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Development of a new rotary ultrasonic spindle for precision ultrasonically assisted grinding
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 49 (2009) 933–938
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Development of a new rotary ultrasonic spindle for precision ultrasonically assisted grinding Yongbo Wu a,, Syota Yokoyama b, Takashi Sato a,1, Weimin Lin a,2, Toru Tachibana c,3 a
Department of Machine Intelligence and Systems Engineering, Akita Prefectural University, Tsuchiya-ebinokuchi 84-4, Yurihonjo, Akita 015-0055, Japan Suzuki Motor Corporation, Kosai, Shizuoka, Japan c Micron Machinery Co. Ltd., Zao-uwano 578-2, Yamagata 990-2303, Japan b
a r t i c l e in fo
abstract
Article history: Received 1 May 2009 Received in revised form 22 June 2009 Accepted 22 June 2009 Available online 30 June 2009
In our previous work, a new method for inducing a machine spindle to ultrasonically vibrate was proposed in which the axial ultrasonic vibration is excited by a fluctuating electromagnetic force applied to the spindle. The validity of this method was also confirmed experimentally. In this paper, focusing on the development of a new rotary ultrasonic spindle based on this new method, an actual ultrasonic spindle unit was designed and constructed in accordance with the previous work. The unit consists of an ultrasonic spindle, two rotary bearings and their housings for holding the spindle, eight electromagnets and their supporters for the generation of a fluctuating electromagnetic force, and a base plate. Its performance was also investigated experimentally for different exciting conditions. The results showed that an ultrasonic vibration with a sub-mm order amplitude is generated on the produced spindle and that the applied electromagnetic force affects the spindle performance significantly. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Machine spindle Ultrasonic vibration Internal grinding Electromagnetic force
1. Introduction Rotary ultrasonic machining [1–3] has been used in drilling, cutting, milling, and threading operations of advanced engineering materials such as silicon, ceramics, glass and various metals in manufacturing industries; the tool is rotated around its axis and ultrasonically vibrated along its axis. This is because the rotary ultrasonic machining has many advantages compared with the conventional machining processes. For example, in our previous works [4,5] an ultrasonic vibration-assisted grinding technology has been developed for the internal precision grinding of small holes. The obtained results revealed that applying ultrasonic vibration to the grinding wheel decreases the normal and tangential grinding forces by more than 65% and 70%, respectively, due to the grinding chips becoming smaller and fracturing more easily under ultrasonication, and the work-surface roughness by as much as 20%. Our further work [6] has been carried out to investigate the effect of ultrasonic vibration in truing and dressing of small CBN grinding wheel, showing that the truing force
Corresponding author: Tel.: +81184 27 2144; fax: +81184 27 2165.
E-mail addresses: [email protected] (Y. Wu), [email protected] (S. Yokoyama), [email protected] (T. Sato), [email protected] (W. Lin), t-tachi@ micron-grinder.co.jp (T. Tachibana). 1 Tel.: +81184 27 2157; fax: +81184 27 2165. 2 Tel.: +81184 27 2143; fax: +81184 27 2165. 3 Tel.: +81 23 688 8111; fax: +81 23 688 7476. 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.06.012
decrease by more than 22% and better truing accuracy is attained owing to the presence of ultrasonic vibration. In rotary ultrasonic machining, an ultrasonic vibration spindle is employed, and the tool is attached to the end face of the spindle in order to ultrasonically vibrate the tool. Conventionally, there are two types of transducers for inducing the spindle to vibrate ultrasonically [7]. One is made of piezoactive materials, generally industrial piezoceramics (PZT), and the other of magnetostrictive materials, usually ferrite. Although the former has the advantage of low electrical energy losses, a slip-ring and carbon brushes must be employed for the application of AC voltage, and poor contact between these elements occasionally occurs, leading to failure. On the other hand, the ferrite transducer is subject to high electrical energy losses because of the high thermal generation in ferrite materials. Given the drawbacks of the conventional methods, a new technique was proposed for inducing ultrasonic spindle vibration by utilizing a fluctuating electromagnetic force in our previous work [8]. In order to confirm the proposed new method, a prototype spindle was designed and constructed, and its performance was previously examined experimentally. On the basis of the previous work, an ultrasonic spindle unit, consisting of an ultrasonic spindle, two rotary bearings and their housings for holding the spindle, eight electromagnets and their supporters for the generation of a fluctuating electromagnetic force, and a base plate, was designed and constructed. In the design, the detailed structure and dimensions of the spindle were
ARTICLE IN PRESS Y. Wu et al. / International Journal of Machine Tools & Manufacture 49 (2009) 933–938
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Fig. 1 schematically shows the structure of the proposed magnetic force-induced ultrasonic spindle unit and the generation principle of the axial ultrasonic vibration of the spindle. The spindle, formed from a cylindrical ultrasonic body, two driving discs and two flanges, is fabricated by cutting off a metal rod, and is supported by two housings on a base plate via the flanges and the bearings. A tool such as a small grinding wheel is attached on the front end face of the spindle, and a motor is connected to the spindle’s rear end face via a coupling to drive the spindle rotationally at a rotational speed of nt. The 3rd longitudinal vibration (L3 mode) is selected as the spindle resonant vibration mode, so that the spindle has three vibration nodes along its length where the amplitude of vibration is assumed to be zero. Consequently, the front and rear flanges can be positioned at the front and rear nodes, respectively, in order to support the spindle at two different points. Since the role the driving discs play is to receive the fluctuating magnetic forces generated by the electromagnets and to excite the axial ultrasonic vibration in the spindle, the two discs are, respectively located at the two abdomens of the L3 mode so that the ultrasonic vibration can be induced easily. In order to generate the fluctuating magnetic force, four electromagnets fixed on their respective holders are installed for each driving disc with a gap of d. A spacer is installed between the base plate and the magnet holders and used to adjust the vertical and horizontal positions of the electromagnets. When the same AC voltage V(t) is applied to all the electromagnets simultaneously at a frequency close to that of the L3 mode of the spindle, fluctuating magnetic forces are generated and act on the driving discs, thus resulting in the ultrasonic vibration of the spindle along its axis and eventually ultrasonicating the tool. Varying the frequency and amplitude of the applied AC voltage and the gap changes the spindle vibration amplitude.
