Ultra-high Precision Machining Research Articles

Nonlinear Effects of Pulsed Ion Beam in Ultra-High Resolution Material Removal.

Ion beam sputtering is widely utilized in the area of ultra-high precision fabrication, coating, and discovering the microworld. A pulsed ion beam (PIB) can achieve higher material removal resolution while maintaining traditional ion beam removal performance and macro removal efficiency. In this paper, a 0.01 s pulse width beam is used to sputter atom layer deposition (ALD) coated samples. The nano-scale phenomenon is observed by high-resolution TEM. The results show that when the cumulative sputtering time is less than 1.7 s, the sputtering removal of solid by ion beam is accompanied by a nonlinear effect. Furthermore, the shortest time (0.05 s) and lowest thickness (0.35 nm) necessary to remove a uniform layer of material were established. The definition of its nonlinear effect under a very small removal amount guides industrial ultra-high precision machining. It reveals that PIB not only has high removal resolution on nanoscale, but can also realize high volume removal efficiency and large processing diameter at the same time. These features make PIB promising in the manufacturing of high power/energy laser optics, lithography objective lens, MEMS, and other ultra-high precision elements.

Effect of Tool Geometry and Turning Parameters on Cutting Force during Ultra-Precision Diamond Machining of Rapidly Solidified Aluminum (RSA) Alloy 6061

The most important goal of advanced diamond machining is to improve precision and product integrity. Cutting force could have a strong effect on diamond machining performance and hence, it can be used as an indicator for monitoring tool condition for achieving successful machining. Moreover, diamond machining process parameters such as cutting speed, depth of cut and feed rate may strongly influence the process outcomes in terms of surface roughness and tool failure. Diamond machining parameters have a significant role on the cutting force values, and machine-tool stability during ultra-high precision machining that makes use of a natural diamond cutting tool. The main objective of this research work is to investigate the effect of cutting speed, feed rate, depth of cut and nose radius on the cutting force generated during diamond turning of a rapidly solidified aluminum alloy grade called RSA 6061. The effect of nose radius and cutting parameters on cutting force during the ultra-high precision diamond turning process was monitored. The study shows that low speed, low depth of cut and an increase in feed rate at large nose radius consequently results in increase in the cutting force.

Research on high-speed synchronous motor direct torque control

High-speed motor is increasingly used in ultra-high precision machining and high-performance machinery. It is necessary to achieve closed-loop control of high-speed motor in order to realize high-performance motor control. The high-frequency and high-speed characteristics of high-speed motor have brought great difficulties for its realization of closed-loop control. The performance of the hexagonal flux direct torque control (DTC) method that can reduce the switching frequency of power devices is researched for high-speed synchronous motor in this paper. After adopting this control method, the harmonics in the motor current and torque can be suppressed by adopting an appropriate filtering method. The same effect as the conventional DTC can still be achieved. It provides a basis for the application realization of the high-speed synchronous motor control system.

Profile Measurement Using Confocal Chromatic Probe on Ultrahigh Precision Machine Tool

An on-machine measurement (OMM) system is an effective apparatus for achieving an efficient profile compensation and improving machining conditions in ultrahigh precision machining. Herein, we report a new OMM system with a confocal chromatic probe on a five-axis ultrahigh precision machine tool constructed using a real-time position capturing method. The probe and machine tool positions are captured synchronously using a personal computer to generate profile measurement data. Long- and short-term stability, micro step response, and repeatability tests using an optical flat indicates that the system has a precision of approximately ±10 nm. The profile measurement test using a reference sphere indicates that the precision of the OMM system deteriorated at a large slope angle of ±45°. However, the overall accuracy is estimated to be within ±100 nm at a slope angle within ±15°. The linearity test at various slope angles indicates that the decrease in reflected light from a mirror-like surface deteriorates the performance of the probe.

RSM and MD—a roughness predictive model and simulation comparison of monocrystalline optical grade silicon

Silicon is a major material for manufacturing high-quality optical components, a process requiring silicon roughness to be below 8 nm. Lately, the popular choice for manufacturing these high-quality components has been the use of single-point diamond on ultra-high precision machines. Diamond properties such as clear grain structure, low coefficient of friction, and well-defined grain structure are well harnessed in this process. To achieve the demanded roughness level, there is a need for an appropriate choice of machining conditions which does not only ensure ductile regime machining but also form a good starting point for an improved prediction. The present study varied three well-known strong determinants of surface roughness, namely, rake angle, nose radius, and feed rate, as design input factors to facilitate Ra prediction. One important discovery made that constitutes the novelty of this research which to the best of our knowledge has not been reported before is that high negative rake angle reduces the negative influence of high feed rate on surface roughness. Simulation was carried out using MD and the result was compared with the experiment. High form accuracy with Ra values between 1.8 and 7 nm, close approximation of actual turning/prediction results, and the conformity of MD and experimental results attested to the success of the research.

