Fast Tool Servo Research Articles

Study of the surface roughness and optimization of machining parameters during laser-assisted fast tool servo machining of glass-ceramic

Glass-ceramic is a typical hard and brittle material that is difficult to machine. In order to improve the surface quality of laser-assisted fast tool servo machining optical free-form surface of glass-ceramic, the effects of spindle speed, feed speed, piezoelectric frequency and laser power on the surface roughness were investigated. Firstly, the Taguchi method (TM) was used to establish the orthogonal experiment, and the contribution rate of each machining parameter to the surface roughness was obtained through variance and signal-to-noise ratio (S/N) analysis. The order of the influence degree of each parameter on the surface roughness is as follows: laser power > spindle speed > feed speed > piezoelectric frequency. The optimal machining parameter combinations obtained for the TM experiment are as follows: spindle speed 50 rpm, feed speed 0.01 mm rev−1, piezoelectric frequency 8 Hz, laser power 75 W. The range of surface roughness reduction obtained by comparing laser-assisted machining (LAM) with pure fast tool servo (FTS) machining is 38.75%∼58.77%. The Box-Behnken Design (BBD) in response surface methodology (RSM) was used to design experiments and a regression model for surface roughness was established through RSM. The deviation between the surface roughness predicted by the regression equation and the experimental value is less than ±6%. The influence law of various machining parameters on surface roughness was studied through three-dimensional response surface. RSM optimized the minimum surface roughness with a desirability of 99.43%. The optimal combination of machining parameters optimized through RSM is as follows: spindle speed 53.71 rpm, feed speed 0.02 mm rev−1, piezoelectric frequency 6.73 Hz, laser power 72 W. This paper is the first to combine LAM with FTS for machining optical free-form surface of glass-ceramic. This study provides a reference for laser-assisted fast tool servo machining and the research methods of surface quality.

A Review of Advances in Fabrication Methods and Assistive Technologies of Micro-Structured Surfaces

Micro-structured surfaces possess excellent properties of friction, lubrication, drag reduction, antibacterial, and self-cleaning, which have been widely applied in optical, medical, national defense, aerospace fields, etc. Therefore, it is requisite to study the fabrication methods of micro-structures to improve the accuracy and enhance the performance of micro-structures. At present, there are plenty of studies focusing on the preparation of micro-structures; therefore, systematic review of the technologies and developing trend on the fabrication of micro-structures are needed. In present review, the fabrication methods of various micro-structures are compared and summarized. Specially, the characteristics and applications of ultra-precision machining (UPM) technology in the fabrication of micro-structures are mainly discussed. Additionally, the assistive technologies applied into UPM, such as fast tool servo (FTS) technology and slow tool servo (STS) technology to fabricate micro-structures with different characteristics are summarized. Finally, the principal characteristics and applications of fly cutting technology in manufacturing special micro-structures are presented. From the review, it is found that by combining different machining methods to prepare the base layer surface first and then fabricate the sublayer surface, the advantages of different machining technologies can be greatly exerted, which is of great significance for the preparation of multi-layer and multi-scale micro-structures. Furthermore, the combination of ultra-precision fly cutting and FTS/STS possess advantages in realizing complex micro-structures with high aspect ratio and high resolution. However, residual tool marks and material recovery are still the key factors affecting the form accuracy of machined micro-structures. This review provides advances in fabrication methods and assistive technologies of micro-structured surfaces, which serves as the guidance for both fabrication and application of multi-layer and multi-scale micro-structures.

A Review on the Design and Application of Compliant Mechanism-Based Fast-Tool Servos for Ultraprecision Machining

The compliant mechanism (CM)-based fast-tool servo (FTS) is used in ultraprecision machining contexts to produce high value products for technically advanced applications. Far too often, the FTS’ machined products are expected to be geometrically complex with minimal form tolerance and surface roughness. Since the FTS’ enclosing CM is responsible for guiding the cutting motion, its design is of utmost importance in determining the quality of the machined product. The objective of this paper is therefore to review specifically the design and structural related aspects of CM-based FTS that affects its ultraprecision machining performance. After a brief introduction, the fundamentals for designing ultraprecision capable CMs such as flexure hinge modelling, actuator selection and isolation and CM designing are comprehensively explained. In the subsequent section, the various configurations of CM-based FTSs that exist so far and their functionalities are listed. The critical factors which impact the CM-based FTS’ ultraprecision machining performance are identified and mitigating measures are provided wherever possible. Before concluding, the research questions that should be investigated for raising the state of the art of CM-based FTSs are presented as food for thought. With this review article, not only can practitioners have a clearer picture of how better to design their CMs for their FTSs, but they can also improve upon existing FTS designs from leading researchers so that products of higher quality than before can be made for the future.

