Machine Tool Motion Research Articles

A Feedrate-Constraint Method for Continuous Small Line Segments in CNC Machining Based on Nominal Acceleration

When the CNC machining of continuous small line segments is performed, the direction of the machine tool movement will change abruptly at the corner of adjacent line segments. Therefore, a reasonable constraint on the feedrate at the corner is the prerequisite for achieving high-speed and high-precision machining. To achieve this goal, a feedrate-constraint method based on the nominal acceleration was proposed. The proposed method obtains the predicted value of acceleration during the machining process by the machining trajectory prediction and acceleration filtering. Then, the feedrate at the corner is constrained, according to the predicted acceleration. Specifically, for any corner of adjacent line segments, the proposed method assumes that the CNC machining of a short path centered on the corner is carried out at a constant feedrate. First, the actual machining trajectory is predicted according to the transfer function of the servo system. Then, the nominal acceleration, when the CNC machining is carried out to the corner, is calculated and processed by a low-pass FIR filter. Last, the feedrate-constraint value at the corner is obtained according to the nominal acceleration and the preset normal acceleration. The advantage of the proposed method is that it can be used for different machining paths consisting of long segments or continuous small segments and it has no special requirement for the accuracy of the machining path. As a result, the feedrate-constraint value obtained is reasonable and the smooth machining process can be ensured. The simulation results in both 2D and 3D machining paths show that the proposed method is insensitive to the length of the line segment and the angle of the corner, and the calculated feedrate-constraint value is close to the theoretical value, which has good stability and versatility. In contrast, the feedrate-constraint values obtained by conventional methods change abruptly along the machining path, especially in the 3D simulation, which will damage the machining quality. The experiment was performed on a three-axis CNC machine tool controlled by a self-developed controller, and a free-form surface workpiece was machined by a conventional feedrate-constraint method and the proposed method, respectively. The experimental results showed that the proposed method can make the feedrate of the machining process higher and more stable. Then, machining defects such as overcutting and undercutting can be avoided and the machining quality can be improved. Therefore, the article proposes a new method to constrain the feedrate at the corner of continuous small line segments, which can improve the machining efficiency and quality of the CNC machining.

Condition for machining feasibility for a five-axis machining center

Recently, five-axis machining centers with non-orthogonal rotation axes have been developed to achieve a high stiffness and large machinable area. Although such machine tools can expand the manufacturing field, adopting not only orthogonal axes, but also non-orthogonal axes, requires generalization of conventional form-shaping theory. Moreover, since it is not possible to imagine simultaneous five-axis movement of machine tools with non-orthogonal axes, it is important to analyze the validity of the developed machine tools with non-orthogonal axes using generalized form-shaping theory by a structure analysis method. In the present paper, generalization of the form-shaping function is first performed, and the conditions whereby machining centers with non-orthogonal axes can achieve every position and posture are clarified for an index showing the validity of the machine tool structure. Finally, the machining feasibility is verified for a machine tool structure with non-orthogonal axes using the derived conditions.

Time-Varying Error Prediction and Compensation for Movement Axis of CNC Machine Tool Based on Digital Twin

Currently, it is a challenging issue to predict the thermal-induced time-varying error for the movement axis of the computer numerical control (CNC) machine tool. Subsequently, it is a challenge to predict the machining accuracy for a certain workpiece when the machine tool and numerical control program are determined. In order to resolve this shortcoming, the time-varying error prediction and compensation for the movement axis of the CNC machine tool was studied in this article, and the advantage of digital twin in predicting the changing trend for the physical entity was applied. The framework of time-varying error prediction and compensation for the CNC machine tool's movement axis was proposed based on the digital twin concept, which includes the physical entity layer, data transmission layer, function execution layer, and application service layer. The digital twin of the movement axis, containing a time-varying error model based on heat transfer theory and visual model, was created and packaged as a ready-to-use software for predicting and compensating time-varying error. The prediction experiment of the hole-pitch time-varying error for a workpiece was carried out. It was found that the difference between prediction error and actual error is only −0.2–2.6 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m. The fluctuation range of hole-pitch error is reduced by 69.19% with the time-varying error compensation in real-time. The experimental results show that the method of time-varying error prediction and compensation based on a digital twin is an effective and feasible scheme.

