Profile Error Compensation Research Articles

Profile error compensation in ultra-precision grinding of aspherical-cylindrical lens array based on the real-time profile of wheel and normal residual error

Profile error compensation in ultra-precision grinding of aspherical-cylindrical lens array based on the real-time profile of wheel and normal residual error

Ultra-precision raster grinding biconical optics with a novel profile error compensation technique based on on-machine measurement and wavelet decomposition

Abstract Biconical optics with excellent profile accuracy and bright surface finish are increasingly applied in the fields of aviation, aerospace, biology, consumer electronics, etc. However, the extreme demand of micron/sub-micron scaled profile accuracy of the biconical optics poses a great challenge for the manufacturing process. In this study, a novel compensation strategy was developed based on the on-machine non-contact laser displacement sensor and wavelet decomposition technique for depressing the final profile error of biconical optics machined by raster grinding. Firstly, a profile error evaluation model was established to restructure the error surface measured by the laser sensor, and the low-frequency profile error was separated successfully from the reconstructed errors using the proposed technology. Secondly, the momentous error sources that affect the profile accuracy were identified and analyzed, and a wheel-error model was established based on the correspondence between the error sources and the profile errors, which is then applied for the wheel path compensation. Finally, the compensation experiments were undertaken. The profile errors along the workpiece length (220 mm) and workpiece height (105 mm) decreased from 15.425 μm to 1.678 μm and 18.6 μm to 1.6948 μm, respectively. The PV-values of the machined surface with profile error compensation reduced from 21.6 μm to 1.5486 μm in a 220m × 105 mm measurement area. This demonstrates the effectiveness of the proposed profile error compensation methods for the biconical surface.

Profile error compensation in precision grinding of ellipsoid optical surface

Optical glass elements with the ellipsoidal surface are used in many critical aerospace instruments, such as satellites, telescope and cameras. Their optical performance is mainly affected by profile accuracy and surface quality. In this paper, a rectangular ellipsoid surface is precisely ground on a BK7 optical glass blank by grating scanning grinding path with a three-axis CNC precision surface grinder. A profile error compensation procedure for ellipsoidal grinding is proposed based on the error analysis about the primary error sources in the XY and YZ projection planes during the grinding process. The mathematical prediction models of the wheel arc profile error, the measurement and wear error of the grinding wheel radiuses are established. By applying the proposed error compensation procedure, the profile accuracy of the ellipsoidal surface was improved from 4 μm to 2 μm in the XY plane, and improved from 15 μm to 5 μm in the YZ plane.

Tool electrode wear compensation in block divided EDM process for improving accuracy of diffuser shaped film cooling holes

Diffuser shaped film cooling holes are increasingly being applied in the turbine blades of advanced aeroengines. It is a difficult problem to machine this kind of cooling holes due to their complicated shape and superalloy material. Currently, die-sinking electrical discharge machining (EDM) is regarded as an available process for machining the diffuser holes. However, the change of tool electrode shape caused by the electrode wear and difficult renew of dielectric fluid affect the processing precision. The worn tool electrode has to be replaced with a fresh one after machining one or several holes. The replacing procedure is time consuming to break the machining process. In the previous work, a novel block divided EDM process was proposed to efficiently machine the diffuser holes, but the machining accuracy needs to be further improved. In this research, the block divided EDM process firstly is analyzed and simulated in detail to know the key factor affecting the machined accuracy. Furthermore, a self-repair method and a profile error compensation method of the tool electrode wear are proposed for improving the processing accuracy. The self-repair method is to plan and control the rotation motion of the tool electrode for maintaining its shape. The profile error compensation method is to modify the Z coordinate of the tool electrode for decreasing the machining error. Machining experiments verified that the proposed methods can meet the accuracy requirements of the film cooling holes with diffusion angle error < 0.1° and average profile error < 10 μm.

A profile error compensation method in precision grinding of screw rotors

In the operation of positive displacement screw machine, the profile accuracy of the screw rotor has a significant effect on the meshing stability between male rotor and female rotor, which is the main factor for vibration, noise, sealing, and wear resistance. This sets extremely strict requirements of the profile accuracy of screw rotor. Therefore, in order to obtain precise screw rotor profile, form grinding is generally used as a finishing process in the manufacture of screw rotors. Unfortunately, the grinding wheel has to be frequently dressed due to inevitable grinding wheel wear. This will cause a substantial waste of time and thus lead to inefficiency. It is therefore desirable to be able to predict the profile error caused by grinding wheel wear in order to make it controllable. In this paper, the influence of installation angle, center distance, and grinding wheel wear on rotor profile error is investigated. A novel method has been proposed for prediction profile error of the screw rotor in precision form grinding. The proposed method has been employed to compensate profile error caused by grinding wheel wear and improve grinding efficiency. Verification experiments were conducted with several screw rotors produced and profile error measured. The comparison shows that the compensation results agree well with the numerical simulation results, showing the effectiveness of the proposed compensation method.

