With the 2011 launch of Industrie 4.0, a German project aiming to promote the computerization of manufacturing, the integration of physical or actual manufacturing systems with cyber-physical systems (CPS) using various technologies, such as the Internet of things (IoT), industrial Internet of things (IIOT), and artificial intelligence, is considered to be more important than ever before. One of the goals of the Industrie 4.0 is to realize smart factories or smart manufacturing using advanced digital technologies. However, the core component in the manufacturing systems is still machine tools.

This special issue, composed of eleven excellent research papers, focuses on the latest research advances in machine tools and manufacturing processes. It covers various topics, including machine tool control, tool path generation for multi-axis machining, and machine tool components. Furthermore, this special issue includes innovative machining technologies, including not only cutting and grinding processes but also the EDM process and burnishing process connected effectively with force control techniques. All the research contributions were presented at IMEC2018, a joint event with JIMTOF2018, held in Tokyo, Japan in 2018.

The editors would like to sincerely thank the authors for their dedication and for their well written and illustrated manuscripts. We are also profoundly grateful for the efforts of all the reviewers who ensured their quality. Finally, we sincerely hope that studies on machine tools and related manufacturing technologies will further contribute to the development of our global society.

The present study introduces a novel tool orientation angle optimization method for improving the machining accuracy of robotic milling systems. The proposed approach considers the intrinsic properties of serial mechanisms and their relationship with robotic stiffness to select optimal robot postures in the generation of tool orientation angle for finish cut. The evaluation of the robotic stiffness is carried out with two performance indices presented in this study: the volumetric stiffness performance index, which measures the overall robot stiffness, and the unidirectional stiffness performance index, which measures the robotic stiffness along a specific direction. As machining errors are reduced by optimally selecting the tool orientation angle without modifications to the tool path itself, the proposed method is significantly less convoluted than conventional optimization methods. The efficacy of the proposed method is validated experimentally using a purpose-designed multi-axis milling robot. Experimental results show that the robotic milling system is capable of machining three-dimensional shapes with a fine surface, and reducing the twist caused by the displacement of the cutting tool towards the direction of lowest robotic stiffness.

As the motion accuracy of 5-axis machining centers directly influences the geometrical shape accuracy of the machined workpieces, accuracy enhancement of the 5-axis machining centers is strongly needed. To improve the shape accuracy during the machining by a 5-axis machine tool, a method that modifies the CL-data based on the motion trajectory errors normal to the machined surface at each command point has been proposed. In this study, the proposed method is applied to simultaneous 5-axis controlled machining to improve motion accuracy. A normal vector calculation method for the simultaneous 5-axis controlled motion is newly proposed, and the compensation method is applied to turbine blade machining by 5-axis controlled motion. Measurement tests of the cutting motion for blade shape machining by a ball-end mill were carried out with a different control mode of NC. The CL-data for the machining tool path was also modified based on the calculated trajectory of the tool center point. Experimental results reveal that the feed speed and machining accuracy significantly depend on the control mode of NC, and that the shape accuracy can be improved by applying the proposed compensation method without any decrease in motion speed.

ISO 10791-7, the test standard for machining centers, was revised in 2014 to add the test method for five-axis machining centers. However, an S-shaped test was additionally proposed as an accuracy test of aircraft parts from China immediately before the establishment of the test standard. In an ISO meeting, various problems such as creating three-dimensional models and evaluation items have been indicated for the proposed test method. By revising these problems, the standard was finally completed and will be introduced as an informative annex soon. However, it is still an inappropriate test method from the viewpoint of performance inspection for machine tools. In this research, the S-shaped test method draft proposed in September 2016 is tested using two types of five-axis machining centers and commercial CAM software. Consequently, a hidden problem is revealed, that is, an abrupt movement that affected the final result is added to the machine because the rotation direction of the rotary axes is not ideal. This is attributed to the performance of the CAM software’s post processor that converts from CL data to NC program. This study provides some insights into avoiding the problem and obtaining better test results.

