Precision surface finishing plays an important role in product quality owing to its direct effects on product appearance. As a result, automated precision surface finishing processes (APSFPs) are key technologies for industrial products and molds for forming and shaping processes. APSFPs can be divided into three main categories, namely, mechanical processes, electrochemical processes, and high energy beam processes. The objective of this special issue is to collect the cutting-edge research works focused on APSFPs. This issue includes 11 papers on APSFPs covering the following topics:

- Review of ultraprecision surface finishing processes.

- Ultraprecision surface machining and finishing with compensated feeding mechanisms.

- Ultrasonic assisted cutting of unidirectional wetting surfaces and polishing of mold steels.

- Vibration-assisted polishing of glass lenses.

- Magnetic-assisted polishing of mirror surfaces.

- Chemical-mechanical polishing of single-crystal SiC and GaN wafers.

- Direct transfer of smoothing Au surfaces.

- Plasma surface finishing of narrow channel walls of X-ray crystal monochromators.

- Analysis and characterization of finished surfaces.

It is expected that this issue will be helpful for readers to understand the recent developments in APSFPs and will lead to further research on APSFPs.

We deeply appreciate the contributions of all authors and thank the reviewers for their incisive efforts.

Surfaces of different complex shapes are aspirated part of many scientific measuring devices, medical, astronomical, and other precision activity utilizations. Components at miniaturized level should meet required surface roughness for the intended applications. Surface finishing of freeform and miniaturized components are always difficult and need to look for a new way out. In this study, an attempt was made to improve surfaces roughness of selected, most frequently used, engineering materials using different innovative processes, which can be integrated with CNC machine centers. An advanced automated surface finishing tools such as ball burnishing embedded with load cell, vibration assisted polishing, and self-propelled abrasive multi-jet polishing tools are proposed. Ball burnishing is advantageous for pre-machining process of ball polishing. Using the polishing device embedded with load cell, the constant force polishing is achieved. To reduce the volumetric wear of a polishing ball, vibration assisted polishing device is also integrated. Moreover, self-propelled abrasive multi-jet polishing tool, which achieves 93.33% improvement of surface roughness for lapped optical glass of BK7 has been subjugated from Ra 0.300 μm to 0.020 μm. These tools can be miniaturized and applicable in small micro CNC machining centers.

Waviness tends to be generated on cut surfaces even when an ultraprecision milling machine with a single-crystal diamond tool is used. The present study deals with the reduction of waviness by controlling the feeding mechanisms of the milling machine. A machining experiment on a spherical surface of a mirror element in a mirror array showed that the machined surface exhibited periodic waviness with a height of 30 nm and a wavelength of 300 μm. To investigate the reason for such waviness, a slope was machined under simultaneous multiaxis motion control of the feeding mechanisms of the milling machine. This proved that the interpolation errors of the encoders used in the milling machine produce the waviness on the machined surface when machining is carried out under simultaneous multiaxis motion control. To reduce such interpolation errors, the positioning accuracy of the machine stages was measured using a laser interferometer. On the basis of the measured results, the feeding mechanisms were compensated such that the positioning errors including the interpolation errors were corrected. Using the machine with the compensated feeding system, a mirror element was shaped. Consequently, waviness was reduced and the surface smoothness was less than 10 nm, demonstrating that such compensation can produce superior optical surfaces.

Surface microstructures can provide various functionalities, and wettability is a typical surface property that can be controlled by surface textures. Unidirectional wetting properties (UWPs) have been garnering attention as a useful wetting function for industrial functions. Thus, in this study, developing UWPs using surface microstructures has been tested. First, UWPs were calculated with the thermodynamic analysis of contact angle (CA). The analytical results predicted that an increased oblique angle of the microstructures, ω₂, can increase the advancing CA; the receding CAs could not be calculated, and might exhibit the pinning effect. Ultrasonic-assisted cutting was subsequently employed to fabricate hierarchical microstructures for providing UWPs to a workpiece. Although many burrs have been observed on the edges of the structures, microstructures with different oblique angles, ω₂=5^◦, 10°, and 15°, were fabricated in the designed scales. Finally, the UWPs were verified by measuring the CAs and sliding angles (SAs). The anisotropy of CA hysteresis was indicated in each oblique angle structure, and the anisotropy of SAs was confirmed when ω₂=10^◦ and 15°. The retention force ratio of a droplet, r, which indicates the UWPs, was subsequently estimated with two different approaches, and both approaches led a similar value of the attrition rates of r from ω₂=10^◦ to 15°.

