Improvement Of Machining Accuracy Research Articles

Energy Saving in Biaxial Feed Drive Systems Using Adaptive Sliding Mode Contouring Control with a Nonlinear Sliding Surface

Energy saving and producing highly accurate products are major essential demands in computer numerical control (CNC) machines. These machines operate all day and night for a long time, therefore they largely consume energy all over the world. This paper proposes adaptive sliding mode contouring control (ASMCC) with a nonlinear sliding surface (NSS) to reduce consumed energy and further improve machining accuracy for a biaxial feed drive system. The control gain of the proposed scheme is formulated to be mainly relied on a contour error. Once the contour error is changed, the control gain is adjusted simultaneously to generate appropriate control signal. ASMCC constantly reduces consumed energy and improves machining accuracy by reducing the resultant contour error. In order to verify the effectiveness of ASMCC, simulation and experiment are carried out on a biaxial feed drive system using a circular trajectory. Results show that the adaptive algorithm significantly reduces the mean contour error by 37.62% and 34.65% compared to sliding mode control (SMCC) without any additional energy on simulation and experiment, respectively. In addition, the proposed approach reduces experimentally consumed energy and control input variance by 8.20% and 17.29%, respectively.

Thermal deformation prediction of high-speed motorized spindle based on biogeography optimization algorithm

A thermal deformation prediction model of a high-speed motorized spindle is important for improving machining accuracy and reducing the thermal error in the spindle. The convective heat transfer coefficient reflects the internal heat exchange capacity of a motorized spindle. In the finite element thermal analysis of a motorized spindle, the heat transfer coefficient is used as the boundary condition to calculate the temperature field and thermal deformation. The accuracy of the convective heat transfer coefficient has the most evident effect on the thermal deformation prediction of the motorized spindle. In this paper, a method for optimizing the heat transfer coefficient based on the biogeography optimization algorithm is proposed for the 100MD60Y4 motorized spindle. The proposed method is used to develop a thermal deformation prediction model of an intelligent accurate motorized spindle. Accurate prediction of the thermal deformation of the motorized spindle is realized using the experimental data of the surface temperature of the motorized spindle. Experimental results show that the average prediction error after the optimization of the spindle bearing temperature is 0.22 °C. The average prediction error in the thermal deformation of the spindle is 0.72 μm. The developed model is more accurate compared with the conventional thermal deformation prediction model of the motorized spindle.

IMPROVING MACHINING ACCURACY OF CNC MACHINES WITH INNOVATIVE DESIGN METHODS

The article considers achieving the machining accuracy of CNC machines by applying innovative methods in modelling and design of machining systems, drives and machine processes. The topological method of analysis involves visualizing the system as matrices of block graphs with a varying degree of detail between the upper and lower hierarchy levels. This approach combines the advantages of graph theory and the efficiency of decomposition methods, it also has visual clarity, which is inherent in both topological models and structural matrices, as well as the resiliency of linear algebra as part of the matrix-based research. The focus of the study is on the design of automated machine workstations, systems, machines and units, which can be broken into interrelated parts and presented as algebraic, topological and set-theoretical models. Every model can be transformed into a model of another type, and, as a result, can be interpreted as a system of linear and non-linear equations which solutions determine the system parameters. This paper analyses the dynamic parameters of the 1716PF4 machine at the stages of design and exploitation. Having researched the impact of the system dynamics on the component quality, the authors have developed a range of practical recommendations which have enabled one to reduce considerably the amplitude of relative motion, exclude some resonance zones within the spindle speed range of 0...6000 min-1 and improve machining accuracy.

Establishment and analysis of a time-varying dynamic model of the large-pitch external thread during high-feed turning process

High-feed cutting, such as that used in the machining of high-precision large-pitch thread, is expected to find wide application in the fabrication of key parts of large-scale equipment. The static and dynamic precision of a large-scale press is directly affected by the machining accuracy and surface quality of the large-pitch thread. We propose a new time-varying dynamic model of a large-pitch external thread subjected to a tridirectional moving load and an algorithm for vibration offset calculation. The model has also been validated experimentally and has enabled us to study in detail the phenomenon of chatter in the machining processes. Prediction models for dimensional error and surface morphology, which take into account the vibration offset of the workpiece, are proposed and the results obtained from them agree well with the experimental observations. Finally, the effects of vibration of the workpiece alone and the effects of coupled vibration of the cutter and workpiece on machining accuracy and surface quality are analyzed using the prediction models. Results indicate that there are differences in dimensional accuracy and surface morphology between the left and right flanks of the thread. Furthermore, the degree of variation in dimensional error is more obvious in the left flank than in the right; the wave curve density associated with the machined surface morphology of the left flank is large and the spacing of the wave curves is small. The results also show that the dimensional error in the intermediate stage is larger than those in the initial and the end stages. The study builds a theoretical basis for suppressing workpiece vibration during the cutting process and provides technical assistance for ensuring high-quality machining with a long stroke of the thread, improving machining accuracy, and making sure of consistent surface quality.

