Surface Texturing Technique Based on Ultrasonic Turning for Improving Tribological Properties

Tribological properties such as lubrication, friction, and wear resistance greatly affect machine operation efficiency, performance, and service life. Surface texturing methods such as scraping can be used to improve these properties. Scraping creates many small depressions on the target surface. These depressions, which are evenly distributed, function as oil holes and thus improve lubrication performance. This paper describes a surface texturing technique based on ultrasonic vibration-assisted turning (UVAT) that simultaneously improves tribological properties and machinability. In UVAT, the cutting tool is oscillated mainly in the principal direction. Vibration in the radial direction, which is induced by Poisson deformation, periodically digs up or pushes the workpiece surface in the radial direction, creating a textured surface. A surface subjected to UVAT has periodic depressions along the workpiece rotation direction. The texturing rate of UVAT is up to 6700 mm2/min, which is higher than that of manual scraping. To evaluate the tribological performance of a surface textured by UVAT, the friction coefficient between a stainless steel pin and the surface was measured under oil dipping conditions. The results of friction experiments show that the friction coefficient of the UVAT-treated surface and its fluctuation were lower than those of a conventional turned surface. The UVAT-treated surface had stable friction properties.

Effect of Grain Size on Change in Surface Roughness of Carbon Steels Through Polishing Processes

A mirror-like reflecting surface is an important characteristic in many industrial metallic parts. Polishing is done to form a mirror surface on metals. However, the effect of the grain size of metals on surface roughness through polishing processes is not clear. Specifically, mirror surface formation of ultrafine grained materials is still unknown. Ultrafine grained steels and coarse grained steels with 0.02, 0.10, and 0.60 wt% carbon contents were prepared by warm caliber rolling and annealing. Average grain sizes were 1–2 μm and 4–40 μm. The changes in surface roughness, Sa, were measured with an atomic force microscope (AFM) via eight polishing steps, using emery papers of type #600, #1000, #1500, #2000, #2500, #4000, and free abrasive grains of 3 μm and 1 μm diamond. As the polishing process progressed, the surface unevenness was removed and the surface roughness, Sa, decreased in all steels. The differences of Sa at each polishing step were analyzed from the point of carbon content, Vickers hardness, and grain size. Carbon contents and Vickers hardness have little effect on Sa. However, grain size has a considerable effect on Sa in all steels. Ultrafine grained steels have smaller Sa in all polishing steps in all steels. This is because ultrafine grained steels have very small work hardening rate. After final polishing, Sa is 2.5–3.6 nm in coarse grained steels and 2.0–2.6 nm in ultrafine grained steels. To obtain a mirror surface with smaller Sa, grain size control is important.

Evaluation of the Relationship Among Dressing Conditions Using Prismatic Dresser, Dressing Resistance, and Grinding Characteristics

In the grinding process, the grinding wheel surface condition changes depending on the dressing conditions, which affects the ground surface roughness and grinding resistance. Several studies have been reported on the practical application of dressing using prismatic dressers in recent years. However, only a few studies that quantitatively evaluate the effects of differences in dressing conditions using prismatic dresser on the ground surface roughness and grinding resistance have been reported. Thus, this study aims to evaluate quantitatively the effect of the difference in dressing conditions using the prismatic dresser on the ground surface roughness and grinding resistance by focusing on the dressing resistance. In the experiment, dressing is performed by changing the dressing lead and the depth of dressing cut with a prismatic dresser, and the ground surface roughness and grinding resistance are measured. Consequently, by increasing the dressing lead and the depth of dressing cut, the ground surface roughness increased, and the grinding resistance decreased. This phenomenon was caused by the increase in dressing resistance when the dressing lead and the depth of dressing cut were increased, which caused a change in the grinding wheel surface condition. Furthermore, the influence of the difference in dressing conditions using the prismatic dresser on the ground surface roughness and grinding resistance can be quantitatively evaluated by using the dressing resistance.

