Fabrication of a Peristome Surface Structure of Nepenthes alata by Elliptical Vibration Cutting
The phenomenon of continuous directional water transport on the peristome surface of Nepenthes alata (N. alata) has been found to be of great significance to the applications of microfluids, anti-adhesion surface texture, lubrication and so on. Various methods are used to fabricate the bionic structure of the peristome surface. However, the size of processing area and the fabrication material are limited in the previous methods, which results in the limitation of the bionic structure applications. In order to solve the remained problems of small-scale and limited materials, the mechanical machining is creatively applied to the fabrication of bionic structure of peristome surface of N. alata in this paper. An elliptical vibration cutting system (EVC) driven by mechanical structure is designed and built to satisfy the size requirements of the bionic structure. The surface topographies corresponding to the tool trajectories of cutting and extrusion are obtained, respectively. The results demonstrate that both the two methods can realize the fabrication of bionic inclined microcavities while few defects can be seen on the extruded surface. According to the measured structure dimensions, it can be found the EVC system keeps a superior machining repeatability. As a consequence, the availability of the newly proposed method for the large-area fabrication of the bionic structure is verified.
KeywordsElliptical vibration cutting Bioinspired microcavity structure Cutting Extrusion Nepenthes alata
Tool displacement along cutting direction, μm
Tool displacement along direction of depth of cut, μm
Tool vibration amplitude in cutting direction, μm
Tool vibration amplitude in direction of depth of cut, μm
Cutting velocity, μm/s
Phase shift between vibrations of x-axial and y-axial directions, rad
Tool vibration frequency, Hz
Instantaneous direction angle of the tool motion, rad
Nominal clearance angle of tool, rad
Wedge angle of tool, rad
Cutting length, μm
Maximum cutting depth, μm
Mechanical machining is the most common large-area processing method and has a wide range of machining materials. In mechanical machining field, specialized surface texture can be fabricated through elliptical vibration cutting (EVC), which was proposed by Shamoto and Moriwaki . In this method, the tool motion path can be implemented on workpiece surface [11, 12]. By controlling the depth of cut, cutting direction and vibration amplitude of EVC, the desired surface topography can be obtained. EVC is usually used in fabrication of ultra-precision micro/nano-structures, freedom surfaces and optical glass parts [13, 14, 15, 16]. In general, the EVC system is driven by piezoelectric (PZT) actuators, in which the amplitude is not more than 100 μm . When the size of structure is larger than the amplitude, the PZT-driven EVC system is obviously not applicable.
Therefore, to solve the remained problems of small-scale and limited machining materials, the mechanical machining is creatively applied to the fabrication of bionic structure of peristome surface of N. alata in this paper. An EVC system driven by mechanical structure is designed and built to satisfy the size requirement of the bionic structure. The rest of this paper is organized as follows. The tool trajectories of cutting and extrusion are determined in Sect. 2. Composition of the EVC system and construction of experiment platform are introduced in Sect. 3. The surface topographies obtained by the EVC are analysed and the actual machining contours are compared with the theoretical ones in Sect. 4. Finally, the conclusions are given in Sect. 5.
2 Elliptical Vibration Machining Mechanism
As depicted in Fig. 2a, the tool clearance angle is larger than the instantaneous direction angle at M1. As the tool cuts into the workpiece, the distance between the flank face and the machined surface gradually increases. When the tool separates from the workpiece at M4, the sum of wedge angle and the clearance angle is smaller than the instantaneous direction angle. Therefore, there is no interference between the tool rake face and the machined surface. Different from the cutting process, the fabrication of the final surface topographies completely relies on the flank face interference in the extrusion method. As clear from Fig. 2b, the sum of the wedge angle and the clearance angle is smaller than the instantaneous direction angle at M1. The flank face maintains contact with the workpiece until the tool clearance angle is equal to the instantaneous direction angle after the largest extruded area is obtained. Then, the tool separates from the workpiece, and the structure is fabricated.
3 Experimental Set-up
Clearance angle (°)
Vibration frequency (Hz)
Amplitude in x-axial direction (μm)
Amplitude in y-axial direction (μm)
Phase shift (rad)
Cutting velocity (μm/s)
The EVC experiment platform is set up as depicted in Fig. 3b. The EVC device, which is used to stimulate the cutting tool to perform elliptical vibration, is fixed inside the spindle of a milling machine (BV100, China). The cutting velocity (v) is set as 400 μm/s. The workpiece is fixed on the milling machine worktable. Then, the EVC system is used to fabricate the structure. The surface topographies are observed by a digital microscope (HIROX RH-2000, Japan). A vertical section of the structure is obtained by burying the workpiece in epoxy resin before a polishing process. The structure dimensions are measured by the built-in measuring software of digital microscope. All of the results will be discussed in the following section.
4 Results and Discussions
A low-frequency EVC system driven by mechanical structure is designed and built to fabricate the bionic structure of peristome surface of N. alata in this paper. The tool trajectories of cutting and extrusion are determined by controlling the tool vibration amplitudes, phase shift and geometric parameters. The results demonstrate that both two methods can realize the fabrication of the bioinspired inclined microcavities while few defects can be seen on the extruded surface. And the EVC system keeps a superior processing repeatability. Therefore, the EVC provide the possibility of large-scale manufacturing of bionic structure. But due to the limitation of experimental conditions, the existence of experimental error makes a great influence on the machining precision. The study needs to continue to improve the experimental conditions and processing accuracy.
The authors would like to thank the 4th CIRP Conference on Surface Integrity (CSI) Organizing Committee for the support of this research.
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