Fabrication of curved aspheric compound eye microlens array with high surface quality by precision glass molding

The curved compound eye microlens array (CEMLA) glass optical element is widely used in optoelectronic detectors because it affords a high image quality and a large field of view. However, fabrication of high-precision glass optical elements with a small cell size and many arrays on hard and brittle glass materials is challenging. Here, we report a fabrication method to address molding challenges for high quality CEMLA glass optical elements at molding temperatures up to 500 °C. First, an Ni-P with good turning performance and low adhesion to glass is chosen as the plating material, the CuNi is selected as the mold substrate for their close CTEs. To prevent the diffusion of P to the surface of optical elements and to improve the hardness and wear resistance of mold, a Ti-Ta-C coating is deposited on the Ni-P mold. By single-point diamond turning on the Ni-P surface using a fast tool servo, an aspherical CEMLA mold with a form accuracy PV less than 0.4 μm and surface roughness Sa less than 4 nm was machined. Finally, based on the creep testing of microdeformation compression, an accurate viscoelastic constitutive model of glass was established. Optimization of the molding process and accurate prediction of molding profile deviation were simulated. The aspheric CEMLA glass optical element with a form accuracy PV less than 0.5 μm and surface roughness Sa less than 4.5 nm was obtained by precision glass molding (PGM).

Compensation machining of freeform based on fast tool servo system.

With the advantages of large field of view, low cost and simple structure, the freeform optical system has extensive requirements in space exploration and other fields. However, the current machining methods for freeform are difficult to meet the requirements of optical use. Based on a developed fast servo tool (FTS) device, this paper proposes an error compensation turning method for freeforms. Firstly, the Zernike polynomial fitting method is used to reconstruct the freeform surface shape error obtained from off-line measurement, and the offset compensation is used to correct the tool path. Then, the compensation processing physical system is built to simulate the off-line compensation processing of the workpiece to verify the feasibility of compensation processing. Finally, the turning compensation processing of convex freeform aluminum mirror is carried out, and the surface accuracy of the workpiece meets the requirements of visible band. The research results have important practical significance for realizing the fast response machining of free-form surface mirror.

Development of a novel fast tool servo using topology optimization

Micro/nanostructured surfaces have wide applications in optics, electronics, and aerospace. However, the current fast tool servo (FTS) cannot satisfy the large demand for micro/nanostructured surfaces in the industry, because their low working frequency influences the machining efficiency. To solve this problem, this paper develops a novel FTS with the high working frequency. To improve the working frequency, a topology optimization algorithm is proposed to design its mechanical structure. Then, the numerical models of the FTS, including the input stiffness, amplification ratio, natural frequency, and maximum stress, are established to analyze its specifications. Performance tests show that the stroke and maximal working frequency of the developed FTS are 15 μm and 1250 Hz, demonstrating superior performance. Besides, to improve motion accuracy, a closed-loop control system is designed based on the proportional-integral controller and feed-forward compensation methods. Experimental results show that errors of motion trajectories in three cases are lower than ± 50 nm, demonstrating the ultrahigh motion accuracy of the developed FTS. Therefore, this research develops a new FTS, which not only has a high working frequency but also has an ultrahigh motion accuracy.

