Hydrostatic Guideway Research Articles

Fabrication of Microstructured Surfaces by Five-Axis Ultra Precision Machine Tool

In this paper, a five-axis ultra precision machine tool for fabrication of microstructured surfaces is presented. This machine consists of two rotary axes (C&B) and three linear axes (X&Y&Z). High precision aerostatic bearing and torque motor are adopted in C axis (main spindle) and B axis. X axis and Z axis use the hydrostatic guideway and are driven by linear motors. Y axis is driven by torque motor and precision ball screw. This machine is able to realize multiple processing methods, including ultra precision diamond turning, ultra precision milling, fly-cutting, fast tool servo and slow tool servo diamond turning.Furthermore, a large number of experiment researches are carried out. Some typical microstructure surfaces are manufactured, for sinusoidal grid surface, the surface roughness Ra is 11.9nm, which is machined by slow tool servo diamond turning. Micro pyramid array surface is fabricated by using fly-cutting, which performs well both in the profile accuracy and the repeatability. These experiment researches prove that this ultra precision machine is superior in accuracy and system reliability.

Analysis on Dynamic Characteristics of Closed Hydrostatic Guideway Throttled by Capillary

Hydrostatic guideway is widely used in heavy and ultra-precision machine tools due to their high bearing capacity, low coefficient of friction and high capacity of precision-remaining. So it is necessary to carry on theoretical research on its dynamic characteristics. Firstly, the oil-film damping coefficient of closed hydrostatic guideway throttled by capillary is derived. Secondly, a new dynamic mathematical model, considering the oil damping effect, the extrusion effect and the volume effect, of the closed hydrostatic guideway is proposed. Thirdly, the frequency characteristics and step characteristics of it are simulated in Matlab Simulink through the transfer function. Finally, the paper investigates the influence of damping coefficient, the oil pressure, volume effect and the initial oil-film thickness on the dynamic characteristics of closed hydrostatic guideway throttled by capillary.

Relationship on Oil Cavity Depth and Oil film Flow State of the Large Size Hydrostatic Guide-way

Relationship on Oil Cavity Depth and Oil film Flow State of the Large Size Hydrostatic Guide-way

Measuring the Rail Profiles of a Long Hydrostatic Guideway in a Precision Roll Lathe

A system for measuring the rail profiles of a long hydrostatic guideway is proposed and applied to a precision roll lathe. The rail profiles, consisting of rail form errors and a parallelism error, are measured with four capacitive probes placed on the two rail surfaces, and a laser interferometer with angular optics. The form error of each rail is measured using a mixed sequential two-probe method, while the parallelism error between the rails is measured with two opposing capacitive probes. To eliminate the necessity for additional guidance mechanisms, theprobe table is guided by air bearings and the rails being measured. To verify the measurement results, two self-verification techniques are suggested and tested. Firstly, the linear motion errors of the probe table on the two rails are compared, yielding a difference of 0.79 μm in 2σ. Secondly, the sum of the rail form errors is compared with the slope-removed sum of the two capacitive probes used for the parallelism measurement, yielding a difference of 0.81 μm in 2σ. According to the self-verification results, the accuracy of the proposed measurement method is about 1 μm for a rail length of 2,825 mm.

Influencing of Fluid-structure Interactions on Static and Dynamic Characteristics of Oil Hydrostatic Guideways

Oil hydrostatic guideways are widely applied in ultra-precision machine tools, the structural parts are often considered as rigid bodies without deformations in the process of design and calculation. In other words, the effects of the structural parts on the oil film thickness and the pressure distribution are ignored in the course of working. The fluid-structure interactions model of oil hydrostatic guideways with annular slit restrictors is established. This model is based on the dynamic Reynolds equation on the land, the elastic deformation equation of the upper slideway and the table dynamic equilibrium equation. The effects of fluid-structure interactions on the deformations, static and dynamic stiffness and settling time under the step load are explored. The results show that the deformations of the upper slideway increase linearly with recess pressure, the static and dynamic stiffness are decreased by the deformations, the settling time is extended by the deformations. Measured values of the static and dynamic stiffness agree well with the theoretical values with the deformations.

