Polishing Of Aspheric Surfaces Research Articles

A spiral path generation method for achieving uniform material removal depth in aspheric surface polishing

A spiral path generation method for achieving uniform material removal depth in aspheric surface polishing

Tilt angle correction based forming algorithm for high steepness aspheric surfaces polishing

According to the varying characteristics of removal function in steep aspheric surface polishing, the calculation algorithm of removal function at arbitrary polishing position was given in the best fitting plane of steep aspheric based on the experiment of removal function with several inclined angles and the feature of polishing surface. The forming model was then discretized into the form of large sparse matrix product, and the dwell time was solved efficiently by SBB algorithm. The discrete matrix model describing nonlinear polishing process of steep aspheric surface was established, and finally the process flow was verified by experiment. The experiment was conducted on a 200 mm aperture off-axis aspheric surface, finally the root mean square error of the surface was 0.022 λ and the processing convergence rate was 3.23 after two polishing runs. The research results show that the forming model with plane projection and tilt-angle-correction removal function is reasonable, and the dwell time solving algorithm based on SBB principle is a fast and efficient surface shape control technology, which could fulfill deterministic and accurate shaping for steep aspheric surface.

Evaluation and compensation of a kinematic error to enhance prepolishing accuracy for large aspheric surfaces by robotic bonnet technology.

To obtain an economical uniform polishing method, it is effective to combine robot technology with bonnet polishing technology and apply it to the precision processing of large aspheric surfaces. However, the large robotic kinematic error causes significant deterioration to the surface shape of aspheric optics during robotic bonnet polishing (R-BP). To address this problem, research on the evaluation and compensation of the kinematic error was conducted to enhance the prepolishing accuracy on large aspheric surfaces by R-BP. Firstly, a precession control model of R-BP applied to the polishing of large aspheric surfaces was proposed. Then, an evaluation and compensation method of a robot kinematic error was presented based on machining errors. At last, it had been verified by experiments that at the removal depth of 2.5 μm, the variation of surface shape was reduced from 0.42 μm to 0.11 μm after compensation. Meanwhile, the change rate was reduced from 16.8% to 4.4%. Hence, the effectiveness of evaluation and compensation method to improve the prepolishing accuracy was verified, which is beneficial to implement mass production of high-precision large aspheric surfaces with low cost.

Experimental study on the polishing of aspheric surfaces on an S-136 mould steel using magnetic compound fluid slurries

Experimental study on the polishing of aspheric surfaces on an S-136 mould steel using magnetic compound fluid slurries

Experimental study on the polishing of aspheric surfaces on an S-136 mould steel using magnetic compound fluid slurries

S-136 (STAVAX ESR) as a mould material has been extensively used in the optical lens manufacturing industry. In this study, the aspheric surface made of S-136 fixed by a six-degree of freedom (DOF) manipulator was polished using magnetic compound fluid (MCF) slurry. The effects of experimental parameters included rotational speed of workpiece and MCF slurry composition on surface roughness Ra and shape retention were investigated. The results demonstrated that the cutting marks on the initial surface reduced obviously under the given experimental parameters. Therefore, the smoothest surface with Ra 20 nm was obtained with MCF3 containing micro-sized Fe3O4 particles with lower saturation magnetisation at nw = 1,000 rpm. The material removes uniformly during the polishing process and the Pearson correlation coefficient (Pcc) of the aspheric surface was kept above 0.994. The MCF slurry polishing was demonstrated as a potential method to smooth the S-136 aspheric surface.

Trajectory planning of optical polishing based on optimized implementation of dwell time

This paper presents a study on the trajectory planning of optical polishing based on an optimized implementation of dwell time to improve the polishing quality and efficiency of optical surfaces. The large-scale, sparse, and ill-posed linear equation solution is transformed into a quadratic programming problem with boundary constraint, and the monotone projection gradient method is adopted to obtain an optimal dwell time solution by considering its fluctuation characteristic. Then, parametric polishing paths are constructed using cubic B-spline curves to guarantee one-to-one correspondence between each curve segment and dwell time of a removal spot. Finally, an interpolation process of polishing trajectory is proposed on the basis of the equal–proportional feed rate adjustment strategy to improve the implementation accuracy of dwell time. Simulation and experimental results show that the proposed dwell time algorithm and spline interpolation method can considerably improve the solution accuracy of dwell time and the convergence rate of the form error during polishing. These results provide important scientific basis and technical support for the high-efficiency and high-precision polishing of large-aperture aspheric optical surfaces.

