A computer numerically controlled (CNC) milling center is a machine tool for the production of parts with planar, cylindrical and shaped surfaces. The milling center analyzed here includes an open frame — a structure resembling the shape of the letter C. The main cutting motion is performed by a tool clamped in the spindle. Secondary motion can be linear, rotary or a combination of these. Linear movements in three axes are performed by the tool by means of linear motion components (i.e. motion screws and linear guide rails). Rotary motion is performed only when the workpiece is clamped to the rotary table which is mounted on the mounting plate.

The basic demands placed on the structure of a milling center include high static and dynamic stiffness during machining processes. This article is primarily aimed at evaluating the response of the frame of the CNC milling machine to the excitation caused by the fluctuation of cutting forces due to step changes in the number of engaged cutting edges. To ensure optimum machining conditions it is important to set suitable cutting conditions for a frame structure with sufficient stiffness. Unsuitable cutting conditions and low stiffness of the machine frame may lead to dimensional inaccuracies of the workpiece, to decreased quality of the machined surfaces or even to the destruction of the tool cutting edges.

The aims of the study include the determination of the static deformation, modal analysis to assess the dynamic properties of the frame, and harmonic response analysis, taking into consideration the amplitudes of the loading forces specified in accordance with the recommended operating conditions of the individual tools.

Finite element method (FEM) analyses of the frame were performed using MSC.Marc software. Due to the high structural complexity of the computer aided design (CAD) model, the computational model for the FEM analysis had to be simplified. Only the major structural parts and the connecting parts were meshed in detail, combining both structured and unstructured mesh. Geometrically complicated cast parts with large changes of thickness were meshed with linear tetrahedral elements (tetra4) with full integration. Rotationally symmetrical parts, plates and linear guide rails components were meshed with linear brick elements (hex8) with full integration. The overall number of elements was approximately 1,400,000. Tools, including the clamping head and the spindle, are represented by approximate meshes of brick elements. However, a detailed FEM model of the spindle and the tool would be needed for the analysis of the self-excited oscillations during machining, which is the subject of a large number of scientific publications.

Increased attention was paid to the incorporation and set-up of the springs between corresponding pairs of nodes of the meshed linear motion components. As a computational model for modal and harmonic response analyses needs to be strictly linear, only linear elastic material properties and linear springs were defined in the analyses presented here.

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