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Analytical and experimental study of topography of surface texture in ultrasonic vibration assisted turning
Analytical and experimental study of topography of surface texture in ultrasonic vibration assisted turning S.A. Sajjady, H. Nouri Hossein Abadi, S. Amini, R. Nosouhi PII: DOI: Reference:
S0264-1275(15)30978-3 doi: 10.1016/j.matdes.2015.12.119 JMADE 1143
To appear in: Received date: Revised date: Accepted date:
14 October 2015 20 December 2015 21 December 2015
Please cite this article as: S.A. Sajjady, H. Nouri Hossein Abadi, S. Amini, R. Nosouhi, Analytical and experimental study of topography of surface texture in ultrasonic vibration assisted turning, (2015), doi: 10.1016/j.matdes.2015.12.119
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ACCEPTED MANUSCRIPT Analytical and experimental Study of topography of surface texture in ultrasonic vibration assisted turning
Manufacturing Department, Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran 3
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Ultrasonic Lab., Manufacturing Department, Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran
Department of Mechanical Engineering, Faculty of Engineering, Najafabad branch, Islamic Azad University, Najafabad, Iran
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S.A. Sajjady1, H. Nouri Hossein Abadi1, S. Amini2, R. Nosouhi3
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Abstract
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Ultrasonic vibration assisted turning(UAT) is a machining method for creating precision surfaces that because of advantages such as increased tool life, decreased cutting force, high surface quality, and increasing the machinability of hard cutting materials is widely used. In this method, optimal choice of machining parameters has a significant effect on the obtained surface texture. This paper examines the parameters that influence surface texture in the UAT. Therefore, an algorithm was provided to simulate surface textures in the process of ultrasonic vibration assisted face-turning in three modes of one-dimensional, two-dimensional and three-dimensional. To validate this algorithm, experimental tests were performed on Al7075-T6. Comparing the results of the algorithm and experimental tests shows that the surface texture resulted from simulation algorithm is well-matched with the results of experimental tests. Finally, the effect of machining parameters of cutting speed and feed rate are investigated in a variety of vibration modes applied to the tool. Keywords: Ultrasonic vibration Assisted turning; Surface texturing; Ultra-precision surface; Face-turning, Simulation modeling.
1. Introduction In recent decades, with the development of industries such as micro-electromechanical systems, (MEMS), biotechnology, nanotechnology and so forth, mass and precise production of small and light mechanical parts has been widely considered. In construction of such parts, traditional machining methods do not meet the needs of today’s modern industries. In addition, non
Corresponding author. Address: Kashan university, Kashan, Iran .Tel.:+98(0)9133126014 E-mail address: [email protected]
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traditional machining methods such as ECM, EDM, LBM, etc., because of increased costs, high energy consumption, restrictions in machining certain materials, creation of heat distortion on machined surface and so on, are used less. Therefore, to meet these needs, combining modern production methods with the traditional ones has been considered and investigated. Combining traditional machining with ultrasonic vibration is one of these methods. This process, through applying vibration components to the tool, results in the tool’s periodic vibrations during the machining process and ultimately creates some textures on the surface of the workpiece (Fig. 1). Using these vibrations, high surface quality, high dimensional precision, and intermittent cutting conditions are achievable [1] and, thus, result in reduced machining force, reduced tool wear, temperature decrease during the machining process and increased tool life [2-9].
Fig. 1. Schematic form of surface texture obtained from ultrasonic vibration assisted machining operation.
Ultrasonic vibration cutting is generally performed in two ways of one-dimensional vibration cutting and two-dimensional (elliptical) vibration cutting. For both methods, the path of the tool fluctuates in a few micrometers range and vibrational frequencies are in the ultrasonic range (usually above 20 kHz). Tool actuator is used through the tool’s vibrational excitation in different directions. The first method is a resonance system which operates in natural frequency. Resonance actuators benefit a vibration structure to fluctuate the tool. The movement of the tool tip is obtained through assembling the tool to a designed acoustic structure. A transducer with a low amplitude and high frequency stimulates the acoustic structure in its natural frequency. This structure is designed in such a way to strengthen the transducer’s vibrations and ultimately leads to the tool’s vibration. Fig. 2a shows a 1D-VAT system. In this system, the ultrasonic generator, using a piezoelectric or limited magnetic actuator, results in a reciprocation harmonic motion with high frequency and low amplitude. The second method is a two-dimensional resonance system that is shown in Fig. 2b. In this method, the system is designed and made in such a way that the tool in resonance frequencies vibrates in two dimensions. Therefore, an elliptical path is created at the tip of the tool. The third method is a two-dimensional non-resonance system in which piezoelectric actuator is stimulated by sinusoidal voltage signals and causes them to expand and contract at a frequency lower than the first natural frequency of the system and,
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finally, using a mechanical lever, piezoelectric linear motion is converted into elliptical motion. Fig. 2c shows non-resonance 2D-VAM system [10].
Fig. 2. Vibration actuators (a) the resonance 1D-VAT system using a ultrasonic generator, (b) the resonance 2D-VAT system, (c) the non-resonance 2D-VAT system [10].
Shamoto et al. [1, 11], by applying linear ultrasonic vibrations to the tip of the cutting tool, concluded that this process, compared with traditional machining, has more capabilities and benefits including the creation of mirrored quality surface, the low rate of ablation threshold at clearance surface of the tool and so forth. Then, two-dimensional vibrations with small amplitude and high frequency were added to cutting movement of the tool to create elliptical
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ultrasonic vibration assisted machining process. In order to generate elliptical vibration motion, many efforts have been so far conducted to create and develop high-performance transducers. Shamoto and Moriwaki [3] used a new vibrator to create elliptical vibration with ultrasonic frequency in cutting tool. Lee et al. [12], used an asymmetrical structural model of ultrasonic elliptical vibration transducer driven by single actuator, which had only a linear excitation, to create an elliptical vibratory motion. Shamoto et al. [13] offered an elliptical ultrasonic vibrator with a number of piezoelectric (PZT) which were orthogonal to each other. In this model, when driving voltages are applied, longitudinal and bending vibrations are generated in the second and fifth resonance modes. Then, combining these two resonance vibrations, the vibrator fluctuates elliptically with an ultrasonic frequency in a very limited range. Kurosawa et al. [14] designed a transducer consisted of two sandwich-type vibrators which intersected each other orthogonally at their end. This transducer is used to create two symmetrical and asymmetrical vibrations. Zhang et al. [15], based on the concept of Kurosawa’s work, developed a transducer consisted of two Langevin vibrators with π/2 angle to each other to create elliptical vibration motion. Asumi et al. [16] miniaturized a V-shaped transducer of ultrasonic motor (VSM). The size of their miniaturized VSM was significantly less than the former one, while its speed and precision was desirable compared with the former VSM. Gua et al. [17], based on Kurosawa’s works, offered a new design that added vibrations to the tip of tool both in cutting and feed directions. In this design, a third vibration motion in radial direction is added to the tip of the tool in the elliptical vibration cutting process that, by changing the direction of the tip of the tool, can be easily adapted for various elliptical vibrations. Kim and Loh [18] designed a piezoelectric actuator for ultrasonic elliptical vibration cutting process which consisted of two parallel piezoelectric actuators. When these actuators are stimulated by sinusoidal voltages, they expand and contract alternately. Moreover, controlling the phase difference between the applied sinusoidal voltages permits the tool to transfer or rotate in the determined elliptical path. Surface texturing is an essence of the surface which is determined by three parameters of topography, roughness, and waviness. In other words, surface texturing consists of very minor local asperities compared with an ideal flat surface, and is one of the influential factors in the control of friction and displacement of layer structure during wear operation. Thus, according to the studies conducted so far, it can be concluded that surface texture is fundamentally important in the study of tribological properties (wear and friction) [19]. In fact, surface texture is studied in order to evaluate its effect on wear and friction properties during the sliding condition as the most important feature of this type of surface. The process of creating surface texture by using elliptical ultrasonic vibration has been inspired by the surface-shaping system proposed by Hong and Ehmann [20]. This process, by adding a tertiary motion component as a higher-order motion to cutting and feed movements, creates a controllable and predictable vibration between the tool and workpiece. In this case, cutting conditions are chosen in such a way that the ratio of cutting speed (the ratio of cutting speed to vibration speed) is less than one. As illustrated in Fig. 3, cutting begins in tb moment; then, at t0 moment, the tool reaches to the lowest point of its elliptical path and, finally, the contact ends at te moment. Therefore, the tool and workpiece in each cycle of vibration cutting are in contact with each other at te-tb moment, and then at te-t0 moment, the chip formed during the tool
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movement is drawn up in the direction of chip flow. During T- (te-tb), the tool, without removal, is returned to the cutting point in each vibrational cycle [21, 22].
Fig. 3. Schematic shape of elliptical ultrasonic vibration process.
