Ultra-precision Flycutting Research Articles

Tool‐tip vibration prediction based on a novel convolutional enhanced transformer

AbstractSuperior surface finish remains a fundamental criterion in precision machining operations, and tool‐tip vibration is an important factor that significantly influences the quality of the machined surface. Physics‐based models heavily rely on assumptions for model simplification when applied to complex high‐end systems. However, these assumptions may come at the cost of compromising the model's accuracy. In contrast, data‐driven techniques have emerged as an attractive alternative for tasks such as prediction and complex system analysis. To exploit the advantages of data‐driven models, this study introduces a novel convolutional enhanced transformer model for tool‐tip vibration prediction, referred to as CeT‐TV. The effectiveness of this model is demonstrated through its successful application in ultra‐precision fly‐cutting (UPFC) operations. Two distinct variants of the model, namely, guided and nonguided CeT‐TV, were developed and rigorously tested on a data set custom‐tailored for UPFC applications. The results reveal that the guided CeT‐TV model exhibits outstanding performance, characterized by the lowest mean absolute error and root mean square error values. Additionally, the model demonstrates excellent agreement between the predicted values and the actual measurements, thus underlining its efficiency and potential for predicting the tool‐tip vibration in the context of UPFC.

Modelling and simulation of the 3D surface topography in ultra-precision flycutting machining

A simulation model of the 3D surface topography in ultra-precision flycutting machining is built in this paper. The influences of the cutting parameters, the cutting interference phenomenon, the relative vibration between the tool tip and the workpiece and the workpiece shape are analysed. The formation mechanism of the stripes along the cutting direction and the feed direction is explained. The simulation model can be used to optimize the cutting process of the ultra-precision flycutting machining, which avoids time and money consuming cutting experiments, and can also provide new ideas for tracing the source of the stripes on the surface topography.

Study on traceability and suppression method of medium-frequency error for ultra-precision machining optical crystals

The medium-frequency error on the surface of ultraprecision flycutting has an important effect on the performance of the optical crystal. In this paper, firstly, the characteristic phenomenon of "knife-like grain" in the medium frequency surface of the square and circular optical crystal machined by ultraprecision fly-cutting is revealed. Besides, the error traceability is realized and the results show that the periodic low-frequency fluctuation of 0.3 Hz between the tool tip and the workpiece is the cause of the medium frequency error of "knife-like grain". Secondly, through the frequency domain waterfall diagram of vibration signal and the analysis of spindle speed signal, it is proved that the surface shape characteristic is caused by the fluctuation of spindle speed during the cutting process. Then, the variation rule of the cutting amount caused by the fluctuation of spindle speed is explored theoretically and experimentally, and the formation mechanism of medium frequency error in flycutting is revealed. Finally, in order to reduce the medium frequency error of "knife-like grain", the control parameters of the aerostatic spindle are reasonably optimized based on the analysis of the mechanical and electrical coupling control performance of the spindle, so that the RMS values in the medium frequency band of the workpiece are greatly improved after machining.

Investigation on dynamic performance of ultra-precision flycutting machine tool based on virtual material method

In the field of ultra-precision machining, the dynamic performance of ultra-precision equipment contributes a lot to the processing accuracy. In this paper, in order to study the dynamic performance of the flycutting machine tool, the virtual material method was adopted creatively to build a machine tool model. This method overcame the complexity of actual structure and obtained more accurate results than traditional methods. Subsequently, the finite element method was applied to analyze the dynamic performance of the virtual material model. Furthermore, the modal test for the flycutting machine tool was performed to verified the simulations by means of the one-point hitting and multi-point measurement. The simulation results indicate that the virtual material model has high comprehensive accuracy, of which the average error of natural frequency is 5.9% and errors are all less than 6% excepting the first order. Moreover, combined with the flycutting experiments, it can be found that the fifth order mode of machine tool contributes a lot to processing quality, which leads to the waviness of 53 mm on the machined surface directly because of the weak stiffness of beam-column joints. By increasing the stiffness of the key joints, the waviness can be eliminated, which greatly improves the surface quality of the workpiece.

