Precision Machine Tools Research Articles

Optimal tolerance allocation for precision machine tools in consideration of measurement and adjustment processes in assembly

The high geometric accuracy requirement of precision machine tools represents a challenge for tolerance design and assembly process planning that guarantee the final assembly accuracy. Component tolerances should be allocated in association with assembly processes. However, tolerance design and assembly process planning are usually considered separately and lack quantitative analysis. In this paper, to integrate the geometric tolerance of components and variation propagation in assembly process, a state space model is developed. The measurement and adjustment process are expressed as observation matrix and control inputs. An optimal control problem is formulated to determine the adjustment process in consideration of the loss of final assembly accuracy and costs of remachining adjustment process. Tolerances of components can be optimally allocated based on the variation propagation in this deterministic assembly process. The generality and effectiveness of this approach are validated by applying the model on a four-axis horizontal machining center.

Static Characteristics of Aerostatic Thrust Bearings With Multiple Porous Inlet Ports

Aerostatic porous bearings have been applied successfully to various precision devices, such as precision machine tools and precision measuring equipment, to achieve higher accuracy of motion. Recently, large aerostatic porous thrust bearings have been used as essential components in a lithography machine for large liquid crystal display (LCD) glasses. Conventional aerostatic porous bearings are made of porous material that covers the whole of the bearing surface area, requiring a large piece of such material for large bearings. However, making large pieces of porous material requires the use of a large electric furnace, which is a very expensive part of the bearing's manufacturing cost. To overcome this problem, this paper proposes an aerostatic thrust bearing with multiple porous inlet ports. The proposed bearing has a number of small porous inlet ports on the bearing surface, thereby avoiding the need for large electric furnaces. The static characteristics of the proposed bearings are investigated numerically and experimentally. The results show that the proposed aerostatic thrust bearing is potentially very useful for the manufacture of large aerostatic thrust bearings, where it would have advantages over conventional aerostatic porous thrust bearings.

Thermal simulation modeling of a hydrostatic machine feed platform

Hydrostatic guideways are widely applied into precision and ultra-precision machine tools. Meanwhile, the oil film heat transfer causes thermal disturbance to the machine accuracy. Therefore, it is necessary to study the mechanism of the oil film heat transfer and the heat-transfer-reducing method to improve the machine accuracy. This paper describes a comprehensive thermal finite element (FE) simulation modeling method for the hydrostatic machine feed platform to study methods of reducing machine thermal errors. First of all, the generating heat power of viscous hydraulic oil flowing between parallel planes is calculated based on the Bernoulli equation. This calculation is then employed for the simulation load calculations for the closed hydrostatic guideways, which is adopted by the hydrostatic machine feed platform. Especially, in these load calculations, the changing of oil film thickness (resulted from external loads) and the changing of oil dynamic viscosity (influenced by its temperature) are taken into account. Based on these loads, thermal FE simulation modeling of the hydrostatic machine feed platform is completed to predict and analyze its thermal characteristics. The reliability of this simulation modeling method is verified by experiments. The studies demonstrate that the hydrostatic machine thermal error degree is determined by the oil film heat transfer scale, and this scale is mainly influenced by the relative oil supply temperature to ambient temperature (quantitative comparison of oil supply temperature and ambient temperature). Furthermore, the reduction of the absolute value of this relative oil supply temperature can reduce the oil film heat transfer scale and improve the machine accuracy.

Study on Thermal Error Measurement Model of Small Precision Machine Tools

Taking the effect of geometric errors on machining accuracy into consideration in the design of small precision machine tools, the thermal errors become the main factor affecting the machining accuracy of machine tools. Currently, how to quickly and effectively evaluate the impact of thermal error on machining precision, in order to make effective thermal error prediction and compensation method, is needed to solve for small precision machining. Based on stepwise regression algorithm theory, combined with the DBB measured and analyzed on MATLAB in this paper, a better thermal error fitting regression formula is obtained. Meanwhile, this mathematical model approach is also applicable in a variety of other areas, such as machine tool thermal error analysis, compensation and testing etc.

