Research On The Bending Spring-back Of Thin-walled Tube And The Control System Of Tube Bending Machine

Tube has been widely applied in fields of aerospace, aviation and automobile because of its light weight, high strength, precision, high efficiency and some other advantages. However, the quality of bended-tube is determined by the spring-back. Spring-back is an inevitable phenomenon when the bending pipe is unloaded. Spring-back seriously affects the accuracy and production efficiency of tube. When the spring-back exceeds the permissible error, the part becomes defect ware. So spring-back is not a problem that exists in the industry, but also it is a hotspot that the academia has been paying attention to for a long time. For the moment, researches and applications of tube bending equipment and processing technology of our country lag behind the world's advanced level. In order to meet the need of the development of the society, especially to meet the quick rising need of the bended-tube in the industry such as automobile and shipping. It is necessary to enhance manufacture level and the automation of our country's tube bending and also the quality of the bended-tube.First, this paper describes the status quo of the traditional tube bending technics and tube bending equipment. It summarizes the status quo and development trend of spring-back theoretical study based on a great deal of overseas and domestic literature information. And it expounds the key technologies of numerical simulation theory. It points out that tube bending deformation belongs to the plastic deformation,which needs to be solved with the theories of plastic. The paper also introduces large deformation elastic-plastic finite element theory, V.Mises Yield Guidelines, Flow Guidelines and Hardening Guidelines. And especially describes the essential connection of flexibility and plastic and correlative formulas.The plastic deformation theory for thin-walled tube bending is studied. The appropriate elastic-plastic finite element simulation model is also established. When tube is bended to plastic deformation, elastic deformation always exists. When the load is removed, the elastic deformation will come back immediately. As a result, the bending angle and bending radius of the bended-tube will change, which are different with the design dimensions, this phenomenon is called spring-back of bended-tube, which is one of the defects of tube forming. In the field of material forming, spring-back is a key factor to be considered. The final shape of the tube depends on the spring-back, when the spring-back exceeds the permissible error, it affects the geometric accuracy of the tube. So spring-back is an important factor that affects the mold and product quality. In this paper, the author analyses the stress and the strain of thin-walled tube under bending condition, and deduces an approximate calculation formula to represent the spring-back angle, based on the spring-back theory.When the tube is bended, unload the load, if the spring-back is not small, then the actual shape of the workpiece is different with the wanted shape. The difference may result to failure of the manufacture. How to control and compensate the spring-back is an important task. In the process of tube bending, so many factors affect the spring-back and they are very complicated. Also in different manufacture conditions, with the same process parameters, the spring-back may different. The paper expounds the techniques to control spring-back, and points out that the traditional techniques that to control spring-back are Over-bending, Die compensation, Stretch Bending and Pressure Correction Method. Just coming through long-term practice, we know that the same passel tubes, when they are bended at the appropriate angle of bending, the relationship between bending angle and spring-back angle is linear. The result is very important, which is an important fact to control spring-back. The scheme design of spring-back compensating should base on the principle that theoretical analysis and engineering practice are combined. So just do as it is, the spring-back can be solved.Bases on the spring-back rule during tube bending, the control system of the tube bending machine with the spring-back-compensated function is designed, which is feasible to compensate spring-back. The paper expounds the function and characteristic of tube bending machine, and also the principle. The control system is a feasible control system based on industry PC and Windows XP system. The paper also expounds the hardware of the control system. Industrial PC-7484 as a hardware interface board is selected to complete the data acquisition, data output control and other functions. As for software system, In order to realize the good man-machine interface, VB is made use of to program the software system, we can set and amend technologic parameter handily, save data and display the running state of tube bending machine. But VB does not have the capacity to access to I/O interface directly. In this system, Dynamic Link Libraries (DLL) programmed by VC is made use of to solve the problem of accessing input-output interface. The experimental results show that the development of CNC tube-bending machine's control system not only meets actual production requirements, but also improves the efficiency of production and processing quality.