Computer Controlled Optical Surfacing Research Articles

Research on the Optimized Tool-Path Planning of Computer Controlled Optical Surfacing

Computer controlled optical surfacing (CCOS) is widely used in aspheric optical lenses fabrication because of their high convergence rate on surface based on deterministic removal processes since 1963. As an important part of CCOS techniques, reasonable tool-path would increase the polishing speed, decrease the processing time and then improve the efficiency of polishing. Optimized policy combined with improved Prim algorithm is presented in this paper based on the study of the characteristic of aspheric polishing and the tool-paths in common use. The simulated results show that the length of tool-path is reduced so as to decrease the processing time and increase the working efficiency.

Ion beam machining error control and correction for small scale optics

Ion beam figuring (IBF) technology for small scale optical components is discussed. Since the small removal function can be obtained in IBF, it makes computer-controlled optical surfacing technology possible to machine precision centimeter- or millimeter-scale optical components deterministically. Using a small ion beam to machine small optical components, there are some key problems, such as small ion beam positioning on the optical surface, material removal rate, ion beam scanning pitch control on the optical surface, and so on, that must be seriously considered. The main reasons for the problems are that it is more sensitive to the above problems than a big ion beam because of its small beam diameter and lower material ratio. In this paper, we discuss these problems and their influences in machining small optical components in detail. Based on the identification-compensation principle, an iterative machining compensation method is deduced for correcting the positioning error of an ion beam with the material removal rate estimated by a selected optimal scanning pitch. Experiments on ϕ10 mm Zerodur planar and spherical samples are made, and the final surface errors are both smaller than λ/100 measured by a Zygo GPI interferometer.

平转动大磨头加工大口径非圆形球面的粗磨试验

A large tools with orbital tool motion was adopted to upgrade the processing efficiency of the small tools with orbital tool motion based on Computer Controlled Optical Surfacing(CCOS) technology by enlarging the lap size.The edge effect that is common for large laps was simulated and compensated by combination of the first order approximation and the high order compensation to improve the convergence rate and to maintain better remoral efficiency.Therefore,the processing periods were shortened effectively.A round square of 1 100 mm×800 mm SiC mirror was grinded with this method.After 22 runs of 55 hours in all processing,the overall surface error is 122 μm PV before and 5.9μm PV after the process.Comparing with that of previous small lap technology,the efficiency is improved by a factor of at least 2.

The Ultra-Precision Polishing of Large Aperture Reaction Bonded Silicon Carbide Mirror

Problem statement: Silicon Carbide (SiC) optical materials have becom e the first choice for mirrors in space optical systems and large grou nd-based optical systems due to their outstanding mechanical, physical and optical properties. Compar ed with traditional optical glasses, SiC optical materials are provided with the characteristics of high hardness and multiphase, which embarrass the high efficient fabrication of SiC mirrors with ultr a-smooth surfaces and high precision. Approach: We choose some typical polishing parameters and conduct a series of experiments, trying to find out the relationship between polishing parameters and resul ting surface roughness. The polishing parameters for Reaction Bonded Silicon Carbide (RB-SiC) are optimized from the analysis of these experiments. Then we can apply these parameters to the figuring process. Computer Controlled Optical Surfacing (CCOS) is a widely used deterministic polishing met hod and features in low-cost, high-precision and large flexibility. The basic theory and process of CCOS are given in detail. Then we employ CCOS in the ultra-precision machining of RB mirrors. Results: We polish a RB SiC sample with the optimized polishing parameters and a surface roughness better than 1nm (RMS) is obtained. A 475 mm diameter sphere RB-SiC mirror with a relative aperture of 1: 1 is polished by CCOS and the surface error reduces to 0.175 λ(PV)/0.009 λ(RMS) from 0.526 λ(PV)/0.080 λ(RMS). Both results represent a very smooth surface and a very precise figure. Conclusion: The result proves the feasibility of the polishing technology and CCOS method. These works also accumulate experience for the manufacturing of aspheric SiC mirror.