Modeling of Rapid Response Characteristics of Piezoelectric Actuators for Ultra-Precision Machining

Piezoelectric actuators are characterized by high positioning accuracy, high stiffness and a fast response and are widely used in ultra-precision machining technologies such as fast tool servo technology and ultrasonic machining. The rapid response characteristics of piezoelectric actuators often determine the overall quality of machining. However, there has been little research on the fast response characteristics of piezoelectric actuators, and this knowledge gap will lead to low precision and poor quality of the final machining. The fast response characteristics of a piezoelectric actuator were studied in this work. Firstly, the piezoelectric actuator was divided into a no-load state and a load state according to the working state. A fast response analysis and output characteristic analysis were carried out, the corresponding dynamic model was established, and then the model was simulated. Finally, an experimental system was established to verify the dynamic model of the piezoelectric actuator's fast response by conducting an experiment in which the piezoelectric actuator bounces a steel ball. The experimental results verify the correctness of the model and show that the greater the cross-sectional area and height of the piezoelectric actuator, the higher the bouncing height of the ball, and the better the dynamic performance of the piezoelectric actuator. It is believed that this study has guiding significance for the application of the dynamic characteristics of piezoelectric actuators in the machining field.

Two-stage topology optimization for a piezo-actuated fast tool servo based on the velocity field level set method

Nowadays, piezo-actuated fast tool servo (FTS) is very promising for assisting the diamond turning of microstructured surfaces. Topology optimization is effective for the structure and dimension integrated design of compliant mechanisms for the FTS. In this study, the velocity field level set (VFLS) method is employed to achieve the desired performance for the FTS. A two-stage optimization strategy is proposed to improve the convergence stability and modeling accuracy for the VFLS, which combines a gradual constraint relaxation in the first stage and the gradual weak material removal in the second stage. Furthermore, a comparative study with and without the two-stage strategy is conducted to optimize two examples with different objectives and multiple constraints. The optimized results with verification by finite element simulations well demonstrate the superiority of the proposed two-stage VFLS-based topology optimization.

Repetitive-control-based Feedforward Compensation Method for Fast Tool Servo System

Repetitive-control-based Feedforward Compensation Method for Fast Tool Servo System

Optimization of fast tool servo diamond turning for enhancing geometrical accuracy and surface quality of freeform optics

Optimization of fast tool servo diamond turning for enhancing geometrical accuracy and surface quality of freeform optics

A Novel Fast Servo Tool Device with Double Piezoelectric Driving.

The fast tool servo (FTS) technology has unique advantages in the machining of complex surfaces such as special-shaped targets and free-form surfaces. In view of the shortcomings in the performance of the existing FTS device, this paper puts forward a novel FTS which uses two piezoelectric ceramics instead of flexure hinges to provide restoring force. Firstly, the feasibility of the double-drive principle is verified theoretically, and the corresponding mechanism is optimized accordingly. Then, the system control hysteresis model is established and identified, and the appropriate control strategy is designed. Finally, the performances of the proposed FTS device are tested, and a typical microstructure is machined based on the device and ultra-precision lathe. The results indicate that the proposed device effectively improves the performance of the FTS system, which is useful for the processing of microstructures and free-form surfaces.

Dual-stage fast tool servo cascading a primary normal-stressed electromagnetic stage with a secondary piezo-actuated stage

The dual-stage actuation effectively extends the dynamic response and tracking accuracy of conventional fast tool servos (FTS). In this study, a normal-stressed electromagnetic stage is deliberately designed as the primary stage to cascade with a secondary piezo-actuated stage, constructing a new medium-range dual-stage FTS. Analytical models for both the normal-stressed actuator and the flexure mechanisms are established. The mechanical and electromagnetic parameters are selected and verified by finite element simulation, targeting the desired motion stroke and the maximized natural frequency. The prototype using the designed parameters is tested to have an actual stroke of 80μm and a first resonant frequency around 814 Hz. The dual-stage FTS employs a decoupled control strategy to track the desired trajectory precisely. As for the main controller, an additional disturbance observer is designed to work with the typical PID controller to compensate for both the internal nonlinear dynamics and external disturbances for each stage. The practical result demonstrates that the closed-loop bandwidth can be improved from 220 Hz to 500 Hz by cascading the secondary stage. As for the tracking of a complex trajectory with multiple frequencies, a motion error of around ±0.8μm was achieved, which is less than 30% of that obtained by the primary stage alone.

Prediction Model of Three-Dimensional Machined Potassium Dihydrogen Phosphate Surfaces Based on a Dynamic Response Machining System.

