The demand for high-precision and high-efficient machining of high-performance materials and components has increased in various industries such as optical, automotive, communication, life sciences, and medical sciences. Certain difficult-to-machine materials can be reliably machined using deterministic precision cutting processes. However, hard and brittle materials such as ceramics, carbides, hardened steel molds, glassy materials, or semiconductor materials, require precision abrasive technologies with super abrasives like diamond, cBN, or new tool materials for machining. However, machining high-precision components and their molds/dies using abrasive processes is considerably more difficult due to their complex and non-deterministic nature and textured surfaces. Furthermore, high-energy processes such as laser technology can assist abrasive technologies in ensuring higher precision and efficiency. Precision grinding and polishing processes are primarily used to generate high-quality and functional components, typically made of difficult-to-machine materials. Thus, the surface quality achievable by precision grinding and polishing processes becomes more important for reducing machining time and costs.

This special issue features 10 papers on the most recent advances in precision abrasive technologies. These papers cover the following topics:

- Ultrasonic grinding of micro holes using cemented WC tools

- Drilling holes in CFRP aircraft using cBN electroplated ball end mill

- In situ evaluation of drill wear using tool images

- Grinding belt based on modified information entropy

- Radial directional vibration-assisted grinding of Al₂O₃ ceramics

- Chatter vibration suppression using fixed superabrasive polishing stone

- Free abrasive finishing of internal channels with different cross-sectional geometries

- High-quality machining of cemented carbide using PCD ball end mills

- Fixed-abrasive machining with magnetic brush for Ti-6Al-4V ELI alloy

- High-efficient polishing of polymer surfaces using catalyst-referred etching

This issue will provide an understanding of the recent developments in abrasive technologies, leading to further research.

We deeply appreciate the careful work of all the authors, and we thank the reviewers for their incisive efforts.

Although grinding is a widely employed method for hard and brittle materials, drilling microholes requires the use of an ultrasmall-diameter grinding wheel that is difficult to fabricate and breaks easily when grinding force is applied, resulting in a high tool cost. To solve this problem, cemented WC micropins were fabricated by electrical discharge machining, and microholes were drilled using them as micro-grinding tools with the assistance of ultrasonic oscillation. The micropin tools can be employed in grinding because the convex parts of the electrical discharge craters formed on their surfaces serve as cutting edges of abrasive grains in grinding wheels. To clarify the drilling conditions necessary for ultrasmall-diameter tools to drill holes with a diameter less than 5 µm, the relationships between drilling conditions and drilling characteristics were investigated. The drilling conditions included the tool rotation speed, tool feed speed, ultrasonic oscillation amplitude, and use of grinding fluid. The drilling characteristics included the hole diameter and grinding force evolution. The investigation showed that a high tool rotation speed prevented the increase in the grinding force because of the faster grinding speed. A low tool feed speed was favorable to lower grinding force. At a high tool feed speed, most likely, the tool was fed before the workpiece material was sufficiently removed, thereby exhibiting a large grinding force. The ultrasonic oscillation amplitude had no noticeable effect on the grinding force. The hole diameter was not affected by the tool rotation speed, tool feed speed, or ultrasonic oscillation amplitude. Although the use of grinding fluid reduced the grinding force, the hole diameter increased, probably because enhanced lubrication caused the tool to slip on the workpiece surface at the beginning of drilling, resulting in an increased rotational runout. Under the favorable conditions obtained, a hole of 3.2 µm in diameter was successfully drilled in crown glass.

