1. Introduction
In recent years, the requirement for and application of microdevices with complex structures have been rapidly increasing in the national defense and civil fields. The deep-and-narrow microgrooved structure has become a research hotspot in mechanical field because it can reduce workpiece weight and material consumption under the premise of ensuring its stiffness and strength. Deep-and-narrow micro-groove is a critical functional structure of the Terahertz (THz) slow-wave structure [
1,
2], microforming die and micro-heat exchanger [
3], etc. The common characteristics of these structures are [
4,
5]: (1) material diversity (including metal, ceramics, and composite materials); (2) complex geometric structure with large-aspect-ratio and small scale; (3) high machined quality and consistency; and (4) strict machining accuracy and high sidewall perpendicularity, etc. The service performance of miniature parts is dominated by the machined quality and precision of this kind of microstructure, but the fabrication difficulty is still a problem.
At present, the alternative processing technologies for the deep-and-narrow micro-grooved structures primarily include electromachining technology (EDM and ECM), LIGA/UV-LIGA, grinding, micromilling, etc. In contrast, micromilling is one of the most reliable micromanufacturing methods for these micro-grooved structures, because of the advantages of various materials, high efficiency, controllable machined accuracy and low-cost [
6]. Bang et al. [
7] fabricated a group of 200-μm-width wall microstructures with an aspect ratio greater than three on a 5-axis micromilling machine. Llanos et al. [
8] researched the surface quality, milling strategies and tool paths of thin walls by the commercial carbide micromilling cutters. Parameter optimization experiments were conducted and the machining capability was verified by machining 750-μm-deep and 50-μm-thick thin walls on CuZn36Pb3 brass. Two-fluted coated carbide microcutters were used for machining the Beryllium bronze, and a 14-μm-thick thin-walled structure was obtained by adopting an auxiliary support [
9]. A 16-blade aluminum alloy 1100 micro-impeller with average blade thickness of 11.96 μm was manufactured by 200-μm-diameter two-fluted carbide micromilling tools [
10]. However, the commercial coated carbide cutter with large-aspect-ratio has obvious shortcomings in machining the softer metal microstructures, for instance, serious coating-shedding, premature tool failure and uneven milling phenomenon, which further deteriorates the machined surface quality. Meanwhile, frequent tool replacement not only increases the tool consumption, but also introduces unnecessary repeated positioning error and tool clamping error to the manufacturing process [
11,
12].
Microcutter technology is one of the most critical elements to the extensive application of micromilling technology, the machined surface quality and machined dimensional accuracy of microstructures is determined by the micromilling cutter [
13]. Hence, the preparation of super-hard diamond cutters with high aspect ratio and long service life is the critical factor to solve the above difficulties. Due to the high hardness, high compressive strength and wear-resistance, polycrystalline diamond (PCD) compounded by microdiamond powders with metallic or ceramic binders becomes the most popular and promising diamond material for micromilling cutters and shows superior performance in micromilling [
6]. For instance, Nakamoto et al. [
14] machined tungsten carbide material with a PCD micromilling cutter, micro-grooves processed with a depth-to-width ratio of 5 were obtained and the peak–valley surface roughness was below 40 nm. Precision fabrication of PCD micromilling cutters is a very challenging task, because the feature dimensions and rigidity are very limited. The available preparation techniques for PCD micromilling cutters mainly include the pulse laser [
15], conventional grinding [
16,
17], micro electric discharge machining (micro-EDM) [
18] and multiple process manufacturing technology [
19]. The diamond-wheel grinding and polishing was used to prepare 20-edge PCD micromilling cutters by Suzuki et al. [
20]. A PCD end-mill with a designed hexagonal geometry was fabricated using WEDM [
21]. The surface roughness and cutting-edge radius achieved were about 0.11 μm and 1.8 μm, respectively. The femtosecond laser was further utilized to fabricate a binderless polycrystalline diamond (BLPCD) cutter [
22]. The cutting-edge radius and surface roughness obtained were 0.8 μm and 22 nm, respectively, and no graphite residue was found on the tool surfaces.
Single conventional grinding was the primary fabrication technique for the non-ultra-hard micromilling cutters before 2010 due to the low efficiency, intensive wheel wear, and insufficient accuracy. Whereas, for the ultra-hard micromilling cutters, single conventional grinding is no longer recommended, nonconventional technologies such as wire electro discharge machining and pulse laser ablation have become more popular. An additional postprocessing operation is needed due to the recast generated in micro-EDM, which makes the preparation process of micromilling cutters more complex. Limitations of using the pulse laser are the expensive price for picosecond and femtosecond lasers and substantial preparation efforts and repeated parametric trials for different tool material [
19]. With constant advances in the fabrication technology, a new multiple-process fabrication (MPF) method is also proposed. Based on the combination of a picosecond laser and FIB, a cemented carbide ball micro milling cutter with the cutting edge radius of less than 1 μm was fabricated [
23]. By combining a nanosecond laser and grinding, a CVD diamond micro milling cutter with a cutting edge radius of 1.957 μm was also obtained [
24].
At present, very few mature investigations are efficiently applied in preparing the required PCD micromilling cutters with large-aspect-ratio, which hinders the development of micromilling technology in machining the complex microstructures. Moreover, inferior surface quality and low machining efficiency seriously restrict the application of deep-and-narrow micromilled grooves in the fields of the aerospace, biomedicine, and space communication.
In this work, a PCD micromilling cutter with a large-aspect-ratio and sharp cutting-edge was firstly fabricated by the developed hybrid method. To verify the cutting performance, comparative experiments of micro-grooves were conducted using the LAR carbide and self-manufactured PCD micromilling cutters. The influence of the milling parameters on the cutting forces and specific energy was explored. Furthermore, the surface quality and machined accuracy of the micromilled grooves with an aspect ratio of 2.5 were investigated, and the tool wear was analyzed.
3. Experimental Setup and Procedures
Micromilling experiments were carried out on a multifunction high-precision machine, as shown in
Figure 5. The platform mainly included a marble structure bed, PMAC control system, air floating high-speed motorized spindle, etc. The maximum spindle speed and rotation accuracy were about 100,000 rpm and 1 μm, respectively. The CCD microscope system was equipped to realize accurate tool setting and realtime monitoring of the machining process. The micro dynamometer (Kistler
® 9256C1) was fixed on the lower side of the fixture system. The minimum force measuring threshold and the maximum sampling frequency were 0.002 N and 30 kHz, respectively. The milling force signal was saved by NI-DAQ software after the amplifier system.
The workpiece material employed was No.1 oxygen-free copper (OFC-TU1) with a size of 30 mm × 10 mm × 5 mm, and the main physical properties are provided in
Table 2. The self-manufactured LAR PCD cutter was employed, and commercially available two-fluted carbide cutter of 0.5 mm in diameter was included in the comparative experiments, as shown in
Figure 6. The carbide cutter was coated with CrTiAlN and the coating hardness was about 3600 HV. The cutter parameters are given in
Table 3.
The first part of the experiments was to explore the changes of cutting forces and specific energy, and the scheme is shown in
Table 4. The second was the machining of the 1.0-mm-depth micromilled grooves (
Table 5), and surface quality, machined accuracy, and tool wear were compared and analyzed. A Hitachi Scanning Electron Microscope (SEM) (S3400N, Tokyo, Japan) was used to observe the surface morphology, microburr formation, and tool wear. The areal surface roughness (
Sa) was measured by a 3D confocal microscope (OLS500, Tokyo, Japan).