Size effect on surface generation of multiphase alloys in ultra-precision fly cutting

Abstract Ultra-precision fly cutting (UPFC) is a typical discontinuous cutting process where a cutting tool flies simultaneously with the rotation of a spindle and cuts the surfaces of a workpiece intermittently. To obtain good surface roughness with an acceptable productivity, it is crucial to investigate the influence of cutting chips and tooling movement during the cutting process. In UPFC, it is found that the surface rough patterns (SRPs), which are affected by submillimetre-size cutting chips, are generally formed at the tool-out area of each tool feed imprint on the machined surface. The formation of SRPs in UPFC is assumed to be affected by the size effect that is critical in new surface generation in the micromachining process. In this process, the tooling cuts through both the surface layer and inner layer grains of the workpiece during chip formation, thus resulting in the formation of an SRP. In this research, the influence of cutting parameters on the SRP is investigated; a hybrid constitutive Johnson Cook model is established and a finite element simulation using the established constitutive model is conducted to analyse the generation of the SRP. Experimental and simulation results indicate that when surface grain ratio is larger than 35 %, the inconsistent fracture strain between the surface and inner layers is the primary reason of the fracture inside the uncut chip in UPFC. SRPs are thus formed owing to the void formation and tooling movement marks on the machined surfaces. Increasing the spindle speed and reducing the feed rate minimise the occurrence probability of the SRP. Upon addressing the issues described above and with informative findings, this research provides an in-depth understanding of SRP generation affected by size effect in UPFC, and further presents a basis for improving the quality of machined surfaces.

Investigation on the formation mechanism and controlling method of machined surface topography of ultra-precision flycutting machining

The three-dimensional topography of machined surface induced by machine tool vibration has a great impact on the optical performance of ultra-precision optical components. This paper aims to explain the formation mechanism of the wavy topography of the machined surface of ultra-precision flycutting. Firstly, a dynamic model of the machine tool was built based on finite element method (FEM) considering the fluid-structure interaction (FSI) effect. Then, a mathematic model for simulating the machined surface topography was built based on the FEM model of the machine tool. The machined surface topography was simulated, and the spatial periods of waviness errors were analyzed based on power spectral density (PSD) method. The reason of waviness errors on machined surface was revealed by investigating the internal relationship between the spatial period of waviness error and the dynamic performance of the machine tool. Moreover, the method for controlling the spatial period of waviness error was discussed, which provides guidance for improving the optical performance of large size optical component.

Dynamics Modelling and Simulating of Ultra-precision Fly-Cutting Machine Tool

Dynamics modelling and simulating are the significant process to improve the machining accuracy of the machine tool. This paper is aimed to model and simulate the ultra-precision fly-cutting machine tool (UFMT) and find the relations between structure parameters and machined surface. In this paper, the multi-rigid-flexible-body dynamics model of the UFMT is firstly built by using transfer matrix method for multibody systems. After deducing overall transfer equation, overall transfer matrix, eigenfrequency equation and dynamics equations, the vibration characteristics and dynamics response of tool-tip are simulated and validated by tests. The machined surface is simulated by transferring displacement between the fly-cutting tool-tip and the workpiece into 3D curve. According to the simulation results, both the air-bearing stiffness of the flying-cutting head and cutting process parameters have effects on the machined surface.

Feasibility investigation on ductile machining of single-crystal silicon for deep micro-structures by ultra-precision fly cutting

Single-crystal silicon is a widely used brittle material in infrared optics and optoelectronics industries. However, due to its extremely low fracture toughness, it is difficult to obtain deep micro-structures on single-crystal silicon with ultra-smooth surface quality using previous ductile machining models based on plunge cutting, diamond milling and grinding. Current methods to enhance the machinability of silicon include laser-assisted machining, ion implantation modification and vibration-assisted machining. However, the increase of the ductile machining depth using these methods is still very small in the fabrication of deep micro-structures with a depth over tens of micrometers on silicon. This paper proposes a novel ductile machining model for ultra-precision fly cutting (UPFC) to efficiently fabricate deep micro-structures on silicon. The modeling results show that through configuring a large swing radius, much deeper ductile machining depth can be reached by UPFC. To confirm this proposed model, micro-grooves with different depths were machined, and the surface micro-topographies, form error, tool wear patterns and material phase transformation were analyzed and compared with that acquired by diamond sculpturing method. The experimental results demonstrated that much deeper micro-grooves (over tens of micrometers) with better surface quality were acquired by UPFC. Moreover, compared with the sculpturing method, UPFC prolonged the tool life, and generated less amorphous silicon on the machined surface.