High-quality die and mold production is becoming increasingly important in modern mass production. Surface quality is one of the most frequent and stringent customer specifications for machined parts, of which the major consideration and indication of quality is their surface roughness. In this study, a novel ball-ended polishing tool made of polyurethane impregnated with micro cubic boron nitride (CBN) was developed. The polishing tool was mounted on a three-axis machining center; the rotary polishing action was achieved via ultrasonics. Polishing experiments were conducted on specimens of hardened Stavax stainless mold steel. Four types of polishing tools – containing 10 wt% of pure CBN particles and Al₂O₃ coated CBN with two different degrees of hardness (Shore 25 and 45) – were fabricated; the results of the experiments were compared with those obtained using a traditional elastic-ball polishing tool. It was found that the surface quality achieved using the Al₂O₃ coated CBN tool was superior to that with pure CBN particles because the hardness of the polishing tool increased with the increase in the surface roughness of the workpiece. In addition, ultrasonic-assisted polishing yielded a better surface finish.

In recent years, owing to the advent of mobile phones, product miniaturization and multifunctionalization have rapidly progressed. However, the large-sized machine tools for the manufacture of small products waste a considerable amount of space and power. The present study aimed at applying a magnetic-polishing method using a ball-end mill-type tool to examine the optimum processing conditions. This was done to apply a mirror finish for the integration of the cutting and polishing processes by using the small machine tool. The magnetic-polishing effect was evaluated from the point of view of the polishing amount, surface roughness, specimen shape, and mirror-surface condition. In addition, the movement of the paste during polishing was observed through images obtained through a high-speed camera. The movement of the paste is considered for effective polishing and other cases. Accordingly, various magnetic-polishing techniques were used for irregularities and step shapes. Various conditions were also examined, and a stable condition was determined. The results reveal that the amount of polishing paste significantly influences the polishing movement. In addition, a sufficient polishing effect could be obtained by duplicating the polishing course by using a sine wave course.

The demand for improving the image quality of cameras has increased significantly, especially in industrial fields such as broadcasting, on-vehicle, security, factory automation, and medicine. The surface of glass lenses as a key component of cameras is formed and finished by polishing using small tools. The existing polishing technologies, however, exhibit serious problems including an unstable material removal rate over time. In our previous work, the mechanism of time variation in material removal rate was clarified. Based on the findings, a vibration-assisted polishing method using polishing pads containing titanium dioxide particles was developed for improving the stability of the material removal rate with the accumulated polishing time. Our experiments revealed that the proposed polishing method suppressed the time variation significantly in the material removal rate. The developed polishing pads, however, possessed a short life because of their poor wear resistance; as such, they could not be applied to the mass-production process of lenses. In this study, we applied the vibration-assisted polishing method to the polishing process using commercial polishing pads that exhibit sufficient wear resistance for practical use. To investigate the effect of vibration on the stability of the material removal rate, polishing experiments and the observation of slurry flow on the surface of the polishing pads during the vibration-assisted polishing process were conducted. Based on the findings, a new polishing method utilizing a large-amplitude high-frequency vibration applied to the polishing pressure was developed. In addition, a new polishing method utilizing the overhang of a polishing pad, where the polishing pad was moved to hang over the edge of the workpiece for incorporating periodic dressing processes of the polishing pad surface during the polishing process, was also developed. Our polishing experiments revealed that both the proposed polishing methods improved the stability of the material removal rate significantly over the course of the polishing process.