자동차 파워트레인 샤프트 가공용 8축 복합가공기 베드의 유한요소 해석에 관한 연구

To build a high precision machine tool and increase its productivity, structural analysis needs to be carried out for vibration and stiffness of the machine tools before any detailed design. Therefore, in this paper, static and dynamic analysis is carried out to evaluate 8-axis multi tasking machining beds for automotive power train shafts; then, selection of an appropriate device is made for application to bed design. The results of structural and modal analysis confirmed the structural characteristics of the 8-axis multi tasking machine for automotive power train shaft beds: and the second shape bed is the safest is considered secure.

Precision ring rolling technique and application in high-performance bearing manufacturing

High-performance bearing has significant application in many important industry fields, like automobile, precision machine tool, wind power, etc. Precision ring rolling is an advanced rotary forming technique to manufacture high-performance seamless bearing ring thus can improve the working life of bearing. In this paper, three kinds of precision ring rolling techniques adapt to different dimensional ranges of bearings are introduced, which are cold ring rolling for small-scale bearing, hot radial ring rolling for medium-scale bearing and hot radial-axial ring rolling for large-scale bearing. The forming principles, technological features and forming equipments for three kinds of precision ring rolling techniques are summarized, the technological development and industrial application in China are introduced, and the main technological development trend is described.

Research on Detection of Machine Stiffness

Machine tool stiffness is a principal factor affecting machine tool precision, traditional methods can only be used to detect limited categories of machine tools. The paper introduces a new scheme to detect machine tool stiffness on the basis of dynamic detection of machine tool stiffness considering its characteristics and stress state during processing. An experiment conducted in turn-milling machining center CH7516GS indicated by comparison that statics analysis of finite elements matched the experimental result well, which provided precise original data for design of improved machine tool precision and access to design of precision detection equipment for other types of machine tools.

Hard turning and machine tool: a review

Hard turning is a precise machining operation that imposes strict requirements on both the cutting and machine tools. During the last few decades, the application of hard turning has been increasing in various automotive industrial areas. With the development of new cutting tool, the precision and rigidity of machine tools have been improved to allow hard turning to become a viable process. The objective of this paper is to report a survey of the recent research progress in hard turning with cubic boron nitride tools in regard of machine tool, cutting tool, tool wear, tool geometry and surface issues. This paper report a survey of all the factors that affect the performance of hard turning process, in an attempt to achieve better understanding of hard turning process. A summary of the hard turning techniques is outlined and further a comparison of hard turning and grinding is discussed in brief.

Materials in machine tool structures

A broad variety of materials can be found in modern machine tool structures ranging from steel and cast iron to fiber reinforced composite materials. In addition, material combinations and hybrid structures are available. Furthermore, innovative intelligent and smart materials which incorporate sensor and actuator functionality enable the realization of function integrated structures. Consequently, material design and application discloses manifold degrees of freedom regarding a sophisticated layout and optimization of machine frames and components. This keynote paper presents the current state-of-the-art with respect to materials applied in machine tool structures and reviews the correspondent scientific literature. Thus, it gives an overview and insight regarding material selection and exploitation for high performance, high precision and high efficiency machine tools.

Contour Error Analysis of Precise Positioning for Ball Screw Driven Stage Using Friction Model Feedforward

This paper presents contouring error analysis using various classical feedforward controllers. A circular motion is performed using an XY positioning stage with specified amplitude and velocities. This study applied single Static friction model, Generalized Maxwell Slip (GMS) model and combination of both models together with feedforward Proportional-Integral-Derivative (PID) controller. Contour error in term of quadrant glitch is measured by respective angle in each quadrant of circular motion. Due to stick slip motion during velocity reversal generate glitches near zero velocity. Root-mean-square error (RMSE) is calculated based on radial error of circular motion to show variance of errors towards average. The results are experimentally shown that glitches have higher reduction in lower velocity by comparing between applied with and without friction feedforward controller. Better reduction in contour errors improves precision of machine tools and hence increases productivity.