Forward Kinematics Model for Evaluation of Machining Performance of Robot Type Machine Tool

Robot-type machine tools are characterized by the ability to change the tool posture and machine itself with a wider motion range than conventional machine tools. The motion of the robot machine tool is realized by simultaneous multi-axis control of link mechanisms. However, when the robot machine tool performs a general milling process, some problems that affect the machining accuracy occur. Moreover, it is difficult to identify the motion errors of each axis, which influence machining accuracy. Thus, it is difficult to adjust the servo gain and alignment error. In addition, the machining performance is unidentified because of the rigidity differences when the posture changes. In this study, the focus was on robot-type machine tools consisting of a serial and a parallel link mechanism. A geometric model is described, and the forward kinematics model is derived based on the geometric model. Machining tests were then carried out to evaluate the machining accuracy by measuring the machined surfaces and the simulated motion of the tool posture based on the proposed forward kinematics model to identify the mechanism that affects the machined surface roughness and surface waviness. As a result, it was shown that the proposed model can separate and reproduce the behavior of each axis of the machine. Finally, it was clarified that the behavior of the second axis has a great influence on the tool posture and machined surface.

Vision Based Two-Dimensional Measurement of Machine Tool Motion Trajectory

Vision Based Two-Dimensional Measurement of Machine Tool Motion Trajectory

Synthesising of flexural spindle head micro drilling machine tool in PLM environment

The advantage of building a machine in virtual environment helps in detection of the collision of the machine tool components with the work piece and effective process planning. Further every machine tool is virtually planned, designed and developed in product lifecycle management (PLM) platform before it is actually manufactured. This paper explains the synthesis of flexural spindle head used for micro drilling is in the PLM platform. The systematic procedure is adopted for validating the machine tool design in PLM environment. The first step deals with the hierarchy of assembly components that are designated using various possible kinematic chains. The second step involves assigning the required motions of the assembly components. The final step is the virtual control of the machine tool movements with the help of the virtual controller and respective post processor. On completion of all the steps the validation is carried out on the work piece.

Synthesizing of Flexural Spindle Head Micro Drilling Machine Tool in PLM Environment

The advantage of building a machine in virtual environment helps in detection of the collision of the machine tool components with the work piece and effective process planning. Further every machine tool is virtually planned, designed and developed in product lifecycle management (PLM) platform before it is actually manufactured. This paper explains the synthesis of flexural spindle head used for micro drilling is in the PLM platform. The systematic procedure is adopted for validating the machine tool design in PLM environment. The first step deals with the hierarchy of assembly components that are designated using various possible kinematic chains. The second step involves assigning the required motions of the assembly components. The final step is the virtual control of the machine tool movements with the help of the virtual controller and respective post processor. On completion of all the steps the validation is carried out on the work piece.

A Geometric Accuracy Error Analysis Method for Turn-Milling Combined NC Machine Tool

Turn-Milling Combined NC machine tool is different from traditional machine tools in structure and process realization. As an important means in the design stage, the analysis method of geometric accuracy error is also different from the traditional method. The actual errors and the error compensation values are a pair of "symmetry" data sets which are connected by the movement of machine tools. The authors try to make them more consistent through this work. The geometric error terms were firstly determined by topological structure analysis, then based on homogeneous coordinate transformation and multibody system theory, the geometric error model was established. With the interval theory, the function rule of sensitivity of geometric error sources to spatial errors was analyzed in detail, and the global maximum interval sensitivity of nine geometric error sources was extracted, providing a theoretical basis for error compensation and precision distribution. The geometric error sensitivity analysis method proposed in this paper can accurately evaluate the influence weights of each error term on the machining accuracy, and identify the important sensitive error terms with great influence on the machining accuracy from many error terms.

Machine tool movement control method combining the benefit of software and real-time interpolator for sculpture surface machining

In the traditional sculpture surface machining process, the G01 code is still the mainstream trajectory. Furthermore, real-time feedrate scheduling and corner smooth algorithm in controller constitute the mainstream method to improve the machining process of short line G01 code in sculpture surface machining. However, the G01 code’s discontinuity and the limits of real-time calculation capacity hinder the use of high-speed machine tools and the accuracy of the machined part. In this article, a new method for sculpture surface machining that considers the advantages and disadvantages of both the computer-aided manufacturing software and the real-time controller is presented to promote the use of a continuous curve tool path. The method mainly transfers the computing-intensive feedrate scheduling and trajectory optimization algorithm in the real-time controller to the computer-aided manufacturing software. Furthermore, the computer-aided manufacturing software generates the machining data, which contain the geometry and feedrate information of the machining process. Finally, the real-time interpolator and the mathematical form of computer-aided manufacturing–generated data are designed simultaneously. In the method, the real-time controller can be designed as simple as possible to release more computing resources to the other real-time intelligent modules. The powerful computational capacity of the software guarantees the optimality of the machining process.

