Recent progress of chatter prediction, detection and suppression in milling

An efficient decomposition-condensation method for chatter prediction in milling large-scale thin-walled structures

Self-optimizing machining systems

Excessive static deformations often occur during the end-milling of large thin-walled parts, which play an important role in the aerospace industry. The real-time adjustment of axial cutting depth (ACD) is often adopted to compensate deformation errors. Owing to the coupling effect between the ACD and the force induced deformation, the iterative adjustment of ACD is needed. However, it may not converge with the limited adjustment times especially in high-speed machining with large ACD. We hereby propose an approach to speed up the convergence of the ACD iterative adjustment. Firstly, the real-time full compensation process, which is commonly used in the existing real-time compensation methods, is mathematically analyzed and proved to be only first-order convergent. Then, an approach based on the secant method, which provides a higher order of convergence, is developed and introduced into the real-time compensation process for large thin-walled plate machining. Finally, the superiority and effectiveness of the proposed approach are demonstrated through an ACD iterative adjustment simulation and an experiment of cutting square pockets.

An accelerated convergence approach for real-time deformation compensation in large thin-walled parts machining

Curved contour on large thin-walled skin is difficult to be machined since severe deformations usually occur on real skin. It is critical to determine the real geometry of the deformed contour. Under the assumption that only the normal deformation of the large thin-walled skin occurs and the shear deformation is ignored, a novel isometric mapping based adaptive machining method is developed. While straightness distance or local angle is preserved in traditional surface mapping methods, arc-length is preserved in proposed isometric mapping to improve the contour cutting accuracy. The proposed method consists of three steps. (1) The real geometry of the deformed surface is obtained by using a laser scanner based on-machine measurement (OMM) system. The measured point cloud is then transformed into triangular mesh to represent the deformed surface. Some points on the nominal surface are sampled based on uniform sampling strategy, and the corresponding sampling points in the deformed surface are also allocated. (2) Isometric mapping between the two groups of points is constructed. Matching accuracy between nominal surface and real surface is defined based on the deviation of the geodesic distances of the two groups of points. A surface matching optimization model is developed to adjust the mapping point location in the triangular mesh and achieve a minimum surface mismatch error. (3) The toolpath is adaptively adjusted to compensate the deformation error based on the isometric surface mapping results. Both simulation and machining experiments are conducted to demonstrate the feasibility and validity of the proposed method. The experiment results show that accuracy of the machined curved contour on the deformed skin has been significantly improved.

Adaptive machining for curved contour on deformed large skin based on on-machine measurement and isometric mapping

Force-induced deformation is an inevitable phenomenon in thin-wall milling operations, which makes the actual radial depth of cut deviate from its nominal value and changes tool-part engagement angle boundaries. This paper presents an accurate modeling of dynamic milling system with the force-induced deformation effect. Both the thin-walled part and the cutter are discretized into differential elements, such that the influences of the force-induced deformation and multi-point contact structure dynamics can be simultaneously considered at the contact zones. A detail flexible iteration strategy is first presented to calculate the force-induced deformation and the resulting tool-part engagement angle boundaries. After that, time-varying multi-modal dynamic parameters with actual material removal effect are obtained by simultaneously modifying structural static and dynamic stiffness at each tool feed position. The system involving regenerative dynamic displacements is formulated as a matrix of time-periodic delay differential equation. Chatter stability of the system is predicted by an extended second order semi-discretization method. Milling experiments are conducted to validate the proposed approach, and two types of parts with different wall thickness are designed. The results show that chatter can be well predicted by using the proposed approach.

Shanglei Jiang

Papers

Predictive modeling of chatter stability considering force-induced deformation effect in milling thin-walled parts