Investigation on the polishing of aspheric surfaces with a doughnut-shaped magnetic compound fluid (MCF) tool using an industrial robot

Aspheric elements have become essential optical surfaces for modifying optical systems due to their abilities to enhance the imaging quality. In this work, a novel method employing a doughnut-shaped magnetic compound fluid (MCF) polishing tool, and an industrial robot was proposed for polishing aspheric surfaces. Firstly, investigations on the MCF tool, including the formation process and geometry, were conducted to form an appropriate polishing tool. The distribution of abrasive particles was observed using SEM and EDX mapping. Thereafter, a conic workpiece constructed from 6061-aluminum alloy was selected as the workpiece, which was used to discover the effects of the parameters on the polishing ability of aspheric surfaces. Finally, a polishing experiment was conducted with an aspheric element under the optimized conditions. The obtained results are shown as follows. (1) A relatively regular MCF tool was obtained when the eccentricity (r), amount of MCF slurry supplied (V), revolution speed of the MCF carrier and magnet (nc and nm, respectively) were given at appropriate values. (2) Abrasive particles entrapped in or attached to the clusters were observed abundantly on the MCF tool sample. (3) The surface profile of the conic workpiece after 60 min of polishing indicated that material was removed evenly, and an annular polishing area was attained. Meanwhile, a higher material removal rate and better surface roughness were achieved with a smaller working gap (h) and larger volume of the MCF slurry supplied (V). (4) The roughness (Ra) of the aspheric surface decreased from 49.81 to 10.77 nm after 60 min of polishing. The shape retention obtained a Pearson correlation coefficient (Pcc) of 0.9981, which demonstrated that this novel method is appropriate for polishing aspheric elements.

Time-varying tool influence function model of bonnet polishing for aspheric surfaces.

For bonnet polishing of an aspheric surface, the tool influence function (TIF) is inevitably time varying, induced by the different surface curvatures on the aspheric surface. Accordingly, this paper investigated how the surface curvature affects the bonnet-workpiece contact area, and then presented a time-varying TIF model. The time-varying TIF was modeled based on the finite element analysis and kinematics analysis methods, and validated by experiments. The experimental results exhibited good agreement with the theoretical results. The proposed method can forecast the TIF for different polishing positions on aspheric surfaces, and provide the theoretical foundation for dynamic compensation of aspheric surface polishing.

Fundamental Investigation on the Polishing Aspheric Elements with Doughnut-Shaped MCF Slurry

Previous researches have confirmed that MCF (magnetic compound fluid) slurry shows outstanding performance in the nanoprecision polishing of flat surfaces and V-grooves. However, no investigations have been conducted on the polishing of aspheric surfaces using MCF slurry. In this work, a novel method employing a doughnut-shaped MCF polishing tool and a 6-DOF manipulator has been proposed for the aspheric surface polishing. The time consumption for forming stable polishing tool and its final appearance are investigated. Flat aluminum alloy workpieces that can be considered as a kind of aspheric elements with infinite curve radius were adopted in the investigation of the polished forces under variable parameters. As a typical experimental result, with MCF3 slurry, 2.5ml volume of supplied slurry and work gap 3.5 mm, the surface roughness Ra decreases from 125nm to almost 10nm after 90 min polishing, confirming that the proposed method has the potential to polish aspheric surfaces.