节流器结构参数对Nanosys-1000液体静压导轨承载特性的影响

On the basis of practical machining,influences of structural parameters of a restrictor on the load-bearing characteristics of hydrostatic guideway in the Nanosys-1000 ultra precision machine tool were analyzed.The fluid analysis software was used to establish the finite element model of a carrying oil pad and build a related hydrostatic guideway test bench. The pressure field distributioncharacteristics of the carrying oil pad when its inlet pressure was 1.7 MPa and the oil film thickness was 36μm were researched.By integrating the pressure to get the oil film capacity,the oil film stiffness was obtained by derivation,and the effects of the different structural parameters on the loadbearing characteristics of hydrostatic guideway were revealed.The results show that the orifice diameter has the greatest impact on the load-bearing characteristics of the ultra precision machine tool,followed by the orifice hole length,while the entrance and export chamfers have little effect on it.When the guide load is lower,expanding the orifice diameter and shortening the hole length can help to improve the load-bearing characteristics of the hydrostatic guideway.However,narrowing the orifice diameter and increasing the length of the orifice hole would be more favorable.As compared with the simulation data and the experimental ones,data errors of three capacity measuring points are7.9%,6.9%,8.6%,respectively.Finally,according to research results,the restrictor structural parameters of hydrostatic guideway in the Nanosys-1000 ultra precision machine tool were determined.

An Energy Analysis of Electric and Pneumatic Ultra-high Speed Machine Tool Spindles

There is a growing demand for ultra-high speed precision machine tool spindles to create complex miniature devices and features, in consumer markets such as medical devices, electronics and communications. The key enabling drive technologies for spindle rotational speeds in the 100,000rpm range includes electric motors, both AC and DC powered, and air turbines. In particular for high speed micromachining, turbine spindles have advantages in terms of precision and cost. A widely considered drawback of pneumatic technology though is low energy efficiency, but to date there has been little in depth analysis of commercial/state-of-art turbo-spindle energy performance. This paper provides a holistic comparative analysis of electric motor and turbine powered spindle electrical power requirements, by including the power demand of supporting systems and infrastructure. The analysis indicates that at present the energy usage associated with turbine spindles is significantly higher than electric spindles. However, a number of technically feasible energy efficiency measures are identified for turbine spindles, which would make their energy performance comparable with electro-spindles. The analysis and results will contribute towards an overall life cycle assessment of high speed turbo-spindle technology, and provide impetus to further explore energy optimization approaches and methods.

Manufacturing Accuracy Control of Precision Machine Tool Based on Taguchi Method

In the manufacturing process of precision machine tool, the precision of the machine tool can not simply rely on parts design tolerance to achieve, and also need choose parts assemble for different tolerance values to ensure the accuracy of machine tools. Machine tool assembling deviation accumulation model is established by using the homogeneous coordinate transformation matrix, the quality loss function of taguchi methods apply to mechanical products matching activities. Considering the processing deviation of parts, the quality loss function of assembly cumulative deviation as an effective indicator for measure machine tool accuracy, the assembly precision of machine tools are ensured by optimize matching different geometric deviation parts. An example is given to testify the validity of this method.

A differentiated multi-loops bath recirculation system for precision machine tools

Traditional bath recirculation cooler for precision machine tools always has the uniform and open-loop cooling strategy onto different heat generating parts. This causes redundant generated heat being transferred into the machine structure, and results in unsatisfactory thermal errors of precision machine tools. For the solution of this problem, this paper presents the differentiated multi-loops bath recirculation system. The developed system can accomplish differentiated and close-loop cooling strategies onto machine heat generating parts during its operation. Specially, in order to illustrate the advantages of this system, constant supply cooling powers strategy is presented with its applications onto a certain type of built-in motorized spindle. Consequently, advantages of the proposed strategy based on the differentiated multi-loops bath recirculation system are verified experimentally in the environment within consistent temperature (TR = 20 ± 0.3°C). Compared with room temperature tracing strategy based on the traditional bath recirculation cooler, the constant supply cooling powers strategy is verified to be advantageous in spindle temperature stabilization and thermal errors decrease.