An on-machine error compensation method for an ultra-precision turning machine

On-machine error compensation (OMEC) is efficient at improving machining accuracy without increasing extra manufacturing cost, and involves the on-machine measurement (OMM) of machining accuracy and modification of program code based on the measurement results. As an excellent OMM technique, chromatic confocal sensing allows for the rapid development of accurate and reliable error compensation technique. The present study integrated a non-contact chromatic confocal probe into an ultra-precision machine for OMM and OMEC of machined components. First, the configuration and effectiveness of the OMM system were briefly described, and the relevant OMEC method was presented. With the OMM result, error compensation software was then developed to automatically generate a modified program code for error compensation. Finally, a series of cutting experiments were performed to verify the validity of the proposed OMEC method. The experimental results demonstrate that the proposed error compensation method is reliable and considerably improves the form error of machined components.

New method for calculating face gear tooth surface involving worm wheel installation errors

The machining principle and realization method for the continuous generative grinding face gear by a worm wheel are introduced. Based on a five-axis linked CNC grinding machine, a new method is presented to deprive the equation of face gear error tooth surface by assuming the tool surface as the error surface, where actual tool installation position error is introduced into the equation of virtual shaper cutter. Surface equations and 3-D models for the face gear and the worm wheel involving four kinds of tool installation errors are established. When compared, the face gear tooth surface machined in VERICUT software for simulation based on this new method and the one obtained based on real process (grinding face gear by using a theoretical worm wheel with actual position errors) are found to be coincident, which proves the validity and feasibility of this new method. By using mesh planning for the rotating projection plane of the face gear work tooth surface, the deviation values of the tooth surface and the difference surface are acquired, and the influence of four kinds of errors on the face gear tooth surface is analyzed. Accordingly, this work provides a theoretical reference for assembly craft of worm wheel, improvement of face gear machining accuracy and modification of error tooth surface.

Machining accuracy improvement of non-orthogonal five-axis machine tools by a new iterative compensation methodology based on the relative motion constraint equation

Abstract This paper proposes a new iterative compensation methodology of geometric errors to improve the machining accuracy of a non-orthogonal five-axis machine tool (NOFAMT). Firstly, based on homogeneous transform matrix (HTM) and multi-body system (MBS) theory, the relative motion constraint equations (TRMCEs) of the tool tip position and tool orientation vector related to a NOFAMT with a nutating rotary B axis are established. Then, by utilizing TRMCEs, the mapping relationships between tool path and the numerical control (NC) command without and with considering the geometric errors are constructed respectively. In order to truly reproduce tool motion trajectory of the machine tool driven by the given NC command, the mapping relationship between the NC command and tool cutting trajectory is also established. Meanwhile, procedures of iterative compensation are described by using the aforementioned mapping relationships without the traditional inverse calculation, and the actual NC code is generated in self-developed compensation software. It is not difficult to find that the new approach takes the difference between tool path and tool cutting trajectory as the control objective and can directly obtain the actual NC code controlling the machine tool to achieve the desired machining accuracy. Finally, a cutting test is carried out on the DMU60P NOFAMT. Experimental results show the developed iterative compensation methodology is precise and effective for NOFAMTs. Therefore, compared with the existing methods, the new method is more direct and accurate. And its basic idea can be applied to other type of machine tools.

Optimization of process parameters in micro-EDM of Ti-6Al-4V based on full factorial design