Experimental study on elastic deformation molding process for generating aspheric surface glass

The elastic deformation machining method has been demonstrated to be a novel optical manufacturing approach which can greatly reduce and simplify the production of large aspheric lens. Upon completion of the machining process, the workpiece under process will be shaped into a desired surface form of mold. The method allows the lapping and polishing of aspheric optical surfaces using a large lapping or polishing tools. The subject of this study is to determine the surface shape of the finished glass workpiece after the lapping process of the elastic deformation machining process associated with molding. The experimental and finite element analysis (FEA) results were compared with the shape of the molded surface form. The form accuracy between the optical glass and mold is within 1.6 μm (within radius 32 mm). The conclusions show that the method proposed is effective for machining the aspheric optical glass with proper elastic deformation machining parameters.

Study on the influence of the rotational speed of polishing disk on material removal in aspheric surface compliant polishing

When a soft polishing tool is compressed on a stiff workpiece of curved surface, the contact area is a piece of the curved surface. In the process of aspheric surface polishing, the machining speed is always provided by the rotational speed of the spindle of a computer numerical control lathe. Yet, the polishing tool is usually made rotating to remove cutting scraps and broken abrasives from the contact area. The rotational speed of the polishing tool would change the distribution state of the relative speed in the whole contact area, and it would definitely change the volume of material removal in the surface of the workpiece. This article studies how the rotational speed of the polishing tool changes the distribution state of the relative speed in the whole contact area and material removal in the volume of the surface of the workpiece. The computing results show that the volume of material removal increases with the increase in both the rotational speed of polishing disk and the rotational speed of lathe spindle, but the polishing quality is totally not in this case.

Study on the orthomogonalization for hybrid motion/force control and its application in aspheric surface polishing

In the process of curved surface polishing and buffing, the tools have to be compressed on the workpiece and moved to wear the surface, so the normal force and tangential feed movements should be provided and controlled synchronously. It is a typical kind of compliant control, which would most likely be done under the hybrid motion/force control policy. Although its related theory is already almost perfect, this method could seldom be used in the process of curved surface polishing and buffing, if ever, for it is always not easy to orthogonalize force control space from movement control space. In this paper, a force control subsystem is developed and fixed on a computer numerical control (CNC) lathe to control the normal force independently, and the normal force control space is orthogonalized from the feed movement control space by geometry defining for aspheric surface polishing. Furthermore, the orthogonalization in the domain of time is taken to make the force control not to interpolate with the displacement control of the feed movements. Experiments in controlling and polishing the normal force show that the hybrid motion/force control policy could be used in the process of aspheric surface polishing and buffing by keeping the normal force control and feed movement control independently in both domains of space and time.

Aspheric Surface NC Polishing Force-Position Decoupling Control Technology

Based on MTTSD (Magneto-rheological Torque Transmission Servo Devices) and force-position decoupling control technology, an aspheric surface NC polishing project is put forward. The aspheric surface polishing fundamental theory is researched. Under the condition of a constant material removal rate, a new aspheric surface polishing model is brought forward. In addition, under the condition of a constant material removal rate, this paper brought up a polishing force control model and a workpiece rotating speed control model, according to witch, force and rotating speed change with the coordinates of working points, and variation curves are drawn.CLC Number: TH166 Document Identifier: A

Modeling of Machining Force Error in Aspheric Surface Polishing by Hybrid Movement/Force Control Policy

By the hybrid movement/force control policy, machining force could be controlled in aspheric surface polishing. However, the controller could not eliminate and restrain the errors of position and pose which would disturb the force controll consequentially in real system. In this paper, an angle is defined to express all the errors of position and pose for aspheric surface polishing, and the errors are modeled for machining force controll. By detecting this angle, the errors of position and pose could be used by the system controller to eliminate the disturbance in hybrid movement/force controll system and compensate feed movement controll in aspheric surface polishing. Simulation shows that the model could effectively express the errors of position and pose .

A Novel MRF-Based Torque Servo Device

In mechanical equipments or manufacturing processes, force control is definitely required to achieve desired performance or to ensure product quality. Aiming at this issue, this paper introduced an author-invented torque servo device that utilizes magnetorheological fluids (MRF) to provide a controllable torque. In the paper, the structure and working principle of the MRF-based torque servo device (MRT) were introduced, the constitutive model and the characteristics of the MRF were established and discussed. On the basis, the static and dynamic models of the MRT were both analytically and experimentally studied, and the control of the output torque of the MRT was demonstrated. As an example, the application of the MRT to the polishing of aspheric surfaces was presented in the end.

