Errors Of Machine Tools Research Articles

Identification of kinematic errors of five-axis machine tool trunnion axis from finished test piece

Compared with the traditional non-cutting measurement, machining tests can more accurately reflect the kinematic errors of five-axis machine tools in the actual machining process for the users. However, measurement and calculation of the machining tests in the literature are quite difficult and time-consuming. A new method of the machining tests for the trunnion axis of five-axis machine tool is proposed. Firstly, a simple mathematical model of the cradle-type five-axis machine tool was established by optimizing the coordinate system settings based on robot kinematics. Then, the machining tests based on error-sensitive directions were proposed to identify the kinematic errors of the trunnion axis of cradle-type five-axis machine tool. By adopting the error-sensitive vectors in the matrix calculation, the functional relationship equations between the machining errors of the test piece in the error-sensitive directions and the kinematic errors of C-axis and A-axis of five-axis machine tool rotary table was established based on the model of the kinematic errors. According to our previous work, the kinematic errors of C-axis can be treated as the known quantities, and the kinematic errors of A-axis can be obtained from the equations. This method was tested in Mikron UCP600 vertical machining center. The machining errors in the error-sensitive directions can be obtained by CMM inspection from the finished test piece to identify the kinematic errors of five-axis machine tool trunnion axis. Experimental results demonstrated that the proposed method can reduce the complexity, cost, and the time consumed substantially, and has a wider applicability. This paper proposes a new method of the machining tests for the trunnion axis of five-axis machine tool.

The Error Analysis of Ultra-Precision Diamond Turning Machine Based on Tool Swing Feeding

In the respect of ultra-precision manufacturing of axisymmetric surface, the machine tool with tool swing feeding has less interpolation error sources compared to the conventional ultra-precision diamond turning machine tool. The installation errors of machine tool greatly affect the manufacturing accuracy of parts. Therefore, based on the multi-body dynamics theory , the structure characteristics of this machine tool, and the parts' relative positions of machine tool, in this paper we established the space geometric model and the integrated error model of machine tool integrated with the actual installation error of machine. The analysis results of the error model show that the relationship between the depth of axis (tool shaft and main shaft of machine) parallelism and the profile error is linear function; when the installation error of swing arm was lower than 150′′,the profile error of parts was less than λ/10.

Novel thermal error modeling method for machining centers

Thermal deformation is one of the main contributors to machining errors in machine tools. In this paper, a novel approach to build an effective thermal error model for a machining center is proposed. Adaptive vector quantization network clustering algorithm is conducted to identify the temperature variables, and then one temperature variable is selected from each cluster to represent the same cluster. Furthermore, a non-linear model based on output-hidden feedback Elman neural network is adopted to establish the relationship between thermal error and temperature variables. The output-hidden feedback Elman network is adopted to predict the thermal deformation of the machining center. Back propagation (BP) neural network is introduced for comparison. A verification experiment on the machining center is carried out to validate the efficiency of the newly proposed method. Experimental verification shows that the adaptive vector quantization network clustering algorithm and output-hidden feedback Elman neural network is an accurate and effective method.

An analytical approach for crucial geometric errors identification of multi-axis machine tool based on global sensitivity analysis

Geometric errors directly affect the tool tip position, reduce machining accuracy, and are one of the most important errors of multi-axis machining tool. However, the geometric errors are intercoupling, and the measured values at different points vary and are stochastic. The identification of the most crucial geometric errors and the determination of a method to control them is a key problem to improve the machining accuracy of machine tool. To achieve this goal, a new analytical method, to identify crucial geometric errors for a multi-axis machine tool is proposed here based on multibody system (MBS) theory and global sensitivity analysis. The volumetric error modeling of multi-axis machine tool has been given by MBS theory, which describes the topological structure of multibody system simply and conveniently in a matrix. The stochastic characteristic of geometric errors is taken into consideration and Sobol global sensitivity analysis method is introduced to identify crucial geometric errors of machine tool. A vertical machining center is selected as an illustration example. The analysis results reveal that the analytical method presented in this paper can identify the crucial geometric errors and are helpful to improve the machining accuracy of multi-axis machine tool.

Thermal Error Compensation on Boring & Milling Machining Center Feed System Based on PMAC

Aiming to deal with thermal error of NC machine tool which can cause reduce of machining accuracy, this paper uses an external error compensation which interacts with NC controllers and PMAC multi-axis and then revises the tool path by adding the error tested in real-time by PMAC card. The processing accuracy is improved eventually. This method can compensate machine geometric errors and thermal errors in real-time. Comparing with other methods of error preventing, this method is more effective and affordable.

