The social demands for manufacturing systems are constantly changing with the times. In recent years, environmental and social issues have been complicating these demands. For example, all industrial sectors need to consider environmentally responsible ways to achieve sustainable development goals in a time of paradigm shifts. The demands in the manufacturing field include high productivity, high quality, low energy consumption, and help for aging operators. To meet these various demands for manufacturing systems, we must create innovative manufacturing technologies to realize advanced production systems.

This special issue focuses on state-of-the-art machine tools and manufacturing technologies to accelerate production engineering innovation. This issue consists of eleven research papers covering the following fields.

- Advanced structure and drive systems in machine tools

- Industrial robot applications

- Advanced cutting technologies

- Evaluation and calibration technologies of motion error

- Surface finishing technologies

- Grinding of hard and brittle materials

All of these research contributions were presented at IMEC2022, a joint event with JIMTOF2022, held in Tokyo, Japan in 2022.

I would like to sincerely thank all the authors for their contributions, and I sincerely hope that the papers in this special issue further contribute to the development of our future society.

The objective of this study was to determine the effect of vibration behavior on workpiece surface properties in low-frequency vibration cutting. The effects of the parameters that determine vibration behavior on surface roughness were quantitatively evaluated through a comparison with other cutting conditions. Furthermore, by clarifying how the surface properties of the workpiece, such as roughness, roundness, and cross-sectional curves, change depending on the vibration behavior, a search for optimal conditions for low-frequency vibration cutting was conducted. The best surface properties were obtained under the condition of spindle rotation per vibration E=4.5. By using a value close to the minimum possible spindle rotation R=0.5 when the workpiece is retracted, it is expected to be effective in suppressing the variation in surface roughness at each phase angle; this variation is characteristic of low-frequency vibration cutting. Workpieces machined under low-frequency vibration conditions such as (E=2.5, R=1.0) and (E=3.5, R=1.0) were found to form characteristic surface patterns on the workpiece surface owing to a phenomenon in which the depth of the cut to the workpiece changes.

The different flexibility and diversity requirements for respective manufacturing units have made modern cutting tool management much more crucial and complicated, as a greater variety of tools and more frequent tool changes are required to enhance production efficiency and avoid unplanned manufacturing downtime. Developing in-process anomalous change detection methods has been identified as an essential challenge. Machine learning techniques have been widely applied in tool condition monitoring and anomalous change detection. As anomaly data is rare in manufacturing processes, supervised machine learning approaches (such as regression and classification) are not applied to the anomalous change detection problem. Rather, self-supervised machine learning (a representative type of unsupervised machine learning) is applied. This study describes a variational autoencoder (VAE) neural network and proposes a VAE-based method for tool condition monitoring and change detection in a drilling process using the temperature near a drill edge. The proposed VAE evaluates the drill tool condition based on the reconstruction error between the input temperature and its estimate per a drill unit process through the trained network. Computational simulations demonstrate that the proposed VAE network model can avoid overfitting to the anomaly data and that its expressive power is greater than that of the conventional autoencoder model.

Burnishing is a surface finishing process to produce a smooth surface. Since it can improve the mechanical properties of the material, such as surface hardness, wear resistance, and fatigue strength, in one process, it has been widely used in industry to enhance the surface quality of mechanical parts. However, due to the high burnishing force, it is difficult to use the process of thin materials, because such materials can easily be deformed in the process. In this research, a novel double-sided burnishing (DSB) tool that can process the thin plate material from both sides is designed and fabricated. The developed DSB tool symmetrically moves the burnishing tips on both sides with a single output to facilitate position control. The burnishing forces from the two sides cancel each other, and the moment that causes bending deformation is suppressed during the processing. Different thin plate metallic materials are processed using the developed tool, it is confirmed that deformation during burnishing is suppressed. By investigating the surface properties of the processed specimen, it is found that surface roughness, surface hardness, tensile strength, and fatigue strength can be improved by using the developed DSB tool. This makes it possible to process complicated, thin-walled parts, such as engine and turbine blades, which are supposed to have great potential applications in the automobile and aerospace industries.

