Experimental and Analytical eva‎luation of the Surface and Sub-surface Characteristics in the Grinding Process of Aspheric Lenses

4/20/2025 12:00:00 AM

BK7 optical glasses are extensively used in the high-power laser equipment and aviation industry due to their excellent mechanical and optical properties. Fused silica, on the other hand, is widely used in lenses and other optical elements in UV regions thanks to its low thermal expansion coefficient and remarkable optical properties. However, due to these materials' brittle and hard nature, grinding is one of the most common machining methods. As a result of the dominant mechanism of fracture in removing brittle materials, surface and subsurface defects are often caused. It is possible to reduce or even eliminate defects beneath the surface during the grinding process by predicting the subsurface damage depth. This dissertation explores the impact of pertinent parameters on the depth of subsurface damage and surface roughness caused by grinding flat BK7 elements. In particular, the effect of the direction of movement of the grinding wheel under different contact conditions, including angular, vertical, and face grinding, considering the maximum undeformed chip thickness, the depth of subsurface damage, and the surface roughness has been investigated using theoretical equations and experimental results. According to the results, there is a maximum deviation of 10% in the subsurface damage depth and 15.61% in the estimation of surface roughness. Non-linear relationships for the studied parameters are presented based on experimental eva‎luations. The difference between the relationships and the measured results is 4.4% for the depth of subsurface damage and 10% for the surface roughness and the maximum undeformed chip thickness. A precision grinding machine with four degrees of freedom and a cup grinding wheel was utilized to investigate the effect of feed speed and rotational speed ratio on profile errors and the waviness of spherical fused silica elements during aspherical grinding. The study employed machine learning methods to analyze the grinding process under semi-finishing and finishing conditions. Accurately predicting workpiece profile errors can help correct manufacturing errors. eva‎luating the impact of input parameters on surface waviness errors is a suitable way to optimize the production process. Based on the literature review, it is worth noting that this is the first time that profile error prediction has been performed. A full factorial arrangement was used with three levels of feed speed and rotational speed ratio to determine the sensitivity of modeling and predictions to these variables. The form errors were fitted to profile errors using the trigonometric Fourier series model. The eva‎luations indicated that the rotational speed ratio and the interaction between the feed speed and the rotational speed ratio significantly influenced the workpiece form error. In semi-finishing grinding, reducing the feed speed has decreased profile errors and the surface waviness of spherical elements. Minimum waviness and profile errors were achieved regardless of the feed speed at the rotational speed ratio 50.25. An ensemble learning regression framework was utilized with three experimental datasets to train a model to predict the profile errors. This model was validated using data not part of the training phase. The model's accuracy was then tested using six new datasets of profile errors. The model was found to predict the error of residual profiles significantly, with R-squared values ranging from 0.740 to 0.980 in the grinding tests under finishing conditions. Apart from eva‎luating the surface characteristics of spherical elements, the subsurface damage depth and the surface roughness were also assessed under finishing grinding conditions. A literature review has revealed that the impact of subsurface damages on spherical elements has not yet been studied. Analytical relationships were developed to calculate surface roughness and subsurface damage depth based on grinding process conditions. These relationships were then compared to experimental results. The experiment results were consistent with the predicted values for surface roughness and depth of subsurface damage, which were based on the apex angle of the abrasive particles. The apex angle ranged from 46 to 62 degrees. The maximum deviation of the experimental results from the predicted value for the apex angle of 62 degrees was less than 12%. Based on the test results, the effect of feed speed in the tested range is negligible. A 1 mm/min feed speed is the most suitable parameter because it produces the highest material removal rate. The grinding tests for finishing conditions achieved the best surface roughness and the lowest subsurface damage depth at a rotational speed ratio of 50.50. The difference in subsurface damage depth was lower between the rotational speed ratio of 50.25 and 50.50 compared to the difference in profile error and waviness in similar conditions. Therefore, the most suitable conditions for grinding among the grinding ranges in semi-finishing and finishing conditions were a feed speed of 1 mm/min and a rotational speed ratio of 50.25.

فيوزد سيليكا , BK7 , سنگ‌زني المان‌هاي تخت , سنگ‌زني المان‌هاي كروي , چرخ سنگ كاسه‌اي , عيوب سطحي و زيرسطحي

Bk7 , Fused Silica , Grinding Flat Elements , Grinding Spherical Elements , Cup Grinding Wheel , Surface and Sub-Surface Defects

Gholamali Nasr

Behnam Davoodi