Time-varying positioning error modeling and compensation for ball screw systems based on simulation and experimental analysis

Thermal expansion of ball screw systems affects the machining accuracy of machine tools significantly. This paper intends to provide a comprehensive error compensation method for the time-varying positioning error of machine tools. To confirm the thermal deformation mechanism of ball screw systems, experiments have been designed to study the thermal behaviors of a ball screw system under varying temperature conditions. An exponential algorithm is proposed to predict the temperature variation pattern of the ball screw based on finite element analysis, and the actual thermal boundary conditions of the ball screw system are exactly defined according to the proposed algorithm and the experimental results. Then, a comprehensive compensation model is established based on the decomposition of the initial geometric error and thermal error components. Finally, a real-time error compensation system is developed for machine tools based on the function of external machine original coordinate shift and fast Ethernet data interaction, and satisfactory results have been achieved for the compensation experiments on a machining center.

Modeling for spindle thermal error in machine tools based on mechanism analysis and thermal basic characteristics tests

This paper proposes a method to accurately predict thermal errors in spindles by applying experimental modifications to preliminary theoretical models. First, preliminary theoretical models of the temperature field and the thermal deformation are built via mechanism analysis, which is based on the size of the spindle and the parameters of the bearing. Then, thermal basic characteristics tests are conducted at two different initial temperatures. Finally, the results of the thermal basic characteristic tests are evaluated, and the preliminary theoretical model is modified to obtain the final model. A simulation of axial thermal deformation under different speeds is conducted by finite element analysis. It shows that the relationship between the axial thermal deformation and the speed is approximately linear. The model is validated via some experiments on the spindle of a numerical control lathe. The results indicate that the proposed model precisely predicts the spindle’s temperature field and multi-degree of freedom thermal errors.

A thermal error model for large machine tools that considers environmental thermal hysteresis effects

Environmental temperature has an enormous influence on large machine tools with regards to thermal deformation, which is different from the effect on ordinary-sized machine tools. The thermal deformation has hysteresis effects due to environmental temperature, and the hysteresis time fluctuates with seasonal weather. This paper focused on the hysteresis nonlinear characteristic, analyzing the thermal effect caused by external heat sources. Fourier synthesis, time series analysis and the Newton cooling law were combined to build a time-varying analytical model between environmental temperature and the corresponding thermal error for a large machine tool. A multiple linear regression model based on the least squares principle was used to model the internal heat source effects simultaneously. The two models were united to make up a synthetic thermal error prediction model called the environmental temperature consideration prediction model (ETCP model). A series of experiments were performed using a large gantry type machine tool to verify the accuracy and efficiency of the predicted model under random environments, random times and random machining conditions throughout an entire year. The proposed model showed high robustness and universality, with over 85% thermal error, with up to 0.2mm was predicted. The mathematical model was easily integrated into the NC system and could greatly reduce the thermal error of large machine tools under ordinary workshop conditions, especially for long-period cycle machining.

An identification method for key geometric errors of machine tool based on matrix differential and experimental test

This paper presents a key geometric errors identification method for machine tools based on matrix differential and experimental test. An error model for a machine tool was established by regarding the three-axis machining center as a multi-body system. The sensitivity coefficients of the machining error with respect to the geometric errors were determined using the matrix differential method, and the degree of influence of the geometric errors on the machining accuracy under ideal conditions was discussed. Using the 12-line method, 21 geometric errors of the machine tool were identified, allowing the three-dimensional volumetric error distributions of the machine tool to be mapped. Experimental results allow the degree of influence of the geometric errors on the machining accuracy under actual conditions to be confirmed. Finally, the key geometric errors affecting the machining accuracy were identified by a combination of matrix differential and experimental test. This paper provides guidance for the machine tool configuration design, machining technology determination, and geometric error compensation.

