KDP (potassium dihydrogen phosphate) is often chosen as a frequency doubling component for lasers because of its strong nonlinear optical properties and its possession of a high laser damage threshold. However, due to its inherent physical properties, KDP crystals are not easy to machine. In this paper, we try to polish KDP using fixed abrasive polishing technique. Copper oxide, carbon powder and phenolic resin are selected for the preparation of pellets for polishing. Orthogonal tests on the workpiece without destroying the KDP and the best process parameters were decided for the lowest roughness. The thermodynamic simulation is also used to analyze the heat of the polishing process to ensure that no thermal damage occurs to the workpiece during the machining process.
During the process of ultra-precision diamond fly-cutting machine tool, due to internal heat sources, the thermal deformation of various parts of the machine tool has severely restricted the accuracy of the machine tool. Therefore, in order to improve the accuracy of the machine, it is necessary to study the mechanism of ultra-precision diamond flycutting machine tool. First, an intelligent monitoring system is arranged for the fly-cutting machine tool to collect temperature signals. Then the heat generations at the spindle motor and linear motor are calculated, and the thermal boundary conditions of the machine tool are analyzed. Finally, the finite element model of the flying-cutting machine tool was established, and the thermal-structural coupling analysis and calculation were carried out.The results show that: under the action of the internal heat source, the maximum deformation of the machine tool in the cutting direction is 6.8μm, which occurs at the spindle motor. The maximum deformation of the tool tip is 1.02μm in the cutting direction. Eventually, the machine thermal error under the tool tip and the work bench is 1.42μm. This research provides a basis for the research of machine tool thermal compensation and the improvement of the accuracy of machine tool.
The research of CO2 laser smoothing fused silica to achieve smooth surface was investigated by simulations and experiments. Micro-flow smoothing of fused silica was numerically simulated. In the experiments, the influence of processing parameters, such as P (laser power), v (scanning velocity) and d (scanning path pitch) were taken into account on surface roughness (Ra) after laser irradiation. The results show that the roughness is rapidly reduced from 183.6nm to 14.27nm under P=35W, v=0.2mm/s, d=1.0mm, and thus the smooth surface is obtained. On the other hand, the raster structure will appear on the surface at inappropriate parameters (P=30W or v=0.5mm/s or d=2.0mm). The surface roughness highly influenced by raster structure ranges from ~40nm to ~140nm, 140.9nm for P=30W, 71.6nm for v=0.5mm/s, 41.3nm for d=2.0mm.
A sphere precessions polishing (SPP) method is proposed to perform the precessions polishing function like the bonnet polishing technique. A compliant inflated hollow ball is adopted as the sphere polishing tool, and three motors are used to collaboratively drive the sphere tool rotating around a desired axis tilting with a precessions angle. A prototype is developed to validate the technical feasibility and demonstrate the polishing removal functions. The polishing spots of vertical polishing show a typical W shape-like profile, and tilted polishing results reveal a D shape-like profile. This proposed SPP method can be regarded as a potential candidate technique to realize the precision precessions polishing in the optical precision engineering.
To overcome the randomness of free abrasive polishing, the abrasive waste, the resulting hydration layer and other issues, this paper presents the characteristic of bound abrasive polishing for fused silica glass in anhydrous environment. A stable sintering process with a fine grooved polishing wheel is developed, which is applied to polish the fused silica glass. Macroscopically, the influence of pressure and rotational speed on the removal rate and surface roughness is explored. Microscopically, the mechanism of material removal, the chemical reaction and the mechanical action produced by the removal process are expounded by using EDS, XRD and FTIR analysis. The removal rate does not follow the Preston formula, temperature rise and the capacity of chip removal on the polishing wheel are the key factors. Furthermore, the experimental results show that the Ce-O-Si bond is formed by CeO2 abrasive grains and fused silica under the action of normal force. The bond energy is larger than that of fused silica Si-O-Si bond, so the CeO2 tears away SiO2 out of the glass surface under the action of shear force.