32
2. The structure and operation principle of the new spindle
obtained by FEM analysis and the detailed structure and dimensions of the designed spindle, respectively. Considering that the spindle material needs to react to the applied magnetic field and has a larger Young’s modulus, low carbon steel (S45C in Japanese standard) was selected for the spindle material. The outside dimensions of the spindle were determined to be L330 mm f96 mm in consideration of the installation space in an existing machining rig. The frequency of the L3 mode of the designed spindle was 22.56 kHz. Finally, based on the designed details, a spindle was actually produced as shown in Fig. 2(c).
26
determined by FEM analysis. Then a series of experiments were carried out to examine the operating characteristics of the produced unit. This article describes the design and construction of the unit and the experimental details.
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3. Design and construction of the unit 3.1. Ultrasonic vibration spindle The detailed structure and dimensions of the spindle were determined by finite element method (FEM) analysis under the condition that the spindle is induced to ultrasonically vibrate in L3 mode. Fig. 2(a) and (b) show a typical image of the L3 mode
Fig. 2. An image of the L3 mode obtained by FEM analysis (a), the structure and dimensions of the designed spindle (b), and a photograph (c) of the constructed spindle.
Fig. 1. Schematic drawing of the new ultrasonic spindle unit and the ultrasonic vibration generation principle.
ARTICLE IN PRESS Y. Wu et al. / International Journal of Machine Tools & Manufacture 49 (2009) 933–938
3.2. Electromagnet
Ultrasonic spindle
Impulse hammer
Sensor head
Base plate
Laser doppler vibrometer
0.30 0.25 Resonant point f = 22.84kHz
0.20 0.15 0.10 0.05 0.00 20
In the new ultrasonic spindle unit proposed, the electromagnets, each of which is composed of an iron core and a coil, play a key role in the generation of the fluctuating magnetic force used to induce the spindle to ultrasonically vibrate. Therefore, it is essential to design and produce electromagnets that are capable of generating a sufficiently strong magnetic force to act on the driving discs. As the strength of the magnetic field generated by the electromagnet depends significantly on the permeability of the materials of the iron core and coil [9], pure iron steel and enameled wire (f0.6 mm) were selected for the iron core and the coil, respectively, due to their great permeability, so that the electromagnet could generate as strong a magnetic field as possible. Finally, in consideration of the installation space in the unit, the electromagnet was constructed from a cylindrical iron core with dimensions of f12 mm in diameter and L30 mm in length and a coil wound around the iron core with a winding number of 320. Fig. 4 shows a picture of the electromagnet constructed. In order to investigate whether or not the ultrasonically fluctuating magnetic force is actually generated by supplying AC electric power to the coil, and how the electric power amplitude and the gap d affect the magnetic force, a measurement apparatus was built in-house as shown in Fig. 5. The produced electromagnet was held on a base via a holder. A load cell, on which a carbon steel disc was attached, was fixed on the base through a holder and used to measure the magnetic force. The gap d between the iron core and the metal disc was set up by moving the holder leftward or rightward. The electric power supplied to the coil was generated by amplifying an AC signal from a wave function generator with a power amplifier. Thus, the magnetic force acting on the metal disc was obtained by recording the signal amplified with a strain amplifier using a digital recorder. However, in order for the frequency of the fluctuating magnetic force to be the same as that of the fluctuating magnetic field generated by the electromagnets, the magnetic field direction, i.e., the direction of the electric current running through the coil, should always be oriented in a certain direction. For this purpose, an AC voltage of V(t) ¼ V0[1+sin(2pft)], the minimum value of which is zero, was applied to the coil so that the electric current runs in a fixed direction.
FFT Analyzer
Multipurpose vibrometer
Axial vibration amplitude [μm]
In order to examine whether or not the actual frequency of the L3 mode of the produced spindle is close to that predicted in the FEM analysis, and to obtain a guideline for the determination of the AC voltage frequency, a hammering test was carried out as shown in Fig. 3(a). In the test, an impulse hammer (VH-60 by Rion Co., Ltd.) was used to impact the left end face of the spindle, which is held by the two flanges on the base plate via bearings and housings, while a laser beam from the sensor head of a laser Doppler vibrometer (LV-1610 by Onosokki Co. Ltd.) was focussed on the right end face of the spindle to collect the spindle vibration. The signals from both the hammer and the sensor head were input into a FFT analyzer (CF5220 by Onosokki Co. Ltd.) after being amplified with a multipurpose vibrometer (VM-80 by Rion Co. Ltd.) and the laser Doppler vibrometer, respectively, for the spectrum analysis. Fig. 3(b) shows the frequency characteristics of the spindle in the hammering test. It is obvious that the spindle vibration amplitude at its right end face reached a peak at f ¼ 22.84 kHz. This indicates that the actual L3-mode frequency of the produced spindle is 22.84 kHz, which resembles the value of 22.56 kHz predicted in FEM analysis.
