The demand for high-precision and high-efficiency machining of hard ceramics such as silicon carbide (SiC) for semiconductors and hardened steel for molding dies has significantly increased for power devices in automobiles, optical devices, and medical devices. Certain types of hard metals can be machined by deterministic precision-cutting processes. However, hard and brittle ceramics, hardened steel for molds, or semiconductor materials have to be machined by precision abrasive technologies such as grinding, polishing, and ultrasonic vibration technologies with diamond super abrasives. The machining of high-precision components and their molds/dies by abrasive processes is much more difficult owing to their complex and nondeterministic nature as well as their complex textured surface. Furthermore, high-energy processes with UV lasers and IR lasers, and ultrasonic vibration can be used to assist abrasive technologies for greater precision and efficiency. In this sense, precision grinding and polishing processes are primarily used to generate high-quality and functional components usually made of hard and brittle materials. The surface quality achieved by precision grinding and polishing processes becomes more important to reduce processing time and costs.

This special issue features seven research papers on the most recent advances in precision abrasive technologies for hard materials. These papers cover various abrasive machining processes such as grinding, polishing, ultrasonic-assisted grinding, and laser-assisted technologies.

We deeply appreciate the careful work of all the authors and thank the reviewers for their incisive efforts. We also hope that this special issue will encourage further research on abrasive technologies.

Grinding is difficult to control because abrasive grains are scattered randomly on the surface of the grinding wheel, and the quality of the grinding work is strongly dependent on the skill of the operator. Therefore, automation and optimization technologies should be established immediately for grinding, along with other machining work. From this perspective, we observed the bending vibrations of a diamond wheel during a grinding project and developed a technique to identify the grinding condition by using a microphone to measure the small noises from the vibration (called bending-vibration noise in this paper). In this paper, we report the application of the technique to an ordinary grinding wheel, and our attempt to automate the grinding work of STAVAX and SKD11 metal materials.

During the cylindrical traverse grinding of a slender workpiece, the ground workpiece is easily bent by the normal grinding force owing to its low stiffness. Therefore, it is difficult to finish the slender workpiece with high accuracy. To prevent the elastic deformation of a workpiece during the grinding process, a steady rest is generally used. However, considerable skill of the worker is required to use a steady rest. Therefore, we developed a new traverse grinding method without any steady rest. In this method, the elastic deformation of a workpiece was kept constant by controlling the traverse speed of the workpiece. At the middle of the ground workpiece, where the elastic deformation increased easily, the traverse speed was slowed down. However, this method had a longer grinding cycle time because the average traverse speed decreased compared to that of the conventional method. To shorten the cycle time, the peripheral speed of the grinding wheel was increased to decrease the normal grinding force. Basic grinding experiments were carried out under several grinding conditions by changing the peripheral speed of the wheel. From these grinding experiments, it was confirmed that the normal grinding force and the form error of the ground workpiece decreased as the peripheral wheel speed increased. By using results obtained from basic experiments, grinding experiments involving changes in the traverse speed were carried out at two peripheral wheel speeds. The grinding cycle time was reduced successfully by increasing the peripheral wheel speed without an increment in the form error of the ground workpiece. Furthermore, a form error was observed at the end of the workpiece where the grinding wheel traveled away from the workpiece. The form error occurred because the normal grinding force decreased rapidly when the contact length between the workpiece and the wheel was decreased at the end of the workpiece. To prevent rapid changes in the normal grinding force, the traverse speed of the workpiece was increased at the end of the workpiece. By using this method, a ground workpiece with high form accuracy was obtained.

This study investigates phenomena in ultrasonic vibration-assisted grinding. The appropriateness of a stress visualization method is proven through comparison of a Hertzian contact stress analysis using finite element methods. The stress distribution on soda-lime glass caused by a 3-mm-diameter diamond electro-deposited wheel is visualized using a photo-elasticity method. The study compares the local stress concentrations caused by grains with and without ultrasonic wheel vibration. The global reaction force is measured by a dynamometer. The ultrasonic vibration leads to a reduced fluctuation of force, as well as a reduced time-averaged force. It is thought that the ultrasonic vibration causes a smaller local stress beneath the grains, which generates chips. In contrast, typical photo-elasticity methods are applicable for plane stress conditions. However, the stress distribution in a workpiece under a face grinding condition is distributed three-dimensionally, and the stress distribution cannot be recognized directly from the phase difference. Assuming that the stress distribution is sufficiently stable in a wheel rotation, continuously-captured images can be reconstructed to produce a 3D stress distribution, using computed tomography. The experimental tomographic images show a spatially-dispersed phase difference image caused by the electro-deposited wheel, with several discontinuous diamond grains on the end face of the wheel.

