Xinyong Mao

A measurement strategy and an error-compensation model for the on-machine laser measurement of large-scale free-form surfaces

This study presents a novel measurement strategy and an error-compensation model for the measurement of large-scale free-form surfaces in on-machine laser measurement systems. To improve the measurement accuracy, the effects of the scan depth, surface roughness, incident angle and azimuth angle on the measurement results were investigated experimentally, and a practical measurement strategy considering the position and orientation of the sensor is presented. Also, a semi-quantitative model based on geometrical optics is proposed to compensate for the measurement error associated with the incident angle. The normal vector of the measurement point is determined using a cross-curve method from the acquired surface data. Then, the azimuth angle and incident angle are calculated to inform the measurement strategy and error-compensation model, respectively. The measurement strategy and error-compensation model are verified through the measurement of a large propeller blade on a heavy machine tool in a factory environment. The results demonstrate that the strategy and the model are effective in increasing the measurement accuracy.

An approach based on singular spectrum analysis and the Mahalanobis distance for tool breakage detection

The failure of cutting tools significantly decreases machining productivity and product quality; thus, tool condition monitoring is significant in modern manufacturing processes. A new method that is based on singular spectrum analysis and Mahalanobis distance are combined to extract the crucial characteristics from spindle motor current to monitor the tools condition. The singular spectrum analysis is a novel nonparametric technique for extracting the properties of nonlinear and nonstationary signals. However, because the components are not completely independent, the original singular spectrum analysis eventually leads to misinterpretation of the final results. The proposed method is used to overcome the weakness of the original singular spectrum analysis. The singular spectrum analysis algorithm is adopted to decompose the original signal and the useful singular values that correspond to the tool condition can be extracted. The Mahalanobis distance of the singular values is proposed as a feature that can effectively express the tool condition. The experiments on a CNC Vertical Machining Center demonstrate that this method is effective and can accurately detect the tool breakage in mill process.

A complete methodology for identifying dynamics of heavy machine tool through operational modal analysis

Heavy machine tool work under such high-load conditions that chatter vibrations are prone to occur, which significantly diminishes the machining efficiency and quality. Stability lobe diagrams are commonly used to select appropriate spindle speed and axial depth of cut to get rid of chatter and maximize the material removal rate. However, this needs precise identification of the dynamics of the entire machine tool structure, especially in the low-frequency range. Operational modal analysis has been the proven technique for estimating dynamic characteristics of machine tool structures in operation conditions. In this article, a complete methodology was presented for employing operational modal analysis for heavy machine tool in machining conditions. A random cutting exciting method originally presented by Minis is modified which generates pseudorandom impulse force to excite a heavy vertical lathe structure. And the excitation signal of random cutting force was modeled to analyze the effect of cutting parameters on energy and frequency band of the excitation. One operational modal analysis method, the pLSCF (referred to as PolyMAX) method, was employed to estimate modal parameters during machining. It was also observed in chatter tests that the operational modal analysis results are more accurate than the traditional impact test results in characterizing the dynamics of machine tool structure in machining.

A Novel Vibration Exciting Method for NC Machine Tools

Considering that NC machine tools are difficult to be excited, a novel method of executing the self-exciting of structural vibration by applying a machine tool movement, which is proved to be effective by machine tool testing, is presented in the proposed study. The influence of movement parameters on the vibration response is studied by testing the designed movement under different parameters and comparing the characteristics of vibration response signals in the time and frequency domains. The proposed method is validated by comparison of the testing frequencies and those of experimental modal analysis (EMA). As this method does not need additional vibration-excitation equipment, it is suitable for online testing and analyses of NC machine tools, especially for heavy-duty machine tools. The latter ones are difficult, even impossible, to be excited.