Carbon fiber reinforced plastic (CFRP) is a lightweight material with exceptional mechanical properties such as high specific strength, high specific modulus, and retained fatigue strength. It exhibits outstanding characteristics derived from its carbon content such as electrical conductivity, low thermal expansion, chemical stability, and high thermal conductivity. These unique features make CFRP a highly versatile material. It can be extensively used across various industries, offering advantages over steel, aluminum, and glass fiber reinforced plastic. Moreover, its anisotropic nature allows for innovative design possibilities, providing different mechanical properties for different fiber orientations. The increasing demand for CFRPs, particularly in the aerospace and automotive industries, is attributed to their high reliability and design flexibility. Consequently, the requirement for efficient and high-quality CFRP processing techniques has led to numerous studies focusing on trimming and hole drilling of CFRP parts. Previous research has also highlighted the significant impact of processing temperature on the quality of CFRP and other fiber reinforced plastics, such as aramid fiber reinforced plastic. However, many existing reports are limited to specific processes such as trimming or hole drilling, without addressing broader concerns such as tool wear, burrs, fiber damage owing to heat, or the lack of multi-purpose cutting tools suitable for CFRP when considering tool costs. In addition, the aerospace industry demands precise hole drilling for thousands of holes, facilitating assembly with rivets or screws; this requires high-precision hole drilling processes. To address CFRP hole drilling challenges, this study proposes and develops a cBN electroplated ball end mill to enable an efficient and high-quality hole drilling in CFRPs. As machining demands evolve with diverse workpiece materials, technological innovations are continuously being sought in hole drilling processes, exploring alternatives beyond conventional drilling such as employing end mills and enhancing tool functionality. In this study, we employed a ball end mill and helical interpolation motion to tackle CFRP hole drilling. The delamination occurring at the exit side of the drilled holes was investigated using strain gauges. Additionally, finite element analysis was employed to compare and analyze experimental results, leading to guidelines for an efficient and high-quality hole-drilling approach that balances productivity and workpiece integrity. We achieved high-efficiency hole drilling while maintaining the quality by adjusting the cutting parameters under conditions that prevent delamination. The proposed cBN electroplated ball end mill offers promising potential for advancing CFRP processing methods, addressing the growing demand for this exceptional material in various applications.

Owing to the rise in demand for electric devices, there has been an increase in the need for manufacturing equipment that produces internal control board parts. To operate this machinery, several ceramic components, such as a chuck table and fastening parts, are required. Consequently, the need for efficiently and precisely machining ceramics has increased. However, ceramics are known for their high hardness, which can lead to tool breakage when using a small tool. This is often influenced by the state of the tool wear. If the drill tip breaks off and becomes embedded in the workpiece, it could take time to remove or destroy the workpiece. To prevent such problems, drills are replaced after a certain number of machining processes, or the operator visually inspects the drill’s wear condition. Unfortunately, these methods reduce machining efficiency. Therefore, we propose a device that captures drill images on a machine tool and measures the amount of drill wear to evaluate the drill’s condition. We fabricated a device to acquire drill images and attempted to quantify the drill wear condition, such as the area and width of the worn part, by analyzing the worn shape from an image of the bottom surface of the drill.

Recent studies have shown that the cutting edge spacing and density of abrasive grains on the grinding tool surface affect the accuracy and tool life of grinding tools. However, studies on the effects of dispersion on the abrasive grain distribution have not yet been conducted. In this study, it was shown that the machining ability of a tool can be evaluated and tool life can be determined using an index of normalized information entropy for abrasive grain dispersion. However, it was difficult to continuously obtain transfer images with different loadings during the measurement of the abrasive grain distribution. Additionally, it has been revealed that in entropy evaluation, the evaluation values may remain the same even when the distribution state changes. Therefore, in this study, a device was developed to obtain a transferred image by continuously changing the loading conditions. We also examine the change in the number of divisions in the evaluation region to modify the entropy evaluation. To demonstrate the effectiveness of the proposed method, models with varying abrasive grain numbers and distributions were prepared, and the abrasive grain distributions were evaluated. After studying the entropy evaluation by simulation, an evaluation of a grinding belt in a processing experiment was conducted. It was demonstrated that evaluation using information entropy is possible in all cases by employing a method that decreases the number of divisions in the evaluation region.

This paper proposes a machining method that uses ultrasonic vibration in the radial direction of the grinding wheel. This method is expected to suppress machining heat because the wheel and workpiece are in intermittent contact with each other. The abrasive grains on the working surface of the wheel act dynamically in the direction of cutting into the workpiece. In this paper, the constant-load grinding of Al₂O₃ ceramics, a hard and brittle material, was performed. Ductile-mode surfaces were more easily obtained when vibration support was used. The standard deviation of the brightness distribution of the ground surface can be used to evaluate the ductile-mode surface. In this study, the value was less than 16. The results of the measurement of tangential/normal grinding force ratio and the rear-surface temperature of the workpiece confirmed that the values were higher when ductile-mode machining was performed, compared with brittle-mode machining. Furthermore, a vibration-assisted removal model was used to discuss the results of the ductile-mode surfaces, which were more easily obtained when vibration-assisted machining was used.