Residual Layer Removal of Technical Glass Resulting from Reactive Atmospheric Plasma Jet Etching by Pulsed Laser Irradiation

Ultrahigh-precision machining of glass is indispensable for optical component fabrication and therefore for applications. In this regard, plasma jet assisted chemical etching technologies enable new fabrication processes for enhanced optical functionalities due to their deterministic localized machining capabilities. This technique has been successfully applied to fused silica and silicon. However, applications require specific glass properties are related to complex material compositions of the glass. Hence, reactive plasma etching of these optical glasses is a challenging task. For instance, etching of metal oxide containing glass like N-BK7 by a fluorine-based reactive atmospheric plasma jet (RAPJ) exhibits currently limitations due to the formation of non-volatile reaction products that remain on the glass surface as a layer. Therefore, a procedure consisting of RAPJ etching and laser ablation is proposed for the machining of N-BK7. The capability of laser-based removal of residual layers is compared to water-based solving of the residual layer. After RAPJ etching of N-BK7 using a CF4–O2 gas mixture with an average microwave power of 16 W, the samples are cleaned either by a water-based solvent or by the ablation with a nanosecond-pulsed ultraviolet laser. The laser irradiation with fluences of 2.8 J/cm2 results in a localized removal of the residual layer. It is demonstrated that the roughness of the laser-cleaned N-BK7 surface is similarly low as solvent-based cleaned samples but the pulsed laser enhanced cleaning allows a dry processing at atmospheric pressure as well as a localized processing with a high lateral resolution.

Analysis of diamond nanomachining of contact lens polymers using molecular dynamics

Contact lens polymer-based materials are extensively used in optical industry due to excellent corrosion resistance, possibility of mass production, and the ability to be processed without external lubrication. The demand for high accuracy and least surface roughness in contact lens industry drives the development of ultra-high precision machining technology of single-point diamond turning. Ultra-high precision machining of contact lens polymer involves material removal at a nanometric level that results in high surface finish. Due to the fast growth in optical industries, contact lens manufacture requires high accuracy and high surface quality. In this study, the nanometric machining of roflufocon E contact lens polymer with precise tool specifications and parameter choices have been investigated by a comparative analysis of molecular dynamics (MD) model with experimental study. Simulated MD acting cutting force, machine stresses, and temperature at the cutting region were used as bases of evaluation to observe the similarities of the model with the experimental surface roughness (Ra) nanomachining results. From the analysis, it can be inferred that the observed MD results commemorates with the different Ra as noted in the experiment.

Process parameter selection for optical silicon considering both experimental and AE results using Taguchi L9 orthogonal design

Optical industry has since been employing the use of ultra-high precision machining (UHPM) to manufacture wide range of optical products. One of the major materials used for producing optical product is optical silicon. However, improving surface quality of the product within 10 nm Ra has remained a research focus. Surface roughness during optical silicon turning which is an important quality determinant depends on factors such as feed rate, depth of cut, cutting speed, nose radius, clearance, and rake angles. To achieve required/targeted quality characteristic, there is need for appropriate selection of these process parameters. Therefore, to create a manufacturing industry that is autonomous, a reliable monitoring system for increased production needs to be developed. This research discussed process parameter selection through variation and application of acoustic emission monitoring technique for obtaining optimal surface roughness in ultra-high precision turning (UHPT) of optical silicon using the Taguchi L9 design. The result obtained showed 100% similarity between the experimental and acoustic emission prediction and the superiority of cutting speed over both nose radius and rake angle.

