Research On The Technique Of Fast Milling Aspheric Surface

With the development of science and technology,aspheric surface has been widely used in various fields due to its unique optical properties.In the field of national defense such as space system,researchers often need to expand the aperture of the lens to improve the performance of the system.However,there are many problems in processing large aperture aspheric surfaces,such as long cycle,poor machine tool stability and high cost.As a commonly used method of milling spherical surface,generation method can use long arc line to mill,which can greatly improve processing efficiency.Although some scholars have already proposed the theory of using it to process aspheric surface with specific parameters,there are few concrete process studies and engineering applications.Therefore,this article will start the research on the rapid milling of aspheric surfaces by generation method.For the non-specific parameter aspheric surface,the efficiency of the ball head grinding wheel is also studied.The detailed content is as follows:1.For the kinematic chain of the HAAS UMC-750 five-axis machine tool,the homogeneous coordinate transformation matrix is used to solve the conversion relationship between the machine coordinate system,the tool coordinate system and the workpiece coordinate system.A feed rate control algorithm is also proposed to maintain a relatively stable feed rate while using generation method to machine workpieces.Finally,the testing instrument and the index of evaluating surface quality are introduced.2.In order to apply the generation method to five axis machine tools,the transformation model between tool contact and tool location is firstly established.Secondly,two grinding wheel processing paths are planned,namely,equal-pitch and variable-pitch methods.The theoretical efficiency of the two paths is represented by the discrete points on the aspheric generatrix to compare with efficiency of the end milling method.Then,the process parameters of the equal-pitch method in the finishing stage are optimized through the experiments of grinding depth,feed speed,and grinding wheel speed.Finally,the experiment shows that the optimized generation method only takes 12 minutes to mill the aspheric surface with a certain parameter,while the end milling method takes 85 minutes to complete the milling.It is verified that the generation method can effectively improve the machining efficiency.3.In order to get rid of the generation method’s requirement for the wheel diameter,the outer edge grinding mode and the composite grinding mode by small wheel are proposed.Through comparing the discrete points of the two modes,it can be seen that the theoretical machining efficiency of the composite grinding mode is better than the outer edge grinding mode.In order to realize the composite grinding mode,firstly,the conversion model between tool contact and tool position is established,and then the process parameters are optimized.Finally,the experiment shows this method spend only 6 minutes milling the aspheric surface with a certain parameter,while the end milling method spend 88 minutes.It is proved that the composite grinding mode can quickly mill aspheric surfaces with specific parameters and is more universal.In order to get rid of generation method’s requirement for aspheric parameters,a grinding method with ball wheel is proposed.Firstly,the vector transformation relationship between the tool position and the tool contact is solved,and then the process parameters are optimized.Finally,the ball head grinding wheel grinding mode can be completed in only 26 minutes when milling aspheric surface with a certain parameter,while the end milling mode needs 78 minutes.This experiment shows the ball head wheel with large milling surface can also process quickly.At the same time,in order to make the application of the above method simple,a special aspheric machining software is written in MATLAB,and its correctness is verified in the process experiment.4.The possible error sources of generation machining are analyzed,and the axis offset error,axis inclination error,grinding wheel measurement error,workpiece assembly and adjustment error are explored.Firstly,the influence of axis offset error on aspheric surface is studied theoretically.Secondly,the influence of axis inclination error and wheel measurement error on aspheric surface is studied experimentally.Then,through theoretical analysis,the workpiece assembly and adjustment error will not affect the surface shape accuracy.Finally,it is summarized that machining error of generation method mainly reflects in the curvature radius error,Therefore,the curvature radius compensation method is proposed to reduce the aspheric error.After comparing with existing coordinate error compensation methods through experiments,it is proved that the curvature radius compensation method can effectively reduce the error,and is relatively simple.