Micro-dimple pattern process and orthogonal cutting force analysis of elliptical vibration texturing
Introduction
Elliptical vibration cutting (EVC) is an alternative cutting method that adds a secondary motion to the tool tip in vibration cutting (VC) to establish an elliptical locus motion of the tool tip [1], [2], [3]. The EVC method exhibits superior performance compared to both conventional cutting (CC) and one-dimensional VC (less cutting force [4], [5], better surface finish [6], [7], [8], [9], burr suppression [10], [11], [12], [13], longer tool life [14], [15], [16], [17], etc.). Shamoto and Morikawi [2] proposed firstly the EVC method in which they claimed the cutting force and the chip thickness are reduced effectively by using the EVC. Kim et al. [5] and Shamoto et al. [15] reported that by applying EVC during machining, the cutting force reduction was observed. Brehl et al. [1] and Xu et al. [18] presented the comprehensive review of basic kinematic principle between 1D and 2D vibration-assisted machining (VAM) and the characteristics of VAM were explained such as reducing tool forces, extended tool life for diamond tools, reduced surface roughness, and suppression of burr formation. Zhang et al. [17] and Zhou et al. [19] carried out the EVC method with polycrystalline diamond (PCD) tools. Their reports concluded that the steel machining can be applied effectively using inexpensive PCD tools instead of highly expensive single crystal diamond (SCD) tools. Kim and Loh [8] investigated the characteristics of the EVC in micro-V grooving associated with variations of the elliptical amplitudes and the excitation frequency. Then, the micro precision patterns for micro molds were achieved by applying the EVC in which the cutting resistance, tool wear and machining quality were significantly improved [13].
In previous studies, several achievements of creating textured surfaces using conventional machining systems have been carried out. In 1994, Hong et al. [20] proposed a surface-shaping system model to evaluate and simulate the textured surface of machined components by considering the tool geometry and the machine kinematics. Greco et al. [21] introduced a vibromechanical texturing (VMT) method in which only the tip tool vibrates simultaneously normal to the cutting direction. Kurniawan and Ko [22] studied the textured surfaces in turning system using piezoelectric actuated tool holder in which the accuracy of the micro-dimples was measured and evaluated. In turning system, the sinusoidal motion of the tool relative to the workpiece is created by pushing the tool along the depth of cut direction using the piezo electric actuator. Gandhi et al. [23] demonstrated both the plunging-type and sliding-type texturing method for generation of micro-sized dimples and ribs/fins on the surfaces using conventional machining platforms. Matsumura and Takahashi [24] presented the micro-dimple machining based on the milling process on the cylindrical surfaces at high machining rate with 45° cutter axis inclination. Suzuki et al. [7] proposed micro/nano structure sculpturing method in which the amplitude of the elliptical vibration was controlled, and hence the depth of cut could be varied quickly to obtain the micro/nano structures.
By adopting the EVC method, micro-dimples or textured surfaces could be manufactured with elliptical vibration texturing (EVT). The first EVT method applied the EVC method for texturing in a turning process and was introduced in 2013 by Guo and Ehmann [25], [26]. The EVT method is a fast and accurate way to produce the micro-dimple pattern. The micro-dimple pattern shape depends on texturing parameters such as the feed of the tool tip, the vibration frequency of the tool, the amplitude of the tool path, and the tool geometry. Guo et al. [26] studied the surface generation mechanics of the EVT process through modeling and experimentations. The micro-dimple shapes were established on cylindrical surfaces using EVT method in ultrasonic ranges about 28 kHz with different feed rates and spindle speeds. Their micro-dimple shapes might be suitable for increasing water contact angle [27] instead of for friction reduction purposes because the micro-dimple arrangement is too close to each other. Anisotropic wettability of hierarchical structures of micro-channels enhances if the micro-channels are built using the EVT method [27]. The micro-channels could be formed by overlapping the micro-dimples in which the shape depends on critical parameters such as the spindle speeds, feed rates, and vibration frequencies [28]. Based on the previous EVT studies, the majority of micro-dimples array and micro-channels were built on cylindrical surfaces in ultrasonic frequency ranges. However, in this paper, the micro-dimples was built on planar surfaces in low frequency ranges using non-resonance transducer. Parameters such as oscillation frequencies and cutting speed were selected properly to make the micro-dimple density and shape suitable for friction reduction applications. The micro-dimple density was kept less than 20% so that the distance among neighboring micro-dimples is large enough to avoid the interaction among them [29]. In addition, the accuracy and the cutting forces analysis of the EVT have not been carried out yet in the previous reports.
In the field of tribology, a micro-dimple pattern is used on a planar surface to enhance lubricant performance. The micro-dimples act as lubricant reservoirs that improve lubricant retention and to capture wear debris [29]. The performance of micro-dimples to reduce the friction coefficient has been investigated widely [30], [31], [32], [33], [34], [35]. A laser surface texturing (LST) is a well-known and common method used to manufacture micro-dimple pattern on a planar surface [36]. The LST method is a fast and flexible process and provides excellent control of the shape and dimensions of micro-dimples. However, if the LST process uses a high-intensity laser beam, melting and rapid solidification of the metal cannot be avoided [33]. In addition, expensive equipment and a clean environment are required in the LST method. These weakness do not occur in the EVT method, which is fast, accurate, and controllable.