Predictive modeling and multiobjective optimization of diamond turning process of single-crystal silicon using RSM and desirability function approach

In ultra-high precision machining (UHPM) of Si optical lenses, the prime task is the realization of nanometric surface finish within the reasonable tool wear rates. Thus, study of the influence of different cutting parameters like cutting speed, feed rate and depth of cut on the machinability characteristics is essential. This paper therefore aimed at developing predictive models for surface roughness (Ra) and tool wear and subsequent determination of optimal cutting conditions for the attainment of minimum Ra and tool wear as well as maximum material removal rate (MRR). Desirability function approach was used for the optimization based on response surface methodology (RSM). Also, ANOVA was used to find the statistical significance of the cutting parameters on the two responses. For Ra, the results showed that feed is the most influential factor with higher percentage contribution. The quadratic term of feed (f2) is the second most influential factor on Ra followed by speed. Depth is statistically insignificant. Concerning the tool wear, feed also has greater influence than the other factors, albeit, the percentage contributions are almost comparable. The speed–feed interaction in the tool wear is stronger than that of the Ra. The combined optimization of the cutting conditions, leading to minimum Ra and tool wear with maximum MRR, revealed the optimum cutting parameters to be as follows: v = 679.97 rpm, f = 2.00 mm/min and d = 10.00 μm. This combination is suitable for quality manufacturing and smaller MRR. The equivalent optimum output are as follows: Ra = 2.099 nm and tool wear = 0.665 μm.

Modeling and multiresponse optimization of cutting parameters in SPDT of a rigid contact lens polymer using RSM and desirability function

Amidst different conventional contact lens manufacturing techniques, single-point diamond turning (SPDT) is one of the recently developed ultra-high precision machining techniques employed in the fabrication of advanced contact lenses due to its capability of producing high optical surfaces of complex shapes and nanometric accuracy. SPDT is regarded as an effective process for the generation of high-quality functional surfaces in optical industries. However, despite advances in the ultra-high precision machining, it is not always easy to achieve a high-quality surface finish with maximum productivity. Machining parameters, namely cutting speed, feed rate, and depth of cut, play the lead role in determining the machine economics and quality of machining. The present study focuses on the determination of the optimum cutting conditions leading to minimum surface roughness as well as electrostatic charge and maximum productivity, in SPDT of the polymethyl methacrylate (PMMA) contact lens polymer using monocrystalline diamond cutting tool. The optimization is based on the response surface methodology (RSM) together with the desirability function approach. In addition, a mathematical model is developed for surface roughness (Ra), electrostatic charge (ESC), and material removal rate (MRR) using RSM regression analysis for a rigid contact lens polymer by the Design-Expert software. RSM allowed the optimization of the cutting conditions for minimal surface roughness, electrostatic charge, and maximal material removal rate which provides an effective knowledge base for process parameters, to make its enhancement of process performance in SPDT of contact lens polymer.

Coupled Eulerian-Lagrangian technique for microcutting FE-modelling of AISI1045 steel

Deepen the analysis of the cutting process mechanics beneath the tool edge radius is one of the primary concerns in micro cutting. This paper shows that coupled Eulerian Lagrangian (CEL) technique applied to micro machining process modelling can be a valid alternative comparison to traditional Finite Element (FE) chip formation modelling methods. CEL does not require separation criteria since it implements workpiece modelling by means of plastic deformation and considering it as Eulerian domain - whilst keeping tool in Lagrangian domain - and avoiding mesh topography changes. The developed 3D CEL entails a full set of thermo-mechanical input parameters and is validated by conducting micro orthogonal cutting experiments on ultra-high precision machining. CEL approach resulted capable to predict the process outputs, i.e., namely chip thickness and cutting/thrust forces, within a mean error of around 20%, comparable to existing techniques and showed better accuracy in actual chip thickness prediction.

Coupled Eulerian-Lagrangian technique for microcutting FE-modelling of AISI1045 steel

Deepen the analysis of the cutting process mechanics beneath the tool edge radius is one of the primary concerns in micro cutting. This paper shows that coupled Eulerian Lagrangian (CEL) technique applied to micro machining process modelling can be a valid alternative comparison to traditional Finite Element (FE) chip formation modelling methods. CEL does not require separation criteria since it implements workpiece modelling by means of plastic deformation and considering it as Eulerian domain - whilst keeping tool in Lagrangian domain - and avoiding mesh topography changes. The developed 3D CEL entails a full set of thermo-mechanical input parameters and is validated by conducting micro orthogonal cutting experiments on ultra-high precision machining. CEL approach resulted capable to predict the process outputs, i.e., namely chip thickness and cutting/thrust forces, within a mean error of around 20%, comparable to existing techniques and showed better accuracy in actual chip thickness prediction.

