| Gear,as an important and fundamental component for transmission of motion and power,has been applied in many fields,such as high-end machine tool,automobile,wind power,aircraft carrier and so on.The precision of gear manufacturing seriously affects the transmission performance and quality of the transmission device since the precision gear is a vital component for the transmission device to meet the requirement of high loading capacity,high-efficiency,long-life and high-reliability.Accordingly,it has always been the focus of the scholars and enterprises,which makes it the driving force of the gear research.In recent years,with the rapid growth of the gear performance demands,new requirements have been put forward to the precision and efficiency of the machine tool.Thus the CNC gear manufacturing is developing in the direction of high efficiency,low cost and high precision.Hobbing,as a highly efficient and low-cost process which plays an important role in gear manufacturing,is widely used for machining spur gear,helical gear,chain gear,spline and worm gear.In recent decades,with the development of tool material and electronic technology,the design and manufacturing capability of hobbing machine tool has been greatly improved,and especially the stiffness and ability of CNC hobbing machine tools,which makes it possible to develop high speed,high precision and hard tooth surface hobbing technology.It leads the hard tooth surface gear after hobbing to be used as the final product in the transmission device,so the requirement of hobbing accuracy is getting higher.The precision of present hobbing technology is almost range from rank 7 to rank 9?national standard?,which is not precise enough,so it is always used as rough machining or semi-finish machining.Should the precision level of the hobbing process be improved to rank 5?national standard?,which can meet the precision requirements of most automobile gear reducer,the hobbing process would be used to replace the grinding process partly since it has obvious advantages in the efficiency and cost.Through this process,the cost is reduced and the efficiency is improved in the premise of ensuring the gear machining precision.From above,majoring this problem has important research significance and engineering value for promoting the technological progress of gear manufacturing industry.The precision of gear manufacturing is affected by the multi-source error which is consist of machine tool,hob cutter and work piece,and of which the error produced by machine tool?including geometric error and motion error?,hob gear?including profile error,pitch error and helix error?and the change of cutting force generated during cutting process takes up most[1].This paper,focused on the hobbing CNC machine tool,research on the error of machine tool,the geometric error of hob gear and the error produced by cutting force.A model to mapping the relationship between geometric error of the hob cutter and the geometric error of gear is set up,and based on the model,an error model of the gear generated by the coupled error of machine tool and hob cutter is proposed.The research on the gear error produced by the change of cutting force which considers the error of the machine tool and the geometric error of the hob cutter has done and then the machine parameters are optimized to reduce the geometric error of gear generated by cutting force.An algorithm to identify and compensate the error of the machine tool is proposed to reduce the error of machine tool and the influence of the geometric error of the hob cutter to the geometric error of gear and finally to reach the improvement of the precision of gear machining.Specific content is as follow:?1?The multi-to-multi comprehensive mapping rule of the geometric error of hob cutter and that of the gear is modeled.In the hobbing process,the cutting tool contact with the gear directly,and the error of machine tool,the error of the cutter and the error of the cutting force are all transferred to the gear tooth surface through the hob cutter,so the research on the multi-to-multi comprehensive mapping rule of the geometric error of hob cutter and that of the gear is the foundation of the multi-source error modeling of the hobbing.Based on the gear meshing theory,the mapping rule between the hob cutter profile error and the gear profile error is modeled.Based on the enveloping theory,the multi-to-multi comprehensive mapping rule between the hob cutter geometric error and gear geometric error is established.The geometric error of hob cutter and gear includes the profile error,the helical error and the pitch error and the geometric error of gear also includes the profile error,the helical error and the pitch error.The evaluation of the gear geometric error is proposed to verify the gear geometric errors affected by the hob cutter.This chapter is the theoretical foundation for the modeling of multiple source errors in the following chapter.?2?Model of the coupled error of machine tool and hob cutter.The error model of geometric posture and geometric motion of machine tool is analyzed and established,the mapping rule of the gear geometric error and the hob geometric error which coupled with the machine tool error is established.Considering actual machining process,hobbing is divided into two processes:pre-processing preparation and the envelope movement during machining.During the pre-processing preparation,the cutting tool move to the tip comp by changing the X?Y?Z and A axis of the machine tool,in which process the position and direction of the cutter's axes are only affected by the geometric posture and geometric motion of machine tool.This process affects the final position and direction of the axis of hob cutter and that of the work piece.What's more,envelope movement is the actual machining process,and during which,the gear geometric error is affected by the geometric posture accuracy of machine tool and motion error of machine tool and geometric error of the cutting tool.This process is influenced by the coupled error the axis error of Z and C axis,the error of the motion and the geometric error of the hon cutter.In this part,the motion of part 1 is substituted into the transformation matrix of the axis error of the hob cutter to establish the geometric error model of gear produced by the coupled error the machine tool and the hob gear.?3?Modeling and optimization of gear geometric error generated by hobbing cutting force.Based on the machining theory and homogeneous coordinate transformation equations,and by considering the area of chip equals to the area swept by the hobbing tool,build the model for cutting chip area affected by multi-source which includes the error of the machine tool,error of the cutting tool.Based on the finite element and experimental models of cutting force,cutting force coefficient per unit area is solved.Besides that,the stiffness model of the machining system is established to calculate the deformation of machining system produced by the cutting force.The deformation of the whole workpiece caused by the cutting force through the finite element and hammer excitation method is modeled.Research on the elastic recoil mechanism of the gear surface after machining has also been considered to determine the final geometric error of the gear through calculation of the value of elastic recoil.Based on the improved particle swarm optimization?PSO?&neural network algorithm,an accuracy prediction model of hobbing processing parameters and gear geometric error is established,and the reduced of the gear geometric error generated by the cutting force through the optimization of machining processing parameters is achieved using this model.?4?Research on the identification and compensation for NC machine tool.Based on laser interferometer measurement,a method to trace the motion error of machine tool's linear axis is proposed.Investigating the cone trajectory of the ball bar,a method to identify the error of the motion error of the machine tool's rotating axis is established.Considering the multi-source error,and based on the inverse kinematics method of the homogeneous coordinate transformation,the inverse decoupling model between the data of cutting tool location and the motion values of servo axis of the machine tool is proposed.In the working space,the direction value of the cutting tool location is substituted into the inverse decoupling equation and the motion control value of each servo axis of the machine tool is obtained.The compensation value of each axis's pitch is set up to realize the multi-source error compensation and the correctness of the theory is verified through compensation experiment.In this paper,through the research on the error of numerically-controlled hobbing machine tool,the geometric error of hob cutter and the error generated by the cutting force,the mapping rule of multi-source error of the machine tool to the geometric error of gear is revealed.Focused on the error generated by the cutting force,the optimization model of the processing parameters is proposed and the machining error of the hobbing reduces effectively.It provides theoretical support for the precision improvement of numerically-controlled hobbing process.In additionally,it has important engineering significance and practical value for the research and development of high efficiency and low cost hobbing process and precision evolution mechanism of numerical hobbing machine tool. |
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