NC Machine Position Accuracy Testing and NC System Error Compensation Analysis

The geometric accuracy of linear axes lays a foundation for the machining accuracy of 5-axis machine tools. Although some geometric error identification and compensation approaches were developed for linear axes, the implementation efficiency and cost remain to be improved. In this paper, a novel 13-line identification method by using a laser interferometer is proposed, and the linear axes are compensated through the modification of compensation parameters in the NC system. The verification experiments are conducted through the tri-axial movement accuracy measurement and manufactured National Aerospace Standard test piece inspection, in which three circumstances, i.e., without compensation, with lead screw error compensation, and with error compensation based on the 13-line method, are compared. By using the 13-line method, the average accuracy of the tri-axial movement and test pieces are improved by 82.2 and 41.9 %, respectively, whereas the accuracy stability are improved by 77 and 46 %, respectively. Results show that the proposed identification and compensation method is effective and efficient.

With the continuous development of manufacturing industry, the application range of NC machining technology has been further expanded. The contour accuracy is strongly related to the NC machining quality as a key machine tool performance indicator. Its application efficiency is plainly low as the majority of offline compensation-based contour accuracy adjustments rely heavily on manual experience. Moreover, the isolated research on automatic error compensation and its combination with algorithms does not start with the characteristics of contour accuracy in data processing. Therefore, based on the advantages of strong the robustness of the fuzzy algorithm and the high effectiveness of parameter adjustment, an automatic compensation method for NC machining contour error based on fuzzy control is proposed. The contour error prediction model is designed according to the machining path, and then the automatic compensation strategy for contour error under fuzzy control is designed based on the feed speed. The results showed that under this method, the contour error can reach a maximum of 0.06 and a minimum of 0.025, which was 0.015 lower than the minimum contour error of genetic algorithm. This indicated that the method greatly reduced the CNC machining contour error and improved the contour accuracy, as well as reducing the time cost of contour error compensation, improving the efficiency of contour error compensation, and realizing the automation of error compensation control capability. This is helpful for advancing CNC machining automation technology and supporting the intelligent development of machinery manufacturing.

Virtual machining, based-on the model of a machining system, aims to simulate, evaluate and optimize the actual machining process with high sense of reality. It provides digital off-line optimization tools for NC machining. Taking advantage of virtual machining used in machining process simulation, one can build the framework of optimization system on NC machining so that the processes of reliability verification, cutting parameter optimization and error compensation can be integrated into one system to improve machining processes comprehensively. The optimization is realized via modifying NC programs. Several key issues such as virtual machining, cutting parameters optimization, error prediction and compensation are also highlighted. Optimization systems based on virtual machining have been developed to demonstrate the effectiveness of off-line optimization for different purposes. The results show that the machining process is obviously improved.

One parallel robot had been developed to support the sensor and make it orient precisely in the test of Inertia! Confinement Fusion (ICF). In this paper some methods for improving the accuracy of parallel robot were discussed to guarantee the position accuracy of the sensor in vacuum environment. Firstly, error sources decreasing the accuracy of parallel robot were analyzed and some measures in structure design were adopted to improve the accuracy. Then error model of the robot was set up by the complete differential theory based on the parallel robot inverse kinematics equation. Therefore the position error of the robot could be reduced further by error compensation, which was realized through software. It's proved by the experiments that the error model was of simple and effective, the locating accuracy of parallel robot could be improved through error compensation markedly and the position accuracy reached 1mum, which could satisfy the sensor locating accuracy requirement of ICF research.

Geometric error is the major part of the inaccuracy of NC machine tools. This paper presents an integrated geometric error measuring, modeling and compensation method for NC machine tools. 21 geometric errors are anlysised. Taking three-axis machine tool as example, the positioning error is measured by a laser interferometer. A new method is proposed using MATLAB to fit the curve measured. The efficient methodology is developed and described for the establishment of the geometric error model. Finally, the compensation mechanism is executed based on the model. The experimental results show that the geometric accuracy of the machine tool is significantly improved by the error compensation.

