Ultra-precision Machine Tool Research Articles

Characterization of ultrasonic vibration and polishing force in sapphire ultrasonic vibration-assisted flexible polishing: Insights from in-situ monitoring systems

Sapphire ultrasonic vibration-assisted flexible polishing (UVAFP) is a promising technique for comprehensively improving the surface integrity of machined parts. The technique was performed on an ultra-precision machine tool with the in-situ monitoring systems in this paper, which aims to provide a new perspective for understanding the material removal mechanisms in the sapphire UVAFP process. A Taguchi L9 (43) orthogonal experiment was conducted to investigate the effects of feed distance, spindle speed, ultrasonic vibration (UV), and polishing time on the surface finish and material removal in the process. In addition, the effect of a polyurethane ball tool is not trivial. A single-factor experiment was conducted for exploring it. Based on a laser displacement measurement system and an acoustic emission sensor system, the characteristics of time-dependent ultrasonic amplitude and ultrasonic frequency for the sapphire UVAFP system were analyzed, with the effectiveness of UV demonstrated. Based on a three-component force measurement system, the characteristics of normal force and its relationship with process parameters and tool deformation were analyzed, with macro- and micro-level examined. In conclusion, this paper presents the characterization of UV and polishing force in the sapphire UVAFP process, providing novel insights into understanding the material removal mechanisms of sapphire and even more manufacturing problems.

Controllable diamond cutting of structured surfaces with subnanometric height features on silicon

Diamond cutting with a controllable depth of cut at the subnanometric scale is desired for next-generation electronics and optics. However, due to the limits of positioning accuracy of the current machine tools, diamond cutting at subnanometric scale depths remains in realm of molecular dynamic (MD) simulations instead of engineering realization. Cutting force feedback and control is regarded as a potential method to improve the positioning accuracy of machine tools. In this study, an ultra-precision force feedback loop with the resolution down to submillinewton is employed and integrated on an ultra-precision machine tool to enable the capability of cutting at subnanometric scale depth. By this way, the relationship between the cutting force and such small depth of cut needs to be well studied. MD simulations are conducted in this study to analyze the mechanism of material removal and influence of the crystallographic effect on the actual cutting depth and force caused by varied cutting directions at subnanometric to nanometric scale depth in diamond turning. Then, the crystallographic effect of silicon with the depth of cut from subnanometric to nanometric scale is compensated experimentally for accurate cutting at such an extremely small scale. Controllable diamond cutting of structured surfaces with actual depths and amplitudes ranging from several angstroms to a few nanometers on silicon is successfully realized.

Investigation of the error averaging effect on design and assembly of external slider closed hydrostatic guideways

Hydrostatic guideways are widely used in ultra-precision machine tools. However, in most cases, assemblers must rely on their experience to adjust the manufacturing errors of the guide rail and slider, which indirectly controls the error motion. To ensure assembly accuracy and improve efficiency, an error motion calculation model is established based on the external slider closed hydrostatic guideway. The accuracy of the model is verified through experiments, and the main factors that affect the error motion are analysed. Finally, a traversal algorithm is used to calculate the distribution of the error averaging coefficient. Once the error motion index is clarified, the design and assembly process of the guideway can be constrained by the error averaging coefficient.

Interference-enhanced micro-vision-based single-shot imaging of five degrees-of-freedom error motions for ultra-precision rotary axes

The measurement of five degrees-of-freedom (5-DOF) error motions, including radial, axial, and tilt motions, is crucial for ultra-precision rotary axes, which are key components of ultra-precision machine tools and instrumentation. In this study, we propose an interference-enhanced micro-vision technique to concurrently derive the 5-DOF error motions from a single-shot two-dimensional image, which was captured by a standard industrial camera equipped with an interference objective lens. By consolidating the essential features into a single optical path, the interference-enhanced micro-vision technique ingeniously merges machine micro-vision and modified white-light interference to detect in-plane and out-of-plane motions. Numerical simulations demonstrated, the basic principle for deriving the 5-DOF error motions, and the magnification of objective lens had inconsistent effects on the measurement accuracy for the radial and tilt motions, i.e. higher magnification led to higher radial accuracy but lower tilt accuracy. As practical application, the error motion detection capability was demonstrated by simultaneously measuring the 5-DOF synchronous and asynchronous error motions for a typical air bearing spindle at rotation speeds of 8.33, 108.33, and 308.33 rpm. The synchronous errors were nearly identical at various spindle speeds. However, because of system dynamics, increased vibrations were observed to be superimposed on the basic tilt error motions as the spindle speeds increased, which were verified by the vibration marks imprinted on the turned surfaces. For the 5-DOF motion measurements, the least-square fitting using large-volume edge and greyscale data of the captured image enabled super-high resolutions, despite using a camera with a relatively large pixel size and low bit depth. These results demonstrate that the proposed interference-enhanced micro-vision technique is a simple and effective tool for measuring spatial error motions in ultra-precision rotary axes.