To remove the microroughness and subsurface damage on the SiC and GaN surface efficiently, a surface finishing technique using a magnetic tool holding iron particles in a hydrogen peroxide solution is developed. This technique utilizes OH radicals generated from the iron catalytic particles in a hydrogen peroxide solution, and can be used to preferentially remove the topmost convex part on the surface, resulting in an atomically smooth surface. We employed this polishing technique to finish the surfaces of 2-inch SiC and 2-inch GaN wafers. The surface roughness before and after finishing was measured by scanning white light interferometric microscopy and atomic force microscopy. In addition, the material removal rate was calculated by weight loss due to the finishing process. The results show that the surface roughness on the SiC and GaN wafers is markedly improved. Moreover, the surface waviness and flatness of these wafers before and after finishing did not deteriorate. Atomic force microscope images indicate that an atomically flat SiC surface with a roughness value below 0.1 nm RMS and a GaN surface with atomic step and terrace structures were achieved. Our proposed finishing technique is effective in improving the surface microroughness of SiC and GaN wafers.

We conducted a polishing test to clarify the change in polishing characteristics resulting from the wear of a pyramidal-structured polishing tool, and discuss the polishing mechanism unique to the pyramidal-structured polishing pad. When the pyramidal-structured polishing pad is used for polishing, there exists an initial polishing stage in which the removal rate is high but the finished surface is rough; followed by a steady-state polishing stage in which the wear rate is low, removal rate is stable, and a high-quality finish is obtained. The true polishing pressure is constant in the steady-state polishing stage regardless of the nominal polishing pressure, although it differs with workpiece hardness. Polishing was carried out using the pyramidal-structured polishing pad containing 6 μm alumina abrasive grains for 90 min without any scorching or clogging. This resulted in finished surfaces of 0.4 μm Rz and 0.07 μm Rz for oxygen-free copper and S45C, respectively.

Channel-cut Si crystals are useful optical devices for providing monochromatic X-ray beams with extreme angular stability. Owing to difficulties in the high-precision surface finishing of narrow-channel inner walls of the crystals, typical channel-cut crystals have considerable residual subsurface crystal damage and/or roughness on their channel-wall reflection surfaces that decrease intensity and distort the wavefronts of the reflected X-rays. This paper proposes a high-precision surface finishing method for the narrow-channel inner walls based on plasma chemical vaporization machining, which is a local etching technique using atmospheric-pressure plasma. Cylinder- and nozzle-shaped electrodes were designed for channel widths of more than 5 and 3 mm, respectively. We optimized process conditions for each electrode using commercial Si wafers, and obtained a removal depth of 10 μm with a surface flatness and roughness of less than 1 μm and 1 nmRMS, respectively, which should allow the damaged layers to be fully removed while maintaining the wavefront of coherent X-rays.

Direct transfer of Au films deposited on smooth SiO₂ film with RMS (root mean square) surface roughness of 0.24 nm was investigated with the aim of generating smooth Au surfaces. Deposited Au films with different thicknesses were transferred to rough Au surfaces on target substrates at room temperature with a contact pressure of 50 MPa. Observation of the growth behavior of the deposited films revealed that they formed a continuous structure when their nominal film thickness was around 15 nm or above. The transfer of continuous Au films with a thickness of 20, 51, or 102 nm reduced the RMS roughness of the rough Au surfaces from 1.6 nm to 0.4 nm. In contrast, the transfer of Au films with a thickness less than 5 nm increased their surface roughness. This direct transfer technique should thus be useful for low temperature bonding.

The generation of fine machined surfaces with high gloss is an important topic in mould manufacturing. The surface gloss can be characterized by means of scattered light sensors and a representative parameter such as Aq. In this paper, in-line measurements of scattered light distribution are compared with roughness parameters calculated using a confocal microscope, in order to assess surface aesthetic quality. Several surfaces have been machined by means of high precision milling, producing different surface topographies. Surface characterization has been performed on a machine using a scattered light sensor, and using a confocal microscope in laboratory conditions. The calculated Aq parameter is compared with the amplitude roughness parameters Sa and Sq, and with hybrid parameters Sdq and Rdq representing the average slope of the surface features. Scanning electron microscope (SEM) images are used as visual benchmarks to identify the parameters’ correlation with the visual appearance. A different linear trend of the relationship between Aq, Rdq, and Sdq is observed. The description of the surface quality through Sa or Sq instead is found to be insufficient. This is explained by means of SEM pictures showing a dramatic influence of the smeared material over the machined surface.

