Accuracy Of Machine Tools Research Articles

Theoretical and experimental analysis of the preload degradation of double-nut ball screws

In most applications, preload is applied to double-nut ball screws to improve the positioning accuracy and rigidity. However, the applied preload degrades owing to wear, which lowers the positioning accuracy of the ball screws, and thus affects the accuracy of computer numerical control machine tools. In this study, to obtain the dynamic contact force and sliding velocities on the ball-raceway contact, we established a load distribution model and velocity variation model, respectively. Based on these models, we propose an accurate model for predicting the preload degradation of double-nut ball screws. The degraded preload of three ball screws was measured under high axial load-low speed, medium axial load-medium speed, and low axial load-high speed, with a running stroke of 7.2 million revolutions. The experimental results are in good agreement with the theoretical values calculated by the prediction model, which confirms the validity of the prediction model. Therefore, it was concluded that the proposed preload degradation model can accurately predict the performance and service life of double-nut ball screws.

Special Issue on Machine Accuracy Evaluation

The accuracy of a three-dimensional (3D) positioning system can ultimately be evaluated via measurement of a 3D vector between command and actual end-effector positions at arbitrary points over the entire workspace. This is a simple, yet challenging, metrological problem. The motion accuracy of a machine tool is traditionally evaluated on an axis-to-axis basis, with every error motion of every axis being independently measured as part of a one-dimensional measurement process in a different setup. Toward the ultimate goal of 3D position measurement over the entire workspace, research efforts have offered several new, practical measurement technologies. This special issue covers the technical and academic efforts regarding the evaluation of machine tool accuracy. The papers in this special issue clarify the latest research frontiers regarding machine tool accuracy from a metrological viewpoint. In the first paper, by Montavon et al., error calibration technologies and their management are reviewed within the Internet of production concept. Long-term accuracy monitoring and management are clearly among the most crucial technical challenges faced regarding machine tools, and the work by Xing et al. is related to them. Ibaraki et al. presented machining tests to evaluate the thermal distortion of a machine tool. Peukert et al. studied the dynamic interaction between machine tools and their foundations. Various 3D measurement schemes for determining machine error motions have been investigated by many researchers, and some have been implemented in industrial applications. Kenno et al. and Florussen et al. investigated 3D measurement using the R-test for five-axis machines. Miller et al. studied simultaneous measurement of six-degree-of-freedom error motions of a linear axis. Nagao et al. presented an error calibration method for a parallel kinematic machine tool. The editors appreciate the contributions of all the authors, as well as the work of the reviewers. We are confident that this special issue will further encourage research and engineering work for improving the accuracy and performance of machine tools.

Identification Method of Error Motions and Geometric Errors of a Rotary Axis by R-Test

While evaluating the accuracy of high-precision machine tools, it is critical to reduce the error factors contributing to the measured results as much as possible. This study aims to evaluate both the error motions and geometric errors of the rotary axis without considering the influence of motion error of the linear axis. In this study, only the rotary axis is moved considering two different settings of a reference sphere, and the linear axes are not moved. The motion accuracy of the rotary axis is measured using the R-test device, both the error motions and geometric errors of the rotary axis are identified from the measurement results. Moreover, the identified geometric errors are verified for correctness via measurement with an intentional angular error. The results clarify that the proposed method can identify the error motions and geometric errors of a rotary axis correctly. The method proposed in this study can thus be effective for evaluating the motion accuracy of the rotary axis and can contribute to further improvement of the accuracy of the rotary table.