Dynamic simulation and active vibration control design of an ultra-precision fly-cutting machine tool

Dynamic simulation and active vibration control design of an ultra-precision fly-cutting machine tool

A smart structural optimization method of magnetorheological damper for ultra-precision machine tool

To address the problem of multi-source vibration in ultra-precision machine tools, a vibration reduction stand was designed by replacing passive damping components with magnetorheological dampers (MRDs). In this work, the structural parameters of MRDs were optimized using an improved pelican optimization algorithm (IPOA) to realize the maximum capability in reducing vibration. Firstly, the working principle of MRDs was explained, and the mathematical models of MRDs were established. Then, an IPOA based on singer chaotic mapping, nonlinear inertia weight factor, and Cauchy mutation strategy was proposed to enhance the global search capability and convergence efficiency of the algorithm. Subsequently, the IPOA was applied to optimize key structural parameters of MRDs, including output damping force, controllable damping range, response time, and power consumption. Finally, COMSOL Multiphysics software was used to verify the effectiveness of the proposed algorithm by comparing the magnetic induction intensity distribution of MRDs before and after optimization, as well as the variation of the four performance indexes under the different applied currents. After being optimized using the proposed IPOA, the MRDs can deliver a larger maximum damping force and a wider damping controllable range, with less power consumption and quick response, which could meet the requirement for vibration suppression of ultra-precision machine tools.

Research on geometric error compensation of ultra-precision turning-milling machine tool based on macro–micro composite technology

Research on geometric error compensation of ultra-precision turning-milling machine tool based on macro–micro composite technology

Universal accuracy model for three different types of precision bearings with three basic structures under quasi-static conditions

A universal accuracy model is established under quasi-static conditions to predict motion accuracies and directly compare error averaging abilities among three different types of precision bearings (including fluid hydrostatic, porous air, and cylindrical roller bearings) with three basic structures (including linear (i.e., guideway), journal, and thrust bearing structures) used widely in precision and ultra-precision machine tools. To introduce the universal accuracy model, three types of precision guideways are taken as examples. Uniform generalized coordinate systems are used to evaluate error motions and uniform averaging coefficients are defined to assess error averaging abilities. It is found that the total error averaging abilities consist of basic and layout error averaging abilities characterized by corresponding averaging coefficients. The total averaging coefficients are the products of basic and layout averaging coefficients which are independent of each other. The porous air bearings have strongest basic error averaging abilities, followed by the fluid hydrostatic bearings, and finally followed by the cylindrical roller bearings. The layout averaging coefficients are related to the number and layout of pads or rollers which are working like some structural filters for a certain kind of error motion. A summary of accuracy experiments based on previous studies provides evidence of the existence of layout error averaging abilities. The content discussed in this paper is of significant importance for the selection and design of precision bearings.

Intelligent G-code-based power prediction of ultra-precision CNC machine tools through 1DCNN-LSTM-Attention model

Intelligent G-code-based power prediction of ultra-precision CNC machine tools through 1DCNN-LSTM-Attention model

Fabrication of the curved Fresnel lens array on the spherical surface by 6-axis diamond ruling

The curved Fresnel lens array on the spherical surface owns exceptional optical performance and meets the growing demand for smart home and security alarm systems. However, the intricate three-dimensional microstructure of such optical component is typically considered impossible to be directly fabricated by diamond machining. In this paper, a novel 6-axis diamond ruling method is proposed to fabricate the complex optics utilizing a commercial 6-axis ultra-precision machine tool. The kinematic principle and tool path generation algorithm of this method are presented. A varying-angle sampling strategy is employed in the proposed algorithm to reduce the acceleration and following error in the rotation of the A-axis. Novel trial cutting processes are introduced to figure out the tool setting errors induced by visual inspection, which need to be compensated in the tool path. Based on the proposed tool path generation algorithm and cutting processes, this method enables the efficient processing of precise curved Fresnel lenses. There is no interference between the diamond tool and the curved microstructure in the 6-axis synchronized motions. Finally, the feasibility and superiority of this method are experimentally verified by analyzing the surface roughness and shape accuracy of the machined component. It can indeed generate high-quality curved Fresnel lenses on the spherical surface by the proposed 6-axis diamond ruling method.