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21
22 23 Frequency [kHz]
24
25
Fig. 3. Hammering test method (a) and the obtained frequency characteristics (b) for determining the actual L3-mode frequency of the spindle.
Fig. 4. The produced electromagnet.
Fig. 5. Illustration of magnetic force measurement.
Fig. 6 shows a typical measurement result obtained under the conditions of V0 ¼ 5 V, f ¼ 23 kHz and d ¼ 1 mm. It is evident that the fluctuating magnetic force varies periodically with time, and its variation frequency agreed well with that of the applied voltage; a P-V amplitude of AF ¼ 0.07 N was obtained for the given conditions. The effect of the applied voltage amplitude of V0 on the value of AF was also investigated for different values of gap d, and the obtained relationship between the V0 and the AF are plotted in Fig. 7, where the voltage frequency was kept at
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f ¼ 23 kHz. It can be seen that the value of AF increases linearly as the value of V0 increases and that a larger value of AF can be obtained once a smaller vale of d is given.
3.3. Unit construction
4. Experimental confirmation of the unit performance
Fluctuating magnetic force [N]
Prior to the actual construction of the unit, two commercially available rotary bearings capable of rotating at a rotational speed of up to 10,000 rpm were selected, and two housings and one base plate were produced in-house. Subsequently, the unit was assembled from the designed and produced ultrasonic spindle and the eight electromagnets and their holders in addition to the
0.12 V0 = 5V, f = 23kHz, = 1.0mm
AF
0.09
0.06
0.03
0 0
0.1
0.2
0.3
0.4 0.5 0.6 Time [ms]
0.7
0.8
0.9
1
Fig. 6. A typical fluctuating magnetic force generated with the electromagnet.
Amplitude of fluctuating magnetic force AF [N]
0.06 0.05 f = 23kHz
0.04 0.03
δ = 0.5mm
0.02
δ = 1.0mm
0.01
δ = 3.0mm
0 0
1
bearings, housings and base plate. The detailed structure and a picture of the constructed unit are shown in Figs. 8(a) and (b), respectively.
2
3
4
5
Applied voltageV0 [V] Fig. 7. Effect of applied voltage on the magnetic force AF.
6
Experiments were carried out to confirm the performance of the constructed unit including the axial ultrasonic vibration generation of the spindle. During the experiments, the gap was kept at d ¼ 0.5 mm, and the amplitude V0 and frequency f of the applied AC voltage were changed in their respective ranges. The same laser Doppler vibrometer as that used in the hammering test (see Fig. 3(a)) was employed to measure the ultrasonic vibration where the laser beam was focused on the front end face of the spindle when no tool was attached or on the end face of the metal core (f2.3 mm in diameter) of CBN grinding wheel when a tool was attached. In this work, the installed tool was a small grinding quill used in our research work on the ultrasonically assisted internal grinding of small holes [4–6] as shown in Fig. 9. Fig. 10 shows a typical output of the laser Doppler vibrometer without a tool attachment under the conditions of d ¼ 0.5 mm, V0 ¼ 20 V and f ¼ 22.585 kHz. Obviously, the spindle was induced to vibrate at the same frequency as that of the applied voltage, and the generated vibration amplitude was around 0.012 mmP-V. The effect of the voltage frequency f on the P-V amplitude of the spindle vibration was also investigated, and the results were obtained as shown in Fig. 11. As can be seen, the vibration amplitude rises rapidly with the initial increase of the frequency and then drops precipitously after reaching its peak value at f ¼ 22.585 kHz. This peak corresponds to the resonant point for the L3 mode of the spindle without a tool attachment, at which the P-V amplitude was around 0.012 mm. However, once the tool was attached on the spindle, the resonant frequency of the L3 mode shifted to the smaller value of f ¼ 21.683 kHz, and the P-V amplitude on the tool end face at the resonant point changed to a larger value of 0.018 mm, although the relationship between the frequency and the amplitude was similar to that in Fig. 11. The decease in the resonant frequency is supposed to occur because the vibration system’s mass became larger by attaching the tool. The reason that the P-V amplitude on the tool end face was larger than that on the spindle end face without the tool attachment is dominantly due to the increase of the system’s length with the tool attachment because the vibration amplitude varies along the axis. Concerning a small vibration of the tool may occur in the planes perpendicular to the spindle axis, a test to check this matter was conducted by focusing the laser beam on a thin
Fig. 8. The detailed structure (a) and a photograph (b) of the designed and constructed unit.