Machining time is a major performance criterion when it comes to high-speed machining. CAM software can help in estimating that time for a given strategy. But in practice, CAM-programmed feed rates are rarely achieved, especially where complex surface finishing is concerned. This means that machining time forecasts are often more than one step removed from reality. The reason behind this is that CAM routines do not take either the dynamic performances of the machines or their specific machining tolerances into account. The present article seeks to improve simulation of high-speed NC machine dynamic behaviour and machining time prediction, offering two models. The first contributes through enhanced simulation of three-axis paths in linear and circular interpolation, taking high-speed machine accelerations and jerks into account. The second model allows transition passages between blocks to be integrated in the simulation by adding in a polynomial transition path that caters for the true machining environment tolerances. Models are based on respect for path monitoring. Experimental validation shows the contribution of polynomial modelling of the transition passage due to the absence of a leap in acceleration. Simulation error on the machining time prediction remains below 1%.

A New Kind of Numerical Control Method

In terms of the realization for high accurate and effective machining of complicated shape having under cut shape and free formed surface, five axis controlled machine tools become indispensable because they have any tool position and attitude to avoid the collision between machine tool elements and work piece. These five axis machine tools have a technical problem as machining accuracy which is inferior to that of three axis NC machine tools.In order to estimate the motion accuracy of NC machine tools, various kinds of certificate methods like circular cone cutting test prescribed with NAS and DBB method using the circular motion have been developed and standardized. These methods are mainly used for the motion testing of three axis NC machine tools and therefore new method and measuring equipment for five axis NC machine tools are needed.This study aims at developing the motion accuracy measurement system which can measure various kinds of motions like linier motion, circular motion and continuous curved motion under simultaneous five axis controlling and establishing the compensate method of tool motion. Measuring results using the developed system and ISO230-4 standard and newly arranged tool motion to machine the reverse circular cone and rhombic shape described in this paper shows that the developed system has sufficient practicality and reliability.

The relative position and orientation deviation between grinding wheel and workpiece caused by geometric errors affect the machining accuracy of five-axis CNC gear profile grinding machine tools directly. Therefore, geometric error modeling and compensation are presented according to homogeneous transformation and differential motion matrix based on the multi-body system theory for the accuracy enhancement of the machine tools. Firstly, the open kinematic chain and the ideal homogeneous transformation matrix from workpiece to grinding wheel are established according to the topological structure of the CFXZAY-type, five-axis gear profile grinding machine tools and the basic homogeneous transformation matrix between coordinate frames. Secondly, the homogenous transformation matrices of linear pairs and rotary pairs with geometric errors are calculated, and the relationships of the error propagation from workpiece to grinding wheel are acquired as well on the basis of 37 geometric error components analysis. The geometric error models including position and orientation errors of grinding wheel in workpiece coordinate system are obtained with matrix multiplication by using small-angle approximation and ignoring the second-order and high-order error terms. Then, the Jacobian is obtained by using transforming differential motion matrix of each motion axis to compensate the integrated error components of the grinding wheel, which can make the compensation effective and convenient with using the corresponding homogeneous transformation matrix. Finally, error measurement, error compensation, and machining experiments are carried out on a five-axis CNC gear profile grinding machine tool SKMC-1200W/10 to verify the applicability and effectiveness of the proposed error modeling, error compensation, and research approach.

The CNC Machine Tools with high efficiency, high precision and high stability is the inevitable trend of the modern manufacturing industry development.This development trend puts forward the extremely high request to the CNC machine tool error compensation technology. However, the error of CNC machine tools have a variety of reasons, these errors affect each other, which leads to the inevitable error. At the high efficiency processing, we must ensure the high stability and high precision of the machine tool. Which requires the error compensation function to maintain high stability and high compensation accuracy, when the CNC machine tool is under the action of high speed and dynamic load. The rapid development of computer technology provides us with more possibilities to solve error compensation. CNC machine tool error compensation technology based on computer is an important means to improve the machining accuracy in the manufacturing industry, which reduce the error in the production process and increase the production efficiency. This paper firstly analysis the error classification and error source percentage of CNC machine tools. And then it analysis the movement error when the CNC machine tools is working. Finally, it puts forward the key technique and application of error compensation based on computer.