Measurement of dynamic error motion of 5-axis machine tool with Laser R-test device

Measurement of dynamic error motion of 5-axis machine tool with Laser R-test device

Quality characteristic decoupling planning of different meta-action units for computer numerical control machine tool

Since computer numerical control machine tool is composed of multiple meta-action units to achieve one specific function, including the meta-action units' own quality, it still needs to control the coupling relationships among different meta-action units' quality characteristics to guarantee the whole machine's quality. In this article, a method of quality characteristic decoupling planning based on meta-action unit for computer numerical control machine tool was proposed. Firstly, the coupling constraint models based on meta-action unit were established. Secondly, the comprehensive coupling strengths of meta-action units were calculated and introduced into the design structure matrices. Thirdly, multidisciplinary design optimization method was adopted to obtain the optimized control sequence of different meta-action units' quality characteristics. What is more, automatic pallet changer rotary motion of computer numerical control machine tool was taken as an example to illustrate the rightness and effectiveness of this method.

A Novel Method for the Measurement of Geometric Errors in the Linear Motion of CNC Machine Tools

In order to improve the accuracy of the linear motion of computer numerical control (CNC) machine tools, a novel method based on a new type of 1-D (1-dimensional) artifact is proposed to measure the geometric errors. Based on the properties of the displacement measurement of a revolutionary paraboloid and the angle measurement of plane mirrors, the 1-D artifact can be applied to identify position errors and angle errors. Meanwhile, the concrete 6 degrees-of-freedom error identification method is described in this paper in sufficient detail. Through measuring the 1-D artifact horizontally and vertically using the machine tool, the geometric errors can be obtained by calculating the deviation between the characteristic parameter of the 1-D artifact measured by the machine tool and that measured by a more precise method, for example, laser interferometry. Experiments were carried out on a coordinate measuring machine, and the validity and accuracy of the method were discussed by comparing the result with the identification error measured by a laser interferometer.

Key point selection in large-scale FBG temperature sensors for thermal error modeling of heavy-duty CNC machine tools

Thermal error is one of the main factors that influence the machining accuracy of computer numerical control (CNC) machine tools. It is usually reduced by thermal error compensation. Temperature field monitoring and key temperature measurement point (TMP) selection are the bases of thermal error modeling and compensation for CNC machine tools. Compared with small- and medium-sized CNC machine tools, heavy-duty CNC machine tools require the use of more temperature sensors to measure their temperature comprehensively because of their larger size and more complex heat sources. However, the presence of many TMPs counteracts the movement of CNC machine tools due to sensor cables, and too many temperature variables may adversely influence thermal error modeling. Novel temperature sensors based on fiber Bragg grating (FBG) are developed in this study. A total of 128 FBG temperature sensors that are connected in series through a thin optical fiber are mounted on a heavy-duty CNC machine tool to monitor its temperature field. Key TMPs are selected using these large-scale FBG temperature sensors by using the density-based spatial clustering of applications with noise algorithm to reduce the calculation workload and avoid problems in the coupling of TMPs for thermal error modeling. Back propagation neural network thermal error prediction models are established to verify the performance of the proposed TMP selection method. Results show that the number of TMPs is reduced from 128 to 5, and the developed model demonstrates good prediction effects and strong robustness under different working conditions of the heavy-duty CNC machine tool.

Analytical models for machine tool motion behavior assessment bench mark subjected to great earthquake

Analytical models for machine tool motion behavior assessment bench mark subjected to great earthquake

Motion control analysis of hybrid grinding and polishing machine tool for blade finishing

Blade is the key component in the energy power equipment of turbine, aircraft engines and so on. Research on the process and equipment for blade finishing becomes one of important and difficult points. The motion control of hybrid grinding and polishing machine tool is different from that of traditional machine tool, and there exists motion coupling phenomenon when it moves as a whole. In order to control precisely motion of machine tool, the inverse and forward kinematics solution considering motion coupling of parallel mechanism was solved based on designed hybrid grinding and polishing machine tool for blade finishing in this paper. Firstly, the coupling factor of parallel mechanism was taken into account in the new model, and the inverse kinematics solution was modified to improve the solution accuracy. Then, the forward kinematics solution of machine tool based on inverse kinematics solution was solved. Finally, the motion simulation was made to verify the inverse and forward kinematics algorithm based on kinematic coupling factor of parallel mechanism by simulation software ADAMS. The simulation result shows that the inverse and forward kinematics solution deduced is correct, which provides theoretical basis for the motion control scheme of machine tool calibration and inspection control system.

