Micro-analysis model for material removal mechanisms of bonnet polishing.

There have been many researches concerning the modeling for material removal mechanisms of bonnet polishing (BP) based on the well-known Preston model. However, various parameters involved in the BP process are not formulated and considered in the classical model, such as slurry characteristics, pad properties, bonnet features, and processing conditions. In this paper, a micro-analysis model capturing those parameters is proposed based on the mutual interaction of the slurry, pad, and workpiece among the BP interfaces with the micro-contact theory and the tribology theory. The proposed model is validated by comparison with the experimental data, and good agreement can be obtained. According to the analysis of key parameters, the proposed model is capable of providing some insight into the material removal mechanisms of BP, and even those cannot be explained properly by the classical Preston model.

Improved analysis model for material removal mechanisms of bonnet polishing incorporating the pad wear effect.

Bonnet polishing technology has been widely applied in precision optical machining. Until now, most of the research concerning the modeling for material removal mechanisms of bonnet polishing have been presented based on the well-known Preston model. However, the various parameters involved in the bonnet polishing process are not formulated into that model, such as slurry characteristics, pad properties, bonnet sizes, processing conditions, etc. Recently, several analysis models capturing those various parameters have been developed and are even capable of interpreting non-Prestonian behaviors, but the pad wear effect has still not been taken into account. Hence, the purpose of this paper is to establish an improved analysis model by incorporating the pad wear effect with the cumulative polishing time. Compared with the previous analysis model and Preston model, the predicted results of the improved analysis model are much closer to the experimental data and become more acceptable. According to the analysis of key parameters, the understanding of material removal mechanisms in bonnet polishing is further completed, and the time-dependent pad wear effect should no longer be neglected.

Dwell-time algorithm for polishing large optics.

The calculation of the dwell time plays a crucial role in polishing precision large optics. Although some studies have taken place, it remains a challenge to develop a calculation algorithm which is absolutely stable, together with a high convergence ratio and fast solution speed even for extremely large mirrors. For this aim, we introduced a self-adaptive iterative algorithm to calculate the dwell time in this paper. Simulations were conducted in bonnet polishing (BP) to test the performance of this method on a real 430 mm × 430 mm fused silica part with the initial surface error PV=1741.29 nm, RMS=433.204 nm. The final surface residual error in the clear aperture after two simulation steps turned out to be PV=11.7 nm, RMS=0.5 nm. The results confirm that this method is stable and has a high convergence ratio and fast solution speed even with an ordinary computer. It is notable that the solution time is usually just a few seconds even on a 1000 mm × 1000 mm part. Hence, we believe that this method is perfectly suitable for polishing large optics. And not only can it be applied to BP, but it can also be applied to other subaperture deterministic polishing processes.

Tentative investigation towards precision polishing of optical components with ultrasonically vibrating bound-abrasive pellets.

Ultrasonic vibration has been employed to improve the quality of machined surface in the grinding of brittle materials. In this report, we transplant the philosophy of ultrasonic vibration assisted grinding to chemo-mechanical bound-abrasive-pellet polishing in anticipation of the improvement in either surface roughness or material removal rate. The preliminary experimental results show that the ultrasonic vibration assisted chemo-mechanical pellet polishing can yield desired results that material removal rate can be significantly raised while surface roughness is not degraded. The experimental results also indicate different mechanisms between ultrasonic-vibration-assisted chemo-mechanical pellet polishing and conventional chemo-mechanical bound-abrasive polishing.

A method for evaluating subsurface damage in optical glass.

An alternative method for evaluating subsurface damage (SSD) in ground fused silica is presented. The method can acquire the knowledge of depth and morphology of subsurface damage at the same time. The fundamental support lent to the method is the fact that the depth of field reduces as the numerical aperture (NA)/magnification increases in optical microscopes. Large depth of field without undermining NA is preferred in most applications while the narrow range of focus depth is desired for our method. Using this method, we experimented on fused silica which was ground with bound-abrasive diamond wheels and the results show good agreement with the traditional method. The consistency indicates that the proposed method is practicable and effective in inspecting the subsurface damage in optical components.

The characteristics of optics polished with a polyurethane pad.

The effect of polishing an optical workpiece with a polyurethane pad was studied in this paper, including material removal rate, surface roughness and subsurface damage. Usually, optical polishing pitch is applied to polish optical workpieces, but the material removal rate (MRR) of pitch is quite low, and polyurethane foam is thus substituted for polishing pitch. With the polyurethane pad a much higher MRR was obtained. Surface roughness and subsurface damage of workpieces were also examined. We were gratified to find that there was almost no subsurface damage in the workpieces manufactured with pad polishing and surface roughness was comparable to the result of pitch polishing. Finally, a hypothesis was proposed in an attempt to explain the result that workpieces were defect-free.

Pressure and velocity dependence of the material removal rate in the fast polishing process.

Based on the direct contact between the wafer and the pad, the pressure and velocity dependence of the material removal rate (MRR) in the fast polishing process (FPP) is investigated. There are three assumptions of the FPP material removal mechanism: the normal distribution of abrasive size, a periodic roughness of the pad surface, and the plastic contact between wafer-abrasive and pad-abrasive interfaces. Based on the particular FPP, a novel movement of the wafer is analyzed and a MRR equation is developed. The experiments with parameters of pressure and velocity are shown to verify the equation. Thus, a better understanding of the fundamental mechanism involved in FPP can be obtained.