Effect of the Abrasive Grain Distribution on Ground Surface Roughness

In order to reduce the grinding surface roughness, it is necessary to optimize the grinding conditions; this requires clear understanding of the relationship between the grinding conditions and ground surface roughness. Therefore, various studies have been carried out over the decades on the ground surface roughness and have proposed statistical grinding theory to define the relationship between the grinding conditions and ground surface roughness. However, the statistical grinding theory does not consider a few grinding conditions such as abrasive grain shape and distribution of abrasive grain, which affect the ground surface roughness. In this study, we construct a statistical grinding theory that considers the effect of abrasive grain distribution and improves the accuracy of the theoretical analysis of the ground surface roughness.

ELID Mirror Surface Grinding for Concave Molds by Conductive Elastic Wheel Containing Carbon Black

Elastic grinding wheels have previously been adopted for the development of the mirror surface finishing method for concave spheres. In this study, new conductive elastic grinding wheels, to which electrolytic in-process dressing (ELID) can be applied, are developed; the aim of the study is to address the challenge of maintaining a constant removal rate for rubber bond wheels. When ELID grinding is performed using a non-diene (isobutane isoprene rubber, IIR)-based wheel, a larger removal amount is achieved, and a higher-quality surface is also achieved compared to a diene (acrylonitrile-butadiene rubber, NBR)-based wheel. In addition, to investigate the effect of grinding wheel bond hardness on the removal amount and ground shape accuracy, grinding wheels with various levels of hardness are prepared by controlling the amount of carbon black contained in them, and grinding experiments are conducted. Thus, a larger removal amount is achieved using a harder grinding wheel, but the roughness of the ground surfaces deteriorates. Therefore, in practice, it is necessary to select an appropriate grinding wheel that can achieve both productivity and surface quality. Finally, to obtain a high-quality mirror finish on a concave spherical surface, ELID grinding is performed on the workpieces as is done for spherical lens molds. Thus, high-quality mirror surfaces with roughness Ra < 10 nm were generated. When the work pieces are ground using a grinding wheel of the same radius, excessive removal occurs at the edge of the concave spherical profile, decreasing the form accuracy. Numerical simulation demonstrates that chamfering of the grinding wheel is effective for improving the shape accuracy. The results of this study are expected to contribute to automation and cost reduction in the mirror-finishing process for concave molds.

Polishing Performance of a Recycled Grinding Wheel Using Grinding Wheel Scraps for the Wet Polishing of Stainless-Steel Sheets

The surfaces of large austenitic stainless-steel sheets, which have side lengths of at least 1 m a sheet thickness of at least 6 mm, used for food tanks and sliding plates in seismic isolation devices, must be finished to a mirror surface. Polishing is performed to improve the surface quality of such sheets and dry machining is typically applied. The problems associated with dry machining are the exhaust heat of machining and treatment of chips. A transition to wet machining is required to solve these problems. In our laboratory, we have developed a wet polishing machine and researched the selection of grinding wheels to develop wet polishing technology for large stainless-steel sheets. In this study, to reduce tool cost and reuse resources, we attempted to manufacture a recycled grinding wheel using snippets of grinding wheel scraps. A polyvinyl alcohol (PVA) aqueous solution was used as the bonding agent for the recycled grinding wheel to reduce environmental load. To overcome the ease of dissolution of PVA in water, we attempted to improve the water resistance of the PVA aqueous solution by incorporating an organic titanium compound. This is one of our efforts to contribute to sustainable development goals. The results are summarized below. (1) A recycled grinding wheel was fabricated by kneading crushed pieces of grinding wheel scrap with a bonding agent. (2) The maintenance of the shape of the recycled grinding wheel was controlled by the concentration of the bonding agent. (3) The recycled grinding wheel with a PVA bonding agent was vulnerable to water. In contrast, the recycled grinding wheel to which the organic titanium compound was added exhibited improved water resistance. (4) The polishing of stainless-steel sheets using the plain PVA recycled wheel was relatively ineffective, but polishing using the recycled wheel with the titanium additive was comparable to polishing with a new grinding wheel.