Oil Film Carrying Property Research of Hydrostatic Vertical Guideway of CNC Vertical Lathe

Hydrostatic vertical guideway is the key component of modern CNC equipments which is a common lubrication form of turret. In order to improve the cutting stiffness during processing, the hydrostatic vertical guideway in sliding seat during westbank ram processing. Take count different cutting force to analysis the thickness changing of hydrostatic oil film, and then the relation between cutting force and the thickness of oil film is obtained. Based on FVM, relation model between them is established and the limited condition including cutting force is T=100KN, flow flux of each inlet is 0.52L/min, pump of constant delivery type is applied, film thickness is 0.023mm and oil cavity depth is 2mm is numerical simulated. The Hydrostatic pressure field and film thickness under different condition of hydrostatic vertical guideway is obtained. The relation model between film thickness and custom forces is validated. Whats more, the regularity of cutting force influence that impacted on film thickness is revealed. The study is of vital theoretical significance for the improvement of machining accuracy of numerical control machine and the entire CNC equipment and provides valuable theoretical basis for the design of hydrostatic guide rail in engineering practice.

System for Compensating the Rigidity of the Hydrostatic Guideways of the Column Saddle Subassembly of the Boring and Milling Machines

The improvement of the machining accuracy of boring and milling machines is imposed by the more and more demanding conditions on the accuracy of the work pieces to be machined. The specificity of this family, of boring and milling machines, consists of the horizontal motion of the milling spindle (quill) with implications on the application point of the gravity center. Thus, the variation of the application point of the sum of the component elements weights of the column saddle subassembly leads to modifications of the oil film thickness in the hydrostatic guideways, affecting the geometrical accuracy of the machine. This work is presenting a system of compensation and hydrostatic guiding of the column saddle subassembly. This new compensation system allows the pressure in the pockets to be modified by taking in consideration the position of the application point of the weight of the entire column saddle subassembly. The implementation of this compensation and guiding system on the boring and milling machine model AFP 220 has led to an increasing of two times the geometrical accuracy of the horizontality of the column saddle`s motion, compared to the variant without compensation.

Device for Supplying the Hydrostatic Pockets with Compensation of the Fluid Temperature

The supply devices of the hydrostatic guideway pockets are elements with a major influence on the rigidity of the machine tool guideways. The value of the flow that circulates through these devices depends on the fluid viscosity as well. The researches in this field show that an important increase of the fluid temperature is produced when it passes through various orifices, slits and interstices that belong to the hydrostatic systems. The increase of the working temperature leads to decreasing the kinematic viscosity, with undesirable effects on the functioning of the hydrostatic guideways. In order to minimize this disadvantage, this work is presenting a constructive solution of a fixed supply device (flow control valve) with fluid temperature compensation. This type of device avoids the negative effects of increased fluid viscosity caused by the temperature increase as well, by automatically modifying the flowing cross section that is performed through a compensation bushing. The constructive solution of the new supply device is simple and involves relative low workmanship and costs. This supply device has been experimented on a rotary table model MRD 1500x2000, on the linear axis of 2200 mm travel. Through the comparative analyses of the new solution versus the standard one that was using as supply device a capillary type resistor, an increase of the guideway rigidity by 13% has resulted.

Analysis of Hydrostatic Guideway Error Based on Multi-Head Pump

Base on the force balance theory, a model is set up to analyze the hydrostatic guideway with the closed four pads. Each oil pocket force is calculated, which is caused by the gravity center and the driving center of a slide carriage. According to the hydrostatic theory, the oil film variation that is generated during the movement is obtained. It is composed of linear and angle errors. The results show the driving force center and the geometrical center of the slide carriage have an effect on each oil film thickness, as well as the size and the location of the driving force. The article states their relationship and supplies the theoretical foundation for the design and employment of the hydrostatic guideway.

Optimizing dynamic characteristics of NC rotary table based on electromechanical-hydraulic coupling

Dynamic characteristics of NC rotary table are a guarantee of indexing accuracy, which is composed of static indexing accuracy and dynamic indexing accuracy. Static indexing accuracy can be measured directly by laser interferometer, while dynamic indexing accuracy is too difficult to obtain directly because of load-carrying and work pattern of the table. To improve supporting rigidity and dynamic indexing accuracy of the NC rotary table, ZT20SW driven by a double worm gear pair was taken as the research subject. On the platform for gear milling, preload of the brake worm could be optimized by feedback of dynamic displacement on the working plane. On the basis of load analysis, an electromechanical-hydraulic coupling model in the circumferential direction of ZT20SW was established. Simulation and experiment results altogether indicated that preload on the brake worm could reduce the negative influences of backlash on dynamic characteristic. As a result, dynamic indexing accuracy of the table can be controlled reasonably and wear of the driving worm pair falls to minimum level. On the experiment platform of ZT20SW, frictional force was set to 600N by adjusting flow of the hydrostatic guideway. If preload reached 4400N, dynamic indexing accuracy of the table was less than 0.02mm.