In the material removal mechanism, the cutting force is an important parameter, especially for improving the efficiency of the cutting process, as well as for cutting tool and machine design. The cutting force depends on how much material has been cut [37], [38], [39], [40]. Shamoto et al. [37] reported calculation method based on time instants at various critical tool locations for determining cutting force values in the EVC process. Kim et al. [41] studied the cutting force of V-grooving by varying the tilt angle of elliptical trajectory in EVC process. Bai et al. [38] and Zhang et al. [40] analyzed the orthogonal cutting forces of the EVC by applying thin shear plane theory. The variation of the thickness of cut (TOC) and transient shear angle of the primary shear plane were considered as the main factors for transient cutting forces. Zhang et al. [39] calculated the cross-sectional area of the cut and the removed chip volumes per vibration cycle by integrating the cross-sectional area of cut to predict quantitatively the cutting force of the EVC for micro-groove turning process. In most of previous studies, analyzing the material removal mechanism could reveal variation of TOC and the variation of shear angle in primary shear zone. In the EVT process, the material load is varied for a transient thickness of cut (TOC). However, there is no effect of a transient shear angle in the texturing process, because the shear angle is bigger than the tool path slope angle. The actual shear angle is assumed to be constant, and the actual shear angle is related to the vibration texturing frequency. The actual shear angle was determined from the experimental results by measuring the chip ratio.
In this paper, a mathematical model of the EVT method was developed, and orthogonal cutting force model of the EVT is presented. The principle of the EVT and the surface texturing methodology based on the time instant of the tool position and the tool geometry are presented in Section 2. An analytical force model of an orthogonal cutting mechanism was applied to fully understand the texturing process, material removal, and cutting force of the EVT method, which is correlated with other machining parameter outputs such as tool life and surface finish. The cutting force model is based on the basic mechanics of orthogonal cutting and the parameters such as the constant shear stress, transient cutting area and constant actual shear angle are taken into account. The analytical micro-dimple dimensions compared with experimental results to validate the texturing model is shown in Section 5. The cutting force model is verified with the experimental results in Section 6. The cutting force model has been found to be in complete agreement with the experimental results.
Section snippets
Principle of elliptical vibration texturing
EVC was developed based on one-dimensional VC. The EVC method adds a secondary motion in the vertical direction to the tool tip to give it an elliptical locus. The EVC method has many advantages over the CC method, such as reduced cutting force, decreased tool wear, burr suppression, less surface roughness, and suitability for difficult-to-cut or brittle materials. The EVC method was first introduced by Moriwaki et al. [2] and has been explored widely.
Fig. 1 shows the fundamental concept of the
Orthogonal cutting force analysis
In this study, the EVT process is simplified as orthogonal cutting to which Merchant's model [43] for CC cutting is applied [44], [45]. The first assumption is that the tool tip is ideally sharp, so the effect of the tool edge radius is neglected. The second assumption is that the cutting process is similar to the CC process, although TOC is not constant. The third assumption is that both the shear stress (τ) and the friction angle (β) are constant and equal to those in the CC method. The shear
Experimental setup
EVT experiments were carried out on a planar surface of an Al-6061 workpiece with dimensions of 20×20×20 mm3. In order to obtain a proper planar surface and low surface roughness, a pre-machining process using conventional milling is necessary (feed rate of 20 mm/min and spindle rotation of 500 rpm). The distance between the tool tip and workpiece surface (∆d) was adjusted to obtain a distance of about 5 μm before beginning the texturing process. Al-6061 was used in these experiments because it is
Micro-dimple pattern results
A micro-dimple pattern was successfully built on an Al-6061 surface using a PCD tool with the texturing parameters described in Table 2 and dry cutting. Because of the ductility of the material, the micro-dimple pattern is typically easy to build on an Al-6061 surface. Fig. 14 shows the microscopy results with magnification of 50× and 100× under vibration frequencies f of 80 Hz, 100 Hz, and 120 Hz, respectively. There were scratches on the exit region along the cutting direction that typically
Validation cutting force model and discussion
In this section, the simulated cutting forces are compared to the experimental cutting forces. The simulated cutting forces were calculated using parameters given in Table 2, which were also used in the experiments. The simulated cutting forces were calculated using the orthogonal cutting force analysis described in Section 3. The actual shear angle ϕa provided in Table 4 is used to calculate the simulated cutting forces. In this case, the shear angle is assumed to be constant during the
Conclusions
This paper introduced a method for making a micro-dimple pattern using the EVT method and an analysis of the orthogonal cutting forces. This paper will hopefully be helpful for researchers who are working on the EVT method. Our conclusions are summarized as follows:
- 1.
General principle of the EVT and the surface texturing methodology based on the parameter time of the tool position and tool geometry are presented. Fundamentally, the tool tip is vibrated elliptically and is driven at a speed ratio (
Acknowledgment
This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2014H1C1A1066502).
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