The magnetic compound fluid (MCF) polishing process is a precision finishing method that has been applied to a large variety of materials, from soft optical polymers to hard ceramics. The purpose of this study is to extend the working life of MCF slurry. In this paper, we focus on the drying phenomenon of MCF slurry during polishing, and we develop a new water supply system that uses an ultrasonic atomization mechanism. This system can moisturize the MCF polishing area locally. Polishing experiments involving supplying water to MCF slurry are carried out, and extending the working life of MCF slurry is discussed.

Silicon carbide (SiC) is a next-generation semiconductor material. However, SiC is difficult to machine because it has high mechanical hardness and chemical inertness. Therefore, high-quality processing technology with high efficiency is now required. In our previous study, the authors developed UV-assisted grinding method and clarified that the critical depth of cut is expanded by UV irradiation and that the surface roughness is reduced by applying this method with a composite-abrasive wheel containing diamond abrasive grains with a mean abrasive diameter of 6 μm. On the other hand, it is important to reduce the damaged layer left by lapping or polishing process after grinding. In this study, we developed UV-assisted constant-pressure grinding method as a new alterative processing method to lapping and polishing. In this method, UV-assisted grinding is applied to the constant-pressure grinding method. To obtain a higher-quality mirror surface, we used a superabrasive wheel containing diamond abrasive grains with a mean abrasive diameter of 0.5 μm. By investigating the influence of UV irradiation on processing characteristics, we clarified that the surface roughness and removal height were reduced by UV irradiation. Finally, a high quality surface with few grinding marks was obtained by UV-assisted constant-pressure grinding.

Ring-shaped SiC ceramics used in sliding components were examined herein. Conventional lapping generates many shallow scratches in random directions in the surface of such SiC ceramics, and these starches extend from the inner circumference of the surface to its perimeter. The scratches affect the ability of the surface to prevent liquid from escaping from the perimeter side to the inner-circumference side of the SiC ring, and they can increase the friction force of the finished surface as a sliding material. We propose concentric mutual lapping to remove the scratches caused by conventional lapping. The surface topography of a SiC ring is evaluated quantitatively using a white light interferometer, and a vector and quantitative analysis of the surface profile is proposed. The results show that concentric mutual lapping can quickly remove the scratches caused by conventional lapping, and subsequently, circumferential scratches are generated along the lapping direction. We have also conducted sliding tests to analyze the effect of surface topography on the surface function as a sliding material. The results reveal that concentric mutual lapping suppresses the differences in surface function that occurred depending on the sliding direction.

The industrial demand for wettability control has been increasing because wettability is a key factor for achieving novel anti-contaminant surfaces and related products. In this paper, the potential of angled fine particle peening (angled-FPP) was explored as a method of surface modification to control wettability. Angled-FPP, which is an abrasive jet machining process conducted using a peening nozzle set at an angle inclined to the material surface, is a potential texturing technique. With it, it is possible to create periodically aligned peaks and valleys (“ridges”) on the peened surface. Because control of wettability could possibly be achieved by varying the geometric characteristics of the texture, an attempt was made to vary ridge texture by conducting angled-FPP using three kinds of shot particles: steel, glass, and alumina grit. Thereafter, changes in the wettability owing to creation of a ridge texture on the surfaces were evaluated, with focus on the geometric and morphological features of the ridge texture. The results indicate that the size of the ridges depends on the size and shape of the shot particles, and the angled-FPP conducted using finer angular particles created densely concentrated ridges. The superficial appearance of the ridge texture differed depending on the shot particles used for angled-FPP. The topographies created by angled-FPP using steel particles and alumina grit were “hierarchical” (i.e., the ridge structure was overlapped by finer-scale roughness). In contrast, angled-FPP conducted using glass particles created ridge structure with a quite plain surface. Measurement of the water drop contact angle revealed that the surface became less hydrophilic after creation of the hierarchical topography. It was concluded that the predominant influence on the wettability of the angled-FPP surfaces came from the superficial morphology of the ridge texture.