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The process of creating micro surface texturing has a great impact on reducing friction and increasing resistance to surface wear [23-27]. Moreover, it has a high potential in diffraction grating [28], self-cleaning property [29], and wettability of surfaces [29-33]. Many studies have been conducted so far to analyze and simulate the mechanism of surface texture generation. Ehmann and Hong [20, 34] suggested a model for predicting the topography of the created surface in which errors happened during machining process such as tool impact, vibrations and deformation of the machine are considered. Since in turning, a certain degree of relative vibration is created between the tool and workpiece which reduces the quality of surface, Cheung et al. [35] and Kim et al. [36] offered models for face-turning simulation to evaluate the effect of tool vibration on the topography of the resulting surface. Lin et al. [37] suggested a model for simulation of surface roughness profile of cylindrical turning operations, which included the effect of tool geometry and relative motion between cutting tool and workpiece to investigate the effect of vibrations on the quality of surface. Zhang et al. [38], for better understanding of surface configuration, created an analytical surface generation model for EVC. In this analysis, geometry of the cutting tool is defined by tool parameters such as nose radius, rake angle, clearance angle, and edge radius. Liu et al. [39] offered a model including kinematic and dynamic effects of the process and tool edge serration to simulate surface texture resulted from milling operations. Applying minimum chip thickness parameter in surface texture simulation is the advantage of this model. Gua et al. [40] offered a new model for simulation of surface texture resulted from elliptical ultrasonic vibration assisted machining. This model describes the geometry of cutting tool and how it moves, and considers the parameters of minimum chip thickness and elastic recovery. Since all the simulation models only include the cutting edge of the tool, and the edge radius, minimum chip thickness and elastic recovery does not consider, so they cannot be used directly to simulate the process of elliptical ultrasonic turning.
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This paper suggests a surface texture generation algorithm in order to simulate ultrasonic vibration assisted face-turning in three modes of one-dimensional, two-dimensional, and threedimensional. Therefore, in order to achieve precision surface texture, the effect of cutting tool geometry, the states of vibrational motion applied to inserts, and the effect of elastic recovery on surface texture configuration are examined. Then, through doing tests, the process is examined in practice and compared with the simulation model.
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2. Analysis of the textured surface generation mechanism 2.1. Textured surface generation model
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Ultrasonic vibration assisted machining process, which is used to create micro textures, is more complicated than the traditional machining process. In this research, the process of onedimensional ultrasonic vibration assisted face-turning (LVT) was used in such a way to vibrate the tool tip in the cutting speed direction. In elliptical ultrasonic vibration face-turning process (EVT), the tool tip is stimulated in two directions of cutting depth and cutting direction. Moreover, three-dimensional ultrasonic vibration assisted face-turning process (3D-VT) was applied in such a way to oscillate the tool tip in three directions of cutting depth, feed and cutting. Therefore, it is clear that the more the vibratory directions applied to the tool, the more the interaction will be between the cutting tool and workpiece, and thus, simulation of the textured surface generation model becomes more complicated. In order to reach precision surface texture under the ultrasonic vibration assisted machining process, the effect of cutting tool geometry, the states of vibrational motion applied to inserts, workpiece geometry model and elastic recovery have to be evaluated. Evaluating the effect of three-dimensional cutting tool geometry in terms of achieving micro textures configuration is particularly important. Therefore, three-dimensional geometry of the cutting insert, which includes the tool rake, clearance surface of the tool, and cutting edge of the tool, was modeled according to the insert geometry of experimental tests by using designing software (CAD). Then, using finite element software, the surfaces and edge of the modeled insert were separately meshed. Since the process of ultrasonic vibration assisted machining is efficient in finishing and creation of high-precision surfaces, cutting depth in this process is in micron scale and close to the cutting edge radius of the tool. Accordingly, in simulation model of cutting tool, tool radius is considered. Since tool vibration allows clearance angle to change into large negative values during the operation, a large interference volume is created between the tool clearance surface and workpiece. Furthermore, in order to consider ploughing effect caused by tool rake surface on the topography of the surface texture, tool surfaces including clearance surface and tool rake surface are also considered in simulation model. Another factor that affects surface topography is elastic recovery. In general, elastic recovery depends on the three factors of cutting tool edge radius, material hardness, and modulus of elasticity [39, 40]. In the process of ultrasonic vibration assisted machining, cutting depth is comparable with cutting edge radius of the tool. On the other hand, the lower the hardness of material, the more effective the role elastic recovery plays in the configuration of surface texture. Regarding the relationship between modulus of elasticity and elastic recovery it can be said that the decrease of modulus elasticity, in constant yield stress, will increase elastic recovery. According to these descriptions, since Al7075-T6 has been used in this research as a well-cutting material with low modulus of elasticity, elastic recovery have to be considered in textured surface generation model. Textured surface generation flowchart is shown in Fig. 4.
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Fig. 4. Textured surface generation simulation flowchart in ultrasonic vibration assisted face-turning process in three modes of one-dimensional, elliptical, and three-dimensional.
According to Fig. 4, the first step in creating textured surface generation algorithm is threedimensional geometry modeling of cutting insert which includes tool cutting edge, rake surface, and tool clearance surface. To this end, three-dimensional geometry of cutting insert has been modeled in the modeling software (CAD) in accordance with the specifications of insert
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geometry used in experimental tests. Then, using finite element software, the surfaces and edge of the modeled insert are separately meshed, that the result of surfaces and edge discretization of the modeled insert is the change of geometry insert into two series of points. Then, the nodes extracted from the finite element software are imported into the MATLAB simulation software. The type of ultrasonic vibration assisted face-turning process which includes LVT, EVT, and 3D-VT has been written and described in three separate programs, that depending on the desired type of process, the related program is added to the main program and is applied to the cutting insert. In the next step, workpiece geometry is modeled in the modeling software (CAD). Then, using finite element software, all workpiece volume is meshed that the result of this discretization, is definition of the workpiece geometry as a series of points. Afterwards, the nodes and elements derived from the finite element software are imported into the MATLAB simulation software. The next step is related to time discretization. In this part of algorithm, the time increment required to move the tool during the process is defined. Defining increments of time in ultrasonic vibration assisted face-turning operation depends on three factors of cutting speed, feed rate, and vibration frequency. In order to simulate the process precisely, very small time increments should be considered. Of these three factors, vibration frequency has the greatest influence on shrinking time increment, and two other factors (cutting speed and feed rate) have no significant effect on determining the amount of time increments and can be ignored. Thus, according to vibration frequency factor, Δt is obtained as the equation (1):
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Where f and Nt stand for vibration frequency and vibration sampling point, respectively. In order to make relationship between the tool coordinate system and the workpiece coordinate system a series of matrices are used that at each time increment are expressed as follows:
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Where Vf is the velocity in the feed direction, Vc is the velocity in cutting direction, A is vibration amplitude, DOC is the cutting depth and Zinit is the initial position of the tool in Z direction. (3) (4) Where N is the Spindle speed, Ri is the momentary radius of the workpiece. (5) In the final stage, distance of all mesh points of the tool and workpiece and their interference has to be investigated. To check mesh points interference of the tool and workpiece, the distance between all points of the tool and all points of the workpiece in a time increment is computed peer to peer. If the distance of all points of tool and workpiece is greater than the user-defined
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(6)
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tolerance, it means that no interference has occurred between the tool and workpiece yet. In this state, we return to algorithm’s previous stage, and in the next time increment the distance of mesh points of the tool and workpiece are examined. If the distance of one pair points of the tool and workpiece is less than the defined tolerance, it means that the tool has penetrated into the workpiece. In order to investigate the interactions between the tool and workpiece and in order to separate intersection points of the tool and workpiece from the workpiece, and to assess the effect of elastic recovery on the topography of surface texture in each time increment, rake thickness is first calculated by Eq. (6); then, using δt and the equation that governs elastic recovery [39, 40], surface texture topography of the tool is updated due to the partitioned area of the cutting insert (area of the cutting tool edge and area of the tool surfaces). If MATLAB simulation software detects the interaction between set of meshed points in the area of the tool cutting edge and set points of the workpiece, equations of elastic recovery and updating surface texture are as follows:
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Where t c is the chip thickness, t ce is the elastic deformation limit (workpiece/cutting edge), t c min is the minimum chip thickness, t c max is the limit chip thickness for elastoplastic deformation, pe is the elastic recovery rate (workpiece/cutting edge) and is the elastic/plastic deformation ratio. If MATLAB simulation software detects the interaction between the set of meshed points existing in the areas of the tool surfaces and the set points of the workpiece, and elastic recovery and updating surface texture are calculated using the following equations: (9) (10) (11) Where tve is the elastic deformation limit (workpiece/cutting surface), pe is the elastic recovery rate for the ploughing phenomena (workpiece/cutting surface). All the dimensions in the above mentioned equations are in μm. The magnitudes of the input parameters in the simulation are presented in table 1.
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Table.1. The value of the input parameters Parameters Magnitudes 0.01 (μm) 0.04 (μm) 0.1 (μm) 0.1 0.05 0.1 (μm) 0.5
Finally, by the end of the algorithm, a surface is outlined using the points of algorithm.