ARTICLE IN PRESS Y. Wu et al. / International Journal of Machine Tools & Manufacture 49 (2009) 933–938
0.02 Axial vibration amplitude [μm]
reflection piece attached on the circumference surface of abrasive wheel. As a result, no radial vibration was recognized on the tool. When the gap was kept constant at d ¼ 0.5 mm and the frequencies of the AC voltage were set at 21.683 kHz with a tool attachment or at 22.583 kHz without a tool, the effects of the applied voltage V0 on the vibration amplitudes were experimentally obtained as shown in Fig. 12. Evidently, either in the case with a tool attachment or without a tool, there are linear relationships between the V0 and the P-V amplitude, indicating that stronger electromagnets should be employed in order to induce the spindle to ultrasonically vibrate more powerfully. It also can be seen that the increase rate of the vibration amplitude of the tool end face with a tool attachment is larger than that of the spindle front end face without a tool by around 50%. In addition, although the results shown in Figs. 10–12 were obtained in the tests without the rotation of spindle, a confirmation test was also carried out on a spindle rotating at a slow speed
937
P-V amplitude of tool end face at f = 21.683kHz with tool
0.016
= 0.5mm
0.012 0.008 0.004
P-V amplitude of spindle end face at f = 22.583kHz without tool
0 0
10
20 30 Applied voltage V0 [V]
40
50
Fig. 12. Effects of applied voltage on vibration amplitude with and without a tool.
of 100 min1 to examine the influence of the rotation motion on the vibration. The result showed that little influence was recognized.
5. Conclusion
Fig. 9. Structure and dimensions of the tool.
Axial vibration amplitude [μm]
0.015
δ = 0.5mm, f = 22.585kHz,V0 = 20V 0.01 0.005 0 -0.005 0.012μmP-V
-0.01 -0.015
0.02
0
0.04 0.06 Time [ms]
0.08
0.1
Fig. 10. Typical measurement results for spindle ultrasonic vibration.
Following our previous work in which a new method using a fluctuating electromagnetic force to induce ultrasonic spindle vibration was proposed, an actual ultrasonic spindle unit consisting of an ultrasonic spindle, two rotary bearings and their housings for holding the spindle, eight electromagnets and their supporters for the generation of a fluctuating electromagnetic force, and a base plate was designed and constructed. Its performance was confirmed experimentally for different exciting conditions. The experimental results can be summarized as: (1) the ultrasonic vibration with a sub-mm order amplitude is generated actually on the produced spindle; (2) the attachment of the tool on the spindle front end face decreases the resonant frequency of the spindle and affects the spindle vibration amplitude significantly; the P-V amplitude on the spindle front end face without a tool is smaller than that on the tool end face with a tool by around 50% when the frequency of the applied voltage is set at the respective resonant points; (3) the P-V vibration amplitude varies linearly with the variation of the applied voltage, indicating that stronger electromagnets should be employed in order to obtain larger vibration amplitude. In future research, the structure and dimensions of the spindle should be re-designed and the unit should be re-constructed so that a larger vibration amplitude suitable for practical applications can be obtained.
Axial vibration amplitude [μm]
0.016
δ = 0.5mm
0.012
Acknowledgments
Resonant point f = 22.585kHz
This research was financially supported in part by Grants-inAid for Science Research from the Japan Society for Promotion of Science (Grant no. 19560121). The authors also gratefully acknowledge the financial support from Osawa Scientific Studies Grants Foundation.
V0 = 20V 0.008
0.004 References
0 22.57
22.58 22.59 Frequency f [kHz]
Fig. 11. Frequency characteristics of the spindle.
22.6
[1] D. Ensminger, Ultrasonics: Fundamentals, Technology, Applications, Marcel Decker Inc., New York, 1988. [2] E.J. Weller, M. Haavisto, Nontraditional Machining Processes, Society of Manufacturing Engineers, Michigan, 1984. [3] S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology, Prentice-Hall Inc., New Jersey, 2000.
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[4] Y. Wu, M. Nomura, M. Kato, T. Tachibana, Study of internal ultrasonic vibration assisted grinding of small holes – construction of ultrasonic vibration spindle and its fundamental performances, J. Jpn. Abrasive Tech. 47 (12) (2003) 550–555 (in Japanese). [5] M. Nomura, Y. Wu, M. Kato, T. Tachibana, T. Kuriyagawa, Study of internal ultrasonic vibration assisted grinding of small holes – mechanism of grinding force reduction due to ultrasonic vibration, J. Jpn. Abrasive Tech. 49 (12) (2005) 691–696 (in Japanese). [6] M. Nomura, Y. Wu, T. Kuriyagawa, Investigation of internal ultrasonically assisted grinding of small holes: effect of ultrasonic vibration in truing and
dressing of small CBN grinding wheel, J. Mech. Sci. Tech. 21 (10) (2007) 1605–1611. [7] Society of Japan Electronic Mechanical Industries, Ultrasonic Engineering, Korona Publishing, Co. Ltd., Tokyo, 1993 (in Japanese). [8] Yongbo Wu, Masanori Tamano, Masana Kato, Toru Tachibana, Inducing a machine spindle to ultrasonically vibrate by fluctuating electromagnetic force, Int. J. Appl. Electromagn. Mech. 25 (2005) 621–626. [9] The Institute of Electrical Engineers of Japan, Fundamental of Electromagnetics, Ohmsha Ltd., Tokyo, 1977 (in Japanese).
