Key Engineering Materials Vol. 625

Abstract: Manufacturing systems have been evolving to accommodate the production of various types and volumes of products, as with the flexible manufacturing system (FMS). However, with FMS, it is difficult to change the layout of a factory and to cope with any problems that may arise. Therefore, the importance of automated guided vehicles (AGVs) is increasing because AGVs can flexibly adapt to changes of equipment in a factory. However, there has not yet been much research on systems that can respond to a large number of indefinite elements. One response to this issue might be a mimetic solution, which is the application of knowledge from one field to another. We feel that knowledge of traffic engineering, dealing as it does with the transport of passengers, could be applied to the conveyance mechanisms of AGVs. This paper describes an autonomous conveyance system for AGVs based on taxi transportation strategy. The system focuses on knowledge of a flexible taxi system in traffic engineering. We chose taxis because they operate with higher flexibility in terms of traveling routes and arrival/departure points compared with railways and buses. In this study, the waiting mode of taxis was applied as AGV rules and then tested to determine the effect of the proposed system. We came up with an equation to identify the best way of determining the waiting position. Moreover, AGV collisions are of extreme importance when AGVs run for real: if a collision occurs, we have to change the factory schedule and fix the AGV. However, in the past we have never taken account of an AGV collision, and so here we do. Two assessment functions are treated: matching time, which is the period of time elapsing between when a load is received and when a load is generated, and conveyance efficiency, which is the total distance the AGV travels divided by the total distance the AGV carries the load. Results showed that the proposed system (applying the waiting strategy of taxis as AGV rules) is effective at reducing the matching time and intersection of AGVs without any decrease in conveyance efficiency.

Abstract: Along with the rapid development of CAD/CAM system, the smooth surfaces are widely used in industry design. Especially NURBS surface attracts rising attentions because NURBS can describe flat surface, freeform surface and so on. Currently a lot of tool path generation patterns are proposed for milling process of freeform surface. Nevertheless there are still some problems on generating efficient tool path for freeform surface under the requested surface finish roughness. In this research, in order to resolve these problems, NURBS surface is divided into some patches and the tool path is generated on each patch. This paper proposes a surface divide method and makes a comparison between this method and undivided method. As a result, it is found that dividing the surface is helpful for the making shorter tool path and increase the machining efficiency.

Abstract: Nowadays, numerical simulation technique is very popular to estimate and predict the machining parameters such as cutting forces, stresses distribution, temperature and tool wear. The objective of this study is to determine the 0.45%C steel (JIS S45C) flow stress value under high strain rate and temperature. The Johnson and Cook (JC) material model is used as a constitutive equation to describe the high speed cutting process. Compression test and orthogonal cutting test were carried out in order to obtain the required parameters in JC model. Inverse calculation method was used to determine the strain rate and temperature dependency parameter based on several cutting conditions. As a result, validity of verification of method was completed and the flow stress of S45C had been evaluated.

Abstract: This report presents a study on the performance of the fully porous and the partially porous aerostatic journal bearings. Based on the finite volume method and the pressure-velocity coupling scheme of the SIMPLE algorithm with the standard k-ε turbulent model, this study utilized the CFD software to solve the incompressible three dimensional Navier-Stokes equations to calculate the pressure of the flow field in the bearings. The effects of the size of the porous medium, the bearing gap, the eccentric ratio and the rotational speed of the spindle on the characteristics of the bearing such as the pressure distribution, the load carrying capacity and the stiffness were investigated. The computed results revealed that, when the spindle rotated at high speed, the effect of the dynamic pressure became dominant, while the effect of the static pressure became insignificant. Among the three types of journal bearings under investigation, the partially porous aerostatic journal bearing exhibited the highest ratio of output pressure to air volume flow rate. It indicated that, in terms of operational efficiency, the partially porous aerostatic journal bearing is superior to the fully porous aerostatic journal bearing.