Characteristics of a spindle supported with water-lubricated hydrostatic bearings were experimentally investigated. In particular, this paper focuses on the thermal characteristics of the spindle. The flowrates of water as the lubricating fluid were measured separately for the radial and thrust bearings, in relation to the supply pressure. Fluid power losses owing to pressure losses of the lubricating fluid were then introduced. Furthermore, the power losses owing to the water viscosity were determined by measuring the spindle torque and angular velocity. The experiments revealed that the total power loss of the spindle is approximately 300 W. The cooling effect of the lubricating water was then examined by introducing a temperature increase between the supply and drain water. The experimental results verified that the water temperature increased by approximately 0.8°C, at a spindle speed of 3000 min^-1. Based on the temperature increase of the water, the power removed from the spindle by the water flow was estimated. By comparing the generated total power loss and the power transferred by the water flow, the cooling efficiency of the flow of lubricating-water was defined in this paper. If the cooling efficiency is 100%, the temperature change of the spindle can be zero regardless of the power loss, achieving ideal thermal stability of the spindle. Experimental results revealed that the cooling efficiency of the tested spindle was over 80%. This indicates that the flow of water as a lubricating fluid removes generated heat from the spindle effectively, and achieves improved thermal stability of the spindle.

Lost motion is a phenomenon that often occurs during the motion of a moving table, which is used for machine tools to ensure their precise positioning. Lost motion occurs when the direction of the table’s motion reverses as a result of nonlinear friction characteristics between the feed drive mechanisms such as the ball screw and linear guide. Lost motion directly influences the machining accuracy of a machine tool, because the accuracy of machining depends on the relative motion between the tool and the workpiece. A number of studies have dealt with suppressing the occurrence of lost motion using model-based control. However, nonlinear friction has not been addressed to the same extent, as it is difficult to determine the motion characteristics of and therefore develop a model for the nonlinear friction. Thus, to address these problems, we propose a compensation method for revers motion based on vector control, which is used to control the torque and velocity of the alternating current (AC) servomotor in the moving table. In this study, the current applied to the AC servomotor for a vector with force components in the rotational direction (torque component) and in the direction perpendicular to the axis of rotation (field component) was measured to clarify and establish the relationship between the motion and the control current. The compensation current was then derived as a functional value based on the results of the measured torque at the occurrence of lost motion. Further, tests were carried out using the proposed method, which directly applies the drive current of the AC servomotor by using a field-programmable gate array controller to improve the reverse motion of the table. The results reveal that the motion characteristics of a numerical control (NC) table can be determined by measuring the drive current of the AC servomotor. In addition, it is verified that the proposed method can compensate for the torque command smoothly at the time of velocity reversal, resulting in suppression of the lost motion and reduction of reverse motion of the moving table.

Burnishing is a surface finishing process, in which a very smooth surface finish is obtained by pressing a ball or roller against a machined workpiece. Additionally, owing to the surface plastic deformation caused by the movement of the hardened ball or roller, other surface properties such as hardness, fatigue life, and wear resistance can be improved. Burnishing force is one of the most important factors affecting the surface modification quality. However, methods for precisely monitoring and controlling the burnishing force are rarely investigated. In this research, a novel ball burnishing tool embedded with a load-cell and air-servo system was developed and fabricated for application to a CNC machining center. Using a specially designed control software, the burnishing force in the burnishing process was monitored in real time and controlled constantly and precisely by a force feedback system. Magnesium alloy AZ31 specimens were used to evaluate the performance of the developed system. The experiments were divided into two parts. In the first part, the effects of the processing parameters on the surface roughness were investigated. The results indicated that the surface roughness could be improved from Ra= 1.95 μm to Ra= 0.26 μm. In the second part, the effects of the burnishing parameters on the surface properties were investigated by conducting experiments using Taguchi’s orthogonal array. The results suggested that the burnishing force was the most significant factors affecting the surface hardness and grain size. The Vickers hardness could be increased from HV62 to HV149. The average grain size was reduced after the burnishing process, and a work-hardening layer thickness of 0.75 mm was achieved. X-ray diffraction results indicated that the crystal orientation was modified after burnishing, and the maximum measured compressed residual stress was 186.3 MPa in the tool feed direction and 87.8 MPa in the step over direction.