In this study, an existing superfinishing method used for polishing glass surfaces was refined using a five-joint closed-link compact robot with fixed abrasive grains. In previous studies, a voice coil motor was used to control the constant-pressure pressing force while maintaining the polishing force for a relatively short period. However, maintaining the polishing force for long periods is imperative for achieving a high-quality polished surface. Thus, in this study, a strain gauge load cell was adopted in addition to a conventional piezoelectric force sensor to maintain the polishing force for a long period. First, the amounts of DC drift of the piezoelectric force sensor and strain gauge type load cell were compared to confirm the necessity of signal processing as well as the compatibility of long-term force measurement and high-frequency vibration measurement by application. Further, a method was proposed in which the change in the pressing force was recorded from the connected sensor; when the pressing force fluctuated, chatter vibrations were determined to occur, and the pressing force was temporarily set to 0 N. This method could obtain a better polished surface than the proportional-integral-differential (PID) control, which simply controls the pressing force at a constant value. Finally, chatter vibrations could be determined by detecting high-frequency sounds using a sound level meter. Notably, a finely polished surface could be obtained.

This study investigates the finishing characteristics of internal channels with different cross-sectional geometries using free abrasive grains and evaluates the cooling performance of these channels before and after finishing. Three types of channels with circular, triangular, and hexagram cross-sections were designed and fabricated using laser powder bed fusion (L-PBF). A fluid flow in the channel was evaluated using computational fluid dynamics simulations, and the finishing characteristics and cooling performances of the channels were experimentally investigated. The results indicated that the use of free abrasive grains enabled the improvement in the surface quality as well as the cooling performance of the channel. The cross-section of the channel affected the fluid flow in the channel and finishing progress. The initial surface roughness varied with the cross-section of the channel owing to the limitations of L-PBF, and the triangular section had a relatively uniform surface quality throughout the channel compared with the other cross-sections. The cooling time decreased with the surface area of the channel. To obtain the uniform surface quality, the application of a suitable cross-section is needed for the finishing process. The outcomes of this study demonstrate that a triangular-section channel is suitable for improving both surface quality and cooling performance.

In this study, cemented carbide was machined to a high quality using polycrystalline diamond (PCD) ball end mills characterized by various surface textures. The effect of the surface texture of the tools on the machining characteristics was studied using two types of PCD tools featuring abrasive diamond grains at various protrusion heights. In addition, a single crystal diamond tool with the same shape as that of the PCD tool was fabricated, and the experiment was repeated to study the differences in machining characteristics. The polished PCD tool yielded a high-quality machined surface with an average surface roughness of 1 nm. The polished PCD tool yielded superior sample surface roughness compared to the PCD tool for feed rates of 10–500 mm/min. The use of a polished PCD tool enables the efficient elimination of material through plastic flow, leading to the attainment of a high-quality machined surface while preventing the adhesion of materials on the tool surface. A single crystal diamond tool can also be used for machining cemented carbide within a feed rate range of 10–200 mm/min; however, its performance is inferior to that of a polished PCD tool. Experiments confirmed that the polished PCD tool was the most effective among the tested tools for the precision machining of cemented carbide.

Focusing on the gap between a fixed-abrasive tool and workpiece, the machining characteristics of the fixed-abrasive machining of a Ti-6Al-4V extra low interstitial (ELI) alloy with magnetic brush were evaluated, which removed material through the combined actions of the fixed abrasive and magnetic brush. Machining experiments demonstrated that the material removal was mainly performed by the fixed abrasive, while the magnetic brush removed the swell out residuals associated with this removal. As a result, it was found that fixed-abrasive machining with a magnetic brush was capable of reducing the finished-surface roughness to 50% or less compared to fixed-abrasive machining alone, although the removal depth was also decreased. This proved that fixed-abrasive machining with a magnetic brush is useful for machining materials whose removal involves plastic deformations.

