Research On Ball Screw Whirlwind Hard Milling Thermal Deformation Error And Its Control Tochnology

High speed and hard whirling ball screw (above HRC60) is a new high efficiency and precision machining technique. The work piece machining can be finished by only once milling without cutting fluid. The milling efficiency could be5to8times than grinding method, so it has been widely used in foreign rolling components processing industry. Some domestic manufacturers also have purchased the whirlwind hard milling machine. However, there is a great difference between domestic and foreign ball screw precision, mainly due to the following aspects:First, the lack of a whirlwind hard milling mechanism and experimental studies; Second, there are differences in the workpiece material between domestic and foreign manufacturers,and there are no reference data on the workpiece thermal deformation error controlling. These reasons largely restricted to improve the accuracy of screw whirlwind hard milling. It is an urgent problem to be solved in the current rolling features enterprise that how to control the thermal deformation error of the whirlwind hard milling and further improve the precision of large ball screws.A lot of cutting heat is generated in the contact area of tool and the workpiece, which leads to the deformation of the workpiece. Based on this, this paper analyzes the research status of the high-speed cutting heat and points out that the current study focused on continuous cutting. However, there is still insufficient in the study of the forming tool’s high-speed intermittent cutting. In order to control thermal deformation error of the whirlwind hard milling, establish a parameterized cutting heat simulation model and thermal deformation simulation model,and verify the correctness of the model through the temperature distribution experiment; establish a thermal deformation prediction model of BP neural network, study on the thermal deformation error compensation method, achieve the error compensation based on thermal deformation prediction model,improve the accuracy of the large pitch precision ball screw machining. The main contents are as follows:1. Investigate the whirlwind hard milling technology, such as process parameters, workpiece material, tool material, workpiece clamping way, the motion mode of the workpiece and tool and so on. Analysis the dynamic and static factors that affecting whirlwind hard milling precision, point out that the mainly factor affecting the cumulative pitch error is thermal deformation of workpiece section to be machined. Simultaneously, the workpiece pitch error curve analysis shows that pitch error forming factors also include workpiece’s thermal deformation causede by frictional heat of the workpiece and hydraulic suspension bracket and the elastic deformation caused by the fall and rise of hydraulic suspension bracket.2. Analysis the principle of the whirlwind hard milling, establish a parameterized whirlwind hard milling simulation model in consideration of the workpiece and the process parameters effecting on the milling heat. This model is different from the previous2D or3D orthogonal cutting simulation models, considering the formed cutter and the changes of cutting thickness in the processing. Calculate the temperature of the tool, workpiece and cutting heat distribution ratio under different parts models and different process parameters when PCBN tools whirlwind milling hardened bearing steel, optimize process parameters and lay the load foundation for further analysis of temperature field and thermal deformation.3. Establish workpiece heat conduction equation based on Fourier’s law, analyze the dynamic changes of the workpiece temperature distribution under the effect of moving heat source, on this basis, analyze thermal structure coupling to obtain the thermal deformation of the workpiece. Deformation of the workpiece is caused by cutting heat which moves along the workpiece helix and enables the workpiece temperature to rise. Temperature distribution of the workpiece is changing in time and space during processing, establish workpiece heat conduction equation firstly, establish a parameterized finite element simulation model, determine the boundary conditions, study on the dynamic temperature distribution of the workpiece, especially of segment to be processed; analyze the impact of parameters on the temperature distribution of the workpiece.Measure the axial distribution of the workpiece temperature by TI400infrared thermometer to verify the simulation results. Obtain thermal elongation variation of the workpiece segment to be processed by thermal structure coupling in ANSYS. Point out to divide thermal elongation into dynamic stage and quasi-steady stage.4. For prediction of whirlwind hard milling ball screw thermal elongation law, extract thermal elongation law eigenvalues of the segment to be machined, establish the prediction model based on BP neural network algorithm, and train network by studied thermal elongation data. For thermal elongation error compensation problem, compensate error by pitch compensation function of CNC system for the sample workpiece. For workpiece of new parameters, predict characteristic value of the thermal deformation curves with BP neural network algorithm, fit deformation curves based on characteristic value, and compensate the thermal deformation according to fitting curve.Whirlwind hard milling thermal deformation error compensation results show that the compensation control method makes the ball screw pitch accuracy improved significantly.Research methods and results of this study can be generalized to other types of process control, which provide theoretical and methodological reference for machining error control during the process of precision machining and have an important reference to promote the further development of precision machinery manufacturing technology.