Design of an ultra-high precision machine for form measurement

Companies are steadily developing more complex and small scale products to be more competitive. This leads to an increasing demand for high precision machines for the measurement and characterization of products. Moreover, to improve the characterization through traceability chain, national and international laboratories are regularly increasing the accuracy level of the calibration of standards. This is done through the development of ultra-high precision measuring machines. The LNE (French National Metrology Institute) is developing a new machine for cylindrical and spherical form measurement with a nanometric level of accuracy. The Dissociated Metrology Technique (DMT) has been applied in order to achieve the targeted level of accuracy. This paper aims at providing details on the design process starting from the design requirement to the final system. The design method as well as the used design concepts is described. The developed machine is also presented and its current limitations are discussed.

Development of Neutron Focusing Mirror by Ultrahigh Precision Machining

Development of Neutron Focusing Mirror by Ultrahigh Precision Machining

Molecular dynamics analysis of nanomachining of rapidly solidified aluminium

Rapidly solidified aluminium (RSA) is becoming more recognised as an alternative material to traditional aluminium alloys in the production of optical and structural components. This is due to its improved physical and mechanical properties and ultra-fine microstructure as a result of rapid solidification in the foundry process. The lack of past studies in the molecular dynamics simulation of RSA can be attributed to the lack of sufficient data on the microstructure and interatomic potential limitations. In this paper, an approximation of RSA was modelled to simulate the nanomachining process. The workpiece for the simulation was modelled as an aluminium lattice mixed with the appropriate fractions of the additional elements based on the chemical composition of RSA. The depth of cut was varied from 0.5 to 1.5 nm, and the cutting speed was varied from 5 to 15 m/s. The cutting process was found to be consistent with increase in depth of cut, producing a steady increase in energy, temperature and average cutting forces. Increase in cutting speed produced a slowing increase in energy and temperature while the average cutting forces showed an initial slowing increase followed by a rapid increase. The results showed that cutting speed had a more significant effect on the cutting process that will subsequently affect the desired surface finish. As the surface quality produced in ultra-high precision machining is in the nanoscale, the purpose of this research was to analyse the mechanisms occurring at the subsurface layers and establish a relation to the actual results produced. Experimental evaluations were carried out that agreed with the conclusions made with the MD simulation further proving the eligibility of the MD in nanomachining investigation.

Surface roughness analysis when diamond turning optical grade rapidly solidified aluminium RSA 905

Ultra-high precision machining is used intensively in the photonics industry for the production of various optical components. Aluminium alloys have proven to be advantageous and are most commonly used over other materials to make various optical components. Recently, the increasing demand from optical systems for optical aluminium with consistent material properties has led to the development of newly modified grades of aluminium alloys produced by rapid solidification in the foundry process. These new aluminium grades are characterised by their finer microstructures and refined mechanical and physical properties. However, the machining database of these new optical aluminium grades is limited and more research is still required to investigate their machinability performance when they are diamond turned in ultrahigh precision manufacturing environment. This work investigates the machinability of rapidly solidified aluminium RSA 905 by varying diamond-turning parameters and measuring the surface roughness over a cutting distance of 4 km. In addition, a predictive surface roughness model was developed. The machining parameters varied in this study were the cutting speed, feed rate and depth of cut. The results showed a common trend of decrease in surface roughness with increasing cutting distance. The lowest surface roughness R a result obtained after 4 km in this study was 3.2 nm. This was achieved using a cutting speed of 1750 rpm, feed rate of 5 mm/min and depth of cut equal to 25 µm. The work carried out provides a reference for machinists and researchers using the material allowing expected results to be predicted and appropriate machining parameters to be selected based on the surface roughness requirements.

Investigation of machining mechanism of monocrystalline silicon in nanometric grinding

Monocrystalline silicon is the foundation of the computer industry, so it has a great significance to study the ultra-high precision machining of silicon. Molecular dynamics has been proved as a very effective method for the study of ultra-precision machining in nanoscale. During the grinding of brittle materials in nano-level, there are some unique phenomena such as brittle-ductile transition. To study the machining mechanism in nanometric grinding of monocrystalline silicon, the subsurface damage of < 0 0 1 > oriented Monocrystalline silicon under different grinding speeds were investigated by means of molecular dynamics simulations. The interactions between different atoms are described by the Morse and Tersoff potential. Based on analyzing the mechanism of diamond tool extrusion induced silicon lattice slip and distortion, the grinding process is explained. The movement of atoms and phase transformation are studied. The results show that there is not enough time for atoms beneath the tool to rearrange when increasing grinding speed at a low speed, which leads to the subsurface damage thickness decreases. When the diamond tool radius is small enough but still bigger than the undeformed chip thickness, the brittle-ductile transition can be achieved in the grinding region. And during the grinding process, the normal force is smaller than the tangential force. However, when the radius of the diamond tool increases to a certain value, the normal force could be larger than the tangential force.