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3. Experiments 3.1. Process Preparation
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The samples used in the experiment are made of AL7075-T6. This alloy is one of the most used lightweight metals in shaping process, particularly removal processes such as machining process. This alloy has good mechanical and thermal properties such as medium hardness, heat treatable, good thermal conductivity, high corrosion resistance, high tensile strength and so forth. Moreover, because of its high strength to weight ratio, it has many applications. Chemical composition of this alloy are shown in Table 2.
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Table 2. The chemical compositions of AI7075-T6. Al Zn Mg Cu Fe Cr 88.76 5.6 2.4 1.6 0.5 0.24
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Elements Compositions (%)
Mn 0.3
Ti 0.2
Si 0.4
In these experiments, for facing of aluminum, PCD (Polycrystalline Diamond) tools with technical specification of TCGW 110204 LN-7 were employed. All of the used inserts had the same geometry with rake angle of 7˚ and without chip break. Polycrystalline diamond inserts are used in many non-ferrous machining applications. The experiments related to the process of creating surface texture using ultrasonic vibration were performed on a TNC50 turning lathe made in Machine Sazi Tabriz. In order to apply ultrasonic vibrations on the tool tip, three sets of piezoelectric layouts are used which include a set of full-ring stack for creating linear mode and two sets of half-ring stack for creating flexural mode. Two sets of half-ring stack are stimulated by 180˚ phase difference to enhance the amplitude of flexural vibration. The reason is that during the expansion of one set, another set is contracting and vice versa. For linear mode, the triggering/ stimulating signal has a 90˚ phase difference, compared with the flexural mode, to create an elliptical path at the tool tip. Fig. 5 illustrates UVT tool and experiment’s initial settings.
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Fig.5. Experimental setup for UAT tool used for the machining tests.
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Vibrational amplitudes were measured in two directions by an Eddy-current gap sensor. In order to measure surface roughness, PS1 model, manufactured in Mahr Company, was used.
3.2. Performing Experiments
The number of samples required in this research was determined using full factorial method. Factors include spindle speed at three levels and feed rate at three levels that, in general, 9 experiments were performed for each machining mode; thus, taking into account 4 machining modes (CT, LVT, EVT, and 3D-VT), a total number of 36 samples were prepared. Levels of the listed factors are shown in Table 3. The samples are kept by a special fixture and vibratory tool is installed on the support of the turning lathe. Then, by applying (stimulating) the desired vibration direction or directions, as shown in Fig. 6, tip of the cutting tool is vibrated in terms of machining processes of LVT, EVT, and 3D-VT and machining operation is performed. Cutting depth was same and equal to 0.25 mm in all experiments. Table 3. Factors and levels of the experiment’s design.
Parameters Spindle speed Feed rate Modes of vibrational process
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45, 31.5, 22.4 220, 180, 140 CT, LVT, EVT, 3D-VT
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Fig. 6. Schematic figure of the working planes in machining processes of EVT, 3D-VT and working axis of LVT process.
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By the end of the machining process and samples preparation, some two-dimensional and threedimensional images were taken from surface texture of the samples in order to provide an analysis of surface texture and validate the textured surface generation simulation algorithm. Furthermore, in order to provide analysis of variance for surface roughness, surface roughness experiments were performed three times on each sample and in positions with 120˚ difference to each other.
4. Results and Discussion
In this section, results of the face-turning experiments in three vibrational modes of onedimensional (LVT), two-dimensional or elliptical (EVT), and three-dimensional (3D microfeatures) are evaluated to compare the surface texture resulted from the experimental results with the results of textured surface generation model. To this end, two-dimensional and threedimensional images were provided from surface texture of the samples. Then, the effects of vibration parameters (type of vibrational motion) and machining parameters (cutting speed and feed rate) were investigated on the surface morphology and roughness.
4.1. Comparison of simulation results and experimental tests results Surface texture of some samples resulted from ultrasonic vibration assisted face-turning in three vibrational modes of LVT, EVT, and 3D-VT were photographed using three-dimensional optical microscope. Then, the images were used to compare the surface texture obtained from the experimental results with the results of textured surface generation model, and validate the simulation results. Fig. 7 shows the topography of the surface texture obtained from the experimental results and textured surface generation model in LVT process and under the conditions of 31.5 rpm spindle speed, feed rate of 180 μm/rev, 10 micrometers vibration amplitude and vibration frequency of 16670 Hz.
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Topography of the surface texture obtained from the experimental results and textured surface generation model in EVT process and under the conditions of 31.5 rpm spindle speed, feed rate of 180 μm/rev, 10 micrometers vibration amplitude and vibration frequency of 16670 Hz are shown in Fig. 8a and b, respectively. Topographic image of the surface texture obtained from the experimental results in 3D-VT process is shown in Fig. 9a. Comparable simulation results, under the conditions of 31.5 rpm spindle speed, feed rate of 180 μm/rev, 10 micrometers vibration amplitude and vibration frequency of 16670 Hz, are shown in Fig. 9b.
Fig. 7. Surface topography for LVT process (a) experimental results, (b) simulation results.
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Fig. 8. Surface topography for EVT process (a) experimental results, (b) simulation results.
Fig. 9. Surface topography for 3D-VT process (a) experimental results, (b) simulation results.
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Topography of the surface texture obtained from the experimental results is shown in Figs. 7a, 8a and 9a. Comparable simulation results are shown in Figs. 7b, 8b and 9b. From the comparison of the images it can be understood that topography of the simulated surface texture, with a good approximation, is similar to topography of surface texture obtained from the experimental results. In general, topography of the simulated surface texture well-illustrates the shape of micro dimples, positioning of micro dimples, according to the type of ultrasonic vibration assisted machining process, and surface characteristics, according to machining parameters. Whereas the process of creating surface texture by ultrasonic vibration assisted turning has a great impact on improving the tribological properties, to characterize and compare the results of simulation and experiment quantitatively; the theoretical and experimental results of surface roughness extracted from generated surface texture are compared. The mean values of the asperity points along different positions on the surface texture caused by simulation algorithm were analyzed and the average values were reported. Comparing the theoretical results of the average roughness and experimental results of the average roughness shows that the surface roughness resulted from textured surface of simulation algorithm is in a good agreement with the results of experimental results. The theoretical and experimental results of surface roughness are listed in Table 4.
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Table 4. Comparison of the theoretical and experimental results of surface roughness (spindle speed: 31.5 rpm, feed rate 180 μm/rev. Surface roughness Experimental results Theoretical results Vibration mode LVT 0.981 μm 0.967 μm EVT 0.852 μm 0.834 μm 3D-VT 1.120 μm 1.080 μm
By comparing of the topography of the surface texture obtained from the experimental results and simulation results it can be understood that there are differences between the topography of the surface texture obtained from the experimental results and that obtained from simulation results. These differences can be investigated in two aspects. The first difference is the tool marks created at the peaks of surface texture that can be the result of the adhesion of the tool and material at the peaks of the surface texture which can be the result of the generated heat in machining. The Second difference is the out of flatness of the generated surface texture. As it can be seen in captured images of the surface texture, the samples’ surface are not flat like the topography of the simulated surface texture. Despite these differences between the topography of the surface texture obtained from the experimental results and simulation, the experimental results confirm the simulation results.
4.2 Texturing Results In this section, the effect of feed rate and cutting speed parameters on the texture morphology in each vibration machining mode (CT, LVT, EVT, and 3D-VT) is evaluated separately. Then, the effect of vibrational motion (CT, LVT, EVT, and 3D-VT) on the texture morphology is evaluated. Fig. 10 illustrates some patterns of the surface texture created by linear (onedimensional) ultrasonic vibration assisted turning process under different parameters of machining.
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Fig. 10. Comparison of surface topography for LVT (a) Feed rate of 140μm/rev, spindle speed of 31.5rpm, (b) Feed rate of 140μm/rev, spindle speed of 45rpm, (c) Feed rate of 180μm/rev, spindle speed of 31.5rpm.
According to Fig. 10, dimples array depends on the parameters of cutting speed and feed rate. All machining and vibration parameters are the same in Fig. 10a and b (f=16670; A=10μm; Vf=140μm/rev), except spindle speed which is equal to 31.5 rpm and 45 rpm in Fig. 10a and b respectively. Therefore, as expected and observable in Fig. 10a and b, order of the created dimples is such that in feed direction, because of constant feed rate, the distance between the dimples is similar; while the distance between the dimples in cutting direction is such that by increasing the cutting speed, the distance between the dimples increases as well. Similarly, all the process parameters are identical in Fig. 10a and c (f=16670; A=10μm; N= 31.5 rpm), except feed rate which is equal to 140μm/rev and 180μm/rev in Fig. 10a and c, respectively. As can be observed, the distance between the dimples created in cutting direction is identical that is due to the constant cutting speed; while the distance between the dimples in feed direction is such that
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by increasing the feed rate, the distance between the dimples increases. This trend is also true for two-dimensional (EVT) and three-dimensional (3D micro-features) processes which are shown in Figs. 11 and 12, respectively.