The objective of this study was to investigate the effects of the micro-electro-discharge machining (micro-EDM) process parameters on the machining performance parameters, in order to understand the behavior of each process parameter as well as to find out their optimal values. This research was conducted through a series of experiments using full factorial design. Analysis of variance (ANOVA) and multivariate analysis of variance (MANOVA) were employed to analyze the findings and to determine the significance of each process parameters on the response variables. The process parameters included in this study were voltage, capacitance, electrode rotational speed, and the electrode coating. Voltage and capacitance were studied separately as well as in combination in terms of the discharge energy. The response variables were consisted of machining time, tool wear, crater size, and surface micro-hardness. The findings indicated that the increase in voltage had a decreasing effect on the machining time, while it increased the crater size. The increase in capacitance resulted in decreased machining time and the tool wear, while it had an increasing effect on the surface micro-hardness. It was found that the effects of titanium nitride (TN) coating and the electrode rotational speed were not statistically significant. There was slight improvement of machining time and accuracy for increasing electrode rotational speed due to improved flushing condition, but very high electrode rotational speed was found to be unsuitable for machining accuracy. The overcut of the micro-holes was found to increase gradually with the increase of voltage and capacitance due to increase in spark gap and discharge duration, respectively. The voltage and the capacitance were found to be the two major parameters influencing the micro-EDM performance. Assuming that the importance and the weight of all response variables had been identical, the optimal process parameters for improved micro-EDM of Ti-6Al-4V were found to be TN-coated electrode, 60 V, 4700 pF, and 3000 rpm.

Machining Accuracy Improvement by Compensation of Machine and Workpiece Deformation

For workpieces of regular shape, recently achieved machining accuracy is satisfactory, but for workpieces of complex shape and thin walls form light alloy machining (e.g. helicopters gearbox cases, automotive parts etc.) it is still difficult to find proper environmental conditions without separating the machine form from its ambient temperature impact. A procedure for adjustment and self-testing has been proposed, prepared and successfully verified on existing flexible machining systems. An approach to improve the machining accuracy with high accuracy machining systems by use of special adjustment procedures and additional workpiece distortion compensations are presented in the paper.

An approach of comprehensive error modeling and accuracy allocation for the improvement of reliability and optimization of cost of a multi-axis NC machine tool

Machining accuracy is critical for the quality and performance of a mechanical product, and the reliability of a multi-axis NC machine tool reflects the ability to reach and maintain the required machining accuracy. The objective of this study is to propose a general methodology that will simultaneously consider geometric errors and thermal-induced errors to allocate the geometric accuracy of components, for improving machining accuracy reliability under certain design requirements. The multi-body system (MBS) theory was applied to develop a comprehensive volumetric error model, showing the coupling relationship between the individual errors of the components of this machine tool and their volumetric accuracy. Additionally, a thermal error model was established based on the neural fuzzy control theory and was compared to the common thermal error modeling method called BP neural network. Based on the traditional cost model and the reliability analysis model, a geometric error-cost model and a geometric error-reliability model were established, taking the weighted function principle into consideration. Then, an allocation approach of the geometric errors, for optimizing total cost (manufacture and QLF) and reliability, subject to the geometrical and operational constraints of the machine tool, was proposed and formulated into a mathematical model, in order to perform the optimization process of accuracy allocation by using the advanced NSGA-II algorithm. A case study was also performed in a five-axis machine tool, and the traditional NSGA algorithm was used for comparison. The optimization results for the five-axis machining center showed that the proposed approach is effective and able to perform the optimization of geometric accuracy and improve the machining accuracy and the reliability of the machine tool.

A smooth double spiral tool path generation and linking method for high-speed machining of multiply-connected pockets

Complex pockets have been widely used in industry for the production. At present, the basic forms of tool-path which most commercial CAM software systems used are still limited to direction-parallel and contour-parallel offset. If the pocket contains multiple islands, the tool-path will include many cutter lifting and retraction motions. In this paper, we propose a new double spiral tool-path generation and linking method for complex pockets with multiple islands which can be used for high speed machining. We also provide segmentation rules to divide surface for further improving machining accuracy and efficiency. Taking into account the path interval, step length and other processing parameters, precise milling can be achieved without cutter lifting and retraction operations to ensure optimal processing performance and reduce processing time. The proposed algorithm has been implemented and tested bounded by 2D straight lines and circular arcs of various shapes, and validated successfully through the actual machining of a complicated pocket. The results indicate that this method is superior to other existing machining methods, and it can realize high speed machining of complicate-shaped pocket based on parametric surface with multiple islands.

Comprehensive error measurement and compensation method for equivalent cutting forces

To determine the error compensation for cutting forces with a machine tool, this paper proposes a comprehensive error compensation method that determines an equivalent cutting force. The loading system that applies this equivalent cutting force was designed to load a constant force that imitates the actual cutting force of a machine tool. The equivalent cutting force, when applied to the working table of the machine tool, induces an error that can be directly measured by a laser interferometer. Moreover, a stable measurement environment suitable for laser interferometer is configured. A comprehensive error compensation model of a three-axis computer numerical control (CNC) machine tool was developed using multi-body systems theory and homogeneous transformation matrices. Solutions for the error compensation model and parametric equation were automatically found using MATLAB, based on this model. The effectiveness of this compensation method has been confirmed by machining experiments. The experimental results show that this comprehensive error compensation of the equivalent cutting force can greatly improve machining accuracy.