Chatter stability in ultra-precision fly cutting considering tool wear effect

In ultra-precision flying cutting (UPFC), a vibration is a unique dynamic response due to periodic intermittent excitation of cutting forces, which has a significant impact on surface generation. In UPFC, cutting forces act on the machine tool system intermittently and periodically, which may cause a self-excited vibration. This self-excited vibration will cause chatter, which maybe occurs during machining operations and even catastrophically degrades surface quality and limits productivity. This study has been motivated by the fact that chatter occurrence is generally in relation to the tool wear, which has seldom been investigated comprehensively in literature. In this paper, a chatter model for UPFC that can account for the effects of tool wear and process damping was presented. A number of experiments with different machining conditions were conducted to investigate the effect of the tool wear on the chatter stability. These experiments proved that the stability lobe is closely related to tool wear and with the tool wear increase, which increases the tendency for chatter in UPFC. Finally, it was found that increasing the tool clearance angle and reducing the spindle speed can improve the stability limit.

Steady tool wear and its influence on tool geometry in ultra-precision fly cutting of CuZn30

In ultra-precision fly cutting (UPFC), steady tool wear progresses gradually when cutting ductile materials. The steady wear of diamond tools changes the tool geometry, and further affects the topography of the machined surface. In this study, a theoretical and experimental investigation was conducted on the forms of steady tool wear and their influence on the tool geometry in UPFC. The features of steady tool wear in UPFC were investigated, the displacement of the cutting edge under the effects of tool flank wear was modeled, and factors affecting the cutting edge roundness were investigated. The study results indicate the following: (1) the features of steady tool wear in UPFC include crater wear on the rake face, smooth and flat wear on the cutting edge, and flank wear on the tool clearance face. (2) A smooth wear-land on the cutting edge changes the top rake of the cutting tools, whereas flank wear on the tool clearance face affects the clearance angle and roundness of the cutting edge. (3) The maximum width of the tool material loss zone, the nose radius of the original fresh cutting edge, and the arc angle of the wear-land have a certain influence on the nose radius of the worn cutting edge. The results of this study will provide deep insight into steady tool wear and its influence on the tool geometry, which can potentially be used to predict or evaluate the finish of a machined surface under tool steady wear.

Theoretical and experimental investigation of the effect of the machining process chain on surface generation in ultra-precision fly cutting

Ultra-precision fly cutting (UPFC) is an enabling ultra-precision machining method since it has many advantages such as providing uniform high surface quality, high flexibility necessary for machining freeform or micro-structural surface, etc. However, low machining efficiency is still a problem for UPFC. This paper attempts to increase its machining efficiency by shortening its process chain based on theoretical analysis. A dynamic model considering the whole process chain in UPFC is developed in this study. The dynamic process chain model (DPCM) is basically divided into two parts. The first part is a single UPFC model consisting of geometry parameters and spindle vibration which are primary factors of surface generation. The second part is the preceding surface topography model of the workpiece. An additional coordinate transfer model is developed to combine the above two parts together. Simulation and actual experiments have been conducted to verify the DPCM. The predicted results from the DPCM match the results from the actual machining experiments well. It is found that the parameters are optimized to shorten the process chain in UPFC. In some conditions, the required surface quality is achieved by a single machining step of UPFC directly from traditional turning. On the whole, the process chain model of UPFC provides a feasible way to simulate the dynamic process and observes the change of the surface generation of workpieces when machining parameters change so as to optimize the process chain in UPFC.

