breakage mode
128 Exp Tech (2017) 41:117–130
scale, resulting in the abrasive wear failure pattern as can beseen in Fig. 14(a). As for low grinding speed levels of 40, 60and 80 m/s, the generated energy does not reach the threshold,which makes the speed of energy propagation is sufficientlylow, so the atoms (B or N) cannot be activated and no imme-diate re-bonding occurs inside cBN material. In this way, thebond breaking between atoms (B or N) spreads fast. From amacro perspective, fracture takes place, and more severe loadcondition presented by measured grinding forces and the ex-istence of internal defects intensifies the generation of frac-tures. At the beginning, little cracks leads to a fast wear rate,which is revealed in Ry measurement results. Finally, bigcracks make large volume of cBN abrasive broken from theabrasive substrate, forming abrasive breakage failure pattern,as is shown in Fig. 14(b). Meanwhile, the cBN abrasives un-der this circumstance cannot complete the whole material re-moval process with cutting tracks of less than 300 revolutions.
In the actual grinding process using cBN grinding tools,although the grinding speed is kept as a constant, the cuttingdepth for every individual cBN abrasive varies, leading to thedifference on the amount of acquired energy. Therefore, cBNabrasives fails in different patterns, but one of the failure pat-terns dominates under a certain grinding speed condition. Thediscussion on atomic energy threshold put forward aboveneeds to be validated in the future work bymeans of modelingor simulation considering atomic or molecular mechanics, be-cause current experimental methods cannot solve such prob-lem at an atomic scale.
Conclusions
This paper conducts single cBN abrasive cutting experimentsto investigate the difference of grinding performance and fail-ure characteristics under four grinding speed levels of 40, 60,80 and 100 m/s. Through comparing experimental results interms of ground cBN abrasive morphologies, specific tangen-tial cutting forces and G ratio values, it indicates that the stud-ied cBN abrasives perform differently as the grinding speedchanges.With an in-depth analysis of grinding energy in com-bination with crystal features of cBN material, the cBN abra-sive failure mechanism under low and high grinding speeds isdiscussed. Based on the investigation above, the conclusionscan be drawn as follows.
1) The observation on cBN abrasive morphology underSEM shows that failure patterns change as the increaseof grinding speed for the cBN abrasives studied in thisresearch. At relatively low grinding speeds of 40, 60 and80 m/s, abrasive breakage occurs. When the speed in-creases up to 100 m/s, the failure pattern turns into abra-sive wear. The detailed failure behavior of each testedcBN abrasive can be seen from recorded grinding force
curves, and time dependent cutting features are presentedin Ry measurement results.
2) With the increase of grinding speed, reducing tangentialgrinding forces suggest that the loading condition get im-proved, which might be the main cause for the failureevolution of cBN abrasives from breakage to wear.Consequently, the number of cutting tracks increases withmore workpiece removal volume, thus leading to the riseof G ratio value. This result demonstrates the high speedapplicability of cBN abrasives concerning on highproductivity.
3) An in-depth analysis of energy considering cBN mate-rial’s crystal features is carried out to explain the sensitiv-ity of grinding speed on cBN abrasive failure mechanism.At high grinding speed condition, the applied externalenergy exceeds the energy threshold of cBN abrasivewhich is represented by binding energy between B or Natoms, hence these B or N atoms on cBN abrasive surfacewill be activated, which results in abrasive wear. If theenergy threshold cannot be crossed at low speeds, thefailure might not occur at the cBN abrasive surface.Therefore, partial volume of cBN abrasive is fracturedfrom the abrasive substrate, presenting abrasive breakage.
Acknowledgments The research is supported by National ScienceFoundation of China (NSFC: E51305229), NSAF: U1430116, andTsinghua University Initiative Scientific Research Program. The authorswould also appreciate the support by Saint-Gobain Research (Shanghai)Co., Ltd.
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