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Development of a new rotary ultrasonic spindle for precision ultrasonically assisted grinding Yongbo Wu a,, Syota Yokoyama b, Takashi Sato a,1, Weimin Lin a,2, Toru Tachibana c,3 a
Department of Machine Intelligence and Systems Engineering, Akita Prefectural University, Tsuchiya-ebinokuchi 84-4, Yurihonjo, Akita 015-0055, Japan Suzuki Motor Corporation, Kosai, Shizuoka, Japan c Micron Machinery Co. Ltd., Zao-uwano 578-2, Yamagata 990-2303, Japan b
a r t i c l e in fo
abstract
Article history: Received 1 May 2009 Received in revised form 22 June 2009 Accepted 22 June 2009 Available online 30 June 2009
In our previous work, a new method for inducing a machine spindle to ultrasonically vibrate was proposed in which the axial ultrasonic vibration is excited by a fluctuating electromagnetic force applied to the spindle. The validity of this method was also confirmed experimentally. In this paper, focusing on the development of a new rotary ultrasonic spindle based on this new method, an actual ultrasonic spindle unit was designed and constructed in accordance with the previous work. The unit consists of an ultrasonic spindle, two rotary bearings and their housings for holding the spindle, eight electromagnets and their supporters for the generation of a fluctuating electromagnetic force, and a base plate. Its performance was also investigated experimentally for different exciting conditions. The results showed that an ultrasonic vibration with a sub-mm order amplitude is generated on the produced spindle and that the applied electromagnetic force affects the spindle performance significantly. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Machine spindle Ultrasonic vibration Internal grinding Electromagnetic force
1. Introduction Rotary ultrasonic machining [1–3] has been used in drilling, cutting, milling, and threading operations of advanced engineering materials such as silicon, ceramics, glass and various metals in manufacturing industries; the tool is rotated around its axis and ultrasonically vibrated along its axis. This is because the rotary ultrasonic machining has many advantages compared with the conventional machining processes. For example, in our previous works [4,5] an ultrasonic vibration-assisted grinding technology has been developed for the internal precision grinding of small holes. The obtained results revealed that applying ultrasonic vibration to the grinding wheel decreases the normal and tangential grinding forces by more than 65% and 70%, respectively, due to the grinding chips becoming smaller and fracturing more easily under ultrasonication, and the work-surface roughness by as much as 20%. Our further work [6] has been carried out to investigate the effect of ultrasonic vibration in truing and dressing of small CBN grinding wheel, showing that the truing force
Corresponding author: Tel.: +81184 27 2144; fax: +81184 27 2165.
E-mail addresses: [email protected] (Y. Wu), [email protected] (S. Yokoyama), [email protected] (T. Sato), [email protected] (W. Lin), t-tachi@ micron-grinder.co.jp (T. Tachibana). 1 Tel.: +81184 27 2157; fax: +81184 27 2165. 2 Tel.: +81184 27 2143; fax: +81184 27 2165. 3 Tel.: +81 23 688 8111; fax: +81 23 688 7476. 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.06.012
decrease by more than 22% and better truing accuracy is attained owing to the presence of ultrasonic vibration. In rotary ultrasonic machining, an ultrasonic vibration spindle is employed, and the tool is attached to the end face of the spindle in order to ultrasonically vibrate the tool. Conventionally, there are two types of transducers for inducing the spindle to vibrate ultrasonically [7]. One is made of piezoactive materials, generally industrial piezoceramics (PZT), and the other of magnetostrictive materials, usually ferrite. Although the former has the advantage of low electrical energy losses, a slip-ring and carbon brushes must be employed for the application of AC voltage, and poor contact between these elements occasionally occurs, leading to failure. On the other hand, the ferrite transducer is subject to high electrical energy losses because of the high thermal generation in ferrite materials. Given the drawbacks of the conventional methods, a new technique was proposed for inducing ultrasonic spindle vibration by utilizing a fluctuating electromagnetic force in our previous work [8]. In order to confirm the proposed new method, a prototype spindle was designed and constructed, and its performance was previously examined experimentally. On the basis of the previous work, an ultrasonic spindle unit, consisting of an ultrasonic spindle, two rotary bearings and their housings for holding the spindle, eight electromagnets and their supporters for the generation of a fluctuating electromagnetic force, and a base plate, was designed and constructed. In the design, the detailed structure and dimensions of the spindle were
ARTICLE IN PRESS Y. Wu et al. / International Journal of Machine Tools & Manufacture 49 (2009) 933–938
Max 6.25μm 6 0
5 4 3
300
y
2
200
z x
mm
100
1
0
3
3
3
96
M4
27 21
50
Min 0.0017μm
18
Fig. 1 schematically shows the structure of the proposed magnetic force-induced ultrasonic spindle unit and the generation principle of the axial ultrasonic vibration of the spindle. The spindle, formed from a cylindrical ultrasonic body, two driving discs and two flanges, is fabricated by cutting off a metal rod, and is supported by two housings on a base plate via the flanges and the bearings. A tool such as a small grinding wheel is attached on the front end face of the spindle, and a motor is connected to the spindle’s rear end face via a coupling to drive the spindle rotationally at a rotational speed of nt. The 3rd longitudinal vibration (L3 mode) is selected as the spindle resonant vibration mode, so that the spindle has three vibration nodes along its length where the amplitude of vibration is assumed to be zero. Consequently, the front and rear flanges can be positioned at the front and rear nodes, respectively, in order to support the spindle at two different points. Since the role the driving discs play is to receive the fluctuating magnetic forces generated by the electromagnets and to excite the axial ultrasonic vibration in the spindle, the two discs are, respectively located at the two abdomens of the L3 mode so that the ultrasonic vibration can be induced easily. In order to generate the fluctuating magnetic force, four electromagnets fixed on their respective holders are installed for each driving disc with a gap of d. A spacer is installed between the base plate and the magnet holders and used to adjust the vertical and horizontal positions of the electromagnets. When the same AC voltage V(t) is applied to all the electromagnets simultaneously at a frequency close to that of the L3 mode of the spindle, fluctuating magnetic forces are generated and act on the driving discs, thus resulting in the ultrasonic vibration of the spindle along its axis and eventually ultrasonicating the tool. Varying the frequency and amplitude of the applied AC voltage and the gap changes the spindle vibration amplitude.