The Thermal Characteristics Analysis of a Super Hydrostatic Guideway

This paper established the finite element model of a hydrostatic guideway. Based on the thermal characteristic simulation analysis, the temperature field and thermal deformation of the guideway were obtained. The analysis results show that the oil film temperature increment will cause thermal deformation of the guideway and processing errors. The errors will influence the machining accuracy of machine tools. The analysis results provide certain theory basis for the research on the above influence. And then the results also give certain reference for the research on the thermal error compensation of machine tools.

Precision measurement of rail form error in a closed type hydrostatic guideway

In this paper, a measuring device and measuring method are proposed to measure the rail profiles of a closed type guideway with submicrometer accuracy. The device is applied to hydrostatic bearing table and a mixed two-probe method (MTPM) is applied to measure the rail profile and linear motion error of the device. The effectiveness of the proposed measuring device is experimentally verified in two ways: In the case of the vertical direction, linear motion errors of the device tested on the upper and lower rail, respectively, are compared. In the case of the horizontal direction, the measured rail profiles using the proposed device are compared with directly measured profiles using a straight-edge and two probes (SEM). In the experiments on the vertical rail, both linear motion errors of the measuring device obtained from the upper rail and the lower rail show good agreement and the measurement accuracy is in a range of ±0.11∼±0.15 µm. In the experiments on the horizontal rail, the measured rail profile by the MTPM agreed well with the profile measured by SEM. However, because the rail has an abruptly varied surface, the measurement accuracy in this case deteriorated to a range of ±0.19∼±0.26 µm. From the experimental results, it is confirmed that the proposed measuring device with the MPTM is very effective to measure closed type multiple rail profiles with submicrometer accuracy.

Prediction of the effect of speed on motion errors in hydrostatic guideways

Motion errors of hydrostatic guideways are affected not only by the geometric errors of the guide rails but also by the speed of the table as well as the supply pressure. Currently, the mechanisms causing motion errors are studied under the condition that the table is moved at a low speed or incrementally. To make a more realistic prediction of motion errors, a new model is developed to analyse the straightness and the yaw of closed hydrostatic guideways with four pads. This model is based on the motion equations of the table and the Reynolds equation of the fluid film in the consideration of the fluid compressibility and the squeeze film effect. The results show that the influences of the table speed on the motion errors of the table are different at different wavelengths of geometric errors, and the motion errors are rarely affected by the supply pressure when the table operates at a low speed. Moreover, the influences of the table speed on the motion errors are smaller at higher supply pressure. Increasing the film thickness can reduce the motion errors of the table. Finally, validation experiments are conducted on a hydrostatic guideway with different speeds.

Research on dynamic characteristics of NC rotary table considering leakage factors of its hydrostatic guideway

The rigidity and damping characteristics of a large and heavy NC rotary table have great influence on machining efficiency and the integrated machine’s dynamic accuracy, but test and control methods related to these characteristics are difficult to apply. In the axial direction, the table was supported by a constant current hydrostatic guideway; the guideway’s surface roughness and planeness errors were the main factors contributing to hydraulic oil leakage. Based on the force analysis of a ZT20SW driven by double worm gear pairs, a dynamic model and an electromechanical–hydraulic coupling model in the circumferential direction were established. Simulation and experimental results altogether indicated that the cutting force on the table, leakage characteristics and Coulomb frictional force of the hydrostatic guideway each have great effects on dynamical accuracy. In addition, test methods on the leakage’s equivalent oil film thickness offer a new comprehensive evaluation standard for the hydrostatic guideway’s geometric errors. The hydrostatic guideway’s Coulomb frictional force can be controlled reasonably to improve the table’s dynamic accuracy.

Influencing Factors on Dynamic Response of Hydrostatic Guideways

Stiffness and damping of hydrostatic guideways are calculated by small perturbation method based on Reynolds equation in dynamic regime. The hydrostatic guideway is considered as a system which consists of the mass, the spring and the damper. The effects of some main parameters on stiffness, damping and damping ratio are analyzed which include the supply pressure, the film thickness, the pad dimension, the pressure ratio, the lubricating oil volume and the lubricating oil viscosity. The relationships between the settling time of the hydrostatic guideways and these parameters are investigated under a step load. It is shown that the slide block returns to equilibrium without overshooting under a step load, and the amplitude of the block vibration has not a maximum value under a cyclic load, due to the large damping effect( ξ>1). In addition, the settling time can be shorten with the increase of the supply pressure, the film thickness and the lubricating oil volume, and also with the decrease of the pressure ratio and the lubricating oil viscosity. The settling time get the shortest value when recess parameter( α) is 0.55.

