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INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11, pp. 1873-1879NOVEMBER 2013 / 1873DOI: 10.1007/s12541-013-0253-1Effect of Ultrasonic Vibration in Grinding; Horn Designand ExperimentYoung-Jae Choi1, Kyung-Hee Park2,#, Yun-Hyuck Hong1, Kyeong-Tae Kim2, Seok-Woo Lee2, and Hon-Zong Choi11 Korea Institute of Industrial Technology, 1271-18, Sa-dong, Sangrok-gu, Ansan-si, Gyeonggi-do, South Korea, 426-791 2 Korea Institute of Industrial Technology, 35-3, Hongcheon-ri, Ipjang-myeon, Seobuk-gu, Cheonan-si, Chungcheongnam-do, South Korea, 331-825# Corresponding Author / E-mail: kparkkitech.re.kr, TEL: +82-41-589-8672, FAX: +82-41-589-8460KEYWORDS: Ultrasonic vibration, Longitudinal vibration mode, Ultrasonic grinding, Grinding force, CBN grinding toolUltrasonic vibration can be applied for grinding of difficult to cut materials, such as titanium, ceramics, etc., because the ultrasonic vibration in longitudinal direction can cause reduction of cutting temperature and tool wear. In this paper, a ultrasonic tool horn, which can vibrate longitudinally with a frequency of 20kHz, was designed using finite element method (FEM). And the ultrasonic horn was fabricated for ultrasonic assisted grinding. A grinding test was performed in terms of machining parameters, such as grinding speed, feed rate, etc., in order to study effect of ultrasonic vibration in grinding. The design of experiment (DOE) approach was used for an optimal condition of ultrasonic grinding, which can minimize the grinding forces. The grinding forces were measured and compared between the conventional grinding and ultrasonic assisted grinding. In addition, to characterize the effect of work materials, titanium (Ti6Al4V), FCD700 and S45C were used as work materials. From the experiment, it was found that the grinding forces decreased as the ultrasonic vibration power and the rotation speed of spindle increased while the grinding force was reduced as the feed rate increased. In addition, regression model was formulated for obtaining optimal grinding condition. The ultrasonic vibration provided beneficial effects regardless of work materials, especially for brittle material.Manuscript received: October 3, 2012 / Accepted: August 22, 20131. IntroductionRecently, aerospace and automotive manufacturers have required for materials that have high strength with low density, such as titanium, composites, ceramics, etc., due to their superior mechanical properties. However, these materials are classified as difficult-to-cut materials so various machining techniques for the materials have been applied. For example, cryogenic machining, laser assisted machining and ultrasonic assisted machining techniques have been successful for machining of these materials. Among the machining techniques, it has been reported in a few literature that the ultrasonic assisted machining can be a promising technique for the difficult-to-cut materials. Mult et al. and Akbari et al. reported that the ultrasonic assisted grinding reduces the normal force in grinding of alumina.1,2 Singh and Khamba claimed that the ultrasonic machining could be useful because ultrasonic machining process does not cause thermal damage, high residual stress and microstructure deformation on work material.3Ultrasonic assisted machining technique can be applied for grinding process where high temperature is generated at the grinding surface. By adding oscillating of tool horn with high frequency vibration over20 kHz, the surface temperature generated can be reduced possibly due to intermittent contact mode during axial ultrasonic vibration of rotating tool.4The ultrasonic vibration generated by ultrasonic actuator has to be transferred to tool horn by passing booster at the same frequency. Therefore, the tool horn has to be designed for vibrating at the resonance frequency and modal analysis of the finite element method (FEM) can be used for the design. In this regard, several works have been made for ultrasonic horn design based on finite element method.5,6Using FEM, various horn shapes can be designed and optimized for an exact ultrasonic frequency. Akbari et al. used FEM for geometrical dimension of the horn on top of the analytical model that can determine the horn contour.2 Lee et al. used ANSYS software for optimal design of the conical horn using element type of solid186. On the other hand, for the verification regarding to ultrasonic horn fabricated, frequency and amplitude were measured by using laser vibrometer and impedance measurement system.7-9Recently, many studies have been studied for ultrasonic assisted grinding that uses kinematic overlapping mechanism in which ultrasonic vibration in axial direction is applied on that of typical KSPE and Springer 20131874 / NOVEMBER 2013INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11grinding process where the grinding wheel is only rotated. In literature, it was reported that the ultrasonic grinding can reduce grinding forces, temperature and tool wear, especially for difficult-to-grind materials, such as ceramics and super alloys.10-13 Therefore, in the ultrasonic grinding, higher surface quality of work material and longer tool life can be attainable comparing to the conventional grinding. And these studies have been done by investigating the relationship of machining parameters, such as abrasive types and sizes,14,16,22 ultrasonic conditions and machining conditions,14-17,22 and output variables, such as cutting force,14-17,21,23,25 material removal rate,16,17,20,21,23,24 and surface roughness14-16,18,21,23,25 and edge chipping.19 Kang investigated the ultrasonic effect in grinding of ceramic material. It was observed that the force in ultrasonic