Journal of Advanced Mechanical Design, Systems, and Manufacturing Vol. 11(2017) No. 3

The high temperature and severe plastic deformation could promote workpiece material microstructure evolution in the machining process. The material mechanical properties are functions of the material microstructure attributes. Traditional material flow stress model approximates the material mechanical behavior at a continuum level by ignoring the microstructure effects. In the current study, a microstructure based mechanical threshold stress (MTS) model is proposed for the machining process application. The MTS model takes the material grain boundary and dislocation resistance into account. Within both the analytical and FEA machining process modeling framework, the MTS model is implemented for the machining forces prediction. The validation of the MTS application into machining forces prediction are conducted by comparison with experimental results. Good agreement is found between the experiment and prediction. Additionally, slight force prediction improvement is observed by comparing with the traditional Johnson-cook flow stress model.

In order to solve the problem of the design and fabrication of high contact ratio spiral bevel gears with seventh-order transmission error (TE), an ease-off flank modification method is proposed based on the modified curvature motion method. In this paper, firstly, based on the predesigned seventh-order transmission error, the polynomial coefficients of transmission error curve can be obtained. Secondly, the pinion target tooth surface is obtained by modifying the pinion auxiliary tooth surface along the meshing line with a predesigned modification curve, and the pinion auxiliary tooth surface is obtained with the predesigned seventh-order transmission error through using the gear as a virtual cutter. Thirdly, a new method called modified curvature motion (MCM) method is proposed to improve the adjustability of spiral bevel gear by modifying parts of machine-tool settings in the process of gear manufacturing. Finally, an optimization model solved by using the improved NSGA-II algorithm is proposed to solve the adjustments of pinion machining settings, and we carry out TCA and LTCA to verify the feasibility of the tooth modification method. The results keep in line with the preconditions that transmission error is seventh-order curve and the contact path is located in the middle of the tooth surface. The proposed flank modification methodology can serve as a basis for developing a general technique of flank modification for spiral bevel gears.

Parallel kinematic mechanism has shown its advantages as a high-precision motion stage in a machine tool. In this paper, a RPS-XY hybrid kinematic structure TRIPOD actuated by linear ultrasonic motors is proposed and designed for a five-axis ultra-precision micro machine tool. In the designing stage, both the titling capacity and the consistency of the deflecting abilities in all directions need to be carefully designed to overcome the platform's limitation in the freeform surface machining. This study proposes an optimizer that can automatically search for a set of design parameters of the 3-RPS parallel part to generate large titling and uniform deflecting capacities simultaneously. A specified orientation workspace is calculated to evaluate the ability of rotation. Additionally, a new index is introduced to justify the uniform deflecting capacity. By evaluating those factors as the objective functions, a modified NSGA-II algorithm is employed for the optimum design with the consideration of discretely selected linear guides and spherical joints from a predefined database, which contains the abstracted engineering models of commercial products. Optimal results verify the feasibility and effectiveness of the proposed optimizer in designing a physical platform. The technique is extensible to most of other parallel mechanisms as a standard procedure.

Low-frequency vibration drilling can suppress the drilling temperature and extend tool life. In low-frequency vibration drilling, there are drilling times and non-drilling times in each vibration cycle. A past study clarified that the temperature increases during drilling, and the peak temperature in one vibration cycle is nearly equal to a conventional drilling temperature, but the temperature of the drill corner decreases during non-drilling periods. However, the relationship between the amount of temperature increase/decrease during intermittent drilling and the vibration and drilling conditions and temperature change near the cutting edge has not yet been clarified. In this study, to determine the drilling temperature during the drilling and non-drilling periods of low-frequency vibration drilling, the temperature near the cutting edge was measured experimentally by an embedded K-type thermocouple. To identify the optimum conditions for low-frequency vibration drilling without repeating the experiment, the temperature transition of the cutting edge was simulated based on the heat input caused by the cutting energy, calculated from the principal cutting force and speed. To simulate the temperature change of the drill edge, the principal force acting on the cutting edge was calculated from two-dimensional cutting data. A comparison of the experimental and simulated temperatures showed that the simulated temperature transition agreed well qualitatively with the results measured during low-frequency vibration drilling.

To assist in lifting heavy weight objects from the floor, an assistive instrument that can be worn on the hip and knee joints has been developed. The instrument was driven by coil and flat spiral springs to operate for a long time. At this instrument, the assistive mechanism for the hip joint comprised a plate cam with a “sine-output” oscillating follower and a compression coil spring. The assistive mechanism for the knee joint comprised a non-circular gear, two grooved cams and a flat spiral spring. To eliminate the influence of abduction or adduction and a medial or lateral rotation, two types of freely rotating joints were attached to each of these mechanisms. Based on the analysis of the lifting motion of an object from the floor, it was concluded that the torque applied on the hip and knee joints depends on the individual differences in body height and a body weight, and can be estimated using a two dimensional model. A prototype of the proposed instrument was fabricated and tested. This instrument can be used to reduce the required muscle activities of the hip and knee joints by about 15% compared to while lifting without the instrument when subjected to a force of 98N by an object. Furthermore, the results showed that the assistive instrument exhibits sufficient performance in aiding the lifting motions.

This paper reports the design of an adjustable amplitude vibration system for mechanical linear friction welding (LFW) equipment. The system consists of two crank slider mechanisms that are connected by a phase modulator. The amplitude of the vibration system is determined by the average displacement of the two sliders, which can be changed by adjusting the phase difference between the two cranks. When the vibration amplitude is zero, the vibration system stays at a fixed position called positioning point. This study analyses the cause of the small amplitude vibrations of the system about the positioning point and improve the length of the connecting rod to minimize these small vibrations and ensure positioning accuracy. A kinematic and kinetic model under linear friction welding working conditions is developed based on Simulink. The simulation results indicate that after improvement of the mechanism, the system can satisfy the large loads of mechanical LFW equipment (40 kN frictional force) as well as the rapid amplitude adjustment capability and positioning accuracy requirements.

This paper presents a controller design approach to compensate for the disturbances and achieve robust vibration suppression against variations in the resonant frequency in a piezo-driven stage system. Disturbances such as nonlinearities due to hysteresis and creep phenomena are inherent in piezoelectric actuators, resulting in a low control performance of positioning or low tracking accuracy. Although various types of modeling and model-based approaches have been proposed to compensate for the nonlinearities, these approaches have limitations such as substantial modeling complexity, time-consuming modeling process, and high computational cost required for their evaluation and implementation. Moreover, resonant vibration modes in the mechanism lead to residual vibrations in the positioning or may even destabilize the system. In particular, the vibrational dynamics tend to have a low stability margin in piezo-driven systems because of sharp resonant peaks associated with the low structural damping. In this study, a state observer is employed to estimate the disturbance and vibration mode signals, wherein the observer is designed to suppress the disturbance and the vibration by using the estimated signals. Considering the hysterisis as an input disturbance to the linear plant renders complex modeling processes unnecessary. To compensate for the mechanical vibrations, pole-assignment method is used to achieve the gain-peak reductions and robustness against variations in the resonant frequency. The effectiveness of the proposed approach was verified by conducting experiments on a piezo-driven stage system.