Fig. 11. Comparison of Surface Topography for EVT (a) Feed rate of 140μm/rev, spindle speed of 31.5 rpm, (b) Feed rate of 140μm/rev, spindle speed of 45 rpm, (c) Feed rate of 180μm/rev, spindle speed of 31.5 rpm.
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Fig.12. Comparison of surface topography for 3D micro-features (a) Feed rate of 140μm/rev, spindle speed of 31.5rpm, (b) Feed rate of 140μm/rev, spindle speed of 45rpm, (c) Feed rate of 180μm/rev, spindle speed of 31.5rpm.
Figs. 13a-d, 14a-d and 15a-d illustrate images of the surface texture of machining with constant parameters of 180μm/rev feed rate and 31.5 rpm spindle speed for LVT, EVT, and 3D-VT respectively. Moreover, micro dimples array, comparison of the dimples in terms of their expansion in feed and cutting directions for LVT, EVT, and 3D-VT, and comparison of the textures profile in feed direction are investigated in these figures.
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Fig. 13. Evaluation of surface texture obtained from LVT under the feed rate of 180μm/rev and spindle speed of 31.5 rpm (a) Perspective view, (b) Front view, (c) Top view and (d) Surface texture magnification.
Fig. 14. Evaluation of surface texture obtained from EVT under the feed rate of 180μm/rev and spindle speed of 31.5 rpm (a) Perspective view, (b) Front view, (c) Top view and (d) Surface texture magnification.
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Fig. 15. Evaluation of surface texture obtained from 3D-VT under the feed rate of 180μm/rev and spindle speed of 31.5 rpm (a) Perspective view, (b) Front view, (c) Top view and (d) Surface texture magnification.
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As shown in Fig. 13d, vibration direction for LVT process is linear and along the x axis (cutting direction). Hence, with respect to the feed rate and cutting speed, micro dimples created on the surface are ordered with a very small deviation angle to the horizontal axis. On the other hand, in EVT process, since two vibration directions lead to an elliptical motion along the z and x axes and also considering the feed rate and cutting speed, micro dimples created on the surface are arranged with a greater deviation angle than LVT process. Finally, in 3D-VT process, since in addition to vibration directions along the z and x axes, a tertiary direction is applied along the y axis to create an ellipsoid motion and also taking into account the feed rate and cutting speed, micro dimples created on the surface are arranged with a greater deviation angle than EVT process. Figs. 14d and 15d confirm this. Another noteworthy point is that according to Figs. 13c, 14c and 15c, because of the nature of LVT (vibration along the x axis, cutting direction) and EVT (vibration directions along z axis, cutting depth, x axis, cutting direction) and considering the rotating motion, EVT micro dimples, compared with LVT process, have been developed in cutting direction. According to this argument and the nature of 3D-VT process, in addition to two vibrations in depth and cutting directions, the tertiary vibration direction is added along the y axis (feed direction), micro dimples of this process compared with EVT process have developed only in feed direction, and compared with LVT process have developed in both feed and cutting directions. On the other hand, according to Figs. 13a-b, 14a-b and 15a-b, surface texture profile of EVT in Fig. 14a-b, like LVT, is smooth and symmetrical. However, surface texture profile of 3D microfeatures, according to Fig. 15a and b, like a unique curvature is asymmetrical. As a result, it is obvious that topography of the simulated surface and generated surface texture are in good agreement in terms of the shape of micro dimples, expansion of micro dimples in feed and cutting directions, positioning and orientation of micro dimples according to the type of
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4.3. The Effect of UVAT Process Parameters on Surface Roughness 4.3.1. ANOVA Results for Surface Roughness
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Analysis of variance for surface roughness was performed by Minitab software with 95% confidence level. Contribution percents in Table 5 show each parameter’s degree of influence on the results. It is obvious, that feed rate (75.38%), process type (5.7%), and cutting speed (1%) have effect on surface roughness output parameter. Table 5. ANOVA results of Ra. Seq SS Adj MS F P 0.02627 0.01313 0.78 0.468 1.98371 0.99185 58.91 0.000 0.15019 0.05006 2.97 0.049 0.47146 0.01684 2.63163 -
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Contribution(%) 1 75.38 5.7 17.92 100
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Comparison of surface roughness in machining processes of CT, LVT, EVT, and 3D-VT, in terms of cutting speed parameter, is shown in Fig. 16. As can be observed in this figure, surface roughness in CT process has been more than other processes and is decreased respectively in 3DVT, LVT, and EVT processes.
Fig. 16. The effect of spindle speed on surface roughness in CT, LVT, EVT, and 3D-VT processes (feed rate= 140 μm/rev)
4.3.3. The Effect of Feed Rate on Surface Roughness Comparison of surface roughness for machining processes of CT, LVT, EVT, and 3D-VT, in terms of feed rate parameter with constant cutting speed, is given in Fig. 17. According to this
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figure, in all processes, with an increase in feed rate, surface roughness increases first and, decreases then.
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Fig. 17. The effect of feed rate on surface roughness in the processes of CT, LVT, EVT, and 3D-VT (spindle speed= 31.5 rpm)
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From the Figs. 16 and 17, it can be understood that the highest level of surface roughness occurs in CT process, and the surface roughness values more decrease in 3D-VT, LVT, and EVT processes, respectively.
5. Conclusion
In this research, topography of surface was investigated in ultrasonic vibration assisted faceturning. In order to achieve precise surface texture by the proposed surface texture generation algorithm, the effect of three-dimensional geometry of the cutting tool, the states of vibration motions applied to the insert, geometry of workpiece and elastic recovery in simulation algorithm, were considered. Then, experimental tests were performed to validate simulation algorithm of surface texture generation in ultrasonic vibration assisted face-turning. The results of simulation algorithm were compared with experimental tests and, finally, the following results were obtained: Comparison of the experimental surface texture with that of simulation algorithm shows that they are relatively well-matched. Regarding the vibration direction in LVT process that is linear, micro dimples created on the surface are arranged with a small deviation angle to the horizontal axis. This phenomenon is observable in the photographs taken from surface texture of the samples and the output of surface texture simulation algorithm. Moreover, this deviation increases in EVT and 3D-VT processes respectively. ANOVA results for surface roughness indicates that feed rate has the greatest influence (75.38%) on the surface roughness. Furthermore, analysis shows clearly that the parameters of process type (5.7%) and cutting speed (1%) are the other influential parameters.
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The results of surface roughness show that the highest and lowest surface roughness belongs to CT and EVT processes respectively.
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[21] Shamoto E, Moriwaki T. Study on elliptical vibration cutting. CIRP Annals-Manufacturing Technology. 1994;43:35-8. [22] Nath C, Rahman M, Neo KS. Machinability study of tungsten carbide using PCD tools under ultrasonic elliptical vibration cutting. International Journal of Machine Tools and Manufacture. 2009;49:1089-95. [23] Xing D, Zhang J, Shen X, Zhao Y, Wang T. Tribological properties of ultrasonic vibration assisted milling aluminium alloy surfaces. Procedia CIRP. 2013;6:539-44. [24] Sudeep U, Pandey R, Tandon N. Effects of surface texturing on friction and vibration behaviors of sliding lubricated concentrated point contacts under linear reciprocating motion. Tribology International. 2013;62:198-207. [25] Li K, Yao Z, Hu Y, Gu W. Friction and wear performance of laser peen textured surface under starved lubrication. Tribology International. 2014;77:97-105. [26] Abdollahi A, Alizadeh A, Baharvandi HR. Dry sliding tribological behavior and mechanical properties of Al2024–5wt.% B 4 C nanocomposite produced by mechanical milling and hot extrusion. Materials & Design. 2014;55:471-81. [27] Xing Y, Deng J, Feng X, Yu S. Effect of laser surface texturing on Si 3 N 4/TiC ceramic sliding against steel under dry friction. Materials & Design. 2013;52:234-45. [28] Bruzzone A, Costa H, Lonardo P, Lucca D. Advances in engineered surfaces for functional performance. CIRP Annals-Manufacturing Technology. 2008;57:750-69. [29] Li X-M, Reinhoudt D, Crego-Calama M. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chemical Society Reviews. 2007;36:1350-68. [30] Matsumura T, Iida F, Hirose T, Yoshino M. Micro machining for control of wettability with surface topography. Journal of Materials Processing Technology. 2012;212:2669-77. [31] Lu Y, Guo P, Pei P, Ehmann KF. Experimental studies of wettability control on cylindrical surfaces by elliptical vibration texturing. The International Journal of Advanced Manufacturing Technology. 2015;76:1807-17. [32] Qin L, Lin P, Zhang Y, Dong G, Zeng Q. Influence of surface wettability on the tribological properties of laser textured Co–Cr–Mo alloy in aqueous bovine serum albumin solution. Applied Surface Science. 2013;268:79-86. [33] Yilbas BS, Khaled M, Abu-Dheir N, Aqeeli N, Furquan SZ. Laser texturing of alumina surface for improved hydrophobicity. Applied Surface Science. 2013;286:161-70. [34] Ehmann K, Hong M. A generalized model of the surface generation process in metal cutting. CIRP Annals-Manufacturing Technology. 1994;43:483-6. [35] Cheung C, Lee W. Modelling and simulation of surface topography in ultra-precision diamond turning. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. 2000;214:463-80. [36] Kim D-S, Chang I-C, Kim S-W. Microscopic topographical analysis of tool vibration effects on diamond turned optical surfaces. Precision Engineering. 2002;26:168-74. [37] Lin S, Chang M. A study on the effects of vibrations on the surface finish using a surface topography simulation model for turning. International Journal of Machine Tools and Manufacture. 1998;38:763-82. [38] Zhang X, Kumar AS, Rahman M, Liu K. Modeling of the effect of tool edge radius on surface generation in elliptical vibration cutting. The International Journal of Advanced Manufacturing Technology. 2013;65:35-42. [39] Liu X, DeVor RE, Kapoor SG. Model-based analysis of the surface generation in microendmilling— Part I: model development. Journal of Manufacturing Science and Engineering. 2007;129:453-60. [40] Guo P, Ehmann KF. An analysis of the surface generation mechanics of the elliptical vibration texturing process. International Journal of Machine Tools and Manufacture. 2013;64:85-95.
