Study On The Key Technologies Of Fewer-Axis Arc Envelope Grinding For Large Aspherical Surfaces

Large-scale optical components are widely used in the important projects of na-tional defense and scientific researches.The current difficulties in our process chain are the low form accuracy and large subsurface damage(SSD)in the grinding process and the poor form accuracy convergence and low material removal rate of the computer control optical surfacing(CCOS)technology based on small polishing pad tool.Thus,it's important to improve the form accuracy and the subsurface quality under the high material removal rate in the grinding process.Arc envelope grinding method(AEGM)is commonly used in grinding large-scale mirrors and greatly increases the wheel life and the machining accuracy.As the grinding point shifts on the arc-shaped wheel,a 3-axis machine tool is qualified to manufacture aspherical or off-axis aspherical surfaces.Compared with 5-axis single-point grinding,the whole wheel surface is used in AEGM to match with the varying normal vectors on the target surface,which increases the grinding system stiffness,improves the form accuracy and suppresses the grinding chatter.However,this also makes the calculation of the wheel profile wear and the compensation of the wheel profile error more com-plicated.Moreover,the wheel-workpiece contact area is parabolic surface.The distri-bution of the undeformed chip thickness(UCT)in the wheel-workpiece contact area is important to analyze the surface/subsurface damage and the grinding force on the brittle materials.In the previous studies,the majority of works have been conducted on paral-lel grinding with disk-shaped wheels.Without proper modelling of the grinding-in-duced damage in the arc-envelope grinding,the approach of high material removal rate with low subsurface damage cannot be fully understood and well designed.Therefore,a set of algorithms are designed for simulation,measurement and com-pensation of the aspherical surface form error.And an algorithm based on the calcula-tion of the UCT distribution is proposed to predict surface roughness(SR)/SSD and analyze the grinding force,which reveals the damage formation in the toric grinding.In addition,an online force-controlled grinding strategy is proposed for large-inertia and long-time-delay grinding machine to achieve constant low-frequency normal grind-ing force by self-adjusting feeding velocity through force feedback and force prediction.The main works and contributions are listed as follow:(1)Based on the path planning by AEGM on T~3-configuration machine tools and the calculation of the wheel-workpiece contact area,a set of algorithms for simulation and compensation of aspherical surface form error are proposed.The mainly considered error sources include wheel geometrical errors,tilt angle errors of spindle axis,wheel profile error and wheel profile wear.A 600mm-scale aspherical SiC surface with par-axial curvature radius of 1500mm is taken as an example for analyzing the form error in the grinding process.And the wheel profile error and the wheel profile wear are the major sources of the form error.Combined with the method of measuring and calculat-ing the aspherical surface form error and compensation method with 3-axis linked mo-tion,the decrease of RMS error from 1.44?m to 0.9?m was achieved on an 80mm-scale SiC workpiece with target paraxial curvature radius of 325mm,and the decrease of RMS error from 13.7?m to 1.8?m was achieved on a 580mm-scale glass workpiece with target paraxial curvature radius of 50m.It can be proved that fewer-axis arc enve-lope grinding method with high-stiffness machine tool can significantly increase the efficiency of the compensation process.(2)Based on the segmentation of the wheel-workpiece contact area,the distribu-tion of the undeformed chip thickness in parallel/cross grinding mode is calculated by the material removal rate.And the UCT distribution is revised based on the normal distribution of the grit protrusion height.Combined with the relationship between grit scratching depth and median/lateral crack depth,an analytical model for SR/SSD is established.With the normal force data of grinding SiC ceramic,the parameters in the proposed specific grinding energy function on a single grit can be identified.And the normal force on a single grit can be analyzed.The analysis provides a better under-standing of material removal mechanism and damage formation in the grinding process.The material removal mechanism is co-existing of ductile and brittle removal mode and brittle removal mode is the main part.Moreover,larger wheel-workpiece contact area is better for generating lower SSD and cross-grinding mode with lower feeding velocity and deeper depth of cut is recommended.(3)An on-line force controlled grinding strategy is proposed for a large-inertia and large-time-delay grinding system.The prediction of the normal grinding force is estab-lished based on the specific grinding energy function on a single grit and the UCT dis-tribution in the wheel-workpiece contact area.The model parameters can be identified by the experimental data of a single grinding path and thus monitoring of the grinding status can be realized.The overshoot of the grinding force can be suppressed by feeding predicted force into the controller and the overshoot drops from 87.5%to below 24%.In conclusion,in order to achieve the targets of high form accuracy and low SSD under high material removal rate in grinding large aspherical surfaces of brittle materi-als,a set of compensation algorithms for improving the form error,an analytical model for SR/SSD and grinding force and an online force-controlled grinding strategy are proposed.More importantly,this thesis presents a method for designing grinding pro-cesses or even special purpose machine tool for manufacturing large-scale optical com-ponents with large curvature radius.