An approach to optimize the machining accuracy retainability of multi-axis NC machine tool based on robust design

Machining accuracy retainability is considered to be one of the most important aspects in the process of performance evaluation and optimization design of the machine tools. Reliability based design optimization (RBDO) and robust design optimization (RDO) are the tools that search for safe structural systems with minimal variability of response when subjected to uncertainties in design parameters. The aim of this paper is to propose an approach that simultaneously considers reliability and robustness to allocate geometric accuracy of components for improving machining accuracy retainability under certain design requirements. Based on homogenous transformation matrices (HTMs), a comprehensive volumetric model explaining how individual errors in the components of a machine affect its volumetric accuracy (the coupling relationship) was established. By applying high-order moment standardization technique (HOMST), reliability and sensitivity with single failure mode were obtained and then the reliability model and the sensitivity model with multiple failure modes were developed and common methods such as Narrow bounds, AFOSM and Monte Carlo were used for verification and comparison. Meanwhile, a cost model taking manufacturing cost and present worth of expected quality loss into consideration was introduced. By taking the minimum possibility of failure and cost of machine tools as criterions and taking the reliability of machine tools as constraint of optimizing the basic parameters of machine tools, an approach to optimize the machining accuracy retainability of multi-axis NC machine tool based on robust design was developed and an example of improving the machining accuracy retainability for a five-axis NC machine tool was used to demonstrate the effectiveness of this approach. This study implies that it is possible to obtain the relationships between geometric errors and specify the accuracy grades of main feeding components of mechanical assemblies.

Novel method for position-independent geometric error compensation of five-axis orthogonal machine tool based on error motion

It has been proven that error compensation is a key technique to improve machining accuracy. However, existing iteration and recursive compensation algorithm is difficult to realize. Hence, a simple and rapid compensation method is considerably necessary for engineering application. In this paper, a novel compensation strategy just by algebraic operation was first proposed for machining accuracy improvement. Error motion transformation was introduced to build the position-independent geometric error (PIGE) model according to homogeneous transformation matrix (HTM). Then, the analytical numerical control (NC) code expression with error compensation was derived and used for NC code generation. In addition, the presented method is appropriate for post-processing of non-orthogonal machine tool. At last, simulation and cutting experiment were demonstrated to verify the feasibility and effectiveness of the proposed method. Taking hemisphere surface as the test object, the simulation results showed that the effects of PIGEs could be eliminated by the proposed method. The experiment results with compensation indicated that the machining accuracy improved to about 14 % compared with those without compensation.

Reduction of undercutting in electrochemical micro-machining of micro-dimple arrays by utilizing oxygen produced at the anode

Micro-dimple arrays are widely used as surface textures in tribology to improve friction performance. Through-mask electrochemical micro-machining (TMEMM) is a popular method to generate such arrays. However, machining accuracy is reduced by lateral undercutting of the micro-dimples. In this paper, we present that the oxygen bubbles produced on the workpiece surface during the machining could be employed to reduce undercutting. A thick mask was introduced to prevent escape of the oxygen bubbles from the micro-dimples. Oxygen bubbles accumulating at the micro-dimple edges might protect the edges from etching, thereby reducing undercutting and improving machining accuracy. The experimental results show an undercutting of only about 3μm with a 250-μm-thick polydimethylsiloxane (PDMS) mask. In particular, there was no increase in undercutting when the applied voltage and machining time were increased. An interesting phenomenon was observed in which there was little increase in depth with increasing voltage, whereas the depth increased with longer machining time. Thus, direct or pulsed current can be used to generate different depths of micro-dimple arrays with low undercutting by simply controlling the machining time, regardless of the applied voltage.

NURBS curve interpolation algorithm based on tool radius compensation method

This paper focuses on developing an algorithm that can generate toolpaths in NURBS form for smooth, high speed and accurate machining. The initial toolpaths are obtained by tool radius compensation method which is based on the workpiece boundary offsetting. According to different lengths and the continuous short block (CSB) criterion, these offset linear segments can be regarded as CSBs or long straight segments. Junctions are located where the curvature value is greater than the preset curvature threshold value or where it changes abruptly, or at the two end points of any long straight segment. During machining, the NURBS fitting module first looks ahead several CSBs and converts them into parametric curves in real time. During the conversion, continuities of the position, slope or even curvature at the transition of the parametric curves and unfitted line segments can be guaranteed. Then the acceleration/deceleration feedrate-planning scheme is proposed to determine the transition feedrate at the junction between the fitted curves and unfitted long straight segments, and the corner feedrate within the fitted curve. Simulations and experiments show that the proposed algorithm can significantly improve machining accuracy and reduce cutting time to satisfy today’s high-speed and high-accurate machining requirements.