The spindle error and geometric error are the main sources of inaccuracy in CNC machining. With the rising of the machine tool parts' temperature, the spindle error and geometric error increase continually, and the error curves have a nonlinear distribution. To analyze the thermal effects on spindle error and geometric error, an experiment is carried out. To improve the machining accuracy of a CNC machine, an error model is proposed based on orthogonal polynomials. With the application of the orthogonal polynomials, the polynomial regression can be transformed into multiple linear regressions which are easier to calculate. In order to implement the real-time error compensation for the thermally induced spindle error and geometric error, an error compensation method is proposed based on the external coordinate offset. The thermally induced spindle and geometric error are compensated by 90 % compared with no compensation.

Based on the flexibility characteristics of the NC machine tool, which means that various motions can be performed arbitrarily on NC machine tools, a new algorithm for manufacturing a hypoid pinion is proposed. This gets rid of the limitation of traditional mechanical machine tools and their algorithms. When the cutter tilt method is combined with the modification method, a method of manufacturing the pinion with a prescribed-size cutter on an NC machine tool can be realised. This helps to simplify cutter specifications and realise the error compensation of the cutter size. In this paper the algorithm is derived. According to the equations of cutting, the machine settings are calculated.

Blade which is one of the most important thin-walled parts is widely used in aviation, aerospace and automotive fields. At present, the manufacturing technology of blade has been developed towards a new direction of high efficiency, precision and multi-process composition. However, the production efficiency and machining precision of blade are greatly limited due to the elastic, residual stress deformation and other inevitable distortion from NC machining process. In this paper, an adaptive error compensation approach by on-machine measurement for precision machining of blade is proposed and developed. According to the analysis and comparison between the nominal model and the practical measurement result, the adaptive process geometric model is constructed, which accurately describes the composite error compensation for precision machining of thin-walled blade. Firstly, an adaptive error compensation system structure and solution by on-machine measurement is described. Secondly, the error displacement field and anti-deformation compensation quantity is constructed and calculated by statistical analysis of on-machine measurement data. Thirdly, different from the nominal model, the process geometric model is adaptively constructed to solve the part-to-part variation for the actual NC programing of blade with the curve fairing algorithm. Finally, based on the adaptive process model, tool paths used for NC machining process can then be adaptively generated to implement composite error compensation for precision machining of blade. Examples show that the adaptive error compensation approach by on-machine measurement for thin-walled blade is feasible and the result is with high efficiency and accuracy.

Accuracy enhancement of five-axis CNC machines through real-time error compensation

The article is concerned with cam mandrels used for positioning of asymmetrical blanks such as rings and bushings while machining. It is understood that blanks have high bending stiffness. There is GOST 31.1066.04–97, developed for cam mandrels, but according to manufacturers and scientists, the design proposed in GOST has a number of drawbacks, which can be attributed to simplified technology of mandrel parts manufacture. This has been proven by practical experience. The paper points out the defects of standard cam mandrel design and describes a fragment of the process for manufacture of more accurate mandrels. While designing cam mandrel it is necessary to provide for safe attachment of blanks (this requirement is well addressed), to know the influence of main parameters of its parts on the machining accuracy and to prevent damage to base surface of the blank. Practically, the last two requirements are not addressed. A series of articles, including this paper, covers theoretical and experimental studies to meet these requirements. Six prototype mandrels with different number of cams (from three to twelve), model rings and six batches of ring blanks are designed and manufactured for experimental and production-related studies of accuracy of positioning and machining of blanks in cam mandrels. The model rings and blanks in the batches have different diameter of the base hole and its length in order to study these parameters for accuracy of positioning and machining of blanks. In each batch there are at least 30 blanks which makes it possible to process the results obtained in the experiments by mathematical statistics methods. Four batches of blanks have polished base holes, and two batches have turned holes. The model rings and blanks have been subjected to metrological control.