In this paper, we propose a new plane magnetic abrasive finishing method, applicable to planes, that uses the alternating magnetic fields to solve problems such as the easy deformation and poor recovery of a magnetic brush in conventional magnetic abrasive finishing method. Compared with the magnetic brush used in conventional magnetic abrasive finishing, that in the new method can stably shape a workpiece under an alternating magnetic field. To determine the optimal finishing parameters, we focused on studying the effects of spindle rotational speed, size of diamond particle, and frequency of alternating magnetic field on the finishing surface. Then, according to the obtained optimal finishing parameters, multi-stage finishing experiments were performed with the new method. The results show that surface roughness can be improved from 230 nm Ra to 19 nm Ra in 60 min with the proposed method.

In this work, the tensile characteristics of a 0.2-mm-thick polypropylene (PP) sheet subjected to indentation with virgin and blunt knives (apex angle, α=42^◦; tip thickness, w=6 and 20 μm, respectively), were experimentally investigated. To determine the effect of mechanical condition, such as the notched depth and the profile of the root surface, on the breaking behavior of the half-cut PP specimen, the tensile testing of the half-cut specimen was carried out by varying the indentation depth and tensile velocity. By the experiments, the breakage behavior of the scored (half-cut) zone was determined by varying the indentation depth, tip thickness of the blade, and elongation rate. A kind of crazing or cracking by the blunt knife decreased the tensile resistance and burr elongation for an indentation depth larger than 0.9, whereas the work hardening by the blunt knife increased these properties for an indentation depth less than 0.8. When a blunt knife was used at a high elongation rate larger than 0.01 s^-1, the half-cut zone of the PP sheet exhibited brittle fracture, i.e., the tensile resistance and burr elongation decreased.

During the production process, regular maintenance is necessary and important to maintain high efficiency, because machines inevitably fail with increasing use. However, certain tasks are often neglected due to time and budget constraints, and other factors. In this regard, we propose the unrelated parallel-machine scheduling problem with maintenance and rejection penalties, wherein the ultimate objective is to minimize total cost while identifying the optimal maintenance frequencies, optimal maintenance positions, set of rejected jobs, and optimal scheduled job sequence. Considering resource constraints, the maintenance cost is controlled by the upper bound of the total maintenance frequency. Based on these factors, the optimal polynomial-time solution and its computational complexity with a fixed number of machines are presented. As an illustrative example, it was determined that the scheduling method proposed in this report is effective and practical.

Welding is an essential technology for joining metal plates. In general, gas metal arc welding (GMAW) generates a large amount of fumes in the welding of thick metal plates. In contrast, the butt joining of thick metal plates can be achieved using plasma arc welding (PAW) with a lower amount of fumes. Further, the improvement of the welding environment is critical in welding. In particular, if there are gaps between the base metals, the welding conditions are adjusted based on the gap. A visual sensor, such as a complementary metal-oxide-semiconductor (CMOS) camera, is useful for observing the welding situation. In this study, such a camera was attached to a plasma torch. During welding, we obtained weld pool images using the camera and detected the gaps by processing the images. As the arc light is very intense, it is difficult to obtain a clear image of the weld pool in PAW. In conventional welding, a constant current is used; however, pulsed welding current is used herein to obtain a clear image. The frequency of the current is 20 Hz, which indicates that the interval time is 50 ms. Moreover, the welding current was reduced to 30 A to minimize the effect of the intense arc light while the shutter of the CMOS camera was opened. The exposure time of the CMOS camera is 1 ms. Furthermore, gaps can be detected through image processing. It is necessary to identify the base metals with or without a gap. It was observed that the gap is darker than the solid area of the base metal. Moreover, a gap can be detected through the binarization method. The center area is not dark in the image of the weld pool without the gap. As the image of the weld pool is uneven without a gap, the binarization method can provide a detection result with some errors. Hence, it is challenging to identify whether there is a gap. A convolutional neural network (CNN) is useful for analyzing images. Thus, we applied a CNN to the weld pool image. If the gap is identified using the CNN, the binarization method is used to obtain the gap width. Hence, in PAW, welding conditions are adjusted based on the gap.