Milling surface quality enhancement through encoder micro-cyclic error compensation in an ultraprecision machine tool

Milling surface quality enhancement through encoder micro-cyclic error compensation in an ultraprecision machine tool

Interfacial micromechanics modeling for bolted joints in ultra-precision machine tools

Interfacial micromechanics modeling for bolted joints in ultra-precision machine tools

Assessment of repeatability in newly developed ultra-precision tool setting method using electrical breakdown for ultra-precision machining

Ultra-precision work coordinate system (WCS) setting by measuring the relative distance between the workpiece and the tool is a key element to further improve the accuracy of ultra-precision machine tools. Touch probe is the most widely used WCS method in conventional machining, yet it cannot be used in ultra-precision machining (UPM) applications due to the nature of contact with the workpiece, potentially resulting in surface damage or tool breakage. Furthermore, indirect methods, such as a touch probe, require the sensor to be replaced with a tool after setting the work coordinate. This induces uncertainty. In this paper, the repeatability of the newly proposed non-contact and direct WCS method using electrical breakdown (E.B.) was evaluated for the use in UPM. The standard deviation of the tool positions after performing the method 11 times was under 40 nm. Using a tungsten carbide tool and aluminum workpiece, the proposed method’s repeatability and surface damage were investigated.

Study regarding the influence of process conditions on the surface topography during ultra-precision turning

During ultra-precision turning, the turned surface “copies” a large amount of turning process condition information. In this study, the path equations were combined with the turned surface topography equations, and the condition parameters were superimposed to develop theoretical surface topography models for ultra-precision turned end face, cylindrical, conical, and spherical surfaces influenced by turning condition parameters. The models were validated through a series of experiments. The turned surfaces were analyzed in the spatial frequency domain, and the corresponding relationship between the frequency information in the time domain and the spatial domain was obtained. The corresponding area power values were calculated to determine the quantitative impact of condition parameters on surface roughness. According to the calculation results, the surface roughness under the influence of process conditions was positively correlated with the amplitude of turning process condition parameters. The topography of the simulated surfaces was consistent with that of the experimental surfaces, and the trend of area power values corresponding to different turning process condition parameters in the simulated surface topography was consistent with the experimental results. It was proved that the established relational model, which determined the qualitative and quantitative influence of machining conditions on the machined surface, is tenable. This study established a mapping relationship between the machining condition parameters and their impact on surface topography, and also further provided a foundation for the digitization of ultra-precision machine tools.

Unbalanced vibration suppressing for aerostatic spindle using sliding mode control method and piezoelectric ceramics

The lubricating medium of the aerostatic spindle is gas, with relatively low rigidity, which is easy to cause rotor offset and additional vibration under the action of external load, thus affecting the processing accuracy of the spindle. To solve the problem of unbalanced vibration of the spindle, a new type of controllable radial aerostatic bearing is designed by combining piezoelectric ceramics, which use the inverse piezoelectric effect of piezoelectric ceramics to squeeze the air film to produce corresponding control force, and the radial unbalanced vibration of the spindle is suppressed. A bearing–rotor–piezoelectric ceramic coupling model was established, and the control method of feedforward combined with feedback was used to reduce the hysteresis characteristics of piezoelectric ceramics. On this basis, the sliding mode controller of the entire system, and through research, it is found that the sliding mode controller based on the extended observer has the best vibration suppression effect compared with other sliding mode controllers in the article; this research results can improve the rotation accuracy of the spindle system and the machining accuracy of ultra-precision machine tools.