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Graphical abstract
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Highlights Comparison of the experimental surface texture with that of simulation algorithm shows that they are relatively well-matched. Regarding the vibration direction in LVT process that is linear, micro dimples created on the surface are arranged with a small deviation angle to the horizontal axis. This phenomenon is observable in the photographs taken from surface texture of the samples and the output of surface texture simulation algorithm. Moreover, this deviation increases in EVT and 3D-VT processes respectively. ANOVA results for surface roughness indicates that feed rate has the greatest influence (75.38%) on the surface roughness. Furthermore, analysis shows clearly that the parameters of process type (5.7%) and cutting speed (1%) are the other influential parameters. The results of surface roughness show that the highest and lowest surface roughness belongs to CT and EVT processes respectively.
S0264-1275(15)30978-3 doi: 10.1016/j.matdes.2015.12.119 JMADE 1143
To appear in: Received date: Revised date: Accepted date:
14 October 2015 20 December 2015 21 December 2015
Please cite this article as: S.A. Sajjady, H. Nouri Hossein Abadi, S. Amini, R. Nosouhi, Analytical and experimental study of topography of surface texture in ultrasonic vibration assisted turning, (2015), doi: 10.1016/j.matdes.2015.12.119
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ACCEPTED MANUSCRIPT Analytical and experimental Study of topography of surface texture in ultrasonic vibration assisted turning
Manufacturing Department, Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran 3
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Ultrasonic Lab., Manufacturing Department, Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran
Department of Mechanical Engineering, Faculty of Engineering, Najafabad branch, Islamic Azad University, Najafabad, Iran
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S.A. Sajjady1, H. Nouri Hossein Abadi1, S. Amini2, R. Nosouhi3
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Abstract
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Ultrasonic vibration assisted turning(UAT) is a machining method for creating precision surfaces that because of advantages such as increased tool life, decreased cutting force, high surface quality, and increasing the machinability of hard cutting materials is widely used. In this method, optimal choice of machining parameters has a significant effect on the obtained surface texture. This paper examines the parameters that influence surface texture in the UAT. Therefore, an algorithm was provided to simulate surface textures in the process of ultrasonic vibration assisted face-turning in three modes of one-dimensional, two-dimensional and three-dimensional. To validate this algorithm, experimental tests were performed on Al7075-T6. Comparing the results of the algorithm and experimental tests shows that the surface texture resulted from simulation algorithm is well-matched with the results of experimental tests. Finally, the effect of machining parameters of cutting speed and feed rate are investigated in a variety of vibration modes applied to the tool. Keywords: Ultrasonic vibration Assisted turning; Surface texturing; Ultra-precision surface; Face-turning, Simulation modeling.
1. Introduction In recent decades, with the development of industries such as micro-electromechanical systems, (MEMS), biotechnology, nanotechnology and so forth, mass and precise production of small and light mechanical parts has been widely considered. In construction of such parts, traditional machining methods do not meet the needs of today’s modern industries. In addition, non
Corresponding author. Address: Kashan university, Kashan, Iran .Tel.:+98(0)9133126014 E-mail address: [email protected]
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traditional machining methods such as ECM, EDM, LBM, etc., because of increased costs, high energy consumption, restrictions in machining certain materials, creation of heat distortion on machined surface and so on, are used less. Therefore, to meet these needs, combining modern production methods with the traditional ones has been considered and investigated. Combining traditional machining with ultrasonic vibration is one of these methods. This process, through applying vibration components to the tool, results in the tool’s periodic vibrations during the machining process and ultimately creates some textures on the surface of the workpiece (Fig. 1). Using these vibrations, high surface quality, high dimensional precision, and intermittent cutting conditions are achievable [1] and, thus, result in reduced machining force, reduced tool wear, temperature decrease during the machining process and increased tool life [2-9].
Fig. 1. Schematic form of surface texture obtained from ultrasonic vibration assisted machining operation.
Ultrasonic vibration cutting is generally performed in two ways of one-dimensional vibration cutting and two-dimensional (elliptical) vibration cutting. For both methods, the path of the tool fluctuates in a few micrometers range and vibrational frequencies are in the ultrasonic range (usually above 20 kHz). Tool actuator is used through the tool’s vibrational excitation in different directions. The first method is a resonance system which operates in natural frequency. Resonance actuators benefit a vibration structure to fluctuate the tool. The movement of the tool tip is obtained through assembling the tool to a designed acoustic structure. A transducer with a low amplitude and high frequency stimulates the acoustic structure in its natural frequency. This structure is designed in such a way to strengthen the transducer’s vibrations and ultimately leads to the tool’s vibration. Fig. 2a shows a 1D-VAT system. In this system, the ultrasonic generator, using a piezoelectric or limited magnetic actuator, results in a reciprocation harmonic motion with high frequency and low amplitude. The second method is a two-dimensional resonance system that is shown in Fig. 2b. In this method, the system is designed and made in such a way that the tool in resonance frequencies vibrates in two dimensions. Therefore, an elliptical path is created at the tip of the tool. The third method is a two-dimensional non-resonance system in which piezoelectric actuator is stimulated by sinusoidal voltage signals and causes them to expand and contract at a frequency lower than the first natural frequency of the system and,
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finally, using a mechanical lever, piezoelectric linear motion is converted into elliptical motion. Fig. 2c shows non-resonance 2D-VAM system [10].
Fig. 2. Vibration actuators (a) the resonance 1D-VAT system using a ultrasonic generator, (b) the resonance 2D-VAT system, (c) the non-resonance 2D-VAT system [10].
Shamoto et al. [1, 11], by applying linear ultrasonic vibrations to the tip of the cutting tool, concluded that this process, compared with traditional machining, has more capabilities and benefits including the creation of mirrored quality surface, the low rate of ablation threshold at clearance surface of the tool and so forth. Then, two-dimensional vibrations with small amplitude and high frequency were added to cutting movement of the tool to create elliptical
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ultrasonic vibration assisted machining process. In order to generate elliptical vibration motion, many efforts have been so far conducted to create and develop high-performance transducers. Shamoto and Moriwaki [3] used a new vibrator to create elliptical vibration with ultrasonic frequency in cutting tool. Lee et al. [12], used an asymmetrical structural model of ultrasonic elliptical vibration transducer driven by single actuator, which had only a linear excitation, to create an elliptical vibratory motion. Shamoto et al. [13] offered an elliptical ultrasonic vibrator with a number of piezoelectric (PZT) which were orthogonal to each other. In this model, when driving voltages are applied, longitudinal and bending vibrations are generated in the second and fifth resonance modes. Then, combining these two resonance vibrations, the vibrator fluctuates elliptically with an ultrasonic frequency in a very limited range. Kurosawa et al. [14] designed a transducer consisted of two sandwich-type vibrators which intersected each other orthogonally at their end. This transducer is used to create two symmetrical and asymmetrical vibrations. Zhang et al. [15], based on the concept of Kurosawa’s work, developed a transducer consisted of two Langevin vibrators with π/2 angle to each other to create elliptical vibration motion. Asumi et al. [16] miniaturized a V-shaped transducer of ultrasonic motor (VSM). The size of their miniaturized VSM was significantly less than the former one, while its speed and precision was desirable compared with the former VSM. Gua et al. [17], based on Kurosawa’s works, offered a new design that added vibrations to the tip of tool both in cutting and feed directions. In this design, a third vibration motion in radial direction is added to the tip of the tool in the elliptical vibration cutting process that, by changing the direction of the tip of the tool, can be easily adapted for various elliptical vibrations. Kim and Loh [18] designed a piezoelectric actuator for ultrasonic elliptical vibration cutting process which consisted of two parallel piezoelectric actuators. When these actuators are stimulated by sinusoidal voltages, they expand and contract alternately. Moreover, controlling the phase difference between the applied sinusoidal voltages permits the tool to transfer or rotate in the determined elliptical path. Surface texturing is an essence of the surface which is determined by three parameters of topography, roughness, and waviness. In other words, surface texturing consists of very minor local asperities compared with an ideal flat surface, and is one of the influential factors in the control of friction and displacement of layer structure during wear operation. Thus, according to the studies conducted so far, it can be concluded that surface texture is fundamentally important in the study of tribological properties (wear and friction) [19]. In fact, surface texture is studied in order to evaluate its effect on wear and friction properties during the sliding condition as the most important feature of this type of surface. The process of creating surface texture by using elliptical ultrasonic vibration has been inspired by the surface-shaping system proposed by Hong and Ehmann [20]. This process, by adding a tertiary motion component as a higher-order motion to cutting and feed movements, creates a controllable and predictable vibration between the tool and workpiece. In this case, cutting conditions are chosen in such a way that the ratio of cutting speed (the ratio of cutting speed to vibration speed) is less than one. As illustrated in Fig. 3, cutting begins in tb moment; then, at t0 moment, the tool reaches to the lowest point of its elliptical path and, finally, the contact ends at te moment. Therefore, the tool and workpiece in each cycle of vibration cutting are in contact with each other at te-tb moment, and then at te-t0 moment, the chip formed during the tool
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movement is drawn up in the direction of chip flow. During T- (te-tb), the tool, without removal, is returned to the cutting point in each vibrational cycle [21, 22].