Demand for highly productive machining of thin-walled workpieces has been growing in the aerospace industry. Workpiece vibration is a critical issue that could limit the productivity of such machining processes. This study proposes a machining process for thin-walled workpieces that aims to reduce the workpiece vibration during the machining process. The workpiece compliance is measured using an on-machine measurement system to obtain the cutting conditions and utilize the same for suppressing the vibration. The on-machine measurement system consists of a shaker with a force sensor attached on the machine tool spindle, and an excitation control system which is incorporated within the machine tool’s numerical control (NC). A separate sensor to obtain the workpiece displacement is not required for the estimation of the displacement. The system is also capable of automatic measurement at various measurement points because the NC controls the positioning and the preloading of the shaker. The amplitude of the workpiece vibration is simulated using the measured compliance to obtain the cutting conditions for suppressing the vibration. An end milling experiment was conducted to verify the validity of the proposed process. The simulations with the compliance measurement using the developed system were compared to the results of a conventional impact test. The comparison showed that the spindle rotation speed for suppressing the vibration could be successfully determined; but, the axial depth of cut was difficult to be determined because the simulated vibration amplitude was larger than that found in the experimental result. However, this can be achieved if the amplitude is calibrated by one machining trial.

The demand for micro-end-milling for products in fields such as the medical, optical, and electronics industry is increasing. However, when machining with a small diameter end-mill (micro-end-mill) with diameters such as 0.5 mm, the rigidity of the tool itself is low; hence, the cutting conditions must be set to low values to achieve stable machining. Therefore, we examined various cutting phenomena that occur during actual machining processes to achieve high machining accuracy, high finished-surface quality, and long tool life. Some studies on micromachining achieved high accuracy, high-grade machining by considering the cutting phenomena. In previous papers, we dealt with the side-cutting phenomena in micro-end-milling of hardened die steels using a high-speed air-turbine spindle with rolling bearing. Cutting experiments were carried out by measuring the cutting force and flank wear of a cutting tool to investigate the difference in cutting phenomena caused by cutting direction in high-speed micro-end-milling. Observation of the machined surface and measurement of the profile of the cutting edge and machined surface were demonstrated. It was revealed that machining quality in high-speed up-cut milling was better than that in down-cut milling. Shoulder cutting, in which both peripheral and bottom cutting edges act simultaneously on the workpiece, was also investigated. A novel small diameter end-mill with left-hand helical tool with right-hand cut was developed to avoid damaging the cutting edge in the initial cutting stage. In the present study, high-quality shoulder cutting of a vertical wall using the new tool was proposed and demonstrated.

This study considers an innovative coolant nozzle that enables a remarkable reduction in grinding fluid consumption in cylindrical plunge grinding of chromium molybdenum steel (ISO 34CrMo4 / JIS SCM435) using a vitrified-bonded cubic boron nitride (CBN) wheel. This coolant nozzle has a simple structure consisting of ordinary nylon or polypropylene fiber brushes and an acrylic resin oil pool. This flexible brush-nozzle is suitably placed in contact with the wheel surface, and the grinding fluid is supplied along the brush to the wheel surface to form a fluid film to the wheel surface, while simultaneously scraping the “air belt.” Thus, the grinding fluid adheres to the whole wheel surface by the “Coandă effect,” and grinding fluid consumption is reduced to 0.5 L/min or less without causing any thermal damage to the ground surface. The cooling effect of this coolant nozzle is examined mainly by a grinding temperature measurement by means of a fiber-coupled two-color pyrometer. The nylon fiber brush is more effective than the polypropylene brush because of its high elasticity and good water absorption rate, which is related to the wettability. Even with such an extremely small amount of grinding fluid, the grinding force, surface roughness, and grinding temperature are nearly the same as those in the case of the conventional wet grinding.

This study proposes a new grinding system using grinding fluid containing oxygenic fine bubbles (O₂FBs) to realize high-performance electrolytic in-process dressing (ELID) using a conductive rubber bond grinding wheel. It was found that grinding fluid containing O₂FBs dramatically increases the dissolved oxygen in the grinding fluid. In addition, the O₂FBs in the fluid are drawn to the conductive rubber bond grinding wheel, which is the positive pole, during ELID. These effects are thought to enhance the dressing performance of the conductive rubber bond grinding wheel. Grinding of pure titanium using the proposed grinding system was found to realize mirror surface finishing while increasing the amount of removed workpiece material, compared to when ELID was not applied and to when ELID grinding was conducted using a normal grinding fluid. Effects of ELID grinding on surface modification were also observed, confirming that the proposed grinding system is able to form a thick oxidized film on pure titanium.