To comprehensively obtain the effect of the machining process on the three-dimensional surface topography of machined potassium dihydrogen phosphate crystals, a dynamic response model of a machining system was built to calculate the dynamic displacement variables in the different processing directions. This model includes almost all processing factors, such as cutting parameters, environment vibration, radial and axial runout of the spindle, cutting tool parameters, material parameters, guide way error, fast tool servo and lubrication condition errors, etc. Compared with the experimental results, the three-dimensional topographies and two-dimensional profiles of the simulation surfaces were nearly consistent with those of experimental machined surfaces. As the simulation shows, the cutting parameters, axial runout of the spindle, and the output noise of the fast tool servo can respectively impact the main, low, and high frequencies of the machined surface topography. The main frequency of all the simulated and experimental surfaces in this study was 0.0138 μm-1. The low and high frequencies of the simulation surfaces had slight differences, about 0.003 μm-1 from those of the experimental surfaces. The simulation model, based on dynamic response, can accurately predict the entire machining process and three-dimensional topographies of machined potassium dihydrogen phosphate surfaces.

High-accurate cutting forces estimation by machine learning with voice coil motor-driven fast tool servo for micro/nano cutting

In a machining process, cutting force is one of the most important machining parameters which can directly reflect the state of tool and quality of the machined workpiece. The cutting force in micro/nano cutting is required to be measured with a high accuracy of a few millinewtons. However, traditional cutting force indirect estimation methods are difficult to meet the high-accurate measurement requirement. In this paper, a reliable and high-accurate cutting force estimating method based on machine learning for a fast tool servo driven by Lorentz force without using additional sensors is proposed. A concept of virtual forces, which could enable obtaining sufficient type of forces than that by actual cutting, is developed to increase the data training accuracy. In addition, an effective data filtering method combining the fast Fourier transform and the inverse fast Fourier transform is adopted to further improve the estimation accuracy. Cutting force estimated by the proposed method is compared with that estimated by traditional disturbance observer and Kalman filter, which demonstrates a much better estimation accuracy. The experiment results show that the estimated cutting force is in good consistent with the best commercial dynamometer. Even the details of small cutting force fluctuation on the order of 50 mN can be clearly identified. The experimental results demonstrated that the cutting force estimation method for micro/nano machining process could have a high measurement resolution of 10 mN and relatively measurement error as small as 0.4%.

Surface astigmatism correction using segmented freeform surfaces for a progressive addition lens.

The undesired distribution of irregular surface astigmatism (SA) on the freeform surface has been the major concern of progressive addition lens (PAL) design. Herein, we proposed a segmented freeform surface (SFS) construction method, which relies on the lines of curvature to rule the surface segmentation and then eliminates the difference between principal curvatures to correct the SA. Based on ray tracing and numerical simulation results, the SFS-PAL design has superior performance in image quality within a dynamic field of view over the conventional freeform PAL. To verify the feasibility and the real performance of the new design, we used the diamond turning method with a fast tool servo to realize the rapid prototyping, and then used injection molding for the mass production of the high-quality SFS-PALs.

Fast-tool-servo-controlled shear-thickening micropolishing

Polishing-based post-processing is essential for removing the undesired surface diffraction on diamond-turned microstructured surfaces that is enhanced by periodic tool marks. To overcome challenges in existing micropolishing methods, a fast-tool-servo-controlled shear-thickening micropolishing method was proposed for the non-contact and controllable polishing of microstructured surfaces. The operating kinematics and material removal mechanism are modeled analytically and investigated experimentally. The comprehensive principal stress in front of the rake face of the tool is found to mainly contribute to the material removal. The fast tool servo can tune the principal stress and the viscosity of the slurry by flexibly adjusting the gap width between the surface and the tool edge. Thus, the material removal can be controlled at any operating position. Meanwhile, although the material removal rate is nonlinearly related to the rotation radius and gap width, the constraint between these two factors is linear for achieving a fixed material removal rate. Finally, the feasibility of the proposed micropolishing method is demonstrated by successfully polishing rotationally symmetric and asymmetric microstructured surfaces to achieve improved surface smoothness and conformal surface shapes.

Development of an Optimized Three-Axis Fast Tool Servo for Ultraprecision Cutting

A three-axis fast tool servo (FTS) with a symmetric structure design to reduce the parasitic motion for realizing high tracking accuracy while maintaining high performances in stiffness and open-loop bandwidth is developed for ultraprecision diamond cutting. Modeling, analysis, and optimization of performances by the finite-element method in six degree-of-freedom are conducted for designing the mechanism. Experiments are carried out to test the performances of the three-axis FTS, which show low parasitic motion, high tracking accuracy, and high dynamic response. With the low tracking error of close-loop control for each axis, complicated trajectory tracking in 3-D space is realized with a tracking error as small as 5 nm. Finally, complicated and irregular microstructures are fabricated by the three-axis FTS to demonstrate the practicability of the FTS.

