Grinding Of Optical Glass Research Articles

Crack Forms Sensitivity-Based Prediction on Subsurface Cracks Depth in Ultrasonic-Vibration-Assisted Grinding of Optical Glasses

Subsurface cracks in ultrasonic-vibration-assisted grinding (UVAG) of optical glasses often exhibit diverse forms and proportions. Due to the variety of loads involved in crack formation and propagation, the crack forms and propagation depths have different sensitivities to each process parameter. Predicting the maximum subsurface cracks depth (MSSCD) by considering the varying effects of process parameters plays a key role in implementing effective control of the UVAG process. In this work, the subsurface crack forms and their proportions are investigated by conducting 40 sets of UVAG experiments. The varying effects of the grinding and ultrasonic parameters on the crack form proportions are unveiled by using grey relational analysis. The weighted least square support vector machine (WLS-SVM) prediction model for the MSSCD was developed. Twelve sets of UVAG experiments were carried out to validate the proposed model. The results show that arc-shaped cracks and bifurcated cracks account for 72.5% of all cracks, while ultrasonic vibration amplitude influences most of the proportions of arc-shaped and bifurcated cracks. Compared to other widely used prediction methods, the maximum and average relative prediction errors of the proposed model are 10.54% and 5.59%, respectively, which proves the high prediction accuracy of the model.

Effects of local strain rate and temperature on the workpiece subsurface damage in grinding of optical glass

Subsurface damage (SSD) generated during the grinding process directly affects the optical quality and the service life of optical components. It is of great significance to accurately predict and effectively control the depth of the SSD layer of the ground workpiece. However, most of the existing SSD depth prediction models are based on quasi-static and room temperature conditions. This paper presented a prediction model of SSD depth, which considered the combined effect of local strain rate and temperature on the SSD generation during grinding process. The variation of strain and strain rate in the local shear zone of an average abrasive grit on the grinding wheel was analyzed, considering the dynamic density of abrasive grits; Strain rate effect of the material was described by introducing a dynamic fracture toughness KID, which would increase with the strain rate; Grinding zone temperature and especially the temperature on the abrasive grits were analyzed. Combining the strain rate effect and temperature effect, a thermodynamic fracture toughness KIDT model was further developed. On the basis of classic brittle material crack system and the kinematics of grinding, the SSD prediction model considering the dynamic variation of mechanical properties in grinding process was derived. The effects of local strain rate and temperature in grinding of BK7 optical glass were systematically studied under different grinding wheel speeds. The predicted SSD values using a conventional static model and current models considering the strain rate or grinding temperature effect or both, were compared with the measured values. The predicted SSD values using the theoretical model including both the strain rate and grinding temperature effects, agree well with the measured values. The proposed model therefore is anticipated to be meaningful to understand of the SSD of hard and brittle materials involved in a precision grinding process.

Experimental investigation of subsurface damage of optical glass in precision grinding using a brittle material removal fraction

Our previous works presented experimental investigations of the brittle material removal fraction (BRF) on the machined surface of optical glass. It is related to the area fraction of brittle material removed on the machined surface and is used to assess the surface roughness of optical glass during precision grinding. In general, subsurface damage (SSD) is considered to be one of the most important machining qualities in grinding of optical glass. This work presents an experimental investigation of the relationship between BRF and SSD depth of optical glass during precision grinding to understand the significance of BRF on sensing the SSD of optical glass. A series of precision grinding experiments were conducted on a precision grinder machine to investigate the influence of grinding parameters on SSD depth. Based on an image processing algorithm developed in previous work, the BRF was calculated by using a microtopography image of a machined surface of K9 glass during precision grinding. The experimental results show that the BRF was determined for different machining parameters (cutting depth, wheel speed, and feed rate) and was dependent on SSD depth in precision grinding, which implies that the BRF has potential for application as a rapid and affordable online vision measurement approach to monitor surface and subsurface qualities during the grinding of brittle materials.

Mechanism underlying formation of SSC in optical glass due to dynamic impact of single diamond scratch

During the grinding of optical glass, the abrasion directly affects the morphology and depth of subsurface cracks (SSC). The effect of dynamic impact of grinding abrasives on optical glass is an important issue in the field of optics manufacturing. In this work, a single diamond scratch was used to grind optical glass, and grinding parameters were collaboratively controlled to ensure that the cutting layer remained constant. A dynamometer was used to record the duration of the impact process, and the cross-section of the test piece was polished for scanning electron microscopy (SEM) to determine the depth of the SSCs. The experimental results show that as wheel speed increases, SSC depth tends to decrease. When the wheel speed gradually increases from 500 r/min to 2500 r/min, the probability distribution curve for the maximum SSC depth shifts downward by around 80 μm. The effect of the dynamic impact of single diamond scratch is found to be an important cause of SSC formation in optical glass during grinding, i.e., the faster the grinding, the shallower the SSCs.

Experiment Research on Grinding of Optical Glass with Indigenously Developed Monolayer Brazed Diamond Grinding Wheel

Conventional grinding of optical glass will normally result in not only much lower material removal rate but also short service life of grinding tools, which causes the high fabrication cost of optical glass. This paper focuses on the precision grinding of optical glass with the indigenously developed monolayer brazed diamond grinding wheel. Before grinding process a precision dressing is conduct. The dressed grinding wheel topography is observed. Through precision dressing the grinding process achieves moderate surface finish as well as ensures certain grinding efficiency. The ground surface of the optical glass workpiece is made up of micro-pits and glazing zones and the obtained surface roughness variation with the process parameters is analyzed. The experiment shows that the developed diamond grinding wheel is suitable to the application of optical glass machining.

ELID Assisted Grinding of Optical Glass with Fine and Coarse Grained Copper-Resin Bonded Diamond Wheels

In this paper, a copper-resin bonded diamond wheel was applied to machine the optical glass on a precision grinder. The process of truing and pre-dressing with ELID (electrolytic in-process dressing) were first carried out for the grinding wheels, then the ELID assisted grinding experiments were conducted with the special fine and coarse grained diamond wheels. The experimental results show that the fine and coarse grained wheels can all generated the smooth surface with the surface roughness in nanometer scale and the coarse grained diamond wheel correlates to the slightly more surface damage than the fine grained diamond wheel, which also proves that the high efficient grinding of the optical glass with a good surface quality can be realized with the coarse grained copper-resin wheel on a precision grinder and the grinding wheels were all well conditioned with the conditioning method presented in this paper.

Surface and Sub-Surface Integrity of Ultra- Machined BK7 Using Fine and Coarse Grained Diamond Wheels

This paper aims to evaluate the surface and sub-surface integrity of optical glasses which were correspondingly machined by coarse and fine-grained diamond grinding wheels on Tetraform ‘C’ and Nanotech 500FG. The experimental results show that coarse-grained diamond grinding wheels are capable of ductile grinding of optical glasses with high surface and sub-surface integrity. The surface roughness values are all in nanometer scale and the sub-surface damages are around several micros in depth, which is comparative to those machined by fine-grained diamond wheels.

Effect of Cutting-Edge Shape on Ductile Regime Grinding of Optical Glass in Single-Grit Diamond Grinding

Effect of Cutting-Edge Shape on Ductile Regime Grinding of Optical Glass in Single-Grit Diamond Grinding

Relationship between subsurface damage depth and surface roughness during grinding of optical glass with diamond tools

Relationship between subsurface damage depth and surface roughness during grinding of optical glass with diamond tools