Bonnet Polishing Research Articles

Research on the Residual Error Evaluation Method for Deterministic Polishing of Aspheric Optics

Residual error evaluation method for deterministic polishing of aspheric optics is studied. Two residual error evaluation methods, which are axis-direction error method and normal error method respectively, are researched theoretically. It's inferred that the residual error of aspheric surface should be evaluated by normal error method. A new approach is proposed to calculate normal direction residual error on the basis of the axis-direction residual error of the aspheric surface. There exists difference between these two kinds of error which increases from the center of the aspheric optic to the edge through the comparison of them. Taking bonnet polishing and numerical controlled small tool polishing as examples, experiments are made to quantitatively prove that using axis-direction error method to evaluate residual error in deterministic polishing would introduce different degrees of processing error. It's found that the processing error is positively correlated with the relative aperture of aspheric optics, which is the ratio of the optic's aperture and vertex's curvature radius. Therefore, it is recommended to use axis-direction error method instead of normal error method as the evaluation method of the residual error during deterministic polishing aspheric optics with relatively small relative aperture; the opposite is the other way around.

球面光学零件气囊抛光工艺参数优化的研究

For the spherical optical parts machined by using the bonnet polishing means,and by the orthogonal experimental means,with a view to the material removal rate and the surface roughness,the influencing degree of the five main influencing factors on the material removal rate and the surface roughness is researched.According to the experimental results,the process parameters are priority selected,and the optimal combination of the process parameters can be achieved.According to the process parameters optimized with a view to the material removal rate and the surface roughness,and aiming at controlling the surface roughness,the spherical optical parts are machined by using the discrete precession motion mode,and the ultra-precise smooth surface can be achieved.

Experimental investigation and analytical modelling of the effects of process parameters on material removal rate for bonnet polishing of cobalt chrome alloy

Abstract Cobalt chrome alloys are the most extensively used material in the field of total hip and total knee implants, both of which need highly accurate form and low surface roughness for longevity in vivo. In order to achieve the desired form, it is extremely important to understand how process parameters of the final finishing process affect the material removal rate. This paper reports a modified Preston equation model combining process parameters to allow prediction of the material removal rate during bonnet polishing of a medical grade cobalt chrome alloy. The model created is based on experiments which were carried out on a bonnet polishing machine to investigate the effects of process parameters, including precess angle, head speed, tool offset and tool pressure, on material removal rate. The characteristic of material removal is termed influence function and assessed in terms of width, maximal depth and material removal rate. Experimental results show that the width of the influence function increases significantly with the increase of the precess angle and the tool offset; the depth of the influence function increases with the increase of the head speed, increases first and then decrease with the increase of the tool offset; the material removal rate increases with the increase of the precess angle non-linearly, with the increase of the head speed linearly, and increases first then decreases with the increase of the tool offset because of the bonnet distortion; the tool pressure has a slight effect on the influence function. The proposed model has been verified experimentally by using different Preston coefficients from literature. The close values of the experimental data and predicted data indicate that the model is viable when applied to the prediction of the material removal rate in bonnet polishing.

An investigation of the viability of bonnet polishing as a possible method to manufacture hip prostheses with multi-radius femoral heads

Recent research has indicated that the use of multi-radius femoral heads can be advantageous in terms of the articulating zone lubrication and tribological performance for metal-on-metal (MoM) hip implants. The emergence of this multi-radius femoral head is a new challenge in manufacturing research because conventional machining technologies are not normally agile enough to realize high-accuracy machining of the new design of femoral heads. This paper attempts to demonstrate the manufacture of a multi-radius femoral head from a typical single radius femoral head using a bonnet polishing method. The sample used in this investigation is an 18-mm radius cobalt chrome (CoCr) alloy, the most common material for MoM hip prostheses. The paper outlines the process of producing a target of 19-mm radius zone in the articulating region. Post polishing measurement of the radius of the articulating zone showed the radius to be between 18.98 and 19.017 mm. Additionally, the peak-to-valley (PV) value of the spherical error map was reduced from 103 to 1.36 μm and root mean square (RMS) was from 29.65 to 0.318 μm. The machined surface roughness obtained through the process was Sa = 16.1 nm which is well within the recommendation of ISO7206-2:2011 standard for metal hip joint prosthesis value of 50 nm. The experimental result confirms that bonnet polishing is a potentially viable choice to finish the new design multi-radius femoral heads systems.