32
2. The structure and operation principle of the new spindle
obtained by FEM analysis and the detailed structure and dimensions of the designed spindle, respectively. Considering that the spindle material needs to react to the applied magnetic field and has a larger Young’s modulus, low carbon steel (S45C in Japanese standard) was selected for the spindle material. The outside dimensions of the spindle were determined to be L330 mm f96 mm in consideration of the installation space in an existing machining rig. The frequency of the L3 mode of the designed spindle was 22.56 kHz. Finally, based on the designed details, a spindle was actually produced as shown in Fig. 2(c).
26
determined by FEM analysis. Then a series of experiments were carried out to examine the operating characteristics of the produced unit. This article describes the design and construction of the unit and the experimental details.
12
934
45 64
7
107
52
57.5
330
3. Design and construction of the unit 3.1. Ultrasonic vibration spindle The detailed structure and dimensions of the spindle were determined by finite element method (FEM) analysis under the condition that the spindle is induced to ultrasonically vibrate in L3 mode. Fig. 2(a) and (b) show a typical image of the L3 mode
Fig. 2. An image of the L3 mode obtained by FEM analysis (a), the structure and dimensions of the designed spindle (b), and a photograph (c) of the constructed spindle.
Fig. 1. Schematic drawing of the new ultrasonic spindle unit and the ultrasonic vibration generation principle.
ARTICLE IN PRESS Y. Wu et al. / International Journal of Machine Tools & Manufacture 49 (2009) 933–938
3.2. Electromagnet
Ultrasonic spindle
Impulse hammer
Sensor head
Base plate
Laser doppler vibrometer
0.30 0.25 Resonant point f = 22.84kHz
0.20 0.15 0.10 0.05 0.00 20
In the new ultrasonic spindle unit proposed, the electromagnets, each of which is composed of an iron core and a coil, play a key role in the generation of the fluctuating magnetic force used to induce the spindle to ultrasonically vibrate. Therefore, it is essential to design and produce electromagnets that are capable of generating a sufficiently strong magnetic force to act on the driving discs. As the strength of the magnetic field generated by the electromagnet depends significantly on the permeability of the materials of the iron core and coil [9], pure iron steel and enameled wire (f0.6 mm) were selected for the iron core and the coil, respectively, due to their great permeability, so that the electromagnet could generate as strong a magnetic field as possible. Finally, in consideration of the installation space in the unit, the electromagnet was constructed from a cylindrical iron core with dimensions of f12 mm in diameter and L30 mm in length and a coil wound around the iron core with a winding number of 320. Fig. 4 shows a picture of the electromagnet constructed. In order to investigate whether or not the ultrasonically fluctuating magnetic force is actually generated by supplying AC electric power to the coil, and how the electric power amplitude and the gap d affect the magnetic force, a measurement apparatus was built in-house as shown in Fig. 5. The produced electromagnet was held on a base via a holder. A load cell, on which a carbon steel disc was attached, was fixed on the base through a holder and used to measure the magnetic force. The gap d between the iron core and the metal disc was set up by moving the holder leftward or rightward. The electric power supplied to the coil was generated by amplifying an AC signal from a wave function generator with a power amplifier. Thus, the magnetic force acting on the metal disc was obtained by recording the signal amplified with a strain amplifier using a digital recorder. However, in order for the frequency of the fluctuating magnetic force to be the same as that of the fluctuating magnetic field generated by the electromagnets, the magnetic field direction, i.e., the direction of the electric current running through the coil, should always be oriented in a certain direction. For this purpose, an AC voltage of V(t) ¼ V0[1+sin(2pft)], the minimum value of which is zero, was applied to the coil so that the electric current runs in a fixed direction.