A novel ductile machining model of single-crystal silicon for freeform surfaces with large azimuthal height variation by ultra-precision fly cutting

Ultra-precision fly cutting (UPFC) is an important technology in the fabrication of freeform surfaces on single-crystal silicon. However, the insufficient understanding of the ductile machining mechanisms in UPFC of silicon under a large depth of cut highly limits the practical fabrication of freeform surfaces with large azimuthal height variation (AHV). Especially, no work has been found on studying the ductile machining model in UPFC of silicon considering both feed and step motion of the diamond tool. In this study, a novel ductile machining model in UPFC of silicon is proposed to demonstrate the superiority of UPFC on achieving the deep ductile-cut region. Experimental validation has been conducted by fabricating two kinds of freeform surfaces, namely micro-grooves and an F-theta lens, on silicon. This paper theoretically and experimentally elaborates that the chip thickness of UPFC is not only determined by the machining parameters, but also is the inversely proportional function of the swing distance of the diamond tool. Thus, by employing a large enough swing distance, much thinner chips can be generated by UPFC even when machining under large cutting depths and feed rates. Therefore, a deep ductile-cut region of silicon can be achieved by UPFC with a large swing distance. Freeform surfaces with tens of micrometers of AHV can be successfully fabricated on silicon by UPFC without the generation of brittle fractures.

Dynamic Accuracy Design Method of Ultra-precision Machine Tool

Ultra-precision machine tool is the most important physical tool to machining the workpiece with the frequency domain error requirement, in the design process of which the dynamic accuracy design (DAD) is indispensable and the related research is rarely available. In light of above reasons, a DAD method of ultra-precision machine tool is proposed in this paper, which is based on the frequency domain error allocation. The basic procedure and enabling knowledge of the DAD method is introduced. The application case of DAD method in the ultra-precision flycutting machine tool for KDP crystal machining is described to show the procedure detailedly. In this case, the KDP workpiece surface has the requirements in four different spatial frequency bands, and the emphasis for this study is put on the middle-frequency band with the PSD specifications. The results of the application case basically show the feasibility of the proposed DAD method. The DAD method of ultra-precision machine tool can effectively minimize the technical risk and improve the machining reliability of the designed machine tool. This paper will play an important role in the design and manufacture of new ultra-precision machine tool.

Influence of air-induced vibration of aerostatic bearing on the machined surface quality in ultra-precision flycutting

The performance of potassium dihydrogen phosphate optics in inertial confinement fusion devices is highly affected by its surface topography. This paper theoretically investigates the relationship between the air-induced vibration of aerostatic bearing and high-frequency ripples (amplitudes of around 12 nm, spatial periods in the range of 8–30 mm) on machined surface of potassium dihydrogen phosphate optics. A simulation model, which combines transient computational fluid dynamic analysis and transient dynamic analysis, is proposed to simulate the vibration of tool tip. The three-dimensional flow status of air film is studied based on large eddy simulation method, and air vortex shedding phenomenon is observed, which results in the dynamical change of air film force. The simulated vibration amplitudes of tool tip is 8 nm, and its spatial periods are in the range of 10–24 mm, which agree well with the measured data.

Influence of ultra-precision flycutting spindle error on surface frequency domain error formation

The frequency domain error of the machined surface in the ultra-precision machining attracts more attention for its strong relationship with the functional requirements of the workpieces. As the key component of ultra-precision machine tool, the spindle error largely influences the machined surface errors. However, little attention has been paid to the influence of spindle error on the surface error formation mechanism in the frequency domain for ultra-precision flycutting. This paper deeply studies the influence of spindle error on the frequency domain error formation of machined surface in ultra-precision flycutting. The influence analysis of the spindle error is theoretically and experimentally carried out according to two different evaluation directions of the machined surface by flycutting, i.e., cutting direction and feed direction. The KDP crystal flycutting experiment is designed and conducted for the theoretical verification. The theoretical analysis has been found to agree well with the experimental results. This study is quite meaningful for deeply understanding the influence law of spindle error from the viewpoint of frequency domain. The research results are quite useful for the spindle error control, machined surface error prediction, machining process planning, and also show the potential use for the spindle (machine tool) design.

