Research On Precision Assurance Key Technology Of Ultra-precision Grinding For Concave-convex Alternation Surface

Tungsten alloy widely used in nuclear industry,photoelectric manufacturing,military defense,aerospace,and other important fields because of its physical and chemical properties.As spallation neutron source and the fusion device experiment thorough development,the modern precision physics of tungsten alloy manufacturing put forward higher requirements.For example,a tungsten alloy target is composed of four off-axis pits superimposed by a rotating paraboloid to form a complex free-form surface with off-axis alternating characteristics,and the accuracy is better than 5μm.This complex surface requirement challenges the existing ultra-precision grinding technology and equipment.Firstly,in the grinding,the positioning accuracy,position tracking performance,and antidisturbance ability of the feed system can not meet the requirements,it is difficult to ensure the plastic removal of tungsten alloy material in the grinding process,which has a certain of impact on the surface quality and surface precision of parts.Besides,considering the difficulty of tungsten alloy material and the complexity of the target surface profile,the problem of high precision dressing and measurement of a spherical wheel in envelope grinding becomes more prominent.In addition,grinding can improve the machining accuracy through many iterations of multiple measurement compensation.Ultra-precision machine tools need an appropriate in-position measurement system of the workpiece,which can measure the in-position of machined parts,to facilitate the compensation machining of the complex curved surfaces.Focusing on the above problems,this paper research several key technologies for ultra-precision machining of tungsten alloy complex surface as a guide and carries out relevant research work with the ultimate goal of building a multi-axis ultraprecision grinding machine.The specific research contents are as follows.:Based on the machining demand of the tungsten alloy complex surface,the overall design of machine tools was carried out and the high sensitivity error factors on grinding accuracy were explored.Error sensitivity analysis takes two aspects of the machine tool geometric error and grinding wheel error for the machining accuracy.Firstly,the volumetric error model of the grinding machine tool was established by the homogeneous transformation matrix method,and the geometric errors of the grinding machine tool were measured by the practical used measurement methods.Then,the sensitivity analysis of the geometric errors of the grinding machine tool was adopted by the variance-based Sobol global sensitivity analysis method.Finally,the grinding wheel error of profile error,center height installation error,and tool setting error for machining accuracy have been analyzed.According to the actual condition,this paper proposed two methods of reducing the tracking error of the feeding system and improving the precision of the grinding wheel to ensure machining accuracy.The feeding system of the ultra-precision grinding machine is directly driven by a linear motor,and there is no transmission link in the middle,which is easy to be disturbed.A servo control method with linear extended state observer(LESO)based on 2 degrees of freedom(2DOF)PID controller is proposed.In the servo method,the LESO can observe both external disturbance and internal parameter perturbation,and the 2DOF PID controller is designed for the desired set-point tracking of the closed-loop system.In order to verify the effectiveness of the proposed method,four types of servo controllers with different control structures have been implemented on Matlab/Simulink software and ultra-precision grinding machine respectively.Experimental results show that the 2DOF-PID-LESO servo controller achieves the best tracking accuracy under the action of internal and external disturbances.In order to ensure the machining accuracy of the grinding machine,a set of in-position dressing and measuring methods for the spherical grinding wheel were put forward.Firstly,the in-situ dressing method of resin spherical diamond grinding wheel by using green silicon carbide cup grinding wheel as dressing wheel was proposed.Furthermore,this paper proposed a set of grinding wheel measurement systems based on machine vision and laser sensor.Therefore,a cross-scale in-position measuring method of grinding wheel profile was developed by combining the two above methods.Finally,the dynamic unbalances measurement method for the grinding spindle based on a disturbance observer was proposed.The workpiece in-position measuring device based on a strain gauge probe was developed,including creating a new named G-Code(RS-247)in the NC system and designing the data acquisition and control interface in IPC.Furthermore,the measuring complex surface workpiece methods including measuring path planning,probe radius compensation,and three-dimension data transformations from the Polar coordinate system to the Cartesian coordinate system were provided.The measurement accuracy of the workpiece in-position measurement system was 3.3 μm,and the repeated accuracy was 0.8μm.The machine tool body,auxiliary system,and numerical control system of the ultraprecision grinding machine were designed and implemented.A multi-axis ultra-precision grinding machine was developed,which integrates grind/turning processing,grinding wheel dressing,in-site grinding wheel measurement,and in-site workpiece measurement.Further,to verify the machining accuracy of the machine tool,turning processings were carried out on the end face and cylinder test pieces with single-point diamond tools.The measurement results showed that the machining accuracy of the machine tool was better than 0.3μm within 60 mm of the X-axis and better than 1.1μm within 100 mm of the Z-axis.The surface profile accuracy of the grinding samples reached 4.4μm,which met the machining requirements.