FFT Analyzer
Multipurpose vibrometer
Axial vibration amplitude [μm]
In order to examine whether or not the actual frequency of the L3 mode of the produced spindle is close to that predicted in the FEM analysis, and to obtain a guideline for the determination of the AC voltage frequency, a hammering test was carried out as shown in Fig. 3(a). In the test, an impulse hammer (VH-60 by Rion Co., Ltd.) was used to impact the left end face of the spindle, which is held by the two flanges on the base plate via bearings and housings, while a laser beam from the sensor head of a laser Doppler vibrometer (LV-1610 by Onosokki Co. Ltd.) was focussed on the right end face of the spindle to collect the spindle vibration. The signals from both the hammer and the sensor head were input into a FFT analyzer (CF5220 by Onosokki Co. Ltd.) after being amplified with a multipurpose vibrometer (VM-80 by Rion Co. Ltd.) and the laser Doppler vibrometer, respectively, for the spectrum analysis. Fig. 3(b) shows the frequency characteristics of the spindle in the hammering test. It is obvious that the spindle vibration amplitude at its right end face reached a peak at f ¼ 22.84 kHz. This indicates that the actual L3-mode frequency of the produced spindle is 22.84 kHz, which resembles the value of 22.56 kHz predicted in FEM analysis.
935
21
22 23 Frequency [kHz]
24
25
Fig. 3. Hammering test method (a) and the obtained frequency characteristics (b) for determining the actual L3-mode frequency of the spindle.
Fig. 4. The produced electromagnet.
Fig. 5. Illustration of magnetic force measurement.
Fig. 6 shows a typical measurement result obtained under the conditions of V0 ¼ 5 V, f ¼ 23 kHz and d ¼ 1 mm. It is evident that the fluctuating magnetic force varies periodically with time, and its variation frequency agreed well with that of the applied voltage; a P-V amplitude of AF ¼ 0.07 N was obtained for the given conditions. The effect of the applied voltage amplitude of V0 on the value of AF was also investigated for different values of gap d, and the obtained relationship between the V0 and the AF are plotted in Fig. 7, where the voltage frequency was kept at
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f ¼ 23 kHz. It can be seen that the value of AF increases linearly as the value of V0 increases and that a larger value of AF can be obtained once a smaller vale of d is given.
3.3. Unit construction
4. Experimental confirmation of the unit performance
Fluctuating magnetic force [N]
Prior to the actual construction of the unit, two commercially available rotary bearings capable of rotating at a rotational speed of up to 10,000 rpm were selected, and two housings and one base plate were produced in-house. Subsequently, the unit was assembled from the designed and produced ultrasonic spindle and the eight electromagnets and their holders in addition to the
0.12 V0 = 5V, f = 23kHz, = 1.0mm
AF
0.09
0.06
0.03
0 0
0.1
0.2
0.3
0.4 0.5 0.6 Time [ms]
0.7
0.8
0.9
1
Fig. 6. A typical fluctuating magnetic force generated with the electromagnet.
Amplitude of fluctuating magnetic force AF [N]
0.06 0.05 f = 23kHz
0.04 0.03
δ = 0.5mm
0.02
δ = 1.0mm
0.01
δ = 3.0mm
0 0
1
bearings, housings and base plate. The detailed structure and a picture of the constructed unit are shown in Figs. 8(a) and (b), respectively.
2
3
4
5
Applied voltageV0 [V] Fig. 7. Effect of applied voltage on the magnetic force AF.
6
Experiments were carried out to confirm the performance of the constructed unit including the axial ultrasonic vibration generation of the spindle. During the experiments, the gap was kept at d ¼ 0.5 mm, and the amplitude V0 and frequency f of the applied AC voltage were changed in their respective ranges. The same laser Doppler vibrometer as that used in the hammering test (see Fig. 3(a)) was employed to measure the ultrasonic vibration where the laser beam was focused on the front end face of the spindle when no tool was attached or on the end face of the metal core (f2.3 mm in diameter) of CBN grinding wheel when a tool was attached. In this work, the installed tool was a small grinding quill used in our research work on the ultrasonically assisted internal grinding of small holes [4–6] as shown in Fig. 9. Fig. 10 shows a typical output of the laser Doppler vibrometer without a tool attachment under the conditions of d ¼ 0.5 mm, V0 ¼ 20 V and f ¼ 22.585 kHz. Obviously, the spindle was induced to vibrate at the same frequency as that of the applied voltage, and the generated vibration amplitude was around 0.012 mmP-V. The effect of the voltage frequency f on the P-V amplitude of the spindle vibration was also investigated, and the results were obtained as shown in Fig. 11. As can be seen, the vibration amplitude rises rapidly with the initial increase of the frequency and then drops precipitously after reaching its peak value at f ¼ 22.585 kHz. This peak corresponds to the resonant point for the L3 mode of the spindle without a tool attachment, at which the P-V amplitude was around 0.012 mm. However, once the tool was attached on the spindle, the resonant frequency of the L3 mode shifted to the smaller value of f ¼ 21.683 kHz, and the P-V amplitude on the tool end face at the resonant point changed to a larger value of 0.018 mm, although the relationship between the frequency and the amplitude was similar to that in Fig. 11. The decease in the resonant frequency is supposed to occur because the vibration system’s mass became larger by attaching the tool. The reason that the P-V amplitude on the tool end face was larger than that on the spindle end face without the tool attachment is dominantly due to the increase of the system’s length with the tool attachment because the vibration amplitude varies along the axis. Concerning a small vibration of the tool may occur in the planes perpendicular to the spindle axis, a test to check this matter was conducted by focusing the laser beam on a thin
Fig. 8. The detailed structure (a) and a photograph (b) of the designed and constructed unit.