Bionic design and verification of high-precision machine tool structures

Highly stiff and lightweight machine tool structures are very important for improving machining accuracy. How to reasonably arrange stiffeners is one of the problems that designers must solve to achieve required structural performances. This paper presents a practical approach for stiffener layout design, which is inspired by the adaptive growth phenomenon observed in nature. Firstly, a mathematical model is built to provide theoretical basis for the adaptive growth of stiffeners to the applied load. Then a corresponding algorithm is developed to realize the generation of stiffener layout, during which the growth rates are determined by the generalized heights of stiffeners and the growth directions by branching and degenerating operations. When applying the approach to redesign an actual machine tool bed structure, a simple spring model is introduced, which can greatly reduce computation cost by transforming the 3D stiffener layout design problem into a 2D one. Numerical and experimental results show the stiffness enhancement, and thus, the applicability and validity of the approach are proved.

Improvement of abrasive water jet machining accuracy for titanium and TiNb alloy

The paper is dealing with abrasive water jet (AWJ) cutting of titanium and titanium–niobium alloy. It contains comparison of the experimental results with values calculated from a previously derived theoretical model describing the interaction between the AWJ and cut material using the limit values of traverse speed, limit depth of cut and limit angle. The attention is paid mainly to the angle of striations created on the walls by abrasive water jet cutting. For lower traverse rates, it was found that the angle of striations is greater than in theoretical expectations. The influence of bigger but slower peripheral abrasive particles in the AWJ outside layer is discussed as the most probable explanation of this phenomenon. Comparison of the results calculated from the theoretical model and the experiment show a very good correlation, especially when a correction, resulting from the explanation of the striation angle increase for low traverse speeds, is incorporated into the theoretical model.

Machining tests for identification of location errors on five-axis machine tools with a tilting head

Location errors are considered as one of the fundamental errors of five-axis machine tools. For the improvement of machine accuracy, it is important to propose an efficient and accurate identification method of location errors. This paper proposes an identification method of location errors on a five-axis machine tool with a tilting head by a set of machining patterns. The machining patterns are proposed under the assumption that the influence of motion errors of each axis on the measuring results is sufficiently small. Location errors of both linear and rotary axes can be decoupled and identified. During the measuring of linear axes squareness errors, a new machining method that significantly increases the moving distances of linear axes without additional removal of material or enlarged volume of the workpiece is proposed. Also, a decoupling method of linear and rotary axes squareness errors relevant to this new machining method is introduced. The measuring accuracy of linear axes squareness errors is greatly improved based on these measuring schemes. The machining tests are performed on a commercial five-axis machine tool with a tilting head. The measurement results are acquired on a coordinate measuring machine, and the identification results are validated by comparing the geometric characteristics of workpieces before and after compensation.

预应力下的结构模态计算

本文采用达朗伯原理建立动力学平衡方程，研究了梁类结构预应力下的模态计算方法，证明通过调节预应力，可有效改善结构的动态性能。本文通过有限元法计算比较正负预应力下桁架结构的变形和模态，提出预应力桁架可作为高速数控机床的横梁结构。在横梁内部施加预应力可大幅提高横梁的弯曲刚性。通过对桁架关键构件内应力的控制，可以使桁架内应力做到可调。根据加工参数的变化，优化调节桁架某些杆件的内应力大小及方向，使设备在加工过程中具有一定改变动态性能的能力，从而达到减小振动提高加工精度的目的。 Using D’Alembert’s principle to establish the dynamic equations to study the mode calculating method of class structure under prestressed, evidenced by adjusting the prestressing, can effec-tively improve the dynamic performance of the structure. Deformations and modal comparatively positive and negative prestressed truss structure by the finite element method is proposed as a beam structure of prestressed truss speed CNC machine tools. The bending rigidity of the internal beam can be improved significantly by prestressing the beam. Through the truss key component stress control, it can make the truss internal stress to be adjustable. According to changes of processing parameters, the size and the direction of the internal stress of certain truss rods are optimized and adjusted so that the device has the certain ability to change the dynamic performance to achieve the purposes of reducing vibration and improving machining accuracy.