Analysis of corrective characteristics of various polishing methods for mid-frequency errors

The theoretical analysis of corrective characteristics of three kinds of polishing methods for mid-frequency errors was studied, which was aimed to confirm the possibility that computer control optical surfacing and computer control active-lap can be replaced by bonnet polishing in the machining process. The first step was to calculate the removal functions of three kinds of polishing technologies and use fast Fourier transform to figure out the frequency spectrum of each method. After that, according to the frequency spectra, curves of cut-off frequencies related to the working ranges of spatial frequencies errors were obtained. It revealed that the affected scope of spatial frequencies is determined by the polishing method, diameter size of polishing tool and shape of removal function. Moreover, only low-frequency errors could be modified and mid-frequency errors could not be corrected or created by computer control active-lap, and computer control optical surfacing can correct part of the mid-frequency errors and low-frequency errors in the polishing process, but at the same time can produce some new mid-frequency errors; as for bonnet polishing, it can be computer control active-lap-like in smoothing which only modified and created the low-frequency errors or computer control optical surfacing-like which corrected and created the mid-frequency errors in local polishing. Otherwise, the efficiency of bonnet polishing is higher than the other two methods. As a result, seen from the point of correction ability of mid-frequency or polishing efficiency, bonnet polishing could replace computer control active-lap and computer control optical surfacing for finishing two polishing stages by only one tool, which is significant to extending the application of bonnet polishing in optical manufacturing.

Research on the Normal Error of Large Aspheric Mirror Employed in Bonnet Polishing

The influence functions action orientation of the Bonnet Polishing (BP) process is perpendicular to the contact zone. So the normal error of the aspheric surface should be used as the residual error to calculate the dwell time. But the Z axis direction error is generally adopted as the residual error and few papers on the normal error have been reported hitherto. Its necessary to pay attention to this issue. In this paper, two algorithms which are Asphericity Subtraction (AS) algorithm and Z Axis Direction Error Transformation (ZADET) algorithm are presented to calculate the normal error of the large aspheric surface. Simulations in three different cases are organized to utilize these two algorithms, together with the comparison of them. And the comparison of the normal error and the Z axis direction error is also organized. Its found that there exists difference between AS algorithm and ZADET algorithm. Both of them can be used to calculate the normal error of the aspheric surface when the ratio value of the width to radius is small. And the difference between the normal error and Z axis direction error is considerable. So the normal error should be used as the residual error to calculate the dwell time in BP process.

Automated Finishing of Diamond Turned Dies for Hard X-Ray and EUV Optics Replication

Ultra-precision diamond turning can deliver very accurate form, often less than 100nm P-V. A possible manufacturing method for thin Wolter type-1 mirrors in hard X-ray space telescopes thus involves generating electroless nickel plated mandrels by diamond turning, before coating them with a reflective film and substrate. However, the surface texture after turning falls far short from the requirements of X-ray and EUV applications. The machining marks need to be removed, with hand polishing still widely employed. There is thus a compelling need for automated finishing of turned dies. A two step finishing method is presented that combines fluid jet and precessed bonnet polishing on a common 7-axis CNC platform. This method is capable of finishing diamond turned electroless nickel plated dies down to 0.28nm rms roughness, while deterministically improving form error down to 30nm P-V. The fluid jet polishing process, which consists of pressurizing water and abrasive particles for delivery through a nozzle, has been specially optimized with a newly designed slurry delivery unit and computer simulations, to remove diamond turning marks without introducing another waviness signature. The precessed bonnet polishing method, which consists of an inflated membrane rotated at an angle from the local normal to the surface and controlled by geometrical position relative to the work-piece, is subsequently employed with a novel control algorithm to deliver scratch-free surface roughness down to 0.28 nm rms. The combination of these two deterministic processes to finish aspheric and freeform dies promises to unlock new frontiers in X-ray and EUV optics fabrication.

Research on the Optimization of the Bonnet Polishing Tool

Bonnet polishing is a practical and economical polishing technology for the fabrication of precision optical components. Its removal function is controlled by many parameters, and much research has been performed on them in the past few years. However, most of the previous research has been focused on the inner pressure of the bonnet, the rotation speed of the spindle, the precession angle, the bonnet decrement, etc. The deformation of the bonnet under the inner pressure has received relatively little attention. This paper demonstrates the deformation of the bonnet under the inner pressure and its effect to the contact area and the bonnet decrement. It also proposes two solutions, one is to add an enhancement layer on the bonnet; the other is to design a novel bonnet contour which is aspheric but turns to spherical under the effect of the inner pressure. The latter one is proved that it’s effective to reduce the deformation to the polishing process.

Super-smooth finishing of diamond turned hard X-ray molding dies by combined fluid jet and bonnet polishing

Fabricating super-smooth aspheric optics for future hard X-ray telescopes will require a new process chain. Whilst diamond turning of electroless nickel plating can generate ~100nm P-V aspheric molding dies, associated turning marks must be removed before replication. An innovative two-step freeform finishing method is presented, that combines fluid jet and precessed bonnet polishing on a common 7-axis CNC platform. The removal rate and surface texture relationship of fluid jet with abrasive type and pressure is documented, and form correction demonstrated down to 27nm P-V. A novel bonnet tool-pathing method called “continuous precessing” is then applied, which delivers super-smooth anisotropic surface texture of 0.28nm rms.

Technology and Application of Ultra-precision Machining for Large Size Optic

The ultra-precision machining technology of large size optic is a comprehensive application of various sciences, which promotes the development for civil and national defense use. Under the processing of national optical project, our country makes great progress in this field. The development of ultra-precision machining technology for large size optic in micro/nano machining and measuring laboratory of Xiamen University is introduced, particularly in grinding equipment, grinding process, precise measuring, controlled bonnet polishing, environment monitoring, mid-spatial frequency error assessing and et al. From the requirement of the ultra-precision machining, the laboratory uses the experience of other countries for reference and integrates the above technology, then forms an intellectual property of grinding and polishing system independently for large size optic manufacture.