Rigid conformal polishing tool using non-linear visco-elastic effect

Computer controlled optical surfacing (CCOS) relies on a stable and predictable tool influence function (TIF), which is the shape of the wear function created by the machine. For a polishing lap, which is stroked on the surface, both the TIF stability and surface finish rely on the polishing interface maintaining intimate contact with the workpiece. Pitch tools serve this function for surfaces that are near spherical, where the curvature has small variation across the part. The rigidity of such tools provides natural smoothing of the surface, but limits the application for aspheric surfaces. Highly flexible tools, such as those created with an air bonnet or magnetorheological fluid, conform to the surface, but lack intrinsic stiffness, so they provide little natural smoothing. We present a rigid conformal polishing tool that uses a non-linear visco-elastic medium (i.e. non-Newtonian fluid) that conforms to the aspheric shape, yet maintains stability to provide natural smoothing. The analysis, design, and performance of such a polishing tool is presented, showing TIF stability of <10% and providing surface finish with <10A roughness.

Stitching algorithm for ion beam figuring of optical mirrors

A novel stitching machining method for ion beam figuring of optical mirrors is presented in this paper. Problems that should be dealt with in the stitching process such as shaping principle, algorithm for dwell time calculation, parameter identification and compensation for positioning errors are discussed. Based on Computer Controlled Optical Surfacing (CCOS) principle, the finite field nonlinear model is deduced from the stitching shaping mechanism; with the superposition property in the model, a modified Richardson-Lucy iterative algorithm is presented to deconvolute the dwell time for the stitching process; the effect of the positioning errors and the removal rate on the machining accuracy and shape is modeled and then the identification and compensation algorithm for these parameters is proposed based on this model. With these studies, shaping theory and algorithm are constructed, and then the stitching process for ion beam figuring comes into being. Figuring experiments are made on planar samples; the process convergence ratio is 10, which is similar to that in full aperture figuring. Theoretical and experimental studies indicate that the stitching theory and algorithm in the ion beam figuring process presented in this paper are a novel method for fabrication of precision mirrors with lower cost.

Non-sequential optimization technique for a computer controlled optical surfacing process using multiple tool influence functions

Optical surfaces can be accurately figured by computer controlled optical surfacing (CCOS) that uses well characterized sub-diameter polishing tools driven by numerically controlled (NC) machines. The motion of the polishing tool is optimized to vary the dwell time of the polisher on the workpiece according to the desired removal and the calibrated tool influence function (TIF). Operating CCOS with small and very well characterized TIF achieves excellent performance, but it takes a long time. This overall polishing time can be reduced by performing sequential polishing runs that start with large tools and finish with smaller tools. In this paper we present a variation of this technique that uses a set of different size TIFs, but the optimization is performed globally - i.e. simultaneously optimizing the dwell times and tool shapes for the entire set of polishing runs. So the actual polishing runs will be sequential, but the optimization is comprehensive. As the optimization is modified from the classical method to the comprehensive non-sequential algorithm, the performance improvement is significant. For representative polishing runs we show figuring efficiency improvement from approximately 88% to approximately 98% in terms of residual RMS (root-mean-square) surface error and from approximately 47% to approximately 89% in terms of residual RMS slope error.

Algorithm for ion beam figuring of low-gradient mirrors

Ion beam figuring technology for low-gradient mirrors is discussed. Ion beam figuring is a noncontact machining technique in which a beam of high-energy ions is directed toward a target workpiece to remove material in a predetermined and controlled fashion. Owing to this noncontact mode of material removal, problems associated with tool wear and edge effects, which are common in conventional contact polishing processes, are avoided. Based on the Bayesian principle, an iterative dwell time algorithm for planar mirrors is deduced from the computer-controlled optical surfacing (CCOS) principle. With the properties of the removal function, the shaping process of low-gradient mirrors can be approximated by the linear model for planar mirrors. With these discussions, the error surface figuring technology for low-gradient mirrors with a linear path is set up. With the near-Gaussian property of the removal function, the figuring process with a spiral path can be described by the conventional linear CCOS principle, and a Bayesian-based iterative algorithm can be used to deconvolute the dwell time. Moreover, the selection criterion of the spiral parameter is given. Ion beam figuring technology with a spiral scan path based on these methods can be used to figure mirrors with non-axis-symmetrical errors. Experiments on SiC chemical vapor deposition planar and Zerodur paraboloid samples are made, and the final surface errors are all below 1/100 lambda.