Ultraprecision grinding of small-aperture concave aspheric mould insert with tilt axis method

With the widespread application of various small aspheric optical components, high-precision manufacturing technology of forming molds is deeply demanded. However, the ultra-precision grinding technology of small-aperture aspheric mould is still undeveloped. An ultraprecision grinding technique called tilt axis grinding which is based on the grinding spindle is tilt with the axis of aspheric surface and profile error compensation of aspheric mould are studied. Through constructing geometric model of the tilt axis grinding, the tool path and the interference conditions were calculated. The form error was compensated with the data measured by profiler. An experimental test is conducted to grind an aspheric tungsten carbide mould of aperture 5 mm, and the profile accuracy is gradually improved from 0.652 μm to 89.3 nm in PV after two compensation cycles. The surface roughness is 10 nm in Ra.

A hybrid fabrication approach and profile error compensation for silicon aspheric optics

Aspheric optics is widely used for many optical applications due to their advantages, that is, light weight, cost-effectiveness and efficiency. There are many fabrication challenges which affect the quality of aspheric optics used for infrared-based applications. Diamond turning is one of the most suitable techniques for fabrication of infrared aspheric lens with high profile accuracies, due to its deterministic approach. However, for optics with large sag value, multiple machining cycles are required to make the best fit surface. Repeated machining cycles result in generation of inherent stresses leading to subsurface deformation and poor quality. In this study, hybrid approach of grinding and machining is proposed for fabrication of silicon infrared optics in large volume. The proposed approach results in reduced fabrication time and subsurface deformation with improved surface quality and tool life. The profile accuracy after compensation of profile error ( Pt) is 0.21 µm and surface roughness ( Ra) 10.5 nm is achieved.

Profile error compensation in cross-grinding mode for large-diameter aspheric mirrors

This study proposes a high-precision compensation system using an on-machine noncontact measuring system to improve the manufacturing accuracy and efficiency of large-diameter aspheric mirrors by reducing profile errors arising from tool setting errors and machine positioning errors. By measuring a standard hemisphere, the assembly tilt angle of the measurement sensor can be calibrated. The grinding wheel setting offset can be calculated by comparing the measured profile and the ideal profile, and the profile error caused by wheel offset can be reduced by adjusting the grinding origin coordinate. According to the normal unit vector and residual error in the Z direction of the measuring points, the normal residual errors corresponding to the grinding points could be generated as well as the compensation grinding numerical control (NC) program. An 800-mm-diameter K9 mirror was ground to verify the proposed compensation grinding method. The profile error was reduced from 65 to 35 μm during the semi-finish grinding stage. By using the compensation grinding path, the profile accuracy was improved from 35 to 8 μm in the fine grinding stage. The proposed compensation method effectively improves the profile accuracy and manufacturing efficiency for grinding large-diameter aspheric mirrors.

Figure correction of a metallic ellipsoidal neutron focusing mirror.

An increasing number of neutron focusing mirrors is being adopted in neutron scattering experiments in order to provide high fluxes at sample positions, reduce measurement time, and/or increase statistical reliability. To realize a small focusing spot and high beam intensity, mirrors with both high form accuracy and low surface roughness are required. To achieve this, we propose a new figure correction technique to fabricate a two-dimensional neutron focusing mirror made with electroless nickel-phosphorus (NiP) by effectively combining ultraprecision shaper cutting and fine polishing. An arc envelope shaper cutting method is introduced to generate high form accuracy, while a fine polishing method, in which the material is removed effectively without losing profile accuracy, is developed to reduce the surface roughness of the mirror. High form accuracy in the minor-axis and the major-axis is obtained through tool profile error compensation and corrective polishing, respectively, and low surface roughness is acquired under a low polishing load. As a result, an ellipsoidal neutron focusing mirror is successfully fabricated with high form accuracy of 0.5 μm peak-to-valley and low surface roughness of 0.2 nm root-mean-square.

Profile error compensation in fast tool servo diamond turning of micro-structured surfaces

There is increasing use of ultra-precision components with micro-structured surfaces in a range of areas such as optical, biological, medical, metrology artifacts and data storage. Fast tool servo (FTS) diamond turning is superior in machining precision and complicated micro-structured surfaces with wavelength above tens of microns. However, the component errors in the FTS diamond turning machine cause profile error to the machined micro-structured surfaces. In this paper, the main sources of component errors and their effect on the profile accuracy of the machined micro-structured surfaces are identified and analyzed. Compensation methods are proposed to accordingly modify the tool path for each component error to pre-compensate the induced profile errors. Typical micro-structured surfaces, e.g. sinusoidal wave along radial direction, sinusoidal wave along x direction and micro-lens array, were machined to show the effect of the component errors on profile accuracy and the effectiveness of the proposed compensation methods.