NC polishing of aspheric surfaces under control of constant pressure using a magnetorheological torque servo

In this paper, a method of maintaining a constant polishing pressure is proposed for a NC polishing system by controlling the polishing force during the polishing process. First, the NC polishing system is developed to resolve the force–position coupling problem encountered in common polishing processes. It mainly consists of a force control subsystem based on a magnetorheological torque servo to provide a controllable torque to polishing tool to generate the polishing force and a position control subsystem based on a general CNC lathe to control the position of the polishing tool. Second, a constant polishing pressure model is established by controlling the polishing force according to the variation of the curvature of the aspheric surfaces, and the polishing parameters for model are planned. Then, the control model of the polishing system is proposed, and a PID controller is designed for torque tracking with the actual torque feedback from a torque sensor. Finally, polishing experiments are conducted with constant force and constant pressure, respectively. Experimental results show that the surface roughness is greatly improved, the aspheric surfaces can be polished more uniformly with constant pressure than with constant force, and the PID controller can meet the requirements for the polishing force control.

Partial-Ductile Grinding, Lapping, and Polishing of Aspheric and Spherical Surfaces on Glass

This paper discusses partial-ductile-mode grinding, lapping and polishing of aspheric and spherical surfaces on glass. Industrial manufacture of glass lenses usually involves three operations: grinding (known as milling in the optical industry), tapping, and polishing. The fracture mode of material removal is dominant in the grinding process. While these three operations have been successful for machining spherical lenses, aspheric lenses have been manufactured in the absence of the lapping process, because of the considerable amount of ductile mode of material removal in grinding. The parameters that helped identify and solve problems in manufacturing were surface roughness, micro-fractures and ductile streaks on glass surfaces, and interferometric fringes.

磁気援用研磨に関する研究 (第2報)

This paper describes the effect of the magnetic field distribution on the removal distribution of the workpiece surface in the magnetic field-assisted polishing method. In case of applying this method to the curved (aspheric) surface polishing, the next process is suitable. The designed shapes are first generated precisely by turning or grinding, and then the surfaces are finished only to improve the smoothness, with the previous form accuracy being maintained. Therefore, in order to carry out the uniform removal polishing using the magnetic field-assisted polishing, it is important to clarify the processing mechanism of this polishing method. In this report the relation between the distribution of magnetic field and the polishing pressure distribution was analyzed theoretically and experimentally using several shapes of magnetic poles in the polishing set-up. And, by polishing examination of the flat glass, the relations between the magnetic field distribution, polishing pressure distribution and polishing removal distribution were analyzed experimentally. As a result, the relations between the magnetic field distribution, polishing pressure distribution and polishing removal distribution were clarified.

New Developments in Optical Fabrication Techniques

Because of the demand for better performance in optical systems, there is a continuing effort to find more efficient ways of producing optics. Perkin-Elmer's involvement in developing new techniques for the fabrication of aspheric and flat surfaces has resulted in the generation of three machining methods: (1) computer-controlled polishing of aspheric surfaces, (2) continuous polishing of flat surfaces, and (3) single-point diamond tool machining of spheres and flats by means of the Precision Surface Generator. Optical flats larger than 20 inches in diameter have been produced that are flat to better than A /8 peak to valley. The Precision Surface Generator has successfully produced metal mirrors of aluminum, copper, and brass.

Air Gauge Measurement and Driven Lap Polishing in the Production of Aspheric Surfaces

The increased demand for aspheric surfaces in optical systems has required refinements and advances in optical shop technology. Air gauges have been used to measure aspheric surfaces in prepolished and polished conditions to accuracies on the order of one wavelength of visible light. Small, synchronously driven laps have been used in the polishing and figuring of aspheric surfaces with good results regarding speed, accuracy, and smoothness. Both techniques provide interesting possibilities for future development.