The paper reports on a new integrated methodology for modelling and prediction of surface errors caused by deflection during machining of low-rigidity components. The proposed approach is based on identifying and modelling key processing characteristics that influence part deflection, predicting the workpiece deflection through an adaptive flexible theoretical force-FEA deflection model and providing an input for downstream decision making on error compensation. A new analytical flexible force model suitable for static machining error prediction of low-rigidity components is proposed. The model is based on an extended perfect plastic layer model integrated with a FE model for prediction of part deflection. At each computational step, the flexible force is calculated by taking into account the changes of the immersion angles of the engaged teeth. The material removal process at any infinitesimal segment of the milling cutter teeth is considered as oblique cutting, for which the cutting force is calculated using an orthogonal–oblique transformation. This study aims to increase the understanding of the causes of poor geometric accuracy by considering the impact of the machining forces on the deflection of thin-wall structures. The reported work is a part of an ongoing research for developing an adaptive machining planning environment for surface error modelling and prediction and selection of process and tool path parameters for rapid machining of complex low-rigidity high-accuracy parts.

Milling error prediction and compensation in machining of low-rigidity parts

The rigid body motion of the workpieces and their elastic-plastic deformations induced during high speed milling of thin-walled parts are the main root causes of part geometrical and dimensional variabilities; these are governed mainly from the choice of process plan parameters such as fixture layout design, operation sequence, selected tool path strategies and the values of cutting variables. Therefore, it becomes necessary to judge the validity of a given process plan before going into actual machining. This paper presents an overview of a comprehensive finite element method (FEM) based milling process plan verification model and associated tools, which by considering the effects of fixturing, operation sequence, tool path and cutting parameters simulates the milling process in a transient 3D virtual environment and predicts the part thin wall deflections and elastic-plastic deformations during machining. The advantages of the proposed model over previous works are: (i) Performs a computationally efficient transient thermo-mechanical coupled field milling simulation of complex prismatic parts comprising any combination of machining features like steps, slots, pockets, nested features, etc., using a feature based milling simulation approach; (ii) Predicts the workpiece non-linear behavior during machining due to its changing geometry, inelastic material properties and fixture-workpiece flexible contacts; (iii) Allows the modelling of the effects of initial residual stresses (residing inside the raw stock) on part deformations; (iv) Incorporates an integrated analytical machining load (cutting force components and average shear plane temperature) model; and (v) Provides a seamless interface to import an automatic programming tool file (APT file) generated by CAM packages like CATIA V5. The prediction accuracy of the model was validated experimentally and the obtained numerical and experimental results were found in good agreement. (C) 2007 Elsevier Ltd. All rights reserved.

Finite element method based machining simulation environment for analyzing part errors induced during milling of thin-walled components

This paper gives a bibliographical review of the finite element methods applied to the analysis and simulation of welding processes The bibliography is an addendum to the finite element analysis and simulation of welding: a bibliography (1976–96) published in Modelling Simul Mater Sci Eng (1996) 4 501–33 The added bibliography at the end of this paper contains approximately 550 references to papers and conference proceedings on the subject that were published in 1996–2001 These are classified in the following categories: modelling of welding processes in general; modelling of specific welding processes; influence of geometrical parameters; heat transfer and fluid flow in welds; residual stresses and deformations in welds; fracture mechanics and welding; fatigue of welded structures; destructive and non-destructive evaluation of weldments and cracks; welded tubular joints, pipes and pressure vessels/components; welds in plates and other structures/components

Finite element analysis and simulation of machining: an addendum: A bibliography (1996–2002)

The dimensional tolerance of flexible, thin-walled aerospace parts can be violated by the excessive static deflections during milling. This paper proposes a method to predict the dimensional surface form errors caused by deflections of both flexible workpiece and slender end-mill in five-axis flank milling of thin-walled parts. The end-mill is modeled as a cantilevered beam. The stiffness of the thin-walled part varies as the metal is removed and the tool-part contact location changes. The time varying stiffness of the thin-walled part is predicted by an efficient structural stiffness modification method that only needs the FE model of the initial workpiece and avoids re-meshing the part at each cutter location. The cutting forces are distributed over both the cutting tool and the part in the engagement zone, and the effect of deflections on the immersion is calculated. The effect of radial runout of the tool is considered in chip thickness, hence in the cutting force prediction. Finally, the cutter and the workpiece deflections are considered to predict the surface errors left on the finished part. The proposed method has been proven in five-axis blade milling experiments.

Surface form error prediction in five-axis flank milling of thin-walled parts

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.

Chung-Li Liao

Papers

Finite-element modeling of static surface errors in the peripheral milling of thin-walled workpieces