Study on the Material Removal Mechanism of Glass in SingleDiamond Grain Grinding with Ultrasonic Vibration AssistedJianyun Shen a*, Bin Dai b, Yuan Li c, Xian Wu d, Zhongwei Hu e

Huaqiao University, 668 Jimei Avenue, Xiamen, Fujian, Chinaajianyun@hqu.edu.cn, b565787680@qq.com, cliyuan@hqu.edu.cn, dxianwu@hqu.edu.cn,

ehuzhongwei@hqu.edu.cn

Keywords: Single diamond grain grinding; Ultrasonic vibration assisted; Material removalmechanism; Acoustic emission signal

Abstract. The ultrasonic vibration assisted grinding is widely used in machining of hard andbrittle material. This paper presents a study on the material removal mechanism in ultrasonicvibration assisted grinding of glass. The single diamond grain grinding experiments wereperformed with and without ultrasonic vibration assisted. The ground groove morphology wasobserved and analyzed in detail. The acoustic emission signal was measured as well. Thefollowing conclusions can be obtained: (1) the material removal mechanism is changed fromcontinuous crack propagation in conventional grinding to micro crack breakage in ultrasonicvibration assisted grinding. (2) The addition of ultrasonic vibration in grinding of brittlematerial can reduce the crack propagation distance. (3) Compared with the conventionalgrinding , the energy distribution of acoustic emission signal in ultrasonic vibration assistedgrinding is more concentrated.

1. IntroductionGlass is widely used to produce optical components, such as lenses, prism and mirror. It is

typically brittle material and has poor machinability. The optical glass parts usually requirevery high surface quality, and most of them are manufactured by grinding. With ultrasonicvibration assisted, the cutting force and temperature in grinding can be decreased, then thebetter grinding quality and efficiency can be obtained [1, 2]. A large number of studies haveconfirmed that ultrasonic vibration assisted technology is an effective method for the hard andbrittle material [3, 4]. The ultrasonic vibration assisted technology is more and more popular inmachining of hard and brittle material. Liang et al. [5, 6] applied ultrasonic vibration assisted ingrinding of single crystal sapphire and silicon. The results showed that the surface quality ofaxial ultrasonic vibration was best, and the cutting force was also slightly reduced. Nik et al. [7]found in UAG, higher depth of grinding and feeding can be selected in comparison to CG withstill better surfaces than CG. Unyanin et al.[8] through the experimental approach stated thatapplying USV on the workpiece provided decrease of grinding forces by 10~15 %.

In single diamond grain grinding, the interaction mechanism between the diamond grainand workpiece can be analyzed independently. The involved material removal and surfaceformation mechanism can be observed without the confusion of other diamond grain. It issignificant way to study the complex grinding process accurately. Liu et al.[9] studied the crackpropagation in SiC grinding by simulation of single diamond grain scratching, and thencompared with the grinding experiment. It was indicated the higher grinding speed would leadto an improved surface quality with larger but shallower cracks. Lin et al.[10] analyzed thesurface formation mechanism during rotary ultrasonic grinding of glass BK7 with singleabrasive grain. Feng et al.[11] conducted single diamond grinding experiments on SiC.

Through comprehensive analysis, it was found that ultrasonic vibration can reduce the materialfracture and reduce the grinding force.

This paper conducted single diamond grain grinding experiments on glass with andwithout ultrasonic vibration assisted. The machined groove morphology in ultrasonic vibrationassisted grinding was observed and compared to conventional grinding. The acoustic emissionsignal was recorded and analyzed as well. The potential material removal mechanism inultrasonic vibration assisted grinding of brittle material was revealed.