assisted grinding was only half of the force in conventional grinding. And also the increase rate of the grinding force as increasing the feed rate and depth of cut was substantially lowered in ultrasonic assisted grinding.26 Onikura et al. used ultrasonic grinding for micro cylindrical tools and it was found that the grinding force was reduced and the tool breakage was prevented in ultrasonic grinding.27,28So, they concluded that the ultrasonic vibrations resulted in the decrease of cutting force and extension of tool life. Also, tool wear reduction and surface roughness improvement by ultrasonic assisted grinding was found in.29 On the other hand, it was reported that the ultrasonic vibration can be useful not only grinding process but also milling, drilling, burr removal and grinding wheel dressing.30-34In this work, to investigate the characteristics of ultrasonic vibration in grinding, the grinding test was performed for various materials. First, a grinding tool horn for ultrasonic vibration was designed using FEM, which in turn was fabricated for the ultrasonic grinding test. A resonant design was made of the grinding tool horn oscillating longitudinally with a frequency of 20 kHz. An electroplated CBN grinding tool was used as a grinding tool. The design of experiment (DOE) was used to analyze the characteristics in terms of grinding forces and the regression model was used. In additions, to identify the effect of work material, the various materials, titanium, FCD700 and S45C, were used as the work material.2. Design of Ultrasonic Horn for Grinding Using Finite Element Method2.1. Configuration of the Tooling SystemAn ultrasonic grinding tool consists of a generator, transducer,booster, and tool horn as shown in Fig. 1. The transducer is composed of piezoelectric ceramic (PZT) to generate ultrasonic vibration. The bolt langevin type (BLT) transducer, which is laminated with piezoelectric (PZT) ceramics and fascened by bolts, was used. The vibrational energy generated by the transducer passes though the booster and then it is transmitted to the tool horn. Thus, the tool horn should have the same natural frequency as the transducer. For this purpose, the resonant design of the tool horn is needed.62.2 Design and Fabrication of Ultrasonic HornDepending on type of the ultrasonic horns, different magnitudes of amplitude are generated. Fig. 2 shows amplitudes of the ultrasonic vibration in terms of horn type.6As shown in Fig. 2, the amplitude is the largest at a half of the wavelength (). And also the amplitude can be the largest at a twice of the wavelength because the maximum amplitude can be obtained at n/2 where n=1, 4, 8, etc. Thus, the horn length should be a half or twice of the wavelength. However, longer tool length could cause the chatter and higher grinding force. In this study, therefore, the horn length was designed using a half of the wavelength. For the determination of horn length, the Eq. (1) was used.2,6l = l/2 c/f(1)Where l is a horn length, c is a wave propagation velocity in materials (sound velocity) and f is an ultrasonic frequency generated by transducers. The Eq. (1) is the empirical equation that calculates approximate length of the cylindrical shaped horn simply and provides the geometrical dimension for FEM model. So the length and shape obtained is used for initial FEM model. Finally, the ultrasonic tool horn was designed to provide the longitudinal vibration with around frequency of 20 kHz in the FEM model. In this study, the step and exponential horn shapes were combined and horn shape was optimized to maximize ultrasonic amplitude of the horn.SAMCEF software was used for FEM simulation, in which free-to-free condition at the horn ends was assumed as boundary condition. This is because free to free condition is simpler way to calculate the natural frequency of the horn designed without any consideration for constraint and frequency input conditions that have to be considered in harmonic analysis. And the maximum amplitude location and nodal point also can be obtained exactly using free-to-free condition.The tool horns, whose diameter was 10 mm, was designed and analyzed by FEM, as seen in Fig. 3. The horn length calculated by Eq.(1) was 128 mm where wave propagation velocity c of 5,120 m/s wasFig. 1 Schematic of diagram ultrasonic tool assemblyFig. 2 Schematic of diagram ultrasonic horn and amplitude7INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11NOVEMBER 2013 / 1875used for AISI 1045 steel and frequency f was set as 20 kHz. This horn length was used for initial FEM model. Based on the FEM results, the tool horn lengths of 130.8 mm diameter was obtained at the ultrasonic frequency of 20 kHz.2.3. Ultrasonic Characteristics of the Tool HornsThe horns designed by FEM were fabricated and tested by an oscillation test for the amplitude measurement. The horn was installed at the ultrasonic actuator and oscillated by changing the generator power. As shown in Fig. 4, the amplitude and frequency were measured through laser vibrometer (Polytech OFV-3001) and an oscilloscope (Tektronix TDS1000B). Fig. 4 represents the oscillation test result at ultrasonic generator power of 20%, which shows a 20 kHz vibration inFig. 3 Vibration mode shape of horn by FEMFig. 