Environmental sound recognition (ESR) refers to the recognition of all sounds other than the human voice or musical sounds. Typical ESR methods utilize spectral information and variation within it with respect to time. However, in the case of transient sounds, spectral information is insufficient because only an average quantity of a given signal within a time period can be recognized. In this study, the waveform of sound signals and their spectrum were analyzed visually to extract temporal characteristics of the sound more directly. Based on the observations, features such as the initial rise time, duration, and smoothness of the sound signal; the distribution and smoothness of the spectrum; the clarity of the sustaining sound components; and the number and interval of collisions in chattering were proposed. Experimental feature values were obtained for eight transient environmental sounds, and the distributions of the values were evaluated. A recognition experiment was conducted on 11 transient sounds. The Mel-frequency cepstral coefficient (MFCC) was selected as reference. A support vector machine was adopted as the classification algorithm. The recognition rates obtained from the MFCC were below 50% for five of the 11 sounds, and the overall recognition rate was 69%. In contrast, the recognition rates obtained using the proposed features were above 50% for all sounds, and the overall rate was 86%.

Transfer printing of a thin film is a great candidate technique for micro/nanofabrication for microelectromechanical system (MEMS) elements. The authors propose a technique to apply atomic diffusion bonding to transfer printing of a gold (Au) thin film. When a substrate is previously coated with Au thin film as an adhesive, another Au thin film can be transfer-printed from a h-PDMS stamp to the substrate. It enables 50 μm-wide line patterns of the Au thin film located on the Au-coated Si substrate, whereas the Au thin film cannot be transfer-printed on a bare (uncoated) Si surface. The interface between two Au thin films disappears after transfer printing; hence, the Au atoms can interdiffuse from one to another to make a strong bonding. This process can be performed with a soft contact without any pressure in atmospheric and vacuum conditions. In the case of Au, the atoms can interdiffuse around a contacted area at room temperature. Moreover, one can make 50 μm-wide line patterns by 1 min of transfer printing and that of 24 h. The proposed process makes the line patterns of the Au thin film transfer-printed to be a bridged microbeam over the grooves when a prestructured (grooved) substrate is prepared.

Hybrid structures of single-crystal silicon and high-density polyethylene (HDPE) with high transmittance in the mid-to-far infrared region are used as infrared lens substrates. The hybrids are usually fabricated by high-precision press molding. The Si-HDPE hybrid lens previously fabricated had a low transmittance in the 9–10 μm wavelength region, thereby limiting its application for human body detection. In this study, a Si-polymer hybrid structure was fabricated using a new polymer without any silane coupling agent. Interfacial adhesion between the polymer and the Si substrate was realized with an extremely thin (a few micron thick) layer of an interfacial silane coupling agent. The press molding conditions that led to improved bonding strength and infrared transmittance of the hybrid substrate were investigated. A transmittance similar to that of a single-crystal Si substrate was achieved in the 9–10 μm wavelength range.

To realize autonomous machining, it is necessary to focus on machining tools and also on the automation of process planning in the preparation stage. This study proposes a process planning system that automatically defines the machining region and determines the machining sequence. Although previous studies have explored computer-aided process planning, only a few have considered geometric tolerances. Geometric tolerances are indicated on product drawings to eliminate their ambiguity and manage machining quality. Geometric dimensioning and tolerancing (GD&T) is a geometric tolerance standard applied to a three-dimensional computer-aided design (3D CAD) model and are expected to be used for the digitization of manufacturing. Therefore, this study developed an automated process planning system by using GD&T as a sequencing constraint. In the proposed system, the machining sequence is automatically determined by the geometrical constraints, which indicate whether the tool can approach, and GD&T, which indicates the geometric tolerance and datum in a 3D CAD model. A case study validated the proposed method of automated process planning constrained by GD&T. The result shows that the proposed system can automatically determine the machining sequence according to the geometric tolerance in a 3D CAD model.