Previously, we developed an abrasive-free polishing technique called catalyst-referred etching (CARE) for inorganic materials. In this method, the topmost site of the workpiece surface is preferentially removed via an indirect hydrolysis reaction promoted by a metal catalyst. In this study, we proposed applying the CARE method to polymer material polishing and demonstrated the polishing characteristics. Using the CARE method, polycarbonate, which has an easy cleavage of ester bond via hydrolysis, was polished, resulting in the smoothness of the surface roughness below 1.0 nm. Based on the surface observations, the removal mechanism was estimated as follows. Molecule chains are entangled to form clusters constituting the polymer surface and help determine the surface roughness. In the CARE method, the top of this cluster was selectively removed, thus creating a smooth surface. Polymers with C–C bonds, such as polymethyl methacrylate and fluorinated ethylene propylene, were also smoothed using the CARE method. These results indicate that the CARE method is highly effective in polishing polymer materials.

This paper presents the design and construction of a new optical head with two phase-shifted interference signals in an absolute surface encoder by using a mode-locked femtosecond laser. A series of discrete absolute positions of the scale grating is obtained from a series of peak wavelengths of the spectrum of the +1st- or -1st-order diffracted beam. The two beams at a specific wavelength λ_i interfere with each other to generate an incremental interference signal for high-resolution displacement measurement over a small interpolation range around the corresponding discrete absolute position x_i. In the previous design of the optical head, the two beams were guided by optical fibers into a fiber coupler for the interference. This fiber optics design was simple and stable but could not identify the moving direction of small displacement within each interpolation range because only one interferential signal could be generated. The aim of this study is to develop a new design of the optical head, where two interference signals with a phase difference of π/2 are generated. For this purpose, free-space optics, instead of fiber optics, is adopted in the new optical head. Experiments are conducted to confirm the generation of the two phase-shifted interference signals. A Lissajous figure is plotted to verify the phase difference between the two signals.

Air cylinders are actuators that slide a piston inside cylinders by applying air pressure. We propose an air-cylinder-type artificial muscle that can be flexibly bent by using a flexible tube for the cylindrical part. The actuator output was a string connected to a piston. When the air-cylinder-type artificial muscle bends, the inner wall of the tube and the string come into contact, causing output fluctuations owing to friction. In this study, we investigated the output when an artificial muscle was bent. After describing the structure of the air-cylinder-type artificial muscle, the measurement results of the resistance force at each part of the actuator are presented. A theoretical output inspired by the capstan equation was derived, and its validity was verified by comparison with experimental results.

Wire electrical discharge machining (WEDM) is a process that removes material from conductive workpieces by using sequential electrical discharges. The morphology of the craters formed by these discharges is influenced by various process parameters and affects the quality and efficiency of the machining. To understand and optimize the WEDM process, it is essential to identify and characterize single craters from microscopy images. However, manual labeling of craters is tedious and prone to errors. This paper presents a novel approach to detect and segment single craters using state-of-the-art computer vision techniques. The YOLOv8 model, a convolutional neural network-based object detection technique, is fine-tuned on a custom dataset of WEDM craters to locate and enclose them with tight bounding boxes. The segment anything model, a vision transformer-based instance segmentation technique, is applied to the cropped images of individual craters to delineate their shape and size. Geometric analysis of the segmented craters reveals significant variations in their contour and area depending on the energy setting, while the wire diameter has minimal influence.

Development of a low-invasive microneedle is currently desired in the medical field to mitigate the patients’ stress and pain. We have paid attention to mosquitoes that puncture the skin without giving humans no feelings of pain. We have observed mosquitoes and found that when their proboscis punctures human skin, they make the following three behaviors: apply tension to human skin; rotate their proboscis; vibrate their proboscis. In our previous studies, we developed a bundled set of three microneedle imitating the mosquito’s proboscis and experimentally proved the usefulness of their alternate vibrations, which is one of the mosquito’s puncturing behaviors. However, the setting of three needles with proper clearances from each other was difficult, making their driving system too complex to practically use it. Therefore, we have developed a simplified microneedle by reducing the number of needles from three to two or one. This paper has focused on the effects of the rotations of a single needle. Using our developed microneedle with a diameter of 90 µm and the thinnest commercial microneedle with a diameter of 180 µm, we evaluated the effect of reciprocating rotation, one of the mosquitoes’ puncturing behaviors, by puncture experiments using artificial skin and nonlinear finite element method (FEM) analysis. As a result, it was found that the reciprocating rotation suppresses the puncture resistance force and the skin deflection.

