Grinding Mechanism Of BK7 Optical Glass Using Engineered Wheels

Due to its excellent mechanical and chemical properties,BK7 optical glass is often used in fabrication of optical component in many advanced fields.But the high hardness and brittleness which can lead to microcracks or even brittle fractures makes it extreme challenging for machining of optical glass.In recent years,engineered grinding wheel has receive much attention due to its pristine grinding performance.Engineered grinding wheel is a special type of monolayer grinding wheel,whose grit distribution strictly follows the optimized result of kinematic simulation.However,such high performance and well-designed grinding tools have little application in grinding of brittle materials because the kinematic simulation model is not yet capable of describing and predicting the complicated behaviors of brittle materials.To investigate the influence between grits in grinding optical glass and to provide theoretical support for grit pattern design,a minimal grit distribution unit was extracted,which classifies the interaction sequence of grits as "successive" and "simultaneous".Meanwhile,a new grit distribution description function is proposed to describe the randomness in grit distribution on different grinding wheels.A new kinematic simulation model which is capable of calculating and predicting the residual stress field is proposed.The result of the kinematic simulation shows that the grit arrange angle has significant influence on the grinding performance of the engineered grinding wheel.When the grit arrange angle increases,the ground surface roughness and the residual stress strength continue to decrease.But the grinding force reach its lowest point when the arrange angle is 60 degree.Successive double scratch tests were conducted in brittle regime and the transition phase between brittle and ductile regime.In brittle regime,it was discovered that the influence from nearby scratch can result in further propagation of cracks and deeper subsurface damage.During the successive double scratch in the ductile brittle transition stage,it was discovered that the residual stress from the 1st scratch can lead to premature lateral cracks but can suppressed the median crack of the second scratch too.Since both of these crack behaviors can lower the depth of subsurface damage,it is recommended that grits can be arranged to interact with the surface one by one in close range to utilize the benefit from residual stress field.A double tip tool was first fabricated by FIB micro milling for the simultaneous double scratch test.The result of the double scratch test shows that the strengthening effect of scratch stress field will result in lateral crack and median crack in the middle zone instead of inflicted two separated crack systems.As the separation distance between the double tip increases,the special crack system in the middle zone gradually disappeared and two separated crack systems were found.Therefore,it is not recommended that grits are arranged to interact with the surface simultaneously in close range.To verify the result of the kinematic simulation,a new brazing technique that involved laser selective cleaning of the dielectrical layer was utilized to fabricate the engineered grinding wheel with different distribution parameters.The engineered grinding wheels were then used to conduct surface grinding on BK7 optical glass.The result of the grinding test verified the result of the simulation as well as the grit distribution design from the double scratch tests.The analysis of the undeformed chip thickness in kinematic simulation shows that the increasing arrange angle leads to a more even and smaller undeformed chip thickness.As a result,the grinding wheel with bigger arrange angle has a better grinding performance and a more robust performance under the influence of random deviation of grit position.