Additive manufacturing (AM) has become a major manufacturing technology in recent years. In the fused deposition modeling (FDM) method, two-layered parts with a shell structure and an internal structure with gaps are often manufactured. When cutting is applied to such parts, the internal structure is exposed and the surface texture and strength deteriorate. Therefore, it is necessary to remanufacture the parts to correct the shape or fill the inside with resin for additional machining. However, if parts are remanufactured or filled with resin, the amount of material used increases, along with the processing cost and environmental load. If the characteristics of additional machining can be improved, the amount of material used, the processing cost, and environmental load can be reduced. Therefore, in this study, we proposed a shell forming method to form a shell structure by processing the surface of the exposed internal structure with a rod. Shell forming experiments were then conducted to evaluate the characteristics of the method. It was found that the shell thickness can be increased by increasing the shell forming depth, and the difference from the theoretical shell thickness grows larger when the shell forming depth increases. Increasing the rotation speed of the rod was effective in increasing the shell thickness. In addition, as a result of the additional cutting experiment on an AM part, it was confirmed that the properties of the additional cutting surface can be improved using the proposed method.

ISO 10791-7, which specifies the machining accuracy test standard for machining centers, and ISO 10791-6, which specifies the interpolation motion test standard, were revised in 2014 to include five-axis machining centers (5-axis MCs). In addition, cone frustum test, which has been used as the National Aerospace Standard (NAS), was revised and introduced in these ISO standards. However, prior to the establishment of these standards, an aircraft manufacturing company in China proposed S-shaped machining test to measure the accuracy of the test piece of an aircraft. This was adopted as an informative annex of ISO 10791-7 in 2020. Furthermore, in 2019, China proposed a method related to S-shaped machining test and a method of 8-shaped interpolation motion test. Since the S-shaped test requires high acceleration and deceleration in some sections, the test results depend on the performance of the computer-aided manufacturing (CAM) software; therefore, it is not efficient in determining the accuracy of the machine tools. In contrast, in the 8-shaped motion test, the tip of the spindle moves based on a sine curve; consequently, high acceleration/deceleration does not occur. Since the tip of the cutting tool is fixed, a device called R-test is used to measure the position of the center of reference ball using three displacement sensors. In this study, we have discussed the feasibility and problems of the 8-shaped motion test. First, the motion of the machine in the figure “8” motion test is clarified. Next, the definition of the parameters necessary to create the NC programs is outlined. In addition, we have proposed a method in which suitable conditions are set for simultaneous 5-axis feed on a 5-axis MC. Finally, the results of an actual test are applied to the 5-axis MC to confirm that no major problems exist.

Machining centers have played a central role in the production field for nearly half a century. Accuracy inspections of machining centers are performed as per ISO standards; however, the accuracy performance of machining centers is often not checked directly after delivery to the factory unless there is a collision between a tool and a workpiece or product failure. Although positioning accuracy is fundamental to the performance of machining centers, the need for expensive laser interferometers and the time required for the setup present barriers to owning and inspecting them. This study proposes a simple method for measuring positioning accuracy that does not require a time-consuming setup of the measurement device or expensive devices. Therefore, the user of the machine tool can own and use the proposed method for daily inspection to identify changes in the positioning accuracy of the numerically controlled machine tool more quickly.

Vertically articulated industrial robots are suitable for machining purposes owing to their advantages over multi-axis machine tools, such as larger workspace, easier installation, and lower cost. However, the rigidity and positioning accuracy of industrial robots are inferior to those of machine tools, which renders it difficult to maintain the robot posture required for machining operations. This study focuses on improving the accuracy of robot machining based on posture optimization by exploiting the kinematic redundancy of a six-axis vertically articulated robot. To decrease positioning errors caused by static and dynamic external forces during machining, this study proposes a path generation method for a redundant joint that simultaneously considers the static and dynamic rigidity of the machining robot. The relationships between the static and dynamic mechanical characteristics of the machining robot and the redundant angle are illustrated using two maps: a static stiffness map and a natural frequency map. Using these two maps in the proposed path generation method, the redundant angle that can be selected for the robot posture with arbitrary mechanical characteristics is selected. Experimental results confirm that the proposed path generation method can control the priority of reducing static positioning error and vibration amplitude by changing the weight coefficients. In addition, the proposed method can improve positioning accuracy compared with conventional trajectory generation methods for redundant robots.