Research on error averaging effect of hydrostatic guideways

Hydrostatic guideways can obtain high motion accuracy due to their error averaging ability. In order to optimize the structural and accuracy design of hydrostatic guideways, the mechanism and affecting factors of the error averaging effect are investigated in detail in this paper. A new method is proposed to quantitatively analyze the motion errors of a typical closed hydrostatic guideway with four pads, and the restrictor is also taken into consideration. A hydrostatic guideway in an ultra-precision grinding machine LGF750 is analyzed with the method. The results show that the wavelength of each profile error component is a main affecting factor on the error averaging effect. Fluid films usually show obvious error averaging effect on each profile error component when the wavelength λ is shorter than twice of the pad length 2lu. However, for each component with the wavelength λ over 2lu, the error averaging effect becomes obvious only when the ratio of the pad length to the evaluation length of motion straightness lu/le is greater than 0.5. Furthermore, the instability of the oil supply system is taken into consideration, and possible influences on the error averaging effect due to the supply pressure fluctuation are investigated preliminarily.

Flow Analysis on Hydrostatic Guideway of Precision CNC Lathe Based on FLUENT

In order to improve the machining accuracy of precision CNC lathe, and explore the fluid motion law in the closed hydrostatic guideway, three-dimensional model of hydrostatic oil chamber was established using preprocessor GAMBIT of FLUENT. The Finite Volume Method of Fluent has been used to the simulation of fluid three-dimensional velocity field and pressure field. The fluid velocity vector and pressure distribution graphs are obtained. They reveal the flow field law in the gap fluid. The simulation result of the pressure distribution shows that the flow pressure was not changing in linear gradient all around the gap under moving condition.

Influences of Moisture Expansion on Motion Errors of Granite Hydrostatic Guideways

Granite is a kind of high damping material with low thermal expansion, so it is widely used in aerostatic guideways. However, moisture expansion may occur when the granite works in a moist environment, so the applicability of granite to oil hydrostatic guideways is investigated in this paper. By error modeling for a typical closed hydrostatic guideway with four pads, it is shown that the motion errors are mainly determined by geometric errors of the rails when film stiffness of each pads are equal. Experiments on rails of Jinan Dark Green granite are carried out, and the results show the rail straightness variation by the moisture expansion may be less than 1μm/300mm. According to the previous analysis on the error averaging mechanism of hydrostatic guideways, the motion straightness variation may be less than 0.1μm/300mm. In addition, the thickness variation may be less than one ten-thousandth of the rail thickness and the viscosity of liquid has little influence on the moisture expansion.

Finite Element Analysis of Static Characteristics of a Hydrostatic Guideway and its Effect Factors

Guide ways of ultra-precision machine tools always have sufficient stiffness and guiding precision. But guide way rails deformation caused by oil film pressure and slider offset lead to oil film clearance change which affects static stiffness and guiding precision. An inside-slider hydrostatic guide way is designed, and its static characteristics are analyzed. As a result, the effect of slider offset on stiffness and guiding precisions is obtained. A FE model of guide way is established. An elastomer of virtual material properties is used to simulate oil film, so that the relationship between oil film clearance and pressure can be represented in a certain range. Static stiffness and guiding precision are obtained considering deformation of rails.

Joint stiffness identification of an ultra-precision machine for machining large-surface micro-features

In this study, to more quantitatively evaluate the structural characteristics of an ultra-precision machine for machining large-surface micro-features, the joint stiffnesses of hydrostatic guideways and bearings were identified based on a virtual prototype and compliances of the ultra-precision machine. The virtual prototype of the ultraprecision machine was constructed to include the joint stiffnesses to be identified, and the joint stiffnesses were identified through an optimization problem in order to minimize the error between the compliances measured from the physical prototype and the compliances predicted from the virtual prototype. In particular, the validity of the identified joint stiffnesses was verified by the fact that the loop stiffnesses of the ultra-precision machine predicted from the virtual prototype and the identified joint stiffnesses coincided well with those measured from the physical prototype.