4 The ultrasonic wave form of the horn (10)Fig. 5 Amplitude by the ultrasonic born (10)a longitudinal direction, which is the same frequency as predicted by FEM. The amplitude varies from 7 to 65 m depending on the power of ultrasonic generator from 20% to 100%, as shown in Fig. 5.3. Experimental WorkIn this work, 3-axial CNC machine capable of generating ultrasonic vibration in axial direction was used for comparison of the grinding forces in ultrasonic and conventional grinding. Fig. 6 shows schematic of experimental setup. The ultrasonic actuator attached to the spindles can produce the axial vibration with a frequency of 20 kHz while the tool spindle is rotated. For the grinding test, the tool end of the horn designed was electroplated by CBN abrasive whose grain size was about 90 mm. Titanium (Ti6Al4V), FCD700, and S45C were used for the work materials. For grinding force measurement, tool dynamometer (KISTLERs Type 9256c) was used.To investigate the effect of the machining parameters such as cutting speed, feed rate and ultrasonic amplitude, the grinding test was performed. Two different grinding speeds, 2000 rpm and 4000 rpm, and were used because the tool failure was observed at 6000 rpm. And also the ultrasonic power of 40% in ultrasonic assisted grinding could not be used due to actuator overload. The depth of cut in the radial direction was fixed at 0.002 mm. All experiment was performed at dry condition. And the ultrasonic vibration frequency of 20 kHz was used for the ultrasonic assisted grinding.Fig. 6 Schematic of experimental setupTable 1 Grinding conditionsRotation speed of spindle (RPM)2000, 4000, 6000 (NA)Feed rate (mm/min)20, 30, 40Depth of cut (mm)0.002Ultrasonic Vibration Power20, 30, 40 (NA)(Amplitude gain, %)1876 / NOVEMBER 2013INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11Especially, ultrasonic assisted grinding test was conducted in design of experiment (DOE) approach to characterize the ultrasonic effect. For DOE, three factors (cutting speed, feed rate and ultrasonic power) and there levels were defined and a factorial design was applied to DOE where total 12 times of the experiment were conducted.The grinding forces were obtained by a signal processing of the raw signals measured. For the signal process, low frequency noise was eliminated by high pass filter, which can eliminate drifts of the raw signal measured. And then the absolute values of the filtered signals were taken and the averages of the maximum values were considered as the grinding forces.wear was small at both low and high grinding speeds.25Typically, the tool wear strongly depends on the cutting temperature. This is because hardness of tool materials substantially decreases as cutting temperature increases, which causes to accelerate the tool wear.35 Fig. 9 shows cutting temperature measured by infrared camera (TH9100WLN by NEC, resolution of 0.1oC in range of -40120oC) in conventional and ultrasonic assisted machining. The temperature in ultrasonic assisted grinding was slightly lower than that in conventional case. The test was repeated five times and the average of temperature difference was 4.54oC and lowered in ultrasonic assisted grinding. Even if the temperature measured is not the interfacial cutting4. Results and discussionFig. 7 shows the grinding force measured at ultrasonic vibration on and off conditions in a grinding of titanium (Ti6Al4V). During a transition from ultrasonic grinding to conventional grinding, the normal and tangential forces increased. This shows positive effect of the ultrasonic vibration in grinding. However, it was observed that the axial force was decreased. This could be because an additional energy generated by ultrasonic actuator in the axial direction causes the axial force increment. However, the axial force is not significant factor relatively that affect grinding performance than normal and tangential forces. Therefore, it can be said that ultrasonic vibration can provide beneficial effect in grinding.Fig. 8 show the grinding forces at feed rates and rotation speeds in the conventional grinding and ultrasonic assisted grinding for titanium (Ti6Al4V). In this study, grinding force was considered as resultant of the normal and tangential forces. From the test, it was observed that the grinding forces increased as the feed rate increased while the forces decreased as the grinding speed increased. Overall, the forces in ultrasonic grinding were reduced up to 26% (average 16%) than that in the conventional grinding. This reduction of the grinding forces in ultrasonic assisted grinding could bring out less grinding tool wear. In fact, it was confirmed from the previous work that the ultrasonic assisted grinding showed smaller tool wear than that in the conventional grinding especially at lower grinding speed.25 In addition, as grinding force was reduced at high grinding speed, the tool wear showed less at high speed for the conventional grinding. However, for the case of the ultrasonic assisted grinding, it was found in previous study that the toolFig. 8 Comparison of grinding force between conventional grinding and ultrasonic grindingFig. 7 Grinding force at ultrasonic vibration on and off at feed rate of 40 mm/min and rotation speed of 2000 rpmFig. 9 Cutting temperature measured by infrared camera at rotation speed of 4000 rpm and feed rate of 20 mm/minINTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11NOVEMBER 2013 / 1877temperature, the argument that ultrasonic vibration can reduce the cutting temperature is reasonable because the data is kept consistent. Therefore, it can be claimed that this lowered cutting temperature by ultrasonic vibration could delay the tool wear.Figs. 10 and 11 show main and interaction effects of the grinding force in terms of machining factors in ultrasonic grinding in DOE approach. As shown in Fig. 10, the main effect shows the grinding force was reduced when the rotational speed and ultrasonic power decreased and the feed rate increased. Especially, it was observed that the higher ultrasonic power slightly reduced the grinding force. On the other hand, the interaction effect shows that there are no interactions between three different machining factors as seen in Fig. 11.The reg