Dynamic design and thermal analysis of an ultra-precision flycutting machine tool

This article presents the dynamic design and thermal analysis of an ultra-precision flycutting machine tool. The hydrostatic bearings are used both in the spindle and slide to ensure the stiffness of the machine tool. The fluid–structure interaction method is used to obtain the actual clearance changes of the hydrostatic bearing. The dynamic and thermal performances of the machine tool are analyzed considering the effect of hydrostatic bearings. The influence of the thermal effect on the static and dynamic performance of the machine tool is further studied. The prototype machine tool is built. Preliminary machining trials have been carried out and provided evidence of fluid–structure interaction method being helpful to design the ultra-precision machine tool based on the hydrostatic technology.

Research on surface topography in ultra-precision flycutting based on the dynamic performance of machine tool spindle

The dynamic performance of ultra-precision cutting machine tools is an important factor for its machining accuracy. To improve the precision of the machine tool, a surface topography model is built in this paper and the tool path during the cutting process is achieved by dynamic finite element analysis (DFEA) of the air spindle in ANSYS, surface topography is obtained using MATLAB simulation by combing the model with the tool path, and an experiment is carried out for validation. Results show the simulation surface is highly consistent with the experimental result. Dynamic performance of the spindle is the main factor for texture generation, and optimization of spindle is investigated, DFEA of the optimized spindle shows better performance, and the connection mode between diamond tool and cutter head should be further studied.

An in-process tool wear evaluation approach for ultra-precision fly cutting

Ultra-precision fly cutting (UPFC) is an intermittent cutting process, which is widely used in the fabrication of non-rotational symmetric micro structures with sub-micron form accuracy and nanometric surface roughness. In UPFC, the occurrence of tool wear certainly affects the accuracy of machined micro structures. Aimed at the close relations between tool wear and chip morphologies and the truth that cutting chips are fully formed in a cutting cycle, this research developed a tool wear evaluation approach based on cutting chips. Chip morphology features related to tool failure patterns were identified and then parameterized to predict the tool failure patterns, the surface topography, and surface roughness under the effects of tool failure patterns. Predictions were then verified by experimental results. Research results show that chip morphologies were successfully used to present tool failure pattern, following the well-designed identification procedures, the tool failure patterns are accurately identified. This approach is practical since it can in-process identify tool failure patterns and their effects on surface quality.

Relationships of tool wear characteristics to cutting mechanics, chip formation, and surface quality in ultra-precision fly cutting

Although the occurrence of tool wear can affect cutting forces, cutting chip morphologies, and machined surface quality in ultra-precision fly cutting (UPFC), there has been no research in this area due to the complex cutting mechanism of UPFC. The theoretical and experimental research described in this paper was therefore conducted to explore tool wear characteristics in UPFC and their relationship to cutting forces, cutting chip morphologies, and machined surface quality. Results from the study reveal that tool wear characteristics in UPFC include cutting edge fractures, workpiece material welding, wear land formation, sub-wear land formation, and micro-grooves. The cutting edge fractures lead to the formation of ridges on both the cutting chips and machined surface; the material welding increases the thrust force, crushes cutting chips, and deteriorates the machined surface quality; and the formation of wear land on the cutting edge makes the machined surface burred and fuzzy, and the micro-grooves leave some traces on the machined surface. By analyzing the captured cutting force, it is found that the progress of tool wear in UPFC can increase the cutting force and its power spectral density at the natural frequency of the dynamometer. Findings from the research provide rich insight into the relationships of tool wear characteristics to cutting forces, chip formation, and machined surface quality in intermittent cutting processes.

Dynamic error budget analysis of an ultraprecision flycutting machine tool

This paper presents an error budget methodology for designing and characterizing machines used to manufacture or inspect parts with spatial frequency-based specifications. In this methodology, the static and dynamic error are considered in the error budget; the static error affects the figure accuracy, while the dynamic error introduces the spatial frequency domain error. The error caused by the components of the machine tool, the cutting process and the external interference, are considered in this paper. Using the error budget, it is able to minimize risk during initial stages by ensuring that the machine will produce components that meet specifications before the machine is actually built or purchased. This methodology is used to predict and improve the performance of an ultraprecision flycutting machine tool.