Dynamic Interaction Between Precision Machine Tools and Their Foundations

The manufacturing accuracy of modern machine tools strongly depends on the placement of the machine tool structure on the factory’s foundation. Civil engineering knows a variety of foundation types and factory planners must carefully consider local circumstances such as the size and the properties of the regional subsoil as well as the individual requirements of machine tools. Two of the major reasons for the effect of the foundation onto the machining accuracy are the added stiffness and the increased mass from the installation site’s foundation. A change of these characteristics greatly affects the dynamic characteristics of the overall machine tool and therefore also the machining dynamics. Although some general rules and guidelines exist for the design of foundations, their dynamic interaction with the supported precision machine tool structures is not well understood yet. This paper presents a series of measurements on two different types of machine tool foundations and highlights the characteristic differences in their dynamic interaction. It also proposes a novel approach to validate the conclusions with the use of foundation and machine tool scale models. These results can serve factory planners of precision targeting shop floors as a valuable guide for deciding on a suitable foundation for lowering the individual machine tool vibrations and/or reducing the dynamic interaction between closely located machine tools.

Automating Accuracy Evaluation of 5-Axis Machine Tools

A wireless non-contact 3D measuring head is used to determine the accuracy of 5-axis machine tools. The measuring head is inserted in the spindle by the tool exchanger automating the measurement routine used. For checking the linear machine axes, a cross shaped artefact containing 13 precision balls is introduced, named Position Inspector, enabling the determination of positioning and straightness errors of two linear axes in one setup. The squareness error between both axes is also determined in this setup. This artefact can be mounted on a pallet system for automatic loading and is measured in a bi-directional run. This artefact can be measured in different orientations (i.e., horizontal, inclined, vertical) and is pre-calibrated with a CMM. The measurement sequence using this artefact is executed in eight minutes and its design and support system is addressed in this paper. The location errors and orientation errors of the axis average line (or pivot line) of both rotary axes are determined with the Rotary Inspector using the same measuring head with a single precision ball. For this, kinematic tests are used from ISO10791-6, e.g., the BK1 test, BK2 test which apply for trunnion or swivel table machines. Derived parameters can be used for machine correction resulting in a significantly improved machine accuracy. An example is given where this correction is performed automatically by implementing this measurement system in the machine’s controller. Finally the machine tool is tested using the BK4 test. For this test all 5-axes are moved simultaneously and the measured displacements between the machine’s spindle and table in X-, Y-, and Z-directions are compared to tolerance levels. This final test reveals the machine’s overall accuracy and dynamic behavior.

Overview of Machine Tool Error Detection Technology

With the rapid development of modern mechanical manufacturing technology, precision and ultra-precision processing technology has become the main trend of the development of modern mechanical manufacturing industry. As a basic tool in mechanical manufacturing, the accuracy of CNC machine tools is an important index affecting the accuracy of workpiece processing. Therefore, the research on improving the accuracy of CNC machine tools has received great attention. In all kinds of high-speed and precision machine tools, the problem of the decline of machining accuracy caused by machine tool errors has become increasingly prominent. How to identify and eliminate machine tool errors quickly and efficiently has become an increasingly urgent task. In this paper, the research status of machine tool error detection technology at home and abroad is analyzed, in order to provide a reference for the development of machine tool error detection technology.

Study on chuck performance of ultra-precision machine tool based on CFRP pair bionic structure

Based on the hardness inspiration of the tortoise structure, this study analyzes the innovative application of the parts of ultra-precision machine tools. Aiming at the problems of machining accuracy, efficiency, and machine cost of ultra-precision machine tools, an innovative design and analysis of chuck of ultra-precision machine tools based on carbon fiber-reinforced polymer and its application to turtle shell structure is proposed. Firstly, the G-element model of carbon fiber composites is deduced and analyzed by layered experiments. Secondly, the original parts of ultra-precision machine tool are simulated and analyzed, and the main sixth-order modal of carbon fiber turtle shell structure is deduced. Finally, the structural parameters of carbon fiber composite “turtle shell” were applied to the surface of chuck claw of ultra-precision machine tool, and the results were compared and analyzed. The results show that the maximum displacements of the chuck are all located at the front of the chuck and the maximum displacements and stresses of the chuck are smaller than those of the original chuck. The maximum stress is the most obvious, the difference is about 240.27 MPa, and the first-order mode increases by 23.4% compared with the original mode. The overall relative mass was reduced by 61.2%.