Frequency-domain analysis of computer-controlled optical surfacing processes

Mid-high spatial frequency errors are often induced on optical surfaces polished by computer-controlled optical surfacing (CCOS) processes. In order to efficiently remove these errors, which would degrade the performances of optical systems, the ability of a CCOS process to correct the errors have been investigated based on the convolution integral model in view of the availability of material removal. To quantify the ability, some conceptions, such as figure correcting ability and material removal availability (MRA), have been proposed. The research result reveals that the MRA of the CCOS process to correct a single spatial frequency error is determined by its tool removal function (TRF), and it equals the normalized amplitude spectrum of the Fourier transform of its TRF. Finally, three sine surfaces were etched using ion beam figuring (IBF), which is a typical CCOS process. The experimental results have verified the theoretical analysis. The employed method and the conclusions of this work provide a useful mathematical basis to analyze and optimize CCOS processes.

计算机控制光学表面成形中加权空间反卷积算法研究

Theoretical and experimental research on the deconvolution algorithm of dwell time in the technology of computer controlled optical surfacing (CCOS) formation is made to get an ultra-smooth surface of space optical element. Based on the Preston equation, the convolution model of CCOS is deduced. Considering the morbidity problem of deconvolution algorithm and the actual situation of CCOS technology, the weighting spatial deconvolution algorithm is presented based on the non-periodic matrix model, which avoids solving morbidity resulting from the noise induced by measurement error. The discrete convolution equation is solved using conjugate gradient iterative method and the workload of iterative calculation in spatial domain is reduced effectively. Considering the edge effect of convolution algorithm, the method adopts a marginal factor to control the edge precision and attains a good effect. The simulated processing test shows that the convergence ratio of processed surface shape error reaches 80%. This algorithm is further verified through an experiment on a numerical control bonnet polishing machine, and an ultra-smooth glass surface with the root-mean-square (RMS) error of 0.0088 \mum is achieved. The simulation and experimental results indicate that this algorithm is steady, convergent, and precise, and it can satisfy the solving requirement of actual dwell time.

Deterministic Magnetorheological Jet Polishing Technology

A new deterministic precision polishing technology of magnetic-assisted jet stabilization, magnetorheological jet polishing (MJP), is introduced. In MJP, the jet of magnetorheological (MR) fluid containing abrasives is magnetized by an external local axial magnetic field and becomes a collimated and coherent hard jet, which can polish the surface placed at a relatively far distance in a deterministic process. The mechanism of MJP is introduced. The forming of collimated MR jet is analyzed qualitatively. A series of experiments are conducted to demonstrate the feasibility to polishing steep concave shapes. And the experiment to remove waveness by MJP is also done. The optimization of material removal function is investigated on the basis of the principles of computer controlled optical surfacing (CCOS). The experimental results show that the collimated jet of MR fluid is insensitive to the offset distance and MJP can provide potential advantages to polishing steep concaves and parts with a variety of cavities in a deterministic process.

The Current State and Development Trends of Deterministic Ultraprecision Optical Surfaces Machining Technology

The deterministic ultraprecision machining achieves accuracy and repeatability not possible using conventional optical machining techniques, greatly enhances product quality, providing a quantum leap in throughput, productivity, yield, and cost effectiveness. The deterministic ultraprecision machining technology, involving various ultraprecision process from turning, flycutting, grinding and polishing to finishing, is usually referred to the following technologies such as single point diamond turning (SPDT), deterministic microgrinding (DMG), magneto-rheological finishing (MRF),computer controlled polishing (CCP), and computer controlled optical surfacing(CCOS),etc. This paper discusses mainly the current state and development trends of the deterministic ultraprecision machining technologies at home and abroad. In addition, the paper also elaborates on the technical features of the various deterministic machining technologies mentioned.