Model for cutting force prediction in high precision single-point diamond turning of optical silicon

This paper presents a mathematical model predicated on a finite series convergent scheme and derivative functions of multi-variants obtained from a cutting force constitutive equation for onward prediction of the maximum cutting force at the tip of a single-point diamond tool (SPDT). The three variants amidst other parameters in the model include the tool length, width and strain effect. The model basically operates under two components, namely the dynamic and predictive components. Prior to the predictive analysis, cutting experiments were carried out using an ultra-high precision machine to diamond-turn units of single-crystal silicon workpiece. Results from nine experimental trials are presented. Other supporting devices deployed for signal monitoring and conditioning include the use of a Kistler force sensor, an analog-digital (AD) sensor for data acquisition and an amplifier unit for signals. The cutting parameters adopted for the experimental process includes depth of cut, feed rate and cutting speed. The simulation interval for the investigation of the cutting force was fixed at intervals of 1 mm from the point of sensor insert to the tip of the tool. The dynamical response of the proposed algorithm to each experimental trial, as seen in the displayed results, shows a trend commensurate to the geometry of the diamond-cutting tool and stiffness of the machined material.

REVIEW OF ULTRA-HIGH PRECISION DIAMOND TURNING OF SILICON FOR INFRARED OPTICS

Silicon is an important material in the manufacturing of microelectronics and optical elements, its application in infrared lenses is due to its unique characteristics. However, the traditional method of fabricating it is time-consuming with inherent defects. Ultra-high precision machining has been widely used for the manufacturing of various critical products in applications where surface roughness within the range of 1 – 10 nm is required, such as infrared lenses, but achieving the surface integrity requirement is difficult. Through ultra-high precision diamond turning (UHPDT) technique using a monocrystalline diamond cutting tool at ductile–regime mode, production of freeform optics of various shapes and profiles is possible with proper selection of process parameters, tool geometry and online monitoring techniques. This paper, therefore, aims to survey the current efforts in the UHPDT of single-crystal silicon as an infrared material, the various factors affecting surface roughness, diamond tool wear measuring techniques, surface roughness and its associated metrology. At the end, challenges and prospects of UHPDT technique are discussed and some elaborate conclusions, considering the forecasted huge investments in both the optical elements and the UHP machine tools, in general, are drawn together with some suggestions on the areas that need further research for future improvements. Keywords: Silicon, infrared lenses, ultra-high precision diamond turning, surface roughness, ductile-regime, diamond tool wear.

Profile measurement of a bent neutron mirror using an ultrahigh precision non-contact measurement system with an auto focus laser probe

A bent neutron mirror has been considered as one of the best solutions for focusing neutron beams from the viewpoint of cost-benefit performance. Although the form deviation of the bent profile is expected because of the large spot size, it is difficult to measure due to its geometric limitation. Here, we propose a non-contact measurement system using an auto focus (AF) laser probe on an ultrahigh precision machine tool to precisely evaluate the form deviation of the bent mirror. The AF laser probe is composed of a diode laser, a position sensitive sensor, a charge-coupled device (CCD) camera and a microscope objective lens which is actuated by an electromagnetic motor with 1 nm resolution for position sensing and control. The sensor enables a non-contact profile measurement of a high precision surface without any surface damage in contrast with contact-type ultrahigh precision coordinate measurement machines with ruby styli. In the on-machine measurement system, a personal computer simultaneously acquires a displacement signal from the AF laser probe and 3-axis positional coordinates of the ultrahigh machine tool branched between the linear laser scales and the numerical controller. The acquisition rate of the 4-axis positional data in 1 nm resolution is more than 10 Hz and the simultaneity between the axes is negligible. The profile of a neutron bent mirror was measured from a transparent side using the developed system, and the result proves that the form deviation of the mirror enlarged the the spot size of focused neuron beam.