Abstract: A dynamic model of a mid-convex variable-elliptic piston CNC machining system (XJOC-2 type) was established through theoretical analysis. Its dynamic characteristics and their influence on contour machining errors were studied through vibration experiments. Corresponding improvement measures and compensation strategies were proposed to ensure good machining performance of the system. Finally, actual cutting and burring experiments were conducted on a typical piston, achieving excellent results. Keywords: Piston; Surface machining; Error; Dynamic characteristics; Vibration [b][align=center]Research On system dimensions of piston NC machine tool LIU Tingzhang LI Baoming[/align][/b] Abstract : Pistons with a mid-convex contour and variable-ellipticity cross sections are machined through high-frequency dynamic unidirectional turning. In this paper, based on the XJOC-2 piston NC Machine Tool, the kinetic model is established by theoretical analysis. The dynamics and its influence on the contour machining error are studied by dynamic excitation trials. Then some improving profile and compensation strategies were suggested to improve the performance of XJOC-2 piston NC Machine Tool. It is shown, by experiments of various pistons, that these methods effectively eliminate the contouring error. Key words: piston; profile machining; error; dynamics excitation 1 Introduction Currently, advanced internal combustion engine piston skirts generally adopt a convex-elliptical curved surface, which can greatly improve the fuel economy, emissions, and NVH (noise, vibration, and ride roughness) of internal combustion engines. This skirt has a convex longitudinal profile and an elliptical cross-section, and the ellipticity changes linearly with the skirt height. It can generally be machined by special machining methods, such as the imitation method, the kit method, the eccentric motion control method, and CNC turning. The machining of this complex profile requires: the tool to perform high-frequency reciprocating motion at a frequency higher than twice the spindle rotation to form an elliptical cross-section; the tool to continuously change the depth of cut during the machining process to form a longitudinal nonlinear baseline. This results in complex characteristics in the machining process: the high-frequency motion of the tool and the nonlinear variation of the depth of cut throughout the machining process cause the tool to have different positions and velocities at various machining points. This frequent variation inevitably excites the inherent nonlinear components of the mechanical system (such as friction, backlash, hysteresis, saturation, and dead zones), thus the entire machining process exhibits significant nonlinear dynamic time-varying characteristics. Since high-speed cutting methods are currently mainly used for piston surface machining abroad, the system's operating frequency is often close to its low-order natural frequency. This can induce resonance in the machining system near its natural frequency and affect the amplitude of the tool's reciprocating motion. Therefore, for piston machining systems, dynamic characteristics are a key factor affecting their machining performance and accuracy, making the study of system dynamic characteristics of great significance. Preliminary experiments on a specific piston using our self-developed XJOC-2 type medium-convex variable elliptical piston machining system also show that although the system can achieve medium-convex variable elliptical surface machining, the contour error is relatively large. Therefore, this paper studies the dynamic characteristics of this CNC system and proposes improvement measures and compensation strategies based on this research. [b][align=center]For details, please click: Research on Dynamic Characteristics of Machining System for Medium-Convex Variable Elliptical Piston[/align][/b]