Mechanics, dynamics, and stability of orthogonal turn-milling operation

Official URL: https://risc01.sabanciuniv.edu/record=b2767162_(Table of contents)

As the demand for higher quality and productivity increases in industry, multi-tasking machine tools attract increasing attention due to their ability to produce complex parts in a single set-up. The mill-turn machining center is a multi-tasking machine tool capable of performing a variety of machining operations simultaneously, including turning, drilling, boring, and multi-axis milling. As a multi-axis machining operation, turn-milling is a combination of milling and turning processes, in which the material is removed as a result of simultaneous rotations of the cutter and workpiece and translational feed of the tool. While turn-milling offers several advantages in manufacturing large-scale parts with hard-to-cut materials, it presents specific challenges in terms of surface form errors, process mechanics, and dynamics. Improper selection of process parameters, tool geometry, and eccentricity may result in undesired form errors and excessive cutting forces leading to workpiece, tool, and machine component failures. Moreover, self-excited chatter vibration may occur, leading to poor surface finish and tool failure. In this study, process kinematics and cutter-workpiece engagement are modeled for orthogonal turn-milling. A novel mathematical uncut chip geometry model for the side and minor edges of the tool is presented. Based on the chip geometry and cutting kinematics, a guideline is developed to avoid surface form errors, namely cusps, while increasing productivity. The cutting forces resulting from minor and side cutting edges are calculated analytically and verified through experiments. The effect of eccentricity on v cutter-workpiece engagement and cutting forces is presented. A fully analytical model is developed to predict the stability of orthogonal turn-milling in the discrete-time and frequency domains for the first time in the literature. In this regard, the regenerative dynamic chip thickness in feed, cross-feed, and axial are modeled, and the corresponding directional coefficients are formulated mathematically. A novel approach is proposed to calculate the varying time delay caused by the simultaneous rotation of the tool and workpiece. The stability diagrams are computed by solving the coupled time-varying delayed differential equations using semi-discretization and zero-order approximation methods. The effect of eccentricity on process stability is discussed for both end mills and inserted tools. The process parameter selection approach is proposed to achieve the highest stable depth of cut and cusp-free surface. The verified models for the mechanics and dynamics of orthogonal turn-milling are generalized to implement serrated and crest-cut tools. The cutting forces are calculated analytically using the updated cutter-workpiece engagement model and verified experimentally. The stability of orthogonal turn-milling using crest-cut tools is predicted in both the discrete-time and frequency domains for the first time in the literature. Another novel study is performed to study the effectiveness and performance of standard, variable-pitch, and crest-cut tools on chatter suppression in milling thin-walled parts. The novel stability maps are generated based on varying stability limits caused by in-process workpiece dynamics. Using the obtained stability maps, the performance of different cutting strategies is compared, considering productivity and surface finish quality. As the main contributors to the stability of a process, the dynamics of spindle and workpiece assemblies are modeled analytically and verified through experiments. The spindle shaft dynamics are modeled based on receptance coupling theory. Then a predictive bearing dynamics model is coupled with the shaft’s model using the structural modification technique. The model can predict spindle dynamics at different speeds. A similar approach is used to model in-process cylindrical workpiece dynamics considering contact mechanics. This thesis proposes comprehensive physics-based digital models of orthogonal turn-milling that predict the most productive cutting conditions with improved part quality for different types of tools. The presented models encompass the process parameters as well as the machine tool structural dynamics. The presented models can be used in industry either at the process planning stage to avoid costly physical trials or during the process for monitoring and fault-detection purposes.