Fig. 3. Schematic shape of elliptical ultrasonic vibration process.
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The process of creating micro surface texturing has a great impact on reducing friction and increasing resistance to surface wear [23-27]. Moreover, it has a high potential in diffraction grating [28], self-cleaning property [29], and wettability of surfaces [29-33]. Many studies have been conducted so far to analyze and simulate the mechanism of surface texture generation. Ehmann and Hong [20, 34] suggested a model for predicting the topography of the created surface in which errors happened during machining process such as tool impact, vibrations and deformation of the machine are considered. Since in turning, a certain degree of relative vibration is created between the tool and workpiece which reduces the quality of surface, Cheung et al. [35] and Kim et al. [36] offered models for face-turning simulation to evaluate the effect of tool vibration on the topography of the resulting surface. Lin et al. [37] suggested a model for simulation of surface roughness profile of cylindrical turning operations, which included the effect of tool geometry and relative motion between cutting tool and workpiece to investigate the effect of vibrations on the quality of surface. Zhang et al. [38], for better understanding of surface configuration, created an analytical surface generation model for EVC. In this analysis, geometry of the cutting tool is defined by tool parameters such as nose radius, rake angle, clearance angle, and edge radius. Liu et al. [39] offered a model including kinematic and dynamic effects of the process and tool edge serration to simulate surface texture resulted from milling operations. Applying minimum chip thickness parameter in surface texture simulation is the advantage of this model. Gua et al. [40] offered a new model for simulation of surface texture resulted from elliptical ultrasonic vibration assisted machining. This model describes the geometry of cutting tool and how it moves, and considers the parameters of minimum chip thickness and elastic recovery. Since all the simulation models only include the cutting edge of the tool, and the edge radius, minimum chip thickness and elastic recovery does not consider, so they cannot be used directly to simulate the process of elliptical ultrasonic turning.
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This paper suggests a surface texture generation algorithm in order to simulate ultrasonic vibration assisted face-turning in three modes of one-dimensional, two-dimensional, and threedimensional. Therefore, in order to achieve precision surface texture, the effect of cutting tool geometry, the states of vibrational motion applied to inserts, and the effect of elastic recovery on surface texture configuration are examined. Then, through doing tests, the process is examined in practice and compared with the simulation model.
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2. Analysis of the textured surface generation mechanism 2.1. Textured surface generation model
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Ultrasonic vibration assisted machining process, which is used to create micro textures, is more complicated than the traditional machining process. In this research, the process of onedimensional ultrasonic vibration assisted face-turning (LVT) was used in such a way to vibrate the tool tip in the cutting speed direction. In elliptical ultrasonic vibration face-turning process (EVT), the tool tip is stimulated in two directions of cutting depth and cutting direction. Moreover, three-dimensional ultrasonic vibration assisted face-turning process (3D-VT) was applied in such a way to oscillate the tool tip in three directions of cutting depth, feed and cutting. Therefore, it is clear that the more the vibratory directions applied to the tool, the more the interaction will be between the cutting tool and workpiece, and thus, simulation of the textured surface generation model becomes more complicated. In order to reach precision surface texture under the ultrasonic vibration assisted machining process, the effect of cutting tool geometry, the states of vibrational motion applied to inserts, workpiece geometry model and elastic recovery have to be evaluated. Evaluating the effect of three-dimensional cutting tool geometry in terms of achieving micro textures configuration is particularly important. Therefore, three-dimensional geometry of the cutting insert, which includes the tool rake, clearance surface of the tool, and cutting edge of the tool, was modeled according to the insert geometry of experimental tests by using designing software (CAD). Then, using finite element software, the surfaces and edge of the modeled insert were separately meshed. Since the process of ultrasonic vibration assisted machining is efficient in finishing and creation of high-precision surfaces, cutting depth in this process is in micron scale and close to the cutting edge radius of the tool. Accordingly, in simulation model of cutting tool, tool radius is considered. Since tool vibration allows clearance angle to change into large negative values during the operation, a large interference volume is created between the tool clearance surface and workpiece. Furthermore, in order to consider ploughing effect caused by tool rake surface on the topography of the surface texture, tool surfaces including clearance surface and tool rake surface are also considered in simulation model. Another factor that affects surface topography is elastic recovery. In general, elastic recovery depends on the three factors of cutting tool edge radius, material hardness, and modulus of elasticity [39, 40]. In the process of ultrasonic vibration assisted machining, cutting depth is comparable with cutting edge radius of the tool. On the other hand, the lower the hardness of material, the more effective the role elastic recovery plays in the configuration of surface texture. Regarding the relationship between modulus of elasticity and elastic recovery it can be said that the decrease of modulus elasticity, in constant yield stress, will increase elastic recovery. According to these descriptions, since Al7075-T6 has been used in this research as a well-cutting material with low modulus of elasticity, elastic recovery have to be considered in textured surface generation model. Textured surface generation flowchart is shown in Fig. 4.
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Fig. 4. Textured surface generation simulation flowchart in ultrasonic vibration assisted face-turning process in three modes of one-dimensional, elliptical, and three-dimensional.
According to Fig. 4, the first step in creating textured surface generation algorithm is threedimensional geometry modeling of cutting insert which includes tool cutting edge, rake surface, and tool clearance surface. To this end, three-dimensional geometry of cutting insert has been modeled in the modeling software (CAD) in accordance with the specifications of insert
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geometry used in experimental tests. Then, using finite element software, the surfaces and edge of the modeled insert are separately meshed, that the result of surfaces and edge discretization of the modeled insert is the change of geometry insert into two series of points. Then, the nodes extracted from the finite element software are imported into the MATLAB simulation software. The type of ultrasonic vibration assisted face-turning process which includes LVT, EVT, and 3D-VT has been written and described in three separate programs, that depending on the desired type of process, the related program is added to the main program and is applied to the cutting insert. In the next step, workpiece geometry is modeled in the modeling software (CAD). Then, using finite element software, all workpiece volume is meshed that the result of this discretization, is definition of the workpiece geometry as a series of points. Afterwards, the nodes and elements derived from the finite element software are imported into the MATLAB simulation software. The next step is related to time discretization. In this part of algorithm, the time increment required to move the tool during the process is defined. Defining increments of time in ultrasonic vibration assisted face-turning operation depends on three factors of cutting speed, feed rate, and vibration frequency. In order to simulate the process precisely, very small time increments should be considered. Of these three factors, vibration frequency has the greatest influence on shrinking time increment, and two other factors (cutting speed and feed rate) have no significant effect on determining the amount of time increments and can be ignored. Thus, according to vibration frequency factor, Δt is obtained as the equation (1):
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Where f and Nt stand for vibration frequency and vibration sampling point, respectively. In order to make relationship between the tool coordinate system and the workpiece coordinate system a series of matrices are used that at each time increment are expressed as follows:
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Where Vf is the velocity in the feed direction, Vc is the velocity in cutting direction, A is vibration amplitude, DOC is the cutting depth and Zinit is the initial position of the tool in Z direction. (3) (4) Where N is the Spindle speed, Ri is the momentary radius of the workpiece. (5) In the final stage, distance of all mesh points of the tool and workpiece and their interference has to be investigated. To check mesh points interference of the tool and workpiece, the distance between all points of the tool and all points of the workpiece in a time increment is computed peer to peer. If the distance of all points of tool and workpiece is greater than the user-defined
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tolerance, it means that no interference has occurred between the tool and workpiece yet. In this state, we return to algorithm’s previous stage, and in the next time increment the distance of mesh points of the tool and workpiece are examined. If the distance of one pair points of the tool and workpiece is less than the defined tolerance, it means that the tool has penetrated into the workpiece. In order to investigate the interactions between the tool and workpiece and in order to separate intersection points of the tool and workpiece from the workpiece, and to assess the effect of elastic recovery on the topography of surface texture in each time increment, rake thickness is first calculated by Eq. (6); then, using δt and the equation that governs elastic recovery [39, 40], surface texture topography of the tool is updated due to the partitioned area of the cutting insert (area of the cutting tool edge and area of the tool surfaces). If MATLAB simulation software detects the interaction between set of meshed points in the area of the tool cutting edge and set points of the workpiece, equations of elastic recovery and updating surface texture are as follows:
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Where t c is the chip thickness, t ce is the elastic deformation limit (workpiece/cutting edge), t c min is the minimum chip thickness, t c max is the limit chip thickness for elastoplastic deformation, pe is the elastic recovery rate (workpiece/cutting edge) and is the elastic/plastic deformation ratio. If MATLAB simulation software detects the interaction between the set of meshed points existing in the areas of the tool surfaces and the set points of the workpiece, and elastic recovery and updating surface texture are calculated using the following equations: (9) (10) (11) Where tve is the elastic deformation limit (workpiece/cutting surface), pe is the elastic recovery rate for the ploughing phenomena (workpiece/cutting surface). All the dimensions in the above mentioned equations are in μm. The magnitudes of the input parameters in the simulation are presented in table 1.