“4+2” axis ultra-precision diamond shaping for partially overlapping Fresnel lenses based on an ultra-precision machine tool and two additional rotational axes

Optical surfaces and microstructures with high form accuracy and excellent surface quality are widely utilized nowadays and the diamond machining technology is still preferable for the fabrication of these workpieces. However, when machining surfaces featuring a high aspect ratio or intricate microstructures like monolithic microstructural optics, the machining efficiency is quite low and the cutting tool interference is inevitable for the existing diamond turning techniques. Hence, a novel method is proposed to machine such microstructures by introducing two additional rotational motions into the traditional ultra-precision machine tool. In this way, the arbitrary adjustment of the tool posture may be achieved in the machining process and intricate microstructures like monolithic microstructural optics can also be machined by the side cutting edge with high efficiency. The whole mechanical system and machining principle are presented firstly. Next, the tool path planning and the compensation for the tool tip offset are introduced. Then, the partially overlapping Fresnel lens array consisting of two segmented Fresnel lenses are fabricated and inspected. Finally, the measurement data are analyzed and the proposed method is validated to be able to fabricate such complex microstructures with high profile accuracy and surface quality successfully.

Ultra-precision time-controlled grinding for flat mechanical parts with weak stiffness

High-precision flat mechanical parts are widely used, such as in aerostatic bearing, which requires a rather high machining precision. Due to large size, low stiffness and other machining problems, traditional ultra-precision grinding machine tool cannot directly achieve the final precision, so manual grinding is necessary to meet the manufacturing requirements, resulting in low efficiency. Based on the computer-controlled optical surfacing (CCOS) method and the flexible grinding process, the time-controlled grinding method (TCG) is proposed to achieve quantitative material removal at each position on the workpiece. Combined with the material removal mechanism, the removal function with good linearity and stability is extracted by controlling corresponding parameters. By applying a novel measuring and processing method of surface data with high precision and high efficiency, the flatness error and parallelism error are accurately obtained and used as grinding reference to carry out the time-controlled grinding experiment on one spacer ring with a diameter of 190 mm. The final flatness error converges from 1.5–2 μm to 0.6–0.7 μm and parallelism error converges from ~2 μm to ~0.6 μm, while the roughness converges from 273 nm Ra to 65 nm Ra, achieving the goal of ultra-precision digital machining of submicron level. The machining precision is better than traditional ultra-precision grinding machine tool, and the potential for time-controlled grinding method of ultra-precision machining in flat mechanical parts especially with low stiffness is verified.

A geometric error measurement method for five-axis ultra-precision machine tools

A geometric error measurement method for five-axis ultra-precision machine tools

Complete topographic measurement of the monolithic multi-surface workpiece on a cylindrical cavity

Monolithic multi-surfaces workpieces (MMSWs) are widely used in aerospace, advanced optics, and other fields. The complete topographic measurement of each sub-surface in MMSWs requires high measurement flexibility and strong space constraints since each sub-surface is located in an inner cylindrical cavity. This paper proposes a complete topographic measuring method for MMSWs based on a four-axis measuring system with a rotating single-point probe. Several basic geometric parameters of the measuring system are identified in the calibration. The mapping relation between the theoretical surface equations and measuring system coordinates is established by searching for reference points. The measurement scanning paths with different orientations are designed for each sub-surface. The iterative measurement and optimization are adopted to compensate for the measurement errors from the coupling of the probe and the four-axis measuring system. A standard sphere is measured, and the deviation of the fitting radius is below 0.5 μm. Besides, a freeform four-mirror optical system is selected as the experimental study case, where the complete topographic measurement of each sub-surface is completed on an ultra-precision machine tool. The form error and the standard deviation of each sub-surface can reach the micron level.

Transfer matrix method for linear vibration analysis of flexible multibody systems

Achieving high computational efficiency has been well recognized as a research challenge in the vibration analysis of flexible multibody systems. This paper presents a novel transfer matrix method to model and analyze the linear vibration of flexible multibody systems. The transfer equations, the transfer matrices, and the consistency equations of the general flexible body elements with multi-input ends are deduced for the first time. Subsequently, the automatic deduction theorem of the overall transfer equation is developed for flexible multibody systems. Based on the theorem, the overall transfer equation can be deducted automatically and the natural vibration characteristics can be obtained. The dynamic equations and augmented eigenvectors are formulated to solve the forced vibration response. Further, the proposed method is applied to study the dynamics of an ultra-precision machine tool with flexible body elements. The natural vibration characteristics and the forced vibration response are solved and validated with the data from the modal tests and the working conditions. The proposed method has the following advantages: easy deduction of the overall transfer equation, low order of system matrix, and high computational speed.