Power Consumption Simulation of Servo Motors Focusing on the Influence of Mechanical Vibration on Motor Efficiency

This paper presents a simulation method for the power consumption of servo motors, focusing on the influence of vibrations on the motor efficiency. An apparatus consisting of two servo motors connected through a coupling was specifically designed for this study. The efficiency of the servo motor was experimentally investigated for several torque vibration levels imposed through the selection of the control parameters, and the torque vibration level was quantified through the standard deviation of the torque signal. The efficiency map characteristics for each torque oscillating level were determined. A numerical model of the apparatus clarifying the dependency of the coupling characteristics on the oscillating torque was developed, and the torque oscillation of the system was simulated. A model based on the measured motor efficiency maps and the torque oscillation level was developed to simulate the motor efficiency under several torque vibrating conditions. Finally, the power consumption of the motor was simulated based on the simulated efficiency and mechanical power. A balance of input, output, and loss powers was presented, and the experimental measurements were compared with the simulation results. The power consumption of the motor increased when the torque oscillated owing to vibrations, and the loss of power due to both oscillations and the loss of motor efficiency was quantified.

Rotary Cutting with Ultrasonic Vibration of Hardened Steel

We propose ultrasonic rotary cutting, in which ultrasonic vibrations are imparted to a rotating cemented carbide cylindrical tool to cut hardened steel to reduce the cutting resistance and improve the properties of the machined surface, and investigate the machining characteristics. Machining experiments were conducted under dry and wet conditions to verify the effects of the ultrasonic vibrations. The surface produced via ultrasonic rotary cutting was intermittently machined, which is characteristic of ultrasonic cutting. In dry machining, the cutting resistance was reduced by approximately 20%, and the surface roughness of the machined surface was reduced by approximately 30% when the cutting speed was below the critical speed. We also demonstrated that the surface roughness was improved by ultrasonic vibrations when the cutting speed was equal to or above the critical speed. A similar tendency was observed in wet machining with longer cutting lengths. We then applied ultrasonic rotary cutting to machine a straight R groove in hardened steel and showed that the cutting resistance was reduced, and the tool engagement was improved.

Microscopic Wear Characteristics of Ceramic Grinding Wheel in Creep Feed Grinding

This paper deals with the microscopic wear characteristics of ceramic (Seeded Gel, SG) grinding wheels used in creep feed grinding. Creep feed grinding experiments with SG grinding wheels were carried out compared to rose-pink alumina (RA) grinding wheels. To clear the wear characteristics of the wheel working surface in creep feed grinding, changes in the shapes of grain cutting edges were observed by a field emission-scanning electron microscope (FE-SEM). This is a self-sharpening phenomenon based on micro fractures generated on the top of SG grain cutting edges. On the other hand, large fracture and attritious wear effected RA grain cutting edges. In addition, the features of any grain cutting edges were evaluated using attritious wear flat percentage. Changes in attritious wear flat percentage of SG grits maintained constant value and were stable. From these results, the influence of wear mode of the grinding wheel on grinding characteristics parameter, such as grinding force and workpiece surface roughness, is understood.

Special Issue on High Performance Abrasive Technologies

Demand for the high-precision and high-efficiency machining of hard ceramics, such as silicon carbide for semiconductors and hardened steel for molding dies, has significantly increased for optical and medical devices as well as for powered devices in automobiles. Certain types of hard metals can be machined by deterministic precision-cutting processes. However, hard and brittle ceramics, hardened steel for molds, and semiconductor materials have to be machined using precision abrasive technologies, such as grinding, polishing, and ultrasonic vibration technologies that use diamond super abrasives. The machining of high-precision components and their molds/dies using abrasive processes is very difficult due to their complex and nondeterministic natures as well as their complex textured surfaces. Furthermore, the development of new cutting-edge tools or machining methods and the active use of physicochemical phenomena are key to the development of high-precision and high-efficiency machining. This special issue features 11 research papers on the most recent advances in precision abrasive technologies. These papers cover the following topics: - Characteristics of abrasive grains in creep-feed grinding - Quantitative evaluation of the surface profiles of grinding wheels - ELID grinding using elastic wheels - Nano-topographies of ground surfaces - Novel grinding wheels - Grinding characteristics of turbine blade materials - Polishing mechanisms - Polishing technologies using magnetic fluid slurries - Application of ultrasonic vibration machining - Turning and rotary cutting technologies This issue is expected to help its readers to understand recent developments in abrasive technologies and to lead to further research. We deeply appreciate the careful work of all the authors, and we thank the reviewers for their incisive efforts.