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Table.1. The value of the input parameters Parameters Magnitudes 0.01 (μm) 0.04 (μm) 0.1 (μm) 0.1 0.05 0.1 (μm) 0.5
Finally, by the end of the algorithm, a surface is outlined using the points of algorithm.
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3. Experiments 3.1. Process Preparation
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The samples used in the experiment are made of AL7075-T6. This alloy is one of the most used lightweight metals in shaping process, particularly removal processes such as machining process. This alloy has good mechanical and thermal properties such as medium hardness, heat treatable, good thermal conductivity, high corrosion resistance, high tensile strength and so forth. Moreover, because of its high strength to weight ratio, it has many applications. Chemical composition of this alloy are shown in Table 2.
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Table 2. The chemical compositions of AI7075-T6. Al Zn Mg Cu Fe Cr 88.76 5.6 2.4 1.6 0.5 0.24
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Elements Compositions (%)
Mn 0.3
Ti 0.2
Si 0.4
In these experiments, for facing of aluminum, PCD (Polycrystalline Diamond) tools with technical specification of TCGW 110204 LN-7 were employed. All of the used inserts had the same geometry with rake angle of 7˚ and without chip break. Polycrystalline diamond inserts are used in many non-ferrous machining applications. The experiments related to the process of creating surface texture using ultrasonic vibration were performed on a TNC50 turning lathe made in Machine Sazi Tabriz. In order to apply ultrasonic vibrations on the tool tip, three sets of piezoelectric layouts are used which include a set of full-ring stack for creating linear mode and two sets of half-ring stack for creating flexural mode. Two sets of half-ring stack are stimulated by 180˚ phase difference to enhance the amplitude of flexural vibration. The reason is that during the expansion of one set, another set is contracting and vice versa. For linear mode, the triggering/ stimulating signal has a 90˚ phase difference, compared with the flexural mode, to create an elliptical path at the tool tip. Fig. 5 illustrates UVT tool and experiment’s initial settings.
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Fig.5. Experimental setup for UAT tool used for the machining tests.
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Vibrational amplitudes were measured in two directions by an Eddy-current gap sensor. In order to measure surface roughness, PS1 model, manufactured in Mahr Company, was used.
3.2. Performing Experiments
The number of samples required in this research was determined using full factorial method. Factors include spindle speed at three levels and feed rate at three levels that, in general, 9 experiments were performed for each machining mode; thus, taking into account 4 machining modes (CT, LVT, EVT, and 3D-VT), a total number of 36 samples were prepared. Levels of the listed factors are shown in Table 3. The samples are kept by a special fixture and vibratory tool is installed on the support of the turning lathe. Then, by applying (stimulating) the desired vibration direction or directions, as shown in Fig. 6, tip of the cutting tool is vibrated in terms of machining processes of LVT, EVT, and 3D-VT and machining operation is performed. Cutting depth was same and equal to 0.25 mm in all experiments. Table 3. Factors and levels of the experiment’s design.
Parameters Spindle speed Feed rate Modes of vibrational process
Values
Levels
Unit
45, 31.5, 22.4 220, 180, 140 CT, LVT, EVT, 3D-VT
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(rpm) (μm/rev) -
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Fig. 6. Schematic figure of the working planes in machining processes of EVT, 3D-VT and working axis of LVT process.
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By the end of the machining process and samples preparation, some two-dimensional and threedimensional images were taken from surface texture of the samples in order to provide an analysis of surface texture and validate the textured surface generation simulation algorithm. Furthermore, in order to provide analysis of variance for surface roughness, surface roughness experiments were performed three times on each sample and in positions with 120˚ difference to each other.
4. Results and Discussion
In this section, results of the face-turning experiments in three vibrational modes of onedimensional (LVT), two-dimensional or elliptical (EVT), and three-dimensional (3D microfeatures) are evaluated to compare the surface texture resulted from the experimental results with the results of textured surface generation model. To this end, two-dimensional and threedimensional images were provided from surface texture of the samples. Then, the effects of vibration parameters (type of vibrational motion) and machining parameters (cutting speed and feed rate) were investigated on the surface morphology and roughness.
4.1. Comparison of simulation results and experimental tests results Surface texture of some samples resulted from ultrasonic vibration assisted face-turning in three vibrational modes of LVT, EVT, and 3D-VT were photographed using three-dimensional optical microscope. Then, the images were used to compare the surface texture obtained from the experimental results with the results of textured surface generation model, and validate the simulation results. Fig. 7 shows the topography of the surface texture obtained from the experimental results and textured surface generation model in LVT process and under the conditions of 31.5 rpm spindle speed, feed rate of 180 μm/rev, 10 micrometers vibration amplitude and vibration frequency of 16670 Hz.
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Topography of the surface texture obtained from the experimental results and textured surface generation model in EVT process and under the conditions of 31.5 rpm spindle speed, feed rate of 180 μm/rev, 10 micrometers vibration amplitude and vibration frequency of 16670 Hz are shown in Fig. 8a and b, respectively. Topographic image of the surface texture obtained from the experimental results in 3D-VT process is shown in Fig. 9a. Comparable simulation results, under the conditions of 31.5 rpm spindle speed, feed rate of 180 μm/rev, 10 micrometers vibration amplitude and vibration frequency of 16670 Hz, are shown in Fig. 9b.
Fig. 7. Surface topography for LVT process (a) experimental results, (b) simulation results.
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Fig. 8. Surface topography for EVT process (a) experimental results, (b) simulation results.
Fig. 9. Surface topography for 3D-VT process (a) experimental results, (b) simulation results.
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Topography of the surface texture obtained from the experimental results is shown in Figs. 7a, 8a and 9a. Comparable simulation results are shown in Figs. 7b, 8b and 9b. From the comparison of the images it can be understood that topography of the simulated surface texture, with a good approximation, is similar to topography of surface texture obtained from the experimental results. In general, topography of the simulated surface texture well-illustrates the shape of micro dimples, positioning of micro dimples, according to the type of ultrasonic vibration assisted machining process, and surface characteristics, according to machining parameters. Whereas the process of creating surface texture by ultrasonic vibration assisted turning has a great impact on improving the tribological properties, to characterize and compare the results of simulation and experiment quantitatively; the theoretical and experimental results of surface roughness extracted from generated surface texture are compared. The mean values of the asperity points along different positions on the surface texture caused by simulation algorithm were analyzed and the average values were reported. Comparing the theoretical results of the average roughness and experimental results of the average roughness shows that the surface roughness resulted from textured surface of simulation algorithm is in a good agreement with the results of experimental results. The theoretical and experimental results of surface roughness are listed in Table 4.
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Table 4. Comparison of the theoretical and experimental results of surface roughness (spindle speed: 31.5 rpm, feed rate 180 μm/rev. Surface roughness Experimental results Theoretical results Vibration mode LVT 0.981 μm 0.967 μm EVT 0.852 μm 0.834 μm 3D-VT 1.120 μm 1.080 μm
By comparing of the topography of the surface texture obtained from the experimental results and simulation results it can be understood that there are differences between the topography of the surface texture obtained from the experimental results and that obtained from simulation results. These differences can be investigated in two aspects. The first difference is the tool marks created at the peaks of surface texture that can be the result of the adhesion of the tool and material at the peaks of the surface texture which can be the result of the generated heat in machining. The Second difference is the out of flatness of the generated surface texture. As it can be seen in captured images of the surface texture, the samples’ surface are not flat like the topography of the simulated surface texture. Despite these differences between the topography of the surface texture obtained from the experimental results and simulation, the experimental results confirm the simulation results.
