Research On Cutting Force And Profile Error For Ball-end Milling Of Hardened Steel Molds

With the development of high speed cutting and hard cutting technology, the hardened steel, which has the characteristics of high hardness, high toughness and high wear resistance, can be machined by milling instead of grinding. And with the advantages of high precision and high efficiency, milling process has become the main technology of automobile panel mold processing. Because of the large size structure, complex surface and frequently changing of curvature, high surface quality and machining accuracy, the manufacturing enterprises of automobile panel mold are facing great challenges to control the cutting process and the machining accuracy of mold with free form surface. Due to the changeable directions of the cutting forces and the weak rigidity of the process system, it is easy to result in decreased stability,machined surface quality and accuracy, and the reduction of cutter life in ball-end milling of hardened steel molds with free form surface by three-axis NC machine tool. Therefore, it has an important significance to improve the manufacturing technology of automobile panel mold and the development of the processing technology of the free form surface for investigating the cutting force, the cutter deflection, milling stability, profile error and compensation in ball-end milling of hardened steel with free form surface.The instantaneous cutting forces are unstable, as the changing curvatures and inclination angles, in ball-end milling of hardened steel with free form surface. The variation of cutting forces could result in the cutter deflection and the decreasing of the cutting stability. In addition, it is difficult to meet the requirements of free-form surface morphology and machining accuracy. Therefore, in view of these difficulties,this thesis will focus on the milling forces and profile errors in ball-end milling of hardened steel with the main contents as follow:In the light of the values and the directions of cutting forces changing with the curvature of the workpiece and the tool inclination angle, the curvature of the workpiece is illustrated as the curvature on feed direction and on the cross feed direction, and the tool inclination angle is illustrated as the lead angle and the tilt angle. According to the geometry of the cutting edge of the ball-end milling cutter,the actual three dimensional trochoidal tooth trajectory of the cutting edge is modeled. Based on the trajectory, the model of uncut chip thickness is modified and the effects of the curvature of the workpiece and the tool inclination angle on the uncut chip thickness are obtained. The experiments of ball-end milling of free form surface are conducted to validate the proposed method for predicting the instantaneous milling force, which could be the basis for investigating the cutter deflection and milling dynamic.Based on the change rules of the instantaneous milling force of ball-end milling of free form surface, the finite element method is adopted to analyze the cutter deflection, to obtain the distribution of the deformation of the cutting edge.According to the three dimensional trochoidal tooth trajectory of the cutting edge, the uncut chip thickness is modeled with considering of the deformation along the cutting edge to investigate the effect of the deformation on the uncut chip thickness.Subsequently, the model of flexible milling force for ball-end milling of curved surface is established by analyze the effect of the deformation along the cutting edge on the cutter-workpiece engagement. The proposed model is validated by the experiments and provides the theoretical basis of deflection error.Based on the three dimensional trochoidal tooth trajectory of the cutting edge,two corresponding points on the prior chip surface and the rear chip surface are illustrated. The curvature of the free form surface results in the uncut chip thickness,determined by the distance between the two corresponding points, changing along with the cutting edge. The uncut chip thickness model with considering of the varying time-delay parameters is established by analyzing the time-delay between the time of two adjacent cutting edges pass by two corresponding points on the prior chip surface and the rear chip surface respectively. The ball-end milling dynamic model of free form surface is established by analyzing the effects of the curvature and tool inclination on the varying time-delay and the regeneration dynamic chip thickness. A combination of full-discretization method and numerical method is used to predict the stability of low radial immersion ball-end milling with varying time-delay parameters. The effects of the inclination angle and curvature radius on the milling stability are obtained and validated by the experiments.The profile error is made up of programming errors and machining errors in ball-end milling of free form surface. The distribution of the profile error is revealed by investigating of the deflection error, residual height and chord error. The experiments of the deflection error and surface topography are conducted to validate the model of the deflection error and the residual height. Based on the distribution characteristics of various surface characteristics, the profile errors of different areas with various surface characteristics are compensated by adopting the integrated optimization.