Impact of measurement procedure when error mapping and compensating a small CNC machine using a multilateration laser interferometer

This paper deals with the accuracy of compensation of machine tools using a tracking interferometer using the multilateration method. The measurement strategy and thermal drift compensation of the measurements are studied. It shows that most effects of temperature are accurately compensated by the laser tracking interferometer software. However, thermal drifts of accessories are not taken into account, and are therefore not corrected. To validate the robustness of procedures, the geometrical errors of the same machine tool were measured by five measurement strategies using the same equipment. Each strategy is devised and carried out independently by a different person from several institutions. For each strategy, the geometrical compensations were applied to a set of nominal tool path points. The difference, between the nominal points and the compensated or uncompensated points was calculated. This criterion was used to discuss the procedures employed by the participants.

다축공작기계의 공간오차 예측 및 검증

Article history: This paper describes a method of estimating and evaluatingthe volumetric errors of multi-axis machine tools. The estimation method is based on a generic model that was developedfrom conventional kinematic error models for the geometric and thermal errors to help predict the volumetric error easily in various configurations. To demonstrate the advantages of the model, an application in the early stages of a five-axis machine tool design is presented as an example. The model was experimentally evaluatedfor a four-axis machine toolby using the data from ISO230-6 and R-test measurements to compare the estimated and measured volumetric errors.

Novel compensation of axial thermal expansion in ball screw drives

Novel functional materials in the field of machine tools such as shape memory alloys are capable to convert thermal energy into mechanical energy by generating work. Besides, their self-sensing properties and high energy density make them suitable to compensate thermal deformations. Manufacturing requirements concerning machine tools and machining centers can be summarized in a high productivity, a high reliability (low-maintenance) and a high accuracy/precision. The latter machine design parameter depends almost on linear drive systems, which are responsible for the relative distance between workpiece and tool center point. Their positioning performance is limited, among others, to changes in thermal conditions; even with cooling systems that represent about 90 % of the overall energy consumption of the machine. Therefore, thermal errors in machine tools are currently an issue to overcome. Within the scope of this work, shape memory alloys have been integrated in ball screw drives in order to achieve a thermal stable machine transmission component.

Product of exponential model for geometric error integration of multi-axis machine tools

This paper proposes a product of exponential (POE) model to integrate the geometric errors of multi-axis machine tools. Firstly, three twists are established to represent the six basic error components of each axis in an original way according to the geometric definition of the errors and twists. The three twists represent the basic errors in x, y, and z directions, respectively. One error POE model is established to integrate the three twists. This error POE formula is homogeneous and can express the geometric meaning of the basic errors, which is precise enough to improve the accuracy of the geometric error model. Secondly, squareness errors are taken into account using POE method to make the POE model of geometric errors more systematic. Two methods are proposed to obtain the POE models of squareness errors according to their geometric properties: The first method bases on the geometric definition of errors to obtain the twists directly; the other method uses the adjoint matrix through coordinate system transformation. Moreover, the topological structure of the machine tools is introduced into the POE method to make the POE model more reasonable and accurate. It can organize the obtained 14 twists and eight POE models of the three-axis machine tools. According to the order of these POE models multiplications, the integrated POE model of geometric errors is established. Finally, the experiments have been conducted on an MV-5A three-axis vertical machining center to verify the model. The results show that the integrated POE model is effective and precise enough. The error field of machine tool is obtained according to the error model, which is significant for the error prediction and compensation.

Machine tool probes testing using a moving inner hemispherical master artefact

• The new method of testing the CNC machine tool probes is proposed. • The method is based on the usage of the mobile gauge. • The measurement setup basing on the proposed method was built. • The standard uncertainty of measurement of triggering radius variation is 0.35 μm. • The kinematic probe OMP40-2 was tested to verify the setup's work. The growing popularity of usage of touch probes for CNC machine tools has created an increasing requirement to test their accuracy. Indirect methods used until now, based on the measurement of a material gauge with a machine tool equipped with a probe, made the separation of machine tool errors from probe errors impossible. In this article, a new method of testing the probe accuracy, which does not employ a machine tool, is presented. This method employs a moving master artefact in the form of an inner hemisphere. The standard uncertainty of the determination of triggering radius variation is 0.35 μm.