2. Experimental procedureThe single diamond grain grinding experiments were performed on the precision surface

grinding machine BLOHM-PLANOMAT HP. The used tool was conical diamond indenterwith tip angle of 90° and corner radius of 0.2mm, as shown in Figure 1a, which was fixed onthe aluminum wheel of 360mm diameter. A counterweight was held on the opposite position tokeep dynamic balance. The ultrasonic vibration assisted device was consisted of amplitudetransformer and transducer, and was mounted on the C shape fixture which was clamped onworktable subsequently. The experimental workpiece was ordinary flat glass with a dimensionof 15mm×15mm×5mm. After polishing, the surface roughness of the glass was 0.5~0.8μm,and the main performance parameters of this kind of glass was shown in Table 1. Theworkpiece was fixed on the end of amplitude transformer, as shown in Figure 1b.

Figure 1. The single diamond grain grinding experimental setupThe acoustic emission sensor was fixed on workpiece to acquire the acoustic emission

signal in grinding process. The sampling frequency was 5 MSPS. The line velocity vs was 10m/s, feed rate fw was 8000 mm/min, grinding depth ap was 20µm, and amplitude A was set at1.1 µm. The frequency f was set at 28 and 40 KHz. After experiments, the scanning electronmicroscope was used to observe the grinded groove morphology. The grinded groove depthwas measured by the three-dimensional reconstruction of optical microscope.

Table 1. Flat glass material mechanical propertiesmaterial type Flat glassPoisson's ratio 0.15

Tensile strength / [MPa] 48Bending strength / [MPa] 67

Compressive strength / [MPa] 1100Modulus of elasticity /[GPa] 72

3. Results and discussion

3.1 The ground groove morphology. Figure 2 shows the grinded groove morphology whenthe diamond grain just grinding in the glass workpiece with very small grinding depth. Inconventional grinding, the groove is continuous. Along the grinding direction, at the beginning,the groove morphology is relatively smooth, and has many scratching marks. It indicates thematerial removal mode of plastic deformation. Then, the groove morphology is changed toconsist of obvious crack and breakage defects. This reflects the material removal mode ofbrittle fracture. In contrast, in ultrasonic vibration assisted grinding, the ground groove isdiscontinuous and periodic. The groove morphology is filled of micro cracks withoutscratching mark all at the beginning, middle and end parts, as shown in Figure 2b. The groovelength and width are smaller relatively. This illustrates the intermittent cutting characteristicsof the abrasive grain after the application of radial ultrasonic vibration.

Figure 2. The cutting-in morphology

The grinded groove bottom morphology at the maximum grinding thickness position ingrinding with different ultrasonic vibration frequency is shown in Figure 3. It is seen that thegroove bottom morphology is composed of brittle cracks and material fragments in both theconventional and ultrasonic grinding. The material fragment is formed by brittle crackgenerating, crack propagating and confluence, material breakage. The propagating distanceand density of brittle crack is significantly related with the material fragment size. From theresults, it is found that the material fragments are coarse and brittle cracks are longer inconventional grinding along the grinding direction. In comparison, the material fragments arevery fine in ultrasonic vibration assisted grinding. With the vibration frequency increasingfrom 28KHz to 40KHz, the material fragments even are finer. It is indicated that the materialremoval mechanism in conventional grinding is continuous crack propagation, but changed tomicro crack bursting in ultrasonic vibration assisted grinding.

Figure 3. The ground groove bottom morphology

Figure 4 shows the grinded groove depth and cross-section profile by the morphologyreconstruction. In experiments, the grinding depth which is equal to the groove theoreticaldepth is 20µm. It is observed that the ground groove real depth is far greater than the theoretical

depth both in conventional and ultrasonic grinding. When the glass material occurs brittlefailure, the generated cracks include median crack and transverse crack. The median crackpropagates into workpiece interior under the action of grinding force, and induces the glassmaterial spalling in depth direction. This leads to the groove real depth is greater than thetheoretical depth. As shown in Figure 4a, the groove maximum depth ΔA1 in conventionalgrinding is 140µm. But it is only 100µm in ultrasonic grinding. The groove depth variation ΔB1in conventional grinding is larger than ultrasonic grinding as well. The deeper the median crackpropagates, the larger the grinded groove depth is. The groove depth variation also can reflectthe median crack propagating depth to a certain extent. Based on the results, it is meant that themedian crack propagating distance of ultrasonic vibration assisted grinding is relatively shorterthan conventional grinding.