The multimode frequency analysis and its effect on the position accuracy of the sorting arm during operation

Inertial vibrations are one of the crucial problems in the chip sorting process. High acceleration excites the sorting machine structure up to high frequencies so that the multimode vibration appears. The multiple-input nature of operational excitation depends on the varying machine configurations and running accelerations. In this article, the strain gauges are used to study the dynamic behavior of a light-emitting diode chip sorting arm which runs in high speed with varying configurations. This analysis is done in three stages. In the first step, the structure dynamic characteristics are compared between the static and operation states. It points out that some high frequency may display different characteristics in the states. The second step aims to find the effects of different configurations on the sorting arm dynamic vibration during operation. In the last step, the sorting arm is sorted at different levels of running excitation, and the effects of the multimode vibration on chip accurate position are discussed. This study provides an approach to study the dynamic behavior of high-speed running structure within true boundary conditions and actual force and vibration levels.

Investigation on the dynamic behavior of machine tool with respect to different worktable feed rates

With the rapid application of high speed machining, the feed rate of moving parts has an important influence on the dynamics of high speed machine tool. In this paper, a method of modal decoupling of Operational Deflection Shapes (ODS) was proposed to investigate the dynamic behavior of machine tool with respect to different worktable feed rates. Based on this method, the dominant frequencies vibration of the machine tool at different frequencies and its transformation at different worktable feed rates was identified. Finally, a series of empty running experiments were performed under different worktable feed rates. The experimental results show that (i) the change of worktable feed rate will affect the contribution of different modes vibration to the machine tool vibration; (ii) the contribution of a certain mode vibration to the machine tool vibration is different at different worktable feed rates; (iii) with the worktable feed rate increasing, the change rule of dominant frequency vibration participation is from multimode participation to single mode dominant, and then to the multimode participation. This research will provide valuable information for the selection of machine tool operation parameters and the optimization design of machine tool structure.

A new approach to identify the ball screw wear based on feed motor current

Ball screw drives are one the most important means of delivering high precision motion in the industries. Due to the lubrication condition in the ball screw is usually a mixed or boundary lubrication, and this creates wear at the contact area in a ball screw when the motion of a ball screw is in the acceleration or the deceleration step. And the transmission performance and lifetime of ball screw would be decreased, because the wear of ball screw can increase the backlash of a ball screw. For the inaccurate motion of the feed drive systems, there were some troubles in the production. A predictive maintenance should be take to detect the wear condition of ball screw before the deterioration of the positioning accuracy trouble occurs. Mostly researcher are mainly based on through the vibration signals measured by vibration sensors to research the wear of ball screw, or other methods that were not easy application in industry site. This paper puts forward a method to detect the wear condition of ball screw, Base on the motor current signals monitor the states of the ball screw. The proposed method can not only monitor the wear condition of the ball screw but also avoid the influence of the interference from environment.

A Complete Methodology for Estimating Dynamics of the Heavy Machine Tool Structure

Dynamic properties of machine tool structure under machining operations are different from static results of impact or shaker tests. This paper proposes a complete methodology for applying operational modal analysis (OMA) in the case of heavy machine tool to estimate the dynamic modal parameters of the whole structure during machining. A random cutting technique is presented to meet the white noise excitation requirement of OMA. This technique is realized by interrupted cutting of a specially designed workpiece, of which the surface is modulated with pseudorandomly distributed teeth and channels. The resulting cutting force excites the structure in all three directions. Finally, Machine tool dynamic modal parameters are estimated and verified.

Research and Development of the Thermal Error Compensation Embedded in CNC System

Thermally-induced errors are the major contributor to the inaccuracy of the high-precision machine tools. Limited by CNC system’s close-loop structure, the present thermal error compensations are difficult to be embedded in CNC system and only can be as an additional unit out of the CNC system. This paper designed and developed an embedded error compensation module based on the opening CNC system with field bus. The presented embedded error compensation module can exchange data with the CNC system main control module in real time, share the system resources and provide on-line temperature measurement and thermal model calculation. Then the position-dependent thermal error can be compensated by correct the coordinates command during CNC system interpolation progress. An experiment was carried out to verify the performance of the compensation system.