To suppress the medium-high spatial frequency, error on optical surfaces is still a challenging work to date, and the tool path ripple (TPR) error is the main reason for these errors. With this in view, the effect of the tool influence function (TIF) shape of the semirigid (SR) bonnet to the TPR error is analyzed. The SR bonnet is a recently developed bonnet tool for high efficiency polishing. This tool can generate three kinds of TIF including Gaussian-like shape, trapezoidal shape, and “M” shape. Experimental studies have been conducted to analyze their effect to the root mean square/peak-to-valley value of the TPR error, and discussions have been made on those results. It is found that different shapes of TIF can be implemented through controlling its inflated pressure. The Gaussian-like shape has the highest probability to generate lower TPR error than the trapezoidal shape and “M” shape TIFs, which have been proven by the verification experiments.
Detection of the subsurface damage depth in optical elements has significance on the subsequent material removal amount and improving element surface quality. The paper focuses on the subsurface damage of chemical-mechanical polished K9 specimen, and analyses the chemical-mechanical polishing mechanism and the cause of subsurface damage. A most suitable etchant is chosen and the step-by-step etching method is applied to measure the subsurface damage depth. A microscope is used to detect the damage morphology and the variation trend at different depth. Research shows that the subsurface damage caused by chemical-mechanical polishing is Hertz scratch, and the scratch quantity below surface presents a variation of zero-more-less-disappeared. The K9 specimen is polished for 3 min under the pressure of 2.5 Kgf and the spindle speed of 43139 r/min, thus resulting in a subsurface damage depth 15.3μm.
This paper presents a semirigid (SR) bonnet tool which has the advantages of high efficiency and determinacy for material removal on optical elements and also has the potential to be used on aspheric optics. It consists of three layers: a metal sheet, a rubber membrane, and a polishing pad, from inside to outside. It inherits the flexibility of a normal bonnet but has a higher stiffness. Finite element analysis was performed to determine that the stainless steel is the best-suited material for use as the metal sheet. An SR bonnet with a stainless-steel metal sheet was fabricated and tested. Its tool influence function (TIF) is Gaussian-like, and the TIF stability is more than 90%. The peak-to-valley of its uniform removal area is less than 0.1λ. Tool ripples are highly depressed and the surface profile is well preserved in the prepolishing test. In 12 min, ∼36 mm3 of material is removed.
For extremely high accuracy optical elements, the residual error induced by the superposition of the tool influence function cannot be ignored and leads to medium-high frequency errors. Even though the continuous computer-controlled optical surfacing process is better than the discrete one, which can decrease this error to a certain degree, the error still exists in scanning directions when adopting the raster path. The purpose of this paper is to optimize the parameters used in bonnet polishing to restrain this error. The formation of this error was theoretically demonstrated and will also be further experimentally presented using our newly designed prototype. Orthogonal simulation experiments were designed for the following five major operating parameters (some of them are normalized) at four levels: inner pressure, z offset, raster distance, H-axis speed, and precession angle. The minimum residual error method was used to evaluate the simulations. The results showed the impact of the evaluated parameters on the residual error. The parameters in descending order of impact are as follows: raster distance, z offset, inner pressure, H-axis speed, and precession angle. An optimal combination of these five parameters among the four levels considered, based on the minimum residual error method, was determined.
An on-line condition monitoring and monitoring point position visual system for grinding machine is developed for
monitoring the grinding condition in a grinding operation. This system can set and observe the monitoring point position
in computer, measure the vibration and temperature of the grinding machine to judge the operation state. The
experiments indicate that the visual system described in this paper is more adequate than conventional on-line
monitoring techniques.
Ultrasonic vibration assisted machining with harder abrasives than the material to be machined can improve the quality
of machined surface and manufacturing efficiency. Therefore, we integrated ultrasonic vibration (UV) into a recently
developed technique chemo-mechanical bound-abrasive polishing in anticipation of further increasing the material
removal rate (MRR) and/or surface roughness. The preliminary results indicate that ultrasonic vibration assisted
chemo-mechanical bound-abrasive polishing can lead to increased material removal rate of manufactured optics while
leaving the surface roughness comparable to conventional chemo-mechanical bound-abrasive polishing. The great MRR
is attributed to the superiority of UV-assisted chemo-mechanical bound-abrasive polishing in discharging resultant swarf
during machining.
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