High Efficient Precision Grinding Of Optical Glasses With The Coarse-grained Diamond Wheel

High-precision and high-efficiency grinding of optical glasses has become animportant development orientation for the national optical industry and major projects,such as "Shenguang Ⅲ". Generally, fine-grained diamond wheels (resin/metal bonded)have been used for precision/ultra-precision grinding of optical glasses. But the frequentwheel truing process greatly reduces the machining efficiency. Coarse-grained diamondwheels can implement a better surface accuracy machining with the greater abrasionresistance and the larger grinding ratio. However, efficient precision truing is verycritical to achieve precision grinding. In this paper, the coarse-grained electroplateddiamond wheel is used to grind BK7, fused quartz, fused silica glasses. In order torealize the high-precision and high-efficiency machining, the research works are mainlycarried out as following:By single diamond grit scratching test for BK7optical glasses, wear state of thesharp and passivated diamond grits and plastic flow on the workpiece surface wereanalyzed. According to the measured normal grinding forces, the mechanical residualstress of the ground BK7glasses was simulated. The results showed that compared withthe sharp grain, the passivated grain had a greater abrasion resistance, producing alarger area of plastic flow region. And under the premise of plastic removal mode, withthe coarse-grained diamond wheels, the mechanical residual stress of the ground opticalglasses surface was lower. These results lay a theoretical foundation for the applicationof the coarse-grained diamond wheels in the field of precision grinding optical glasses.The efficient precision truing method for coarse-grained diamond wheels wasproposed, and the abrasive wear morphology and Raman spectra analysis revealed thetruing mechanism. Firstly, according to the simulation results, Cr12steel was chosen forrough truing of the electroplated diamond wheel, with no coolant. The gathering heatspeeded up the diamond wear rate, making the wheel run-out error quickly reduced tobelow10μm. Graphite and C60generated on the wheel surface, as well as the diamondabrasives were worn in the form of passivation, oxidation and diffusion wear, with lightmicro-broken. Secondly, the cup-shaped diamond wheel was applied to precision truingof the electroplated diamond wheel. The injecting cooling liquid reduced the wheelthermal deformation. The abrasives were worn mainly in the form of abrasion wear dueto thermal stress, incidentally with a little micro fracture. Eventually, the wheel run-outerror and the axial gradient error could respectively drop to within5μm and3μm.Optical glasses were surface ground by the trued electroplated diamond wheel. Itwas found that the ground surface roughness Ra value was less than25nm, and sub- surface crack depth was about2μm. Then, the electroplated diamond wheel, thefine-grained resin bonded and metal bonded diamond wheels were respectively used togrind large-sized BK7glasses. The workpiece quality inspection results showed that forthe electroplated diamond wheel, the ground surface roughness was the highest.However the ground PV values were respectively2.28μm,4.17μm parallel andperpendicular to the direction of grinding traces, significantly less than the other twofine-grained wheels.Based on the newly proposed measurement method for wheel wear volume, thegrinding ratio of coarse-grained and fine-grained diamond wheels was compared. Theresults showed that during grinding large-sized BK7glasses using the coarse-graineddiamond wheel, the grinding ratio was up to about350, being50-70times higher thanfine-grained diamond wheels. Evidently the coarse-grained diamond wheel wasprovided with extremely high grinding ratio, which helped to reduce the wheel wear rate,and greatly improve the processing efficiency.With the multi-sensors (force sensor and acoustic emission sensor) integratedmonitoring technology, the impact of the machining parameters and ground quality onthe force and acoustic emission (AE) signals were investigated. Results showed that, thehigher material hardness, single grit un-deformed chip thickness and the wheel-workpiece contact area were, the greater normal grinding force was. And when theductile removal capacity and the wheel loading velocity relative to workpiece weregreater, AE signal was stronger. In addition, wavelet packet transform method was toextraction frequency band signal corresponding to the wheel wear. Consequently, duringthe truing and grinding process, the solved AE signal characteristics threshold couldprovide technical support for achieving the purpose of monitoring wheel dressing andgrinding.