Control Techniques of Bonnet Polishing for Free-form Optical Lenses with Precession

Control Techniques of Bonnet Polishing for Free-form Optical Lenses with Precession

A Novel Method of Bonnet Polishing for Aspheric Surface

On the basis of compliant polishing device for aspheric, a new method of bonnet polishing on magneto-rheological of torque servo driver has been proposed. It has introduced the bonnet polishing composition and polishing principle. Polishing tool using the flexible bonnet, it can better adapt to the different curvature change of parts, and the stable polishing force can be get through MRT. Combining the constant torque device on MRF with CNC machine, this embodies high rigidity, higher precision of NC machine and reflects the MRT with adjustable, controllable, rapid response characteristics. The formulae of stress and relative velocity of polishing are deduced, and the material removal rate model has been set up. The experimental results show that this new method of bonnet polishing can provide stable polishing force, and the model is correct, has good prediction ability.

Research on the Principle of Compiled-Typed NC System

To solve the conflict between OSs real time property and NC systems open and universal property [, a compiled-typed NC system is advanced based on analysis and research of the structure of modern NC systems. This kind of system can diminish the affection of the real-time OSs respond precision to machining with high speed and high precision and improve the frequency of interpolation sampling and respond precision. Motion control experiment and parametric curve direct interpolation experiment are finished by bonnet polishing machine tool with compiled-typed NC prototype system, proving that working principle of compiled-typed NC system is correct and validity.

Research on Flexible Bonnet Polishing Process Method and Related Technology

Bonnet tool polishing is one kind of methods that can obtain higher surface quality of the ultra precision processing for producing flat, aspheric and other mould free surface, etc. The latest development and research results on bonnet polishing technology at home and abroad were reviewed. The core technologies of bonnet polishing were analyzed, including polishing principles, polishing tool, the rate of material removal, edge control, dwell time, path planning etc. The results show that the bonnet polishing method is an economic, efficient, practical and high precision polishing technology.

大口径非球面元件可控气囊抛光系统

大口径非球面元件可控气囊抛光系统

The Application of Taguchi Approach to Optimise the Processing Conditions on Bonnet Polishing of CoCr

This paper applied the Taguchi approach to investigate the effects of polishing parameters to obtain the optimal processing conditions for CoCr alloy polishing using a multiaxis CNC controlled corrective polishing machine. The polishing medium used was 1m diamond paste with a Microcloth(polishing cloth). Surface finish parameter Sa was chosen as the criterion for process optimization. The experimental results indicate that the optimal polishing conditions for CoCr alloy polishing is 50 precess angle, 800 rpm head speed, 0.2mm tool offset and 1.5 bar tool pressure. With this optimal condition, a confirmatory experiment was conducted. The surface roughness Sa reduced from an initial 24nm to 7nm and reduction ratio was 70.8% which was very close to the estimated ratio of 64%.

Modelling and measurement of polishing tool influence functions for edge control

We present progress on our development of edge control for fabrication of mirror segments potentially for the European Extremely Large Telescope (E-ELT). Zeeko’s Bonnet polishing technology has been adopted to achieve the form correction target. Like other Computer Numerical Controlled (CNC) polishing, accurate and stable tool Influence Functions (IFs) are important. Particular challenges arisen when polishing up to edges where the geometry of the IFs created by a bonnet changes. We described a model based on measured IFs data that allowed us accurately to predict the edge profile. To obtain IFs at the edge, data from interferometers and profilometers have been stitched. Numerical models with empirical Ifs as input data have been used to predict edge profiles with some preliminary success.

Research on Parameters of Bonnet Polishing Based on FEA

In this paper, according to the bonnet polishing technology, in order to get the change regularity of the bonnet’s deformation, the inflated deformation model of the bonnet, which is made all of rubber or made of rubber but with a nylon enhancement layer, is established and analyzed by using the FEA simulation. And then, by simulating the process of the bonnet polishing the plane wafer, it achieves the contact pressure curve which varies with the bonnet decrement. According to the Preston equation, k can be acquired through the experiment, v can be achieved through mathematical modeling, and p has been achieved in this paper. Thus, the removal function based on FEA will be acquired.

Modelling and measurement of polishing tool influence functions for edge control

We present progress on our development of edge control for fabrication of mirror segments potentially for the European Extremely Large Telescope (E-ELT). Zeekoś Bonnet polishing technology has been adopted to achieve the form correction target. Like other Computer Numerical Controlled (CNC) polishing, accurate and stable tool Influence Functions (IFs) are important. Particular challenges arisen when polishing up to edges where the geometry of the IFs created by a bonnet changes. We described a model based on measured IFs data that allowed us accurately to predict the edge profile. To obtain IFs at the edge, data from interferometers and profilometers have been stitched. Numerical models with empirical Ifs as input data have been used to predict edge profiles with some preliminary success.

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.