Profile error compensation in ultra-precision grinding of aspheric surfaces with on-machine measurement

A compensation approach based on the on-machine measurement was developed for the grinding of tungsten carbide aspheric moulds. In this approach, the on-machine measurement was employed to eliminate the profile error caused by the re-installation of the workpiece. A new method was proposed to reconstruct the actual ground profile based on the measured profile data. The overall profile error after grinding was obtained by subtracting the target profile from the actual ground profile along normal direction and was then used to generate a new tool path for compensation grinding. The experimental results showed that after three compensation grinding cycles the aspheric surface had a profile accuracy of 177 nm (in PV) with a roughness of 1.7 nm (in R a ). The on-machine measurement was in excellent agreement with the off-machine measurement by commercially available profilometers.

Ultra-precision Single-point Grinding Technique and Profile Error Compensation Method for Machining Aspheric Mould

With the widespread application of various small aspheric optical components,high-precision manufacturing technology of forming molds is deeply demanded.However,the ultra-precision grinding technology of small aspheric moulds is still undeveloped in domestic research.An ultra-precision single-point grinding technique and profile error compensation mode of aspheric mould are studied.Through processing the measured data generated from contact on-machine measurement,the actual ground profile is reconstructed.Normal residual error is obtained through calculating the normal distance between actual ground profile and target profile,subsequently,a compensation method of profile error based on ultra-precision single-point inclined axis grinding is proposed.An experimental test is conducted to grind an aspheric tungsten carbide mould of aperture 6 mm,and the profile accuracy is gradually improved from peak to valley 449 nm to 182 nm after two-time compensation cycle.

Profile error compensation approaches for parallel nanogrinding of aspherical mould inserts

This paper reports a parallel nanogrinding protocol for fabricating aspherical mould inserts. In the protocol, a preliminary grinding cycle was first employed, which significantly reduced the effect of the initial wheel geometry error on the ground profile accuracy. A new compensation approach was developed. The compensation used the ground profile measured from a Talysurf profilometer to modify the NC tool path for the next grinding cycle. The tool path was modified by offsetting the residual profile error along the normal direction of a grinding point, rather than in the horizontal direction commonly used in the conventional method. The performance of the developed method was evaluated by fabricating aspherical mould inserts on cemented tungsten carbide. The result indicated that the residual profile error after two grinding cycles was reduced to approximately 0.4 μm in PV, with average surface roughness of below 10 nm, which is more efficient than the conventional compensation process.

非円形形状ＮＣ輸郭研削加工に関する研究 電気式サーボ機構による高速高精度加工の実現

High-speed and heavy-duty grinding of non-circular workpieces generally requires high rigidity of the overall mechanical system including the wheel feed system and the workpiece rotating system. Conventionally, hydraulic systems, such as the hydraulic copying mechanism and hydraulic servo mechanism, with which high power can be obtained relatively easily were mainly adopted as the drive system for highly rigid machine tools. However, electrical servo mechanisms capable of outputting high power are gradually being adopted by NC system. This paper discusses two problems; (1) improving in profile interpolation accuracy and (2) improving in servo error in high-speed grinding. Both problems are related to control technology and both will arise when an electrical servo mechanism is adopted for high-speed high-accuracy non-circular profile grinding. Solutions are offered for both problems. The solutions proposed by this paper are high speed, automatic generation of minimum linear interpolation data, the feed-forward control method, and the profile error compensation method. Prototype test equipment has been made to evaluate the feasibility of the solutions and to check how they can be adopted.

Electronically controllable primary feed for profile-error compensation of large parabolic reflectors

The paper shows that it is possible to compensate for the effects of mechanical profile errors in large parabolic reflectors, by means of a specially designed multielement feed array situated in the focal plane of the reflector. The factors relating the magnitude and period of these profile errors to the size and complexity of the compensating feed are discussed, and a novel method is described for achieving this compensation by controlling the phasing of the array. The system employs a spatial Fourier-transforming device, and its use is mainly restricted to long-period profile errors. Experimental results of this process, derived from a small-scale laboratory system employing a 1-dimensional 5ft parabolic reflector incorporating an 8-element feed, are presented. These results demonstrate compensation for profile errors up to π/2 radians. The possible application of this technique to existing and new reflectors is discussed.