Toward the Accurate Coverage of Aspheric Surfaces Using Two-Dimensional Scanning Paths for Minimizing Regular Errors

This paper describes a two-dimensional tool-path planning model for minimizing the regularly distributed errors or mid-frequency errors during computer controlled optical surfacing (CCOS) by optimally connecting different tool-path segments. The model was established based on a neuro-fuzzy algorithm, a path neighborhood function which is defined as a victorious output element calculated in a self-organization way, then, the optimum material removal function with a modified weight was derived. The material removal function was studied theoretically and the results of simulation present a Gaussian distribution feature. Discrete removal points and optimized tool-path grid were simulated. Finally, an experiment involving a parabolic mirror was performed for residual error removal and the two-dimensional tool-path planning algorithm was found to be valid.

Study on Anastomosis Feature in Polishing Zone in Aspheric Optics Machining

In order to keep the stability of tool’s removal function, it is required that the anastomosis be tight between the tool and workpiece surface in Computer-controlled Optical Surfacing (CCOS). In this paper, the influence of tool’s character on anastomosis status is firstly studied. The relation model on the ratio of radius to thickness, Young's modulus of the tool, normal asphericity and normal arc height of workpiece surface is established, and the macroscopical condition of tight anastomosis is derived in aspheric optics machining. According to the microcosmic distribution of surface error, the mathematical relation between anastomosis error and removal rate is researched. In the end, the influence rule of anastomosis status on the convergence ratio of residual error is analyzed in machining zone. Based on the conclusion of machining instance, it is found that workpiece material would be fast removed in middle contact zone when the peak value of tool’s removal function locates in its center position.

An alternate approach to free-form surface fabrication

Abstract Surface milling and grinding are widely used for the fabrication of free-form surfaces for molds and dies. The continuous demand for better product quality has led to demand in higher surface accuracy. Limiting factors are the positioning accuracy of the tool and the accuracy of the manufacturing process. Computer controlled optical surfacing (CCOS) has been developed mainly for the fabrication of spherical and aspherical optics. The surface is repeatedly measured and corrected using abrasives until the target accuracy is attained. In principle, sub-micron accuracy is achievable. The accuracy is limited by surface measurement, and not by the processing equipment. The current research investigates the adoption of CCOS for the fabrication of free-form steel surfaces. This paper reports on the development of a test bed and initial experiment results. The test bed is based on a six-axis RX robot as the motion platform with a spindle attached to the wrist of the robot. Surface measurement is carried out using a Talysurf surface profiler. Surface error correction is performed by abrasion of material from the surface. The amount of material to be removed is based on the measured surface error. Following a predetermined tool path, the feed rate along the tool path for surface correction is to be varied according to the required among of material to be removed locally. A free-form surface specimen of mold steel of about 40 mm × 40 mm across is used for the initial experiment. The surface is prepared by CNC surface machining with the initial maximum error over 50 μm. The positioning accuracy of the motion system is estimated to be no better than 0.1 mm. Through surface measurement and correction, the surface accuracy is significantly improved.

The Solving Methods of Dwell Time or Pressure in CCOS for Optical Complex Surfaces

Error correction of optical complex surfaces in computer-controlled optical surfacing (CCOS) is primary important. This paper discusses some methods, i.e. Fourier transform method, Wiener filtering method, directive method and iterative method, for calculating dwell time or relative pressure in the CCOS for error-correcting, which are enlightened from the digital image processing techniques. However, these error correction methods are different from the image processing because they are not a subject of solving the causal signal but a problem of an iterative process. The simulation results show that the Wiener filtering method is good and reliable when the correlative parameters in the fabrication are kept consistent.