Abstract: In this study, a small 6-DOF (degree of freedom) parallel mechanism worktable for machine tool was developed. There are many factors that affect the positioning precision and the accuracy of the machine tool. The uncertainty in position is mainly due to the structural rigidity, the geometric error of the parts and the assembly errors. It is very difficult to estimate the assembly errors and the link parameter of each part. Moreover, it is necessary to make clear the relation of cross talk between each axis for parallel mechanism. In this research, a method to compensate the movement error of worktable based on the measurement result was proposed and discussed. First, in order to measure the positioning accuracy of the worktable a measuring method by using 3-dimensional coordinate measuring machine (CMM) was proposed. After the evaluation, the positioning accuracy of the parallel mechanism worktable, the error of each configuration parameter can be obtained. By the correction of the inverse kinematic program, the improvement of the positioning accuracy of the worktable was confirmed. After calibrating several times, the positioning error became stable within a constant range. It is confirmed that this method to obtain the parameter error is effective, and it is possible to improve the positioning accuracy.

Abstract: It is a FTS with flexible hinge that can be replaceable. It includes flexible hinges, a movable block, piezoelectric ceramic driver and framework. The flexible hinge is installed on inner side of the frame, and the other side is connected with the movable block. The piezoelectric ceramic driver is installed in movable block, and its other end is installed on the end beam of the frame. There is a tool base in the front end of the movable block on which the diamond tools can be fixed. Under the support of the flexible hinges, the tool can move back and front driven be piezoelectric ceramic driver. Through the simulation analyses with finite element technology and experiment, it is certify that the design of FTS is successful and practical to further study.

Abstract: Owing to NAS 979 describes a cutting test for five-axis machine center with a universal spindle, several conditions for C-type machine tool have not been defined yet. This paper proposes a cutting test for a non-orthogonal swivel head and a rotary table type five-axis machine tool (C type) to evaluate its performance. The workpiece consists of 10 machining features. These features include the multi-axis simultaneous machining patterns and the positioning machining patterns. The flat end mill cutters are applied in each machining feature. Cutter location data for the test piece was generated using a commercial CAD/CAM system (UG) and converted to five-axis NC code using a postprocessor created in UG Post Builder. This UG postprocessor is verified through the developed postprocessor utilizing the modified D-H notation. It is also verified using VERICUT^® solid cutting simulation software demonstrated the veracity of the generated five-axis NC code. The machining test is applicable for a variety of five-axis machine tool configurations.

Abstract: Thermally induced errors and geometric errors are two main sources that affect the machine tool accuracy when machining. In the last decade, real time compensation method had received wide attention for its ability to reduce the thermal error cost–effectively. Although real-time thermal error compensation techniques have been successfully demonstrated in laboratories, several difficulties hinder its widespread application. The selection of temperature variables and the setup of the error measurement system are the most critical ones among these difficulties. In this paper, a new on line measurement system and a new model that predicts the thermal error of a turning center are developed. The on-line measurement system using a Renishaw’s LT02S probe system is capable of measuring thermal error of a CNC turning center in real cutting conditions. The neural network uses the cutting conditions as the mapping inputs to avoid problems occurred in the traditional temperature-error mapping model. Results show the proposed measurement system and prediction model can be used to accurately estimate the thermally induced error in real cutting conditions.

Abstract: This study focused on the development of an intelligent motion control system for a nanoscale micro-platform based on the pantograph mechanism. The platform is designed to satisfy the need for achieving high accuracy and process efficiency in the manufacture of increasingly small industrial products. The target platform is mounted on a pantograph and small x-y sliders, and is driven by a traditional X-Y platform with common precision. The goal of this study is to drive the target platform to move in the region of 20mm × 20mm for the positioning, and repeatedly positioning accuracy error is less than 800 nm by a traditional X-Y platform. Due to the different PID parameters will affect platform positioning accuracy and system response for the two axes, the optimal PID parameters by the Genetic algorithms was proposed. The simulation and experimental results indicated that the proposed method is feasible for development of an intelligent motion control system for the nanoscale micro-platform positioning.

Abstract: High precise positioning and high-speed performance are demanded for servo mechanical systems in recent industry. However, the nonlinear friction is a main factor to make the positioning mechanism imprecision. To improve the positioning the positioning precision of stages driven by ball-screws, there are two types of compensations including model and non-model based methods. In this study, the LuGre friction Model is applied to model the nonlinear friction behavior for a ball-screw driven stage. A Modified Charge Search System (MCSS) was proposed to identify the system’s parameter. After the system’s parameters are obtained, a feed-forward control integrated with a disturbance observer is proposed to eliminate the external disturbance and improve its tracking performance.