Reserch On The Hole Machining Of Silicon Nitride Ceramics

In the fields of chemical,pharmaceutical and aerospace,harsh conditions such as high temperature and strong corrosion are widespread.Higher requirements of ball valve and bearing are put forward.Engineering ceramics which have the properties of high hardness,corrosion resistance and low deformation coefficient in high temperature,are suitable for the above parts,and have been widely used.However,owing to the high hardness and high brittleness of ceramic materials,it is difficult to be machined,especially in the processing of hole,which seriously affects popularization of engineering ceramic parts.In this study,gas pressure sintered silicon nitride ceramics was choosed as experimental material.Firstly,the silicon nitride ceramics was drill-grinded by braze diamond thin-wall drill,and the effects of drilling pressure,rotational speed and tool wall-thickness on the thickness of the outlet edge collapse were analyzed.The results show that the edge collapse thickness gradually grow with the increase of drilling pressure,gradually decrease with the rise of rotational speed,and slightly increase with the rise of tool wall-thickness.Secondly,a model for simulating edge collapse thickness was established by finite element software ABAQUS.The influence of drilling pressure and tool wall thickness on edge collapsing thickness was studied.And comparison between the simulation results and the experimental results shows that the relative errors are 7.1% and 5.0%.These are in the allowable range,and the model was proved to be reasonable.The auxiliary support method was proposed to reduce the edge collapse defects.The finite element model was used to predict the edge collapse thickness by using the auxiliary support,and then the actual edge collapse thickness was obtained by experiment.The simulation and experimental results show that the edge collapse thickness can be greatly reduced by the auxiliary support.Then,the silicon nitride ceramics were turned with PCD tools.The removal mechanisms were studied,which include brittle removal and ductile removal,and the brittle removal is the main way.The influences of various parameters on cutting force were studied,and the results show that the back force Fy is greater than the feed force Fx and the main cutting force Fz.The influence of rake angle is not obvious,and that the cutting force slightly increases with the rise of cutting speed,gradually increases with the rise of cutting depth and feed rate.The back force increases rapidly with the rise of cutting depth.Finally,the influences of various parameters on surface roughness were studied.The results show that the tool rake angle has little influence on the surface roughness,and that the surface roughness decreases first and then increases with the rise of cutting speed and cutting depth,gradually increases with the rise of feed rate.According to the influence degree on surface roughness,the order of four parameters in descending order is that feed rate,cutting depth,cutting speed,rake angle.