4.2 Texturing Results In this section, the effect of feed rate and cutting speed parameters on the texture morphology in each vibration machining mode (CT, LVT, EVT, and 3D-VT) is evaluated separately. Then, the effect of vibrational motion (CT, LVT, EVT, and 3D-VT) on the texture morphology is evaluated. Fig. 10 illustrates some patterns of the surface texture created by linear (onedimensional) ultrasonic vibration assisted turning process under different parameters of machining.
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Fig. 10. Comparison of surface topography for LVT (a) Feed rate of 140μm/rev, spindle speed of 31.5rpm, (b) Feed rate of 140μm/rev, spindle speed of 45rpm, (c) Feed rate of 180μm/rev, spindle speed of 31.5rpm.
According to Fig. 10, dimples array depends on the parameters of cutting speed and feed rate. All machining and vibration parameters are the same in Fig. 10a and b (f=16670; A=10μm; Vf=140μm/rev), except spindle speed which is equal to 31.5 rpm and 45 rpm in Fig. 10a and b respectively. Therefore, as expected and observable in Fig. 10a and b, order of the created dimples is such that in feed direction, because of constant feed rate, the distance between the dimples is similar; while the distance between the dimples in cutting direction is such that by increasing the cutting speed, the distance between the dimples increases as well. Similarly, all the process parameters are identical in Fig. 10a and c (f=16670; A=10μm; N= 31.5 rpm), except feed rate which is equal to 140μm/rev and 180μm/rev in Fig. 10a and c, respectively. As can be observed, the distance between the dimples created in cutting direction is identical that is due to the constant cutting speed; while the distance between the dimples in feed direction is such that
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by increasing the feed rate, the distance between the dimples increases. This trend is also true for two-dimensional (EVT) and three-dimensional (3D micro-features) processes which are shown in Figs. 11 and 12, respectively.
Fig. 11. Comparison of Surface Topography for EVT (a) Feed rate of 140μm/rev, spindle speed of 31.5 rpm, (b) Feed rate of 140μm/rev, spindle speed of 45 rpm, (c) Feed rate of 180μm/rev, spindle speed of 31.5 rpm.
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Fig.12. Comparison of surface topography for 3D micro-features (a) Feed rate of 140μm/rev, spindle speed of 31.5rpm, (b) Feed rate of 140μm/rev, spindle speed of 45rpm, (c) Feed rate of 180μm/rev, spindle speed of 31.5rpm.
Figs. 13a-d, 14a-d and 15a-d illustrate images of the surface texture of machining with constant parameters of 180μm/rev feed rate and 31.5 rpm spindle speed for LVT, EVT, and 3D-VT respectively. Moreover, micro dimples array, comparison of the dimples in terms of their expansion in feed and cutting directions for LVT, EVT, and 3D-VT, and comparison of the textures profile in feed direction are investigated in these figures.
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Fig. 13. Evaluation of surface texture obtained from LVT under the feed rate of 180μm/rev and spindle speed of 31.5 rpm (a) Perspective view, (b) Front view, (c) Top view and (d) Surface texture magnification.
Fig. 14. Evaluation of surface texture obtained from EVT under the feed rate of 180μm/rev and spindle speed of 31.5 rpm (a) Perspective view, (b) Front view, (c) Top view and (d) Surface texture magnification.
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Fig. 15. Evaluation of surface texture obtained from 3D-VT under the feed rate of 180μm/rev and spindle speed of 31.5 rpm (a) Perspective view, (b) Front view, (c) Top view and (d) Surface texture magnification.
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As shown in Fig. 13d, vibration direction for LVT process is linear and along the x axis (cutting direction). Hence, with respect to the feed rate and cutting speed, micro dimples created on the surface are ordered with a very small deviation angle to the horizontal axis. On the other hand, in EVT process, since two vibration directions lead to an elliptical motion along the z and x axes and also considering the feed rate and cutting speed, micro dimples created on the surface are arranged with a greater deviation angle than LVT process. Finally, in 3D-VT process, since in addition to vibration directions along the z and x axes, a tertiary direction is applied along the y axis to create an ellipsoid motion and also taking into account the feed rate and cutting speed, micro dimples created on the surface are arranged with a greater deviation angle than EVT process. Figs. 14d and 15d confirm this. Another noteworthy point is that according to Figs. 13c, 14c and 15c, because of the nature of LVT (vibration along the x axis, cutting direction) and EVT (vibration directions along z axis, cutting depth, x axis, cutting direction) and considering the rotating motion, EVT micro dimples, compared with LVT process, have been developed in cutting direction. According to this argument and the nature of 3D-VT process, in addition to two vibrations in depth and cutting directions, the tertiary vibration direction is added along the y axis (feed direction), micro dimples of this process compared with EVT process have developed only in feed direction, and compared with LVT process have developed in both feed and cutting directions. On the other hand, according to Figs. 13a-b, 14a-b and 15a-b, surface texture profile of EVT in Fig. 14a-b, like LVT, is smooth and symmetrical. However, surface texture profile of 3D microfeatures, according to Fig. 15a and b, like a unique curvature is asymmetrical. As a result, it is obvious that topography of the simulated surface and generated surface texture are in good agreement in terms of the shape of micro dimples, expansion of micro dimples in feed and cutting directions, positioning and orientation of micro dimples according to the type of
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4.3. The Effect of UVAT Process Parameters on Surface Roughness 4.3.1. ANOVA Results for Surface Roughness
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Analysis of variance for surface roughness was performed by Minitab software with 95% confidence level. Contribution percents in Table 5 show each parameter’s degree of influence on the results. It is obvious, that feed rate (75.38%), process type (5.7%), and cutting speed (1%) have effect on surface roughness output parameter. Table 5. ANOVA results of Ra. Seq SS Adj MS F P 0.02627 0.01313 0.78 0.468 1.98371 0.99185 58.91 0.000 0.15019 0.05006 2.97 0.049 0.47146 0.01684 2.63163 -
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Source Vc (rpm) Vf (μm/rev) Process Error Total
Contribution(%) 1 75.38 5.7 17.92 100
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S = 0.129761 R-Sq = 82.08% R-Sq (adj) = 77.61% 4.3.2. The Effect of Cutting Speed on Surface Roughness
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Comparison of surface roughness in machining processes of CT, LVT, EVT, and 3D-VT, in terms of cutting speed parameter, is shown in Fig. 16. As can be observed in this figure, surface roughness in CT process has been more than other processes and is decreased respectively in 3DVT, LVT, and EVT processes.
Fig. 16. The effect of spindle speed on surface roughness in CT, LVT, EVT, and 3D-VT processes (feed rate= 140 μm/rev)
4.3.3. The Effect of Feed Rate on Surface Roughness Comparison of surface roughness for machining processes of CT, LVT, EVT, and 3D-VT, in terms of feed rate parameter with constant cutting speed, is given in Fig. 17. According to this
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figure, in all processes, with an increase in feed rate, surface roughness increases first and, decreases then.
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Fig. 17. The effect of feed rate on surface roughness in the processes of CT, LVT, EVT, and 3D-VT (spindle speed= 31.5 rpm)
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From the Figs. 16 and 17, it can be understood that the highest level of surface roughness occurs in CT process, and the surface roughness values more decrease in 3D-VT, LVT, and EVT processes, respectively.
5. Conclusion
In this research, topography of surface was investigated in ultrasonic vibration assisted faceturning. In order to achieve precise surface texture by the proposed surface texture generation algorithm, the effect of three-dimensional geometry of the cutting tool, the states of vibration motions applied to the insert, geometry of workpiece and elastic recovery in simulation algorithm, were considered. Then, experimental tests were performed to validate simulation algorithm of surface texture generation in ultrasonic vibration assisted face-turning. The results of simulation algorithm were compared with experimental tests and, finally, the following results were obtained: Comparison of the experimental surface texture with that of simulation algorithm shows that they are relatively well-matched. Regarding the vibration direction in LVT process that is linear, micro dimples created on the surface are arranged with a small deviation angle to the horizontal axis. This phenomenon is observable in the photographs taken from surface texture of the samples and the output of surface texture simulation algorithm. Moreover, this deviation increases in EVT and 3D-VT processes respectively. ANOVA results for surface roughness indicates that feed rate has the greatest influence (75.38%) on the surface roughness. Furthermore, analysis shows clearly that the parameters of process type (5.7%) and cutting speed (1%) are the other influential parameters.
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The results of surface roughness show that the highest and lowest surface roughness belongs to CT and EVT processes respectively.
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Graphical abstract
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Highlights Comparison of the experimental surface texture with that of simulation algorithm shows that they are relatively well-matched. Regarding the vibration direction in LVT process that is linear, micro dimples created on the surface are arranged with a small deviation angle to the horizontal axis. This phenomenon is observable in the photographs taken from surface texture of the samples and the output of surface texture simulation algorithm. Moreover, this deviation increases in EVT and 3D-VT processes respectively. ANOVA results for surface roughness indicates that feed rate has the greatest influence (75.38%) on the surface roughness. Furthermore, analysis shows clearly that the parameters of process type (5.7%) and cutting speed (1%) are the other influential parameters. The results of surface roughness show that the highest and lowest surface roughness belongs to CT and EVT processes respectively.