Method of Evaluating Dynamic Accuracy of Five-axis Machine Tool Based on Test Piece and Comprehensive Evaluation System

Method of Evaluating Dynamic Accuracy of Five-axis Machine Tool Based on Test Piece and Comprehensive Evaluation System

A geometric accuracy analysis and tolerance robust design approach for a vertical machining center based on the reliability theory

Abstract In the present work, an endeavor was proposed a geometric accuracy analysis and tolerance robust design approach for a vertical machining center based on reliability theory. Based on the establishment of accuracy prediction model and key geometric error traceability, combined the prediction accuracy and processing performance requirements of machine tool, the limit state equation of machining accuracy is obtained, and a calculation model of machining accuracy reliability is derived based on reliability theory. The first-order and second moment method is used to obtain an approximate solution of machining accuracy reliability. Taking key geometric errors as tolerance robust design variables, using relationships with manufacturing costs and accuracy reliability, a machine tool cost-geometric tolerance robust design model was established, which is used to realize the tolerance robust design of a vertical machining center. The results show that the method can achieve economical and rational distribution of geometric accuracy in a vertical machining center.

Body diagonal error measurement and evaluation of a multiaxis machine tool using a multibeam laser interferometer

To comprehensively evaluate the accuracy and performance of machine tools, in this paper, a novel measuring method for the body diagonal error of a machine tool using a multibeam laser interferometer as the measuring instrument is proposed. On this basis, a three-dimensional evaluation method for the body diagonal error of a machine tool is proposed. First, a method of measuring the body diagonal error of a machine tool by using a multibeam laser interferometer is proposed. Six errors for the body diagonal of a machine tool can be measured simultaneously, including three position deviations and three angle errors. The mathematical model of the relationship between the measured data and the volumetric error is established. Second, a new evaluation method for the body diagonal error is presented that takes the position deviation and attitude error into consideration. The advantage of this evaluation method is that it fills in the blanks in the evaluation method for the body diagonal error. The method can also be applied to the evaluation of linear axis error. Finally, the proposed measurement method is applied to a vertical three-axis machining centre, and the experimental results show that this measuring method is quite convenient and effective for measuring the body diagonal error of a machine tool. The proposed method is used to evaluate the body diagonal error from measurement data. The results show that the positioning error of the machine tool is 49.37 ??m; the repeatability of the positioning error is 36.85 ??m; and the angle errors are ??????22.47 ??m/m, ??????70.9 ??m/m and 29.7 ??m/m. The traditional method (ISO230-6) is used to evaluate the body diagonal error from measurement data. The results show that the positioning error of the machine tool is 49.4 ??m. Compared with the traditional method, the information from the proposed evaluation method is more abundant, and the evaluation method is systematic and universal.

A linear model for the machine tool assembly error prediction considering roller guide error and gravity-induced deformation

In assembly, error prediction and control according to the error state of the machine parts are the key links to ensuring the accuracy of the entire machine tool. In this study, an assembly error modeling method considering the error of the roller guide and gravity-induced deformation is proposed. First, based on the Hertz contact theory, a nonlinear error propagation model for the roller guide to the moving part is proposed. With this model, the analytical relationship between the straightness error of the guide rail and the errors of the moving part can be established. Then, after the linearization of the guide error model, a machine assembly error model considering the guide error is proposed. This model expresses the linear relationship between the error of the rolling joint surface and the error of the machine tool after the final assembly. Additionally, considering the influence of gravity deformation of structural parts, the gravity deformation is extracted by the finite element analysis method and added to the model by linear superposition. As a result, the model corresponds more with the practical assembly process. Finally, an assembly error prediction method of the precision horizontal machining center is presented, and a case is studied to demonstrate the validity of the proposed model and method.