Optimization of the material removal in fluid jet polishing

Removal-function models play an important role in computer-controlled optical surfacing. We investigate removal-function models of fluid jet polishing experimentally to obtain an optimum one. We adjust the nozzle and let the slurry jet guided by it impact the workpiece with both normal and oblique incidence angle from different positions. The experimental results indicate that when the slurry jet impacts workpiece obliquely from four positions, the profile of material removal approximates to an ideal one with maximum removal at the center, decreasing to zero at the edges. This new removal-function model avoids the disadvantages of the ring-shaped one, and satisfies the requirement of optical manufacturing to obtain a high degree of precision.

Solving the input data in error correction of optical fabrication by finite Fourier coefficient algorithm

A free-form lens (FFL) is a special surface that is difficult to be fabricated. FFL are usually fabricated by computer-controlled optical surfacing (CCOS) technique. During CCOS, the material removal amount is determined by the unit removal function (URF) convoluting with the input data――the dwell time. When the removal amount and the URF are known, how to solve the input data involves the special algorithm――deconvolution. Usually, the input data are solved by virtue of low pass filter or iterative methods. However, an approximation solution would destroy the machining stability necessary to perform pre-cisely CCOS. In this paper, to solve the input data, a new method that is based on the finite Fourier coefficient is put forward. It can give a continuous and accurate solution for fabricating choice. Several parameters are simulated, which influence the process of CCOS, and evaluating on the effect of the method is also carried out. Experimental results verify that this method is suitable for guiding the manufacturing of high precision FFL.

Surface roughness and removal rate in magnetorheological finishing of a subsurface damage free surface

Abstract Based on computer-controlled optical surfacing, a new technique called magnetorheological finishing (MRF), is presented. The new technique combines the features of conventional loose abrasive machining with a wheel shaped polishing tool. The tool incorporates a host of features and has unprecedented fabricating versatility. The pre-polishing and fine polishing processes can be performed only by adjusting different parameters. The material removal function is studied theoretically and the results of simulation present a Gaussian distribution feature. Based on the established theoretical model, material removal rate experiments involving a parabolic mirror are designed and carried out to determine the effect of controllable parameters on size of the gap between the workpiece and the polishing wheel, rotating speed of the polishing wheel, concentration of volume fraction of non-magnetic particles and polishing time. Further experiments are carried out on the surface microstructure of the workpiece, ...

Design of a six-axis high precision machine tool and its application in machining aspherical optical mirrors

The paper presents the design of a six-axis machining system and its application in fabricating large off-axis aspherical mirrors with sub-aperture lapping techniques. The new system is based on computer-controlled optical surfacing (CCOS), which combines the faculties of grinding, polishing, and on-machine profile measuring, has the features of conventional loose abrasive machining with the characteristics of a tool having multiple degrees of freedom moving in planar model. And a novel dual touch-trigger probe profiler is designed, which is composed of a probe, model METRO-MT60 made by HEIDENHAIN Co., is integrated into the system for measuring the shape accuracy of the tested aspherical surface, another probe modeled METRO-MT12 is designed as a calibrating device for minimizing the cosine error caused by assembly inaccuracy. The new CNC machining system with two kinds of moving coordinate systems, dual tool activities and on-machine measuring is presently developed based on the new concept. The general material removal function during machining is analyzed on the basis of the Preston hypothesis. Further, an alignment test of the measuring profiler is carried out using a leveling rule as a specimen. The accuracy of the optical surfaces measured by the dual probe profiler is found to be within 1 μm PV after removing cosine error and error compensating, achieves to the resolving power of the profiler is about 0.2–0.5 μm, so the developed system can be applied to the shape accuracy measuring of aspheric fabrication with micro precision during fine grinding process according to the calibrating results. Finally, the manufacturing experiments are carried out by virtue of an off-axis oblate ellipsoid mirror with rectangular aperture as 770 mm×210 mm and centered 127 mm. The accuracy of the aspherical mirror improved from the initial form error of 17.648 μm rms to the final one of 0.728 μm rms after grinding for 200 h.