| The ball screw mechanisms are important components of CNC machine tools.The performance of ball screws directly affects the processing quality and production efficiency of machine tools and is closely related to the national manufacturing industry.In the past ten years,ball screws manufactured by the national key enterprises have made significant progress in rigidity and comprehensive property.However,compared with foreign high-end ball screws,there is still a clear gap in precision retaining ability.This is a serious problem that makes it difficult to apply national ball screws in high precision CNC machine tools.Through the research on CNC machine tool enterprises and a large number of reliability experiments carried out by the organizations which have been undertaken major national science and technology projects,wear is one of the reasons for the poor precision retaining ability.Wear increases the rate of precision loss and therefore affects the recognition of products.Solving this important problem is the key to making national products quality the same or higher than the quality of foreign high-end products.This paper is based on the major national science and technology project named The Capacity Building and Generic Technology Research of Measuring and Testing of Rolling Components(2016ZX04004007).The theoretical and experimental research on precision loss characteristics and lapping technology were carried out.First,it is necessary to propose a calculation method of precision loss.The precision loss reflects the process of precision changes over time.Therefore,the study of precision retaining ability can be based on the loss of precision.The precision loss model of a ball screw needs to be established and the precision loss trend of a ball screw can be researched.The other point is to develop a modified ball screw lapping method.The aim of this study is to research how to produce ball screws with higher precision and better surface quality in terms of manufacturing process.This research will provide important technical support for further application of lapping.The works carried out are as follows:(1)Based on the previous studies,the precision loss model of a ball screw was established and a calculation method of precision loss of the ball screw was proposed.The model is used to calculate the precision loss caused by adhesive wear considering the kinetic and force characteristics of the ball,the screw and the nut and the frictional characteristics on the contact areas.The preloaded force of the ball screw will decrease because of wear;therefore,the change of preloaded force was also considered.(2)According to the precision loss model,the effect of structure parameters and working parameters were calculated and analyzed.This kind of analysis can provide the theoretical basis for optimal design of ball screws.Besides,experimental results of precision loss change versus time were obtained from a test machine specifically designed for ball screw precision variation measurement.The calculation results obtained from the model approximately agree with the experimental results.(3)In order to improve the lapping effect,increase efficiency and change the present labor-intensive situation,a modified screw lapping method was proposed in this study.A new lapping tool was designed.It is based on the theory of cylindrical lapping.Compared with the conventional lapping tool,the new lapping tool can solve the problem that the centers of the workpiece and the lapping tool do not coincide.Experiments were conducted to verify the feasibility of the new method.The experimental results showed that the travel variation and the residual compressive stress were improved.(4)A set of orthogonal experiments were designed to capture the effects of four main lapping factors: friction torque,particulate size,lapping time and the rotational speed of the screw.The travel variation,surface roughness,and residual stress were used to evaluate the lapping effect.According to the analysis of the experimental results,a set of optimal parameter combination was obtained.The optimal parameter combination was used to conduct a verification experiment.The results showed that the balance among the travel variation,surface roughness,and residual stress can be obtained by using the combination and therefore,the parameter combination is indeed the optimal combination.(5)Based on the optimal parameter combination,the screw lapped and the screw not lapped were used to carried out a set of precision loss experiments.The friction torque and travel variation of the two ball screws at each stage were measured and compared.The precision loss values of the two ball screws were also calculated in order to analyze the effect of lapping on precision retaining ability.The results showed that lapping can not only improve the travel variation of a ball screw,but also improve the ability of the ball screw to maintain its precision. |
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