Robot links are typically subjected to larger thermal deformations than machine-tool structures. In this study, the thermal effect on the two-dimensional (2D) positioning error of a planar robot arm over its entire workspace was investigated. It was experimentally verified that the Denavit–Hartenberg (DH) parameters, namely the link lengths and angular offset of the rotary axis, could be the key contributors to the thermal variation in the 2D positioning error. The experiment revealed that the variation in the angular positioning deviations of the rotary axes was marginal. This paper presents an on-machine test to identify the link lengths and the angular offset by probing an artifact bar of a pre-calibrated length. To compensate for the thermal influence, it is effective to identify the DH parameters by periodically performing the proposed test.

The temperature control performance of a developed temperature feedback control system was experimentally investigated. The control system was based on a real-time temperature control of a cooling fluid. In particular, this study focused on the temperature control performance of a built-in motor spindle that used the developed temperature fedback control system. The built-in motor used in the study had water cooling jackets. The temperature of the built-in motor spindle was measured and feedback into the developed temperature feedback control system. Temperature control accuracy of the built-in motor spindle under steady state was then assessed. Furthermore, the effects of the time-variant changes in spindle rotation and ambient temperature on the performance of the temperature control system was investigated. The results of the experiments show that the temperature control accuracy of the built-in motor spindle under steady state condition was ±0.03°C. The temperature control performance of the built-in motor spindle under changes in the rotational speed of the spindle was examined. The experimental results show that the temperature change of the spindle could be suppressed to a maximum of approximately 0.3°C under transient state during sudden change in spindle speed. In addition, the effects of the changes in ambient and cooling water temperatures, which simulated actual environmental operating conditions, on the spindle temperature were investigated. The results show that the change in the spindle temperature could be suppressed by approximately less than ±0.1°C. These experimental results indicate that the developed temperature feedback control system achieved high temperature control accuracy and high response for the built-in motor spindle. In particular, the developed control system successfully controlled the time-variant change in the generated heat, thereby improving the thermal stability of the machine tool spindle.

The effects of the ambient temperature and relative humidity on crack propagation behavior during wheel scribing were investigated. A chamber was built to allow dynamic observation of crack propagation behavior in a controlled atmosphere. A developed miniature scriber was installed in the chamber, and the crack propagation behavior was observed from lateral and back sides during wheel scribing under various atmospheric conditions. As a result, the median crack propagation rate increased with relative humidity. We speculated that this was caused by the stress corrosion of glass. Although stress corrosion is considered to be more reactive at higher temperatures, the results of scribing at different temperatures showed that higher temperatures did not necessarily increase median crack propagation. This is due to the formation of lateral cracks before the median cracks have fully propagated. These results suggest that the interaction between multiple cracks should be considered when discussing the effects of temperature and humidity in wheel scribing.

In this study, the effects of critical depth of cut and wheel wear were investigated to realize efficient precision grinding of CVD-SiC by ductile mode grinding at low cost. To compare the results under experimental conditions, Vickers indentation tests and grinding experiments were conducted. As a result of the Vickers indentation test at an applied load of 0.015 N, the minimum indentation load in this study, the indentation depth was 1.3 μm, and cracks were observed at the corners of the indentation isotropically. Additionally, the pile-up was observed around the indentation, suggesting that plastic deformation due to shear flow was relatively large. Grinding experiments were conducted using grinding wheels with different grain sizes. All the grinding conditions in this study resulted in a surface with a mixture of brittle and ductile modes. The proportion of ductile modes was larger, and the surface roughness Ra was smaller when a grindstone with a smaller grain size was used. Additionally, the effect of wear was investigated. As wear progressed, the number of protruding grains decreased, resulting in a smaller surface roughness. These results indicate that the amount of protruding abrasive grains must be controlled to achieve stable ductile mode grinding.