Improving the walking functions of hemiplegia patients after a stroke or brain injury is an important rehabilitation challenge. Recently, walking assist robots have been introduced in advanced rehabilitation facilities as a way to improve the efficiency of patient rehabilitation and restore their walking functions. Expert therapists can apply this device on different patients; however, such application mainly depends on the therapist’s tacit knowledge. Thus, it is often harder for novice therapists to apply such devices on different types of patients. Consequently, effective use of a walking assist robot has become a new patient rehabilitation skill. Taking rehabilitation as a service provided by medical doctors or therapists to their patients, this study aims to improve the quality of the rehabilitation service. In particular, the objective of this study is to abstract the rehabilitation skill of expert therapists in using a walking assist robot by applying a service science methodology known as skill education. Skill abstraction was performed by interviewing an expert therapist. From this interview, it was found that the expert therapist classified hemiplegia patients into four different classes. Using videos of patients walking, further analysis revealed the expert’s tacit knowledge, which was indicated by differences observed among these four groups in particular phases of the patients’ walking patterns. This study shows that by successfully obtaining explicit knowledge of part of a rehabilitation skill by using a walking assist robot (which until now was a tacit knowledge of experts), and then organizing the acquired explicit knowledge, even non-experts can easily reproduce the skill of experts in new patient rehabilitation.

Three-axis ball end mills are used for the finishing of metal molds of complicated curved surfaces. Typically, a tool path of this shape machining is derived from the geometric calculations of a tool used, and a product model that is a computer aided design (CAD)-based polyhedron approximating the shape. The polyhedron is more complicated to approximate a shape with more curved surfaces, as it is highly time consuming. To solve this problem, methods to accelerate geometric calculations using a computer graphics drawing processing mechanism were proposed. However, these methods cannot guard against errors arising from the approximation of an inverse offset shape using a set of polygons. In the present study, we propose a method to generate tool paths accurately based on calculating the crossing points of the tool axis and defining the offset surface as a set of polygons, cylindrical surfaces, and spherical surfaces. With this method, it is expected that the height of an area, which was divided by fine polygons in previous methods, can be derived accurately, and a tool path can be generated with high precision.

Nanofocus X-ray projection imaging technology with a resolution of 0.25 μm has been developed and applied to the estimation of the profile of through-silicon vias (TSVs) several microns in diameter. However, analysis and examination of the uncertainty of the system and the calibration method for measurement have not been properly discussed thus far. These topics should be discussed in consideration of the actual application of the method to the automation of inline inspection and the measurement processes of TSV devices. This study focuses on the quantitative analysis of the uncertainty budget in the measurement of the whole X-ray microscope system. A calibration method using a known, conventionally defined TSV sample as a calibration device is employed. The uncertainties are divided into calibration, mechanical, electrical, and algorithmic factors, and their contributions to the combined standard uncertainty and the expanded uncertainty are estimated. An actual case for the analysis of the uncertainty budget is evaluated, where the profile is estimated for actual images with a signal-to-noise ratio of 2.2.

In this study, a novel design and fabrication method that corresponds to simple and optimized press molding for carbon fiber reinforced plastics (CFRP) is proposed based on CAD data. Specifically, in recent years, CFRP has been widely used for weight reduction of transportation equipment. However, optimization of the production process is required to expand the range of applications of CFRP. To satisfy the aforementioned requirements, this study focused on the press molding technique. It was assumed that partial excessive or partial breakage of the fiber occurs due to the drawing of the fiber by the deformation force. A design and fabrication method was proposed for CFRP preform that exhibits the unfolded diagram shape of an objective three-dimensional (3D) model by using a tow prepreg as a solution for the aforementioned problems. A calculation method to generate the unfolded diagram was also proposed. Furthermore, the validity of the unfolded diagram was confirmed by reproducing the diagram for a 3D shape.

The paper presents a smart modular architecture that allows for a traditional production plant to be transformed into a supervised, fully integrated, and monitored system. The proposed approach is based on using commercial devices to create an industrial Internet of Things network connected to PLCs, CNCs, and sensors on every single plant machinery. The novelty is in the smart architecture, software implementation of the supervision, and monitoring. A base software code, easily adaptable to different applications, allows for highly innovative man-machine interfaces to be created. Therefore, the monitoring of the embedded systems can be obtained through multiple remotable interfaces. The effectiveness of the proposed methodology is demonstrated experimentally on a case study during a mechatronics industry exhibition.