ARTICLE IN PRESS Y. Wu et al. / International Journal of Machine Tools & Manufacture 49 (2009) 933–938
0.02 Axial vibration amplitude [μm]
reflection piece attached on the circumference surface of abrasive wheel. As a result, no radial vibration was recognized on the tool. When the gap was kept constant at d ¼ 0.5 mm and the frequencies of the AC voltage were set at 21.683 kHz with a tool attachment or at 22.583 kHz without a tool, the effects of the applied voltage V0 on the vibration amplitudes were experimentally obtained as shown in Fig. 12. Evidently, either in the case with a tool attachment or without a tool, there are linear relationships between the V0 and the P-V amplitude, indicating that stronger electromagnets should be employed in order to induce the spindle to ultrasonically vibrate more powerfully. It also can be seen that the increase rate of the vibration amplitude of the tool end face with a tool attachment is larger than that of the spindle front end face without a tool by around 50%. In addition, although the results shown in Figs. 10–12 were obtained in the tests without the rotation of spindle, a confirmation test was also carried out on a spindle rotating at a slow speed
937
P-V amplitude of tool end face at f = 21.683kHz with tool
0.016
= 0.5mm
0.012 0.008 0.004
P-V amplitude of spindle end face at f = 22.583kHz without tool
0 0
10
20 30 Applied voltage V0 [V]
40
50
Fig. 12. Effects of applied voltage on vibration amplitude with and without a tool.
of 100 min1 to examine the influence of the rotation motion on the vibration. The result showed that little influence was recognized.
5. Conclusion
Fig. 9. Structure and dimensions of the tool.
Axial vibration amplitude [μm]
0.015
δ = 0.5mm, f = 22.585kHz,V0 = 20V 0.01 0.005 0 -0.005 0.012μmP-V
-0.01 -0.015
0.02
0
0.04 0.06 Time [ms]
0.08
0.1
Fig. 10. Typical measurement results for spindle ultrasonic vibration.
Following our previous work in which a new method using a fluctuating electromagnetic force to induce ultrasonic spindle vibration was proposed, an actual ultrasonic spindle unit consisting of an ultrasonic spindle, two rotary bearings and their housings for holding the spindle, eight electromagnets and their supporters for the generation of a fluctuating electromagnetic force, and a base plate was designed and constructed. Its performance was confirmed experimentally for different exciting conditions. The experimental results can be summarized as: (1) the ultrasonic vibration with a sub-mm order amplitude is generated actually on the produced spindle; (2) the attachment of the tool on the spindle front end face decreases the resonant frequency of the spindle and affects the spindle vibration amplitude significantly; the P-V amplitude on the spindle front end face without a tool is smaller than that on the tool end face with a tool by around 50% when the frequency of the applied voltage is set at the respective resonant points; (3) the P-V vibration amplitude varies linearly with the variation of the applied voltage, indicating that stronger electromagnets should be employed in order to obtain larger vibration amplitude. In future research, the structure and dimensions of the spindle should be re-designed and the unit should be re-constructed so that a larger vibration amplitude suitable for practical applications can be obtained.
Axial vibration amplitude [μm]
0.016
δ = 0.5mm
0.012
Acknowledgments
Resonant point f = 22.585kHz
This research was financially supported in part by Grants-inAid for Science Research from the Japan Society for Promotion of Science (Grant no. 19560121). The authors also gratefully acknowledge the financial support from Osawa Scientific Studies Grants Foundation.
V0 = 20V 0.008
0.004 References
0 22.57
22.58 22.59 Frequency f [kHz]
Fig. 11. Frequency characteristics of the spindle.
22.6
[1] D. Ensminger, Ultrasonics: Fundamentals, Technology, Applications, Marcel Decker Inc., New York, 1988. [2] E.J. Weller, M. Haavisto, Nontraditional Machining Processes, Society of Manufacturing Engineers, Michigan, 1984. [3] S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology, Prentice-Hall Inc., New Jersey, 2000.
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[4] Y. Wu, M. Nomura, M. Kato, T. Tachibana, Study of internal ultrasonic vibration assisted grinding of small holes – construction of ultrasonic vibration spindle and its fundamental performances, J. Jpn. Abrasive Tech. 47 (12) (2003) 550–555 (in Japanese). [5] M. Nomura, Y. Wu, M. Kato, T. Tachibana, T. Kuriyagawa, Study of internal ultrasonic vibration assisted grinding of small holes – mechanism of grinding force reduction due to ultrasonic vibration, J. Jpn. Abrasive Tech. 49 (12) (2005) 691–696 (in Japanese). [6] M. Nomura, Y. Wu, T. Kuriyagawa, Investigation of internal ultrasonically assisted grinding of small holes: effect of ultrasonic vibration in truing and
dressing of small CBN grinding wheel, J. Mech. Sci. Tech. 21 (10) (2007) 1605–1611. [7] Society of Japan Electronic Mechanical Industries, Ultrasonic Engineering, Korona Publishing, Co. Ltd., Tokyo, 1993 (in Japanese). [8] Yongbo Wu, Masanori Tamano, Masana Kato, Toru Tachibana, Inducing a machine spindle to ultrasonically vibrate by fluctuating electromagnetic force, Int. J. Appl. Electromagn. Mech. 25 (2005) 621–626. [9] The Institute of Electrical Engineers of Japan, Fundamental of Electromagnetics, Ohmsha Ltd., Tokyo, 1977 (in Japanese).