Smooth flank milling tool path generation for blade surfaces considering geometric constraints

In practical machining, flank milling path generation methods of blade surfaces should concern machine axes’ motions to improve machined surface quality, as well as the gouging-avoidance with the hub surface and the geometric deviation constraints. The smoothness of the tool orientation in the part coordinate system cannot assure the smooth rotary axis motions of a five-axis machine tool, since the conversion of tool orientation from the part coordinate system to the machine coordinate system is nonlinear. A flank milling path is represented with the cutter reference point trajectory and the displacement curves of the two rotary axes in this research, so that the machine axes’ motions along the tool path can be directly smoothed when generating the path. The point-to-surface distance function is adopted to evaluate the geometric deviation, and an algorithm based on the differential evolution algorithm is presented to solve the distance function robustly. The sum of the squares of the first, second, and third derivatives of the cutter reference point trajectory and the two rotary axes’ displacement curves are adopted as the smoothness indicators. Then, a flank milling path generation model for blade surface smoothing the machine axes’ motions and considering the hub surface gouging-avoidance and geometric deviations constraints is developed. Numerical examples and machining experiments for five-axis flank milling of a blade surface are given to confirm the validity and effectiveness of the proposed approach.

A Gouge-Free Tool Axis Reorientation Method for Kinematics Compliant Avoidance of Singularity in 5-Axis Machining

Abstract When a cutter traverses a region local to the singularity in 5-axis machining, the stability of machine tool motion may be violated and inevitably lead to a reduction in machining quality and accuracy. In this paper, the underlying cause of the singular machine behaviors is first investigated by differentiating tool path motions, on the basis of the tool path motion expressions in part and machine coordinate systems. A further investigation indicates abrupt kinematic changes to be inevitable when the rotary axes approach a singularity. To eliminate such possible singular risks in 5-axis machining, a local tool path modification method is proposed by adjusting the two rotary axes out of a singular configuration. The critical kinematics smoothing and the consequent gouging concerns resulting from reorientation are comprehensively incorporated in the process of singularity avoidance, by means of a novel tool orientation optimization model. Specifically, the algorithm starts with the determination of an appropriate adjustment range in a simple yet effective manner, and then the primary rotary axis is adjusted in a constrained region away from zero, so as to avoid singularity. After that, the second rotary axis is accordingly adjusted, with no gouging requirements being violated. In this way, singularity problems in 5-axis machining are solved, and both the machine axes kinematics and surface gouging errors are under control. Machining simulation and laboratory experiments were conducted to validate the effectiveness of the proposed method.

The Method of Geometric Error Measurement of NC Machine Tool Based on the Principle of Space Vector’s Direction Measurement

To achieve the quick and accurate calibration of the geometric errors of NC machine tool, a new method with laser tracker on the basis of space vector’s direction measurement principle is proposed in the paper. A series of measuring points are mounted on the moving part of the machine tool, and then adjacent measuring points are connected to form a space vector respectively. Due to the motion error of the machine tool, the direction of the vectors composed will be changed. Meanwhile, the deviation of vector’s direction only relates to angular displacement error rather than linear displacement error. Based on the characteristic, the change of vectors’ direction is measured by laser tracker based on the multi-station and time-sharing measurement during the motion of machine tool, and then the angular displacement errors and linear displacement errors of each axis can be accurately identified successively, which reduces the complexity of error identification. By establishing the mathematical model of geometric error measurement of machine tool based on the principle of space vector’s direction measurement, the base station calibration algorithm by measuring the motion of the designed precise turntable, the measuring point determination algorithm and geometric error separation algorithm are derived respectively, and the accuracy of these algorithms are verified by simulations. In addition, the results of the experiments show the feasibility of the proposed method.

Configuration optimization of laser tracker stations for position measurement in error identification of heavy-duty machine tools

Location configurations greatly affect the accuracy of sequential multilateration measurements using a laser tracker. This paper presents an approach that optimizes the locations of a laser tracker to improve the accuracy of position measurement in geometric error identification for heavy-duty machine tools. The approach is based on a physical model of the whole working space with the considered constraints determined by the structures of heavy-duty machine tools. The optimal location configuration of the laser tracker is searched by minimizing the average error magnification factor for all the measurement points on the machine tool movement trajectory. The Monte Carlo method is then used to evaluate the final measurement uncertainty of the sequential multilateration measurement under different configurations considering the influence of the repeatability error. The experimental validations are conducted on a coordinate measuring machine and a realistic floor-type milling machine with a 5 m × 2 m × 1 m working space. The experimental results show that position measurement of the optimized configuration has higher accuracy with an improvement of 57.5% compared with that of another feasible configuration analyzed in the uncertainty evaluation. The validity of the location optimization approach is further verified for identifying geometric error with high accuracy using an error identification experiment based on the nine-line method.

The analysis research on the movement of machine Tools for processing free-form surface by ball cutter

In order to resolve the function design issue of the machine tools for processing free-form surface, the workpiece is divided into n equal parts; each one consists of a generatix and a guideline. Using the generative design principle, the mathematical description matrix of the ball cutter and the machined surface are established, and thus determine the cutter’s positional and gesture matrixes and the possible movement scheme for processing free-form surface. The method is used for processing free-form surface by different tools; the results provide a good theoretical basis for the optimal design for machine tools and the design of combined machine tools.