Figure 4. The ground groove depth

In conventional grinding, the diamond grain acts on glass workpiece continuous. Whendiamond grain contacts the workpiece slightly, the glass occurs plastic deformation due to thevery small cutting thickness. With the diamond grain rotating, the cutting thickness increases,the brittle fracture presents, the brittle crack generates, it will continuously propagate to causematerial removal under the action of cutting force. In ultrasonic vibration assisted grinding, thediamond grain undergoes repeated cutting-in and cutting-out accompanied with the ultrasonicvibration, actually the grinding process is high frequency discontinuous. The instantaneous linevelocity of diamond grain is larger than conventional grinding due to the couple of ultrasonicvibration and rotational motion. When the diamond grain is cutting-in, the crash momentum isvery high, and the local stress on the workpiece is very large as well. Even it is great enough tocause the brittle material bursting immediately and this induces many micro cracks as shown inFigure 5. The dense confluence of so many micro cracks results in very fine material fragments.The glass material is removed only by brittle failure in small scale and no plastic deformation.The material removal mechanism in ultrasonic vibration assisted grinding is micro crackbursting and different to the continuous crack propagation in conventional grinding. It canreduce the brittle crack propagation distance.

Figure 5. The material removal mechanism in ultrasonic vibration assisted grinding

3.2 The acoustic emission signal. Acoustic emission signal is the kind of stress wave causedby the changes of material structure, such as plastic deformation, crack generation andpropagation, material breakage, etc. It is an important factor to study the grinding mechanism.The involved material removal mechanism in grinding can be illuminated from the analysis ofacoustic emission signal.

Figure 6 shows the acoustic emission signal and its spectral analysis measured in grindingexperiments. It is seen that the acoustic emission signal amplitude increases with diamondgrain cutting in, reaches the maximum at the middle position of the largest grinding thickness,and then decreases with diamond grain cutting out. This amplitude change appears only once inconventional grinding, but five periods in ultrasonic grinding. It is indicated that the diamondgrain grinding is continuous in conventional grinding and discontinuous in ultrasonic grinding.From the spectral analysis, it is found that the only peak frequency is about 40.2 KHz inconventional grinding. But there are many peak frequencies in ultrasonic grinding. The firstpeak frequency is about 27.3 KHz and very close to the ultrasonic vibration frequency. Themaximum peak frequency is about 78.7 KHz. The addition of ultrasonic vibration hasinterference on acoustic emission signal in grinding and causes it more complex.

Figure 6. The acoustic emission signal

The energy distribution after wavelet packet decomposition of acoustic emission signal isshown in Figure 7. It is found that the energy distribution is mainly at the first frequency bandof 0-62.5 KHz in conventional grinding. This is consistent with the spectral analysis. Inultrasonic grinding, the energy distribution at the first frequency band is far higher than otherfrequency band. The reason may be the ultrasonic vibration frequency of 28 KHz is in thisfrequency band. The second energy distribution is located at the second frequency band of62.5-125 KHz. And its energy coefficient is close to the first frequency band in conventionalgrinding. In comparison, the energy distribution in ultrasonic vibration assisted grinding ismore concentrated due to material removal mechanism of micro crack breakage.

Figure 7. The energy distribution after wavelet packet decomposition

4. SummaryThis paper presents an investigation on the material removal mechanism in single

diamond grain grinding of glass with and without ultrasonic vibration assisted. The followingconclusions have been drawn:

1. The conventional grinding is continuous. The material removal mode includes plasticdeformation and continuous crack propagation.

2. The ultrasonic vibration assisted grinding is discontinuous. The material removalmechanism mainly is micro crack breakage without plastic deformation stage. It can decreasethe brittle crack propagation distance.

3. With the addition of ultrasonic vibration assisted in grinding, the peak frequency ofacoustic emission signal increases, and the energy distribution is more concentrated.

AcknowledgementThe work was supported by the Science and Technology Project of Fujian Province, P. R. China (Grant No.2018H6013) and National project Fund (Grant No. 51675192).

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