Active Chatter Mitigation by Optimal Control of Regenerative Machining Process Dynamics

Chatter vibrations are a type of self-exited vibrations observed in many subtractive manufacturing processes. They limit achievable material remove rate (MRR), reduce tool life, resulting in poor surface finish, and even damage the manufacturing equipment. This article proposes a novel control system design for machine tool feed drives, which modulates cutting tool motion based on the in-process vibration measurements to mitigate chatter vibrations. The proposed approach has two key contributions. First, as opposed to introducing modal damping to the structure, the proposed approach stabilizes machining process dynamics by controlling its governing delay differential equation. H∞ control framework is used to design the controller and ensures robust process stability. The controller design problem is postulated as a convex optimization problem with linear matrix inequalities, which is solved to global optimality. Second, the proposed controller is implemented as an add-on compensator that works jointly with an existing servo motion control system to realize both positioning and chatter vibration control. Experimental validations are conducted on a lathe equipped with a fast tool servo actuator. It is shown that regenerative chatter vibrations in the orthogonal turning process can be suppressed robustly and up to five times increase in MRR (productivity) can be achieved.

Surface deformation errors and self-adaptive compensation for microstructured surface generation of titanium alloys

In diamond servo cutting (DSC) of microstructured surfaces, the oscillated cutting motion inevitably cause non-uniform surface deformation errors. Although static and dynamic geometric errors for DSC have been studied, few studies focus on the prediction and compensation strategies of the surface deformation errors for microstructured surface generation. This study proposes an microstructured surface generation model of titanium alloys by considering the dynamic cutting forces, material microstructure evolution, thermal-mechanical coupling effect and subsurface stress-strain distribution. Harmonic microstructured surfaces are fabricated to characterize the non-uniform surface deformation errors as well as to validate the proposed model. The results show that the proposed model can accurately simulate the variation of surface plastic and elastic deformation errors as a function of undeformed chip thickness and cutting direction. Compared with the downslope cutting direction, the machined surface in upslope direction is subjected to greater plastic strain and plastic deformation errors even at the same depth of cut, due to cutting direction effect. To compensate surface deformation errors, a cooperative servo cutting (CSC) strategy is proposed by incorporating a self-adaptive fast tool servo into conventional servo cutting. The instantaneous compensation motion of CSC is jointly determined by the curvature radius deviation, instantaneous cutting depth and cutting direction. The experimental results show that CSC can make the error distribution more uniform, and greatly decrease the peak-to-valley surface deformation errors from 1.29 to 0.24 μm for different microstructured surfaces.

Ductile machining of single-crystal germanium for freeform surfaces diamond turning based on a long-stroke fast tool servo

Freeform surfaces of single-crystal germanium are required increasingly in advanced infrared optics, but there are difficulties in processing due to the high hardness, brittleness and low fracture toughness of germanium itself. In the present study, the machining mechanisms and parameters of single-crystal germanium were investigated via ultra-precision diamond turning based on a long-stroke fast tool servo. To realize ductile regime machining, a ductile machining model in ultra-precision turning of germanium considering both feed rate, cutting depth and surface slope of freeform surface was proposed and discussed. A novel taper cutting test was adopted to determine the critical ductile–brittle transition depth. Subsequently, XYPFS were fabricated under different cutting parameters to demonstrate the effectiveness of proposed model. The morphologies of machined surfaces and cutting chips were identified and compared. Finally, a smaller feed rate and cutting depth should be used to reduce its maximum undeformed chip thickness and realize the ductile machining for a freeform surface having a large surface slope. The freeform surface of single-crystal germanium with a form error of ~1258 nm PV and surface roughness of ~1.92 nm Ra was successfully fabricated. In summary, this research provides a comprehensive understanding of ductile machining mechanisms of diamond turning based on a LFTS as well as is meaningful in the manufacturing of freeform surfaces on germanium or other brittle materials with optically qualified surfaces and high-precision.