Ultrasonic Assisted Grinding Mechanisms Of Space Reflector Materials

For larger spatial reflectors,ceramic glass,quartz glass and SiC ceramics are usually considered optimal materials.During its several manufacturing phases such as sintering,rough machining,precise machining and surface coating,the rough machining,which includes the machining of weight lightening structures on the back,endures most of material removal volume.It is quite time consuming for conventional grinding(CG)as well as cost inefficient,hence ultrasonic assisted grinding(UAG)could be a more ideal choice due to its lower grinding force,higher material removal rate(MRR),and acceptable surface quality and subsurface damage.It is vital to achieve optimized UAG parameters specifically for the machining of spatial reflectors.Currently the research on UAG results appears to be rather dispersed,and it is not convenient to achieve solid comprehend on the method.The predicting models of UAG still lacks verification on the level of single grain behavior,and the calculations rely on regression analysis which does not reflect the basic mechanics enough.Hence the math of UAG usually fails to adapt to different conditions.In this dissertation,the content is lay out as:(a)experimental study of ultrasonic assisted scratching(UAS);(b)modelling of single grain cutting behavior;(c)simulation of UAG;(d)optimization of UAG on reflector specimen.The following conclusions are made:(1)UAS tests are conducted using the established equipment.With Vickers indenter as the tool and RB-SiC,ceramic glass as material,a specific experimental platform is established with adequate precision.The initial engage time was determine with the help of the power correlation deduced by Lawn-Evans-Marshall(LEM)model.A software is developed to merge all topography data for each groove with additional function of statistic purpose.Comparing to conventional scratching(CS),the scratching force of UAS declined,while the MRR is far beyond that of CS.(2)A novel model of single grain cutting behavior on brittle materials is established.From further analysis of UAS experimental results,it was concluded by the author that while UAS tended to be more efficient in material removal than CS in order of magnitude,but the specific energy of both UAS and CS tended to remain in similar scale.Hence,the advantage of UAS is attributed to the extra input energy brought by the ultrasonic vibration towards the scratched material,which according to Griffith’s law,should results in larger scaled cracking.With the help of ultrasonic energy correction factor ξU and the mechanism of material rebound,the model succeeds in picturing the coupling relationships between vibrational parameters and scratching parameters.The prediction of scratching force appeared to be accurate while the MRR tended to be smaller than experimental results.Calculated results of the model showed that there is CS-continuous UAS-noncontinuous UAS boundaries,which resembles the boundaries indicated by engagement time ratio.(3)The end-face UAG of brittle material is simulated for the first time.It is worth mentioning that Cellular Automata was used to help surface emerging and shortened the computing immensely.The simulated grinding force was accurate in both value and curve shape,according to the experimental verification.The average cutting depth of the grains tended to be several times smaller than the cutting depth of grinding;the cutting force distribution on the grinding tool varies greatly as the machining process proceeds;the ground surface tended to hit the lowest height at the route of inner-circle’s of the grinding tool,etc.According to the simulated results,although UAG controls the grinding force,but during similar MRR UAG would induce slightly deeper subsurface damage.(4)The simulation is used to optimize the UAG process parameters.On a quartz glass specimen,a set of optimized parameters were proposed on the basis of empirical parameters.Both the grinding force and the subsurface damage were predicted by simulation for the purpose of controlling.The specimen was then damaged to testify if the subsurface damage would exceed the predicted value,which did not.The optimized parameters shortened the machining time over half while achieved smaller subsurface damage,hence improved the efficiency with adequate manufacturing quality.