Thermal error modeling and compensation for a high-speed motorized spindle

To improve the precision of CNC machine tools, a motorized spindle thermal error model based on least square support vector machine (LS-SVM) was proposed. A thermal error compensation method was implemented, which takes the length of cutting tools and thermal tilt angles into account. A five-point method was applied to measure radial thermal declinations and axial expansion of the spindle with eddy-current sensors. This resolves a problem arising out of the three-point thermal error measurement, where the radial thermal-induced angle errors cannot be obtained. Variables sensitive to thermal error were selected by grouping and optimizing temperature variables using a combined fuzzy cluster and correlation analysis. LS-SVM models were established for axial elongation and radial thermal yaw and pitch angle errors. Moreover, a method to test the goodness of prediction for the results based on the model is discussed. The results indicated that the LS-SVM has high predictive ability based on fuzzy cluster grouping, and prediction accuracy reached up to 90 %. In addition, the axial accuracy was improved by 82.6 % after error compensation, and the axial maximum error decreased from 39 to 8 μm. Moreover, the X/Y direction accuracy can reach up to 77.4 and 86 %, respectively, which demonstrated that the proposed methodology of measurement, modeling, and compensation was effective.

Autostereoscopy-based three-dimensional on-machine measuring system for micro-structured surfaces.

Traditional off-line measuring systems find it difficult to measure micro-structured workpieces which have a large volume and heavy weight, such as micro-structured patterned precision roller drums. This paper proposes an autostereoscopy-based three-dimensional (3D) measuring method and develops an innovative measuring system for the 3D on-machine measurement of the micro-structured surfaces, an Autostereoscopy-based Three-Dimensional On-machine Measuring (ATDOM) system. The ATDOM system is compact and capable of fast data acquisition and high accuracy in 3D computational reconstruction of complex surfaces under different measuring environments. A prototype ATDOM system is experimentally verified through a series of measurement experiments conducted on a precision machine tool. The results indicate that the ATDOM system provides an important means for efficient and reliable on-machine measurement of micro-structured surfaces.

Post Processing for Five-Axis Machine Tools with Pose Error Compensation

This paper describes the enhancement of the machining precision of 5-axis machine tools. Most post-processors did not consider the pose errors so that the processing error was increased invisibly. A post processing method with pose error compensation was proposed for table/spindle-tilting type machine tools.12 position errors and 12 rotation errors were compensated by the proposed method. Mathematical model contained pose errors which converted cutter location data into machine motion coordinates was established based on inverse kinematics and the analytical solutions of NC code were derived. The tool paths of the freeform surface machining were posted with the presented method. And the simulation experiment was carried out which verified the feasibility of the post processing method with pose error compensation. Comparison of processing error shows that the post processing with pose error compensation can effectively improve the machining accuracy by approximately 50%.

Research on the Impact Vibration Characteristic of the Spindle System

The components of spindle system are key components in ultra precision machine tool. According to the characteristics of the machining process of vertical hydrostatic spindle system for ultra-precision fly cutting machine, the mass- stiffness model of the spindle system is established. This paper describes a system for the analysis of the spindle system under the action of external shocks. The use of the state space method is discussed and an example is given. The influence law of vibration frequency is obtained for the various mass and stiffness value of the spindle system components.

Influence of Guideways Misalignment on the Positioning Precision of Kinematic Feed Chains Used on CNC Machine Tools

The main goal of the kinematic feed chains used on numerically controlled machine tools is to ensure the relative position between the manufactured part and the cutting tool tip with a high accuracy. Therefore, the positioning precision is a major performance criterion for CNC machine tools. The guidance system of the machine table, together with the actuation system and the closed-loop control system, represents a key factor in manufacturing high precision machine tools. Mounting and alignment errors of guideways, both in vertical and horizontal directions, directly affect the table positioning precision and implicitly, the manufacturing precision of the machine tool. Guideways misalignment also generates stresses, plays and excessive run-out. The present paper contains a method of experimental analysis regarding the influence of guideways alignment errors on the positioning precision of the kinematic feed chains, which takes into account the alignment of the two guide rails, both in vertical and horizontal plans. The feed axis used in the experimental setup is closed-loop controlled, making use of an indirect position measurement system.

Methodology for the Efficient Analysis of Thermal and Thermo-Elastic Behaviour of Machine Tools

Thermally induced deviations become the limiting factor for the precision of machine tools. Structure-based finite-element models of high resolution can estimate these deviations with high accuracy but have also a high computational effort. With model order reduction (MOR) these models can be converted into structure-preserving reduced-order finite-element models (FEM-MOR-models) which can be solved very efficiently in MATLAB/Simulink®. To improve model matching selective thermography is used. Selective thermography is a measurement method providing high structural resolution and minimal instrumentation expense due to the use of thermography and photogrammetric methods.