ACCURACY OF MACHINE TOOLS

ACCURACY OF MACHINE TOOLS

Full-scale measurement of CNC machine tools

Geometric error measurement is crucial for error compensation and machining accuracy of CNC machine tools. However, traditional double-ball bar (DBB) method cannot be used for full-scale measurement of geometric errors due to the constant length of bars. In this paper, a full-scale measurement of geometric errors for CNC machine tools with high efficiency is carried out based on the double ball bar with a ball joint (J-DBB) method. Firstly, the principle of J-DBB is introduced. The J-DBB can be used for a full-scale measurement thanks to its freely measuring radius. Then, the equations of geometric errors which include squareness error, straightness error, roll error, pitch error, and yaw error of the axes are theoretically analyzed. In the end, we compared the theoretical trajectories of circular interpolation with the actual ones. The geometric errors between theoretical trajectories and actual ones can be measured and shown by J-DBB method. The results revealed that the J-DBB can be applied to conduct the full-scale measurement of geometric errors in CNC machine tools. Besides, the measurement range of geometric errors is extended from a two-dimensional circle to a three-dimensional sphere with a radius of 0.259L to 2L.

Self-calibration of rotary axis and linear axes error motions by an automated on-machine probing test cycle

Efficient, precise and automated in-process calibration schemes are essential to improve the accuracy and productivity of five-axis machine tools. This paper presents a new calibration approach, which combines an on-machine measurement cycle and self-calibration techniques, to evaluate the position errors and the error motions of a rotary axis using a touch trigger probe and an uncalibrated cylindrical artefact. This significantly reduces the downtime of machine tools required for the calibration process. In contrast to many common calibration strategies for rotary axes of five-axis machine tools, the presented self-calibration concept does not neglect the positioning errors of the linear axes when identifying the position errors and the error motions of the rotary axis. The self-calibration procedure is able to separate the positioning errors of the linear axes in radial direction, and the radial error motions and the position errors of a rotary axis, as well as the errors related to the uncalibrated artefact. This error separation is realized by a test cycle consisting of four tests in which the measurements are conducted by particular axis movements. Furthermore, an uncertainty analysis of the self-calibration concept is conducted to visualize the uncertainty propagation within the mathematical model. The self-calibration procedure is analyzed by an experimental evaluation, which includes a comparison between the results of the self-calibration approach and an R-Test. This comparison shows that the results of both measurement procedures are consistent.

Stiffness and Damping of Epoxy Granite

The productivity and accuracy of machine tools now became most significant as the cutting conditions changes continuously. Therefore to withstand against these cutting conditions the machine tool structural material must have higher stiffness and damping. This review deals with various research works to study the stiffness and damping of epoxy granite or polymer concrete. It is reported that the epoxy granite shows improved damping and high strength to weight ratio than that of conventional machine tool structures of steel and cast iron.

A review on vibration analysis and control of machine tool feed drive systems

With the development of the modern manufacturing industry, especially that of high-speed and high-precision machining, higher performances of computer numerical control (CNC) machine tools are necessary than ever. For high-precision CNC machine tools, vibration in high-speed machining is an important factor to affect the machining accuracy. As a significant part of CNC machine tools, the dynamic characteristics of the feed drive system have a great impact on the machining performance. Ball screw drive systems are widely applied in the majority of CNC machine tools for the time being. In the process of high-speed machining, the vibration of the ball-screw drive system will affect the stability of the control system and the machining accuracy of CNC machine tools. Thus, further investigation of dynamic characteristics and vibration control methods of the ball-screw drive system will be of great importance to improve the machining performance. A comprehensive review is carried out here to research statuses of vibration analysis and control methods for feed drive systems. Related studies on system modeling, parameter identification, and vibration control technologies are classified and summarized. The advantages and disadvantages of different methods are discussed and compared. Moreover, the linear parameter-varying (LPV) modeling method of ball-screw drive systems and corresponding control technologies are introduced in detail. Finally, related research prospects are systematically presented based on the existing studies, and these research directions which are rarely studied will be of great significance to develop vibration control technologies for CNC machine tool feed drive systems.