The effects of surface modification using an electrical discharge coating are investigated. The electrode used for the machining process is a Ti-type semi-sintered compact electrode. A thin and hard layer is formed on the machining surface. In this paper, the change in surface roughness during electrical discharge coating is described. The accretion layer surface became coarse after 10 min of machining. The peak height also increased. After 30 min of machining, the height of the accretion layer was lower than the original surface level, and the peak height decreased. Furthermore, the core roughness value increased. The surface roughness increased within several minutes from the start of electrical discharge coating using a Ti-type semi-sintered compact electrode. In this paper, we report on the temporal change of surface roughness during electrical discharge coating.

In modern agriculture, many advanced automated devices are used on farms. To improve the working efficiency of agricultural vehicles, fields are expected to be pre-leveled, because the vehicles work more effectively on a flat field. Leveling a field requires the current field elevation map. Some farmers in Japan have begun to use high-precision real-time kinematic Global Positioning System (RTK-GPS)-based self-steering tractors in the fields. This study uses the RTK-GPS information from a self-steering tractor system to generate a digital elevation model (DEM) especially in mountain regions where the fields are not flat. In addition, all of the information is from the self-steering system with the result that farmers can use the method of this study without additional instruments. However, the GPS receiver sometimes cannot obtain high-quality signals from satellites in mountain regions. Therefore, this study focuses on how to create a high-precision DEM even when a GPS signal is unavailable. It proposes a dynamic interpolation method for generating a DEM. In addition, a test was conducted in a field in a mountain region. The test results show that the dynamic interpolation method can provide an accuracy of less than 0.03 m in the test field for creating a DEM.

A 5-axis machining center (5MC) is noted for its synchronous control capability, making it a feasible tool for quickly and accurately machining complicated three-dimensional surfaces such as propellers and hypoid gears as it is equipped with a direct-drive (DD) motor in the rotary axis. The current research work identified the necessity of improving both the accuracy of the machined shape and the consistency of the free-form machined surface. A method for maintaining the feed speed vector at the milling point by controlling two linear axes and the rotary axis of a 5MC to improve the quality of the machined surface was investigated. Additionally, a method was proposed for reducing the shape error of machined workpieces by considering differences in the servo characteristics of the three axes. The shape error was significantly reduced by applying the proposed method using a precedent control coefficient determined via calculations. To maintain the feed speed vector at the milling point in the machining of complex shapes, rapid velocity change in each axis is often required, leading to inaccuracy caused by torque saturation at a DD motor in the rotary axis. The results of this study indicate that torque saturation can be evaluated via simulation and that the machining accuracy and consistency can be improved by accounting for these errors using the proposed precedent control coefficient method.

To overcome the consequences of the 2008 crisis on the real sector, especially manufacturing, Industry 4.0 gives guidelines to drive production by emphasizing technological innovations, such as industrial internet, cloud manufacturing, etc. The proposed paper focuses on cognitive manufacturing within the framework of the emergent synthesis paradigm. Specifically, the structuring process by which the manufacturing assets are organized to provide the finished goods is analyzed. The study is carried out by considering the analogies between manufacturing and other inventive processes supported by formal tools such as formal languages, semantic webs, and multi agent system.

The present study proposed a method to automatically generate a numerical control (NC) program by referring to machining case data for each machine tool with only 3D-CAD models of a product and workpiece as the input data, and to select machine tools for machining the target removal region among several machine tools with different characteristics. The special features of the proposed method are described as follows. The removal volume can be automatically obtained from the total removal volume (TRV), which is extracted from the workpiece and product using a Boolean operation by dividing it on the XY plane. The removal region changed according to the determined machining sequence. The conditions for machining the removal region is automatically determined according to the machining case data, which is stored by linking the geometric properties of the removal region with the machining conditions determined by experienced operators. Furthermore, an NC program is automatically generated based on the machining conditions. The machine tools for machining the target region are selected according to the predicted machining time of each machine tool connected by a network.

A case study was conducted to validate the effectiveness of the proposed system. The results confirm that machining can be conducted using only 3D-CAD models as input data. It was suggested that the makespan would be shortened by changing the machining sequence from the optimized machining sequence when machining a plurality of products.