Study On Influence Mechanisms Of Clamping Force On End Milling Deflection For Sheet Metal Part And Deflection Compensation Method

The thin wall parts with light-weight and low-rigidity are found to easily deform during the machining process. In particular, if the end-milling and point supports are adopted, the final deflection of the workpiece is affected by its initial deflection, milling deflection and rebound deflection. In this paper, the sheet metal parts of aluminum alloy 6061 are investigated. Combining the milling experiment and numerical simulation, the mechanism and the rule of the effect of the clamping force on the final deflection are analyzed. Furthermore, the pre-deforming schemes are proposed and applied to compensate the deflection of the workpiece. This research work provides a theoretical basis and is meaningful for the end-milling of the low-rigid workpieces with the point supports.The surface residual stress of the end-milled sheet metal parts with point supports is studied. The results indicate that the surface residual stresses are approximately distributed in two-dimensional plane stress state and the residual stress at different location of the machined surface is different. The average value of the surface residual stress parallel to or perpendicular to the feed direction increases if the clamping points move towards the workpiece center. Compared with the clamping positions, the magnitudes of the clamping forces will slightly influence the surface residual stress. The distribution of the subsurface residual stresses in the depth direction is also studied when the clamping positions and the magnitude of the clamping forces are changed. The results show that the distribution of the subsurface residual stress in depth direction remains the same tendency wherever the clamping forces locate. Meanwhile, if the clamping positions move towards the central area of the workpiece, the depth of the maximum compressive stress at the midpoint of the machined surface increases. Compared with the clamping positions, the influence of the clamping forces on the depth of the subsurface maximum residual compressive stress along the thick direction is unremarkable. The influence mechanisms of the clamping positions on the surface residual stresses and the subsurface residual stresses are analyzed. If the clamping positions move towards the workpiece central area, the upper surface compressive stress caused by the workpiece deflection in machining process decreases. As a result, the mean value of the compressive residual stress on the machined surface increases. Meanwhile, if the clamping positions move towards the workpiece central area, the machining forces in the central area of the workpiece increases and the depth of the maximum residual compressive stress at the midpoint of the machined surface increases.Adopting the elastic contact model between the workpiece and the fixture, a two-dimension simulation model for the end-milled and point supported sheet metal part is established. The influence of the magnitudes and positions of the clamping forces on the distribution of the internal stresses of the point supported and end milled workpiece is analyzed as well. The results show that the distribution of the internal stress is non-uniform after the point supported sheet-metal part is machined and released from the fixture. The influence of the clamping forces on the internal stress distribution is significant. If the clamping positions locate at the workpiece ends, the stresses at both ends along the center line of the workpiece are larger than those in the center. If the clamping points move towards the workpiece center along the feed direction, the largest values of the internal stresses move towards the center of the workpiece. If the clamping points keep moving towards the center of the workpiece, the internal stress at the machining end of the workpiece increases. If the clamping forces magnitudes are only increased and the clamping positions are unchanged, the mean value of the internal stresses at the center line of workpiece increases firstly and then decreases, but the change in amplitude is smaller than the mean value of the internal stresses at the center line of workpiece. The influence mechanism of the magnitude and locations of the clamping forces on the internal stresses of the workpiece is analyzed. The position of overlapping area of the clamping deflection zone and the elastic deflection zone in front of the tool blade depends upon the clamping position. Meanwhile, the stress distribution of the overlapping area depends upon the magnitude of the clamping forces. As a result, the distribution of the internal stress inside the workpiece changes as well.The influence rule of the clamping forces on the workpiece milling deflection or the rebound deflection is studied. The formulas are respectively established to relate the clamping forces and the workpiece milling deflection or the workpiece rebound deflection. And then the location and magnitude of the clamping force are predicted for the case of the minimum milling deflection and minimum rebound deflection, respectively. Including the initial deflection and the influence of the clamping forces, a formula is established to predict the final deflection of the milled workpiece.A calculation method of the clamping forces for pre-bending the end milling sheet metal parts is proposed. In this method, the deflection compensation is realized by adjusting the clamping forces to pre-bend the sheet-metal parts. A calculation method of the pre-tensioned stress for pre-tensioned fixation deflection compensation of the end-milling sheet-metal parts is also proposed, where the pre-tensioned fixation stress is adjusted to increase the rebound deflection of the workpiece. And then, the final deflection of the milled workpiece decreases.