Thermal error compensation of linear axis with fixed-fixed installation

To improve the machining accuracy of machine tool, thermal error modeling and compensation methods are proposed from the view of formation mechanism of thermally-induced error of ball-screw linear axis with fixed-fixed installation. The relationships among such factors as heat generation, heat dissipation, thermal stress, temperature rise, thermal expansion, and the resulting thermal error are explored. The thermal error is formulated as the product of an amplitude function with temperature of screw shaft as its independent variable and a cosine function with axial position as its independent variable. The independent variables of thermal error model are temperature of screw shaft, axial position, and the running time. The closed-loop iterative thermal behavior modeling method is proposed to determine accurate temperature of screw shaft by updating rate of heat generation, convective coefficient, thermal contact resistance (TCR), lubricant viscosity, and preload in each sub-step. To demonstrate the modeling method, the thermal errors are predicted and compensated at different working conditions. The results show that the repetitive positioning error is reduced by more than 45%, 36%, and 19% compared to that without the compensation, with pitch compensation and multivariate linear regression analysis (MLRA) compensation, respectively.

A novel method for evaluating the validity of dynamic accuracy test pieces for five-axis machine tools

The detection method for direct machining the standard test pieces, which is commonly employed to exhibit the machining precision of five-axis machine tools, can truly reflect the dynamic accuracy of five-axis machine tools in the actual machining process. Existing theories on explaining the phenomenon that the ability of S-shaped test pieces to detect dynamic accuracy for five-axis machine tools are stronger than those of NAS979 test pieces mostly used the qualitative research methods, and they cannot be quantitatively used to reveal the mathematical relationship between machine tools and test pieces. Therefore, this article makes the first attempt to investigate the quantitative evaluation method for the validity of dynamic accuracy test pieces. The dynamic error function of the servo feed system in the frequency domain was derived first by establishing the mechanical system model and control system model. The method of dipole cancellation was used to acquire a simplified transfer function for the dynamic error affected by the input. Based on the zero-order hold property of the discrete input signals, the expression of dynamic error affected by input in the time domain, which intuitively shows the mathematical relationship among the machine tool performances, the test piece characteristics and the dynamic error of the servo feed system, was obtained. Then, the novel evaluation methods of the linear combinatorial value and the combinatorial linear combinatorial value were proposed. A series of comparative analyses between an S-shaped test piece and an NAS979 test piece were carried out based on the proposed new evaluation methods. A machining experiment conducted on a five-axis machine tool is used to verify the evaluation results.

An approach for accuracy enhancement of five-axis machine tools based on quantitative interval sensitivity analysis

Error compensation is a common method to improve the machining accuracy of five-axis machine tools (FAMTs). Before that, in order to identify the key geometric errors that affect the machining accuracy, sensitivity analysis (SA) in entire machining region or a specified point is usually used. However, since the influence of geometric errors on form errors is different in each interval, the result of SA would be affected, which decreases the compensating accuracy. For this reason, a new approach for accuracy enhancement of FAMTs based on quantitative interval sensitivity analysis (QISA) is presented in this paper. First, based on multi-body system theory and homogeneous transform matrix, the volumetric error model of the FAMT is established. Then, the geometric error model of the FAMT is formulated. Second, development of form error model is involved in this paper to clarify the effect law of geometric error on form error. Moreover, the machining region of S-shaped test piece is divided into five intervals. Then, the QISA method is presented to calculate the sensitivity index of each geometric error and identify key geometric errors at different intervals. Finally, simulations and experiments are conducted to validate the feasibility and effectiveness of the presented method.

Method of Tolerance Design and Verification of Parts Based on Follow-up Crankshaft Grinder with Following Double Grinding Wheel Frame

Calculating the parameters of accuracy along each axis by the comprehensive geometric accuracy and then optimizing as well as assigning the accuracy, which is the cutting-edge way of accuracy design of machine tools. However, the guarantee of the comprehensive geometric accuracy of machine tools is ultimately to implement the tolerance of specific moving parts. In view of the present shortage about accuracy design of machine tools, this paper puts forward to a way to design and verifies the tolerance of movement parts based on the comprehensive geometry precision of K2-1080 Crankshaft Grinder with Following Double Grinding Wheel Frame.