Three-dimensional (3D) models of leaves are expected to contribute to a wide range of applications, including the study of plant morphology and leaf design. Leaf boundaries and veins are key factors in determining leaf shape in both botany and design. This motivated us to design a leaf-shape generator that uses leaf boundaries and veins. We propose an algorithm to reconstruct leaf geometry as a surface mesh generated from point clouds of leaf boundaries and veins. First, it determines the interior region of the leaf using the multi-level partition of unity implicits approach. Then, based on the Gaussian mixture model, it expresses the 3D shape of the leaf, where the values vary depending on the distances from the leaf boundary to veins. The use of differentiable functions for leaf shapes realizes smooth underlying surfaces and enables various shape analyses using differential operations.

With continuous advancements in science and technology, manipulators have been widely used in human–computer interactions and other fields. However, they are limited by their insufficient flexible interaction ability and the inability of the control algorithm to adapt to changeable task scenarios. In this study, the flexible control technology of a manipulator was investigated to overcome these shortcomings and improve the intelligent level of the manipulator. Specifically, the basic principles and related technologies were applied in developing a dynamic model and analyzing the impedance control technology. Next, the effects of different impedance parameters on the system response characteristics were analyzed. Finally, the simulation experiment was conducted. The results showed that within 15–35 s, the manipulator returned to the target trajectory and continued to complete two rotations of circular trajectory, and the position control was accurate. This study demonstrates the feasibility of rheostatic control of fuzzy impedance in improving the compliance strength of the manipulator

The extraction of valuable compounds from moringa plants involves complex processes that are highly dependent on various environmental and operational factors. Monitoring these processes using Internet of Things (IoT)-based multivariate time series data presents a unique opportunity for improving efficiency and quality control. Multivariate time series data, characterized by multiple variables recorded over time, provides valuable insights into the behavior, interactions, and dependencies among different components within a system. However, with the increasing complexity and volume of IoT data generated during moringa extraction, the anomaly detection becomes challenging. The objective of this study is to develop a robust and efficient system capable of automatically detecting anomalous patterns in real time, providing early warning signals to operators, and facilitating timely interventions. This paper proposes a novel hybrid unsupervised anomaly detection model combining density-based spatial clustering of applications with noise and k-nearest neighbors for IoT-based multivariate time series data. We conducted extensive experiments on real-world moringa extraction, demonstrating the effectiveness and practicality of our proposed approach. In comparison to other anomaly detection methods, our proposed method has the highest precision value of 0.89, the highest recall value of 0.89, and the highest accuracy value of 0.87. Future research will measure and optimize actuators (relays and motors) from anomaly detection to action. It can also be used with forecasting algorithms to detect anomalies in the coming minutes.

Internet of Things (IoT) is developing rapidly with wider application fields. IoT’s main infrastructure is called a wireless sensor network (WSN). Hence, WSN must be able to operate on various network models. Multi-hop clustering is considered a solution for adapting to various network sizes. Multi-hop clustering must be designed to maintain the balance of energy consumption between nodes, and many algorithms have been proposed for this purpose. However, most clustering algorithms are designed with the assumption that the network is a two-dimensional plane. In many applications, WSN is more appropriately modeled as a three-dimensional (3D) network, for example, the WSN application for structural health monitoring or underwater wireless sensor networks. Here, a clustering algorithm for 3D-WSN is proposed. This algorithm is developed based on an analysis of the balance of energy consumption, such that the network lifetime is expected to be longer. The main novelty of our algorithm is the utilization of multi-hop layered transmission. From the simulation, the performance of the proposed algorithm exhibits a good energy balance compared to an un-balanced analysis.