Measurement and compensation of geometric errors of three-axis machine tool by using laser tracker based on a sequential multilateration scheme

A three-point method based on sequential multilateration is applied to measuring the geometric error components (GECs) of three-axis machine tools (MTs). To meet the accuracy requirement of geometric error mapping, a sequential multilateration scheme is developed for high-accuracy point measurement by introducing four additional targets into the measuring system, and the uncertainty of point measurement is verified by simulation. Three independent targets fixed on the MT’s spindle assembly move along with each axis step by step and their coordinates at each step can be determined by the distance data acquired by laser tracker at all steps based on sequential multilateration. Then the volumetric errors of the three target points can be obtained by comparing the actual coordinates and the corresponding desired coordinates, and nine equations can be established by substituting volumetric errors into the error model of linear axis, so that the six GECs of each axis can be obtained by solving these equations. The three squareness errors can be determined by computing the angles between the average lines of the three axes which are achieved by linear curve fitting. Experiments are conducted to measure these 21 GECs, and the volumetric errors in the three-axis MT’s workspace, which are determined by these measured GECs based on the error model of three-axis MT, are compensated. Finally, the positioning errors of the MT with compensation and without compensation are evaluated by laser interferometer, respectively, the experimental results of which demonstrate that the positioning errors are significantly reduced by the error compensation.

Development of Double-Axis Double-Rest CNC Machining Center

The design for a double-axis double-rest CNC machining center is introduced by describing its key machine elements and machine tool design procedures. Then based on the theory of the multi-body system, the modeling process of the volumetric error model for the CNC machining center is expatiated and the result of the modeling is given. It is verified that the error of the machine tool is from three line error and three angular errors between structural units as well as the vertical error between axes. Finally experiments are carried out to evaluate the machining performance of this designed machine tool.

Sensitivity Analysis of Tool Axis Errors in Large Mill-Turn Machine Tools

This paper proposes an approach to carry out sensitivity analysis of tool axis errors caused by component geometric errors, in order to meet the high precision requirement in holes series machining with mill-turn machine tools. Firstly, ideal kinematic model and real kinematic model considering geometric errors of the mill-turn machine tools are built respectively based on homogeneous transfer matrix and multi-body system theory. Secondly, tool axis errors caused by component geometric errors are simulated using an orthogonal test. Finally, sensitivity analysis of tool axis errors is implemented by means of range analysis and variance analysis.

A Machining Test to Evaluate Geometric Errors of Five-axis Machine Tools with its Application to Thermal Deformation Test

This paper proposes a machining test to calibrate position-dependent geometric errors, or “error map,” of rotary axes of a five-axis machine tool. At given sets of angular positions of rotary axes, a simple straight side-cutting using a straight end mill is performed. By measuring geometric errors of the machined test piece, position and orientation of rotary axis average lines (location errors), as well as position-dependent geometric errors of rotary axes, can be numerically identified based on the machine's kinematic model. Furthermore, by repeating the proposed machining test consequently, one can quantitatively observe how the position and the orientation of rotary axes change with respect to the tool spindle due to thermal deformation induced mainly by tool spindle rotation. Experimental demonstration is presented.

Evaluation of Linear Axis Motion Error of Machine Tools Using an R-test Device

The existing accuracy test standard for machining centers (ISO10791) is being revised in order to add a test method for five-axis machining centers. In the revised part of the standard (ISO10791-6), simultaneous three-axis interpolation motion accuracy tests for two linear axes and one rotary axis are proposed as an interpolation accuracy test for five-axis machining centers. These tests enable the measurement of the motion error in three directions, e.g., X, Y, and Z. Two devices can be used as the measurement instrument in these tests. One is a ball bar device, and the other is a combination of a ball and a displacement sensor. An R-test device that incorporates three displacement sensors has been designed for the latter. Using the ball bar device, measurement in three directions can be performed independently, and the radii of circular interpolations for each measurement are different. Using the R-test device, time is shortened by measuring three measurement directions at once, and the acquired data can be analyzed in various ways. The original goal of the measurement method using the R-test device was to measure the dynamic accuracy of the rotary axes. In the present study, the R-test device was actually prototyped and several machines were measured. In addition to the dynamic error of the rotary axes, incorrect pitch error compensation settings for the numerical controller, straightness, or squareness of the linear axes can be well detected.

High precision grey-box model for compensation of thermal errors on five-axis machines

Thermally induced errors of machine tools cause up to 75% of the geometric errors on workpieces. Research carried out in the last decades focused on influences by the environment, spindles and linear axes. With the increasing demand for five-axis machining, the rotary/swivelling axis units have to be checked and compensated for thermal errors. The R-test set-up is a proper measuring device to characterise these errors. This paper introduces a compensation approach to reduce up to 85% of the thermally induced location errors of rotary/swivelling axis units based on internal NC signals, like power supplied to drives.