Modular design and multi-objective performance optimization of an extended mechanical spindle for DH horizontal machining centers

Conventional horizontal machining centers face critical limitations in machining large-scale complex workpieces due to insufficient spindle extension capability. Existing extended spindle designs often suffer from severe stiffness degradation, increased vibration, and thermal deformation, which significantly compromise machining accuracy and stability. While extensive research has been conducted on spindle dynamics, few studies have systematically addressed the coupled stiffness-dynamic-thermal performance trade-offs in extended mechanical spindles for heavy-duty applications. To fill this gap, this paper presents a modular extended mechanical spindle design for DH series horizontal machining centers. A stepped hollow spindle shaft structure is proposed to balance bending stiffness and rotational inertia, and a combined bearing arrangement with optimized preload is adopted to enhance support rigidity. A multi-objective optimization framework integrating finite element analysis (FEA) and response surface methodology is developed to optimize key geometric parameters, including spindle diameter distribution, bearing spacing, and extension interface dimensions. Comprehensive simulations and experimental tests demonstrate that the optimized spindle achieves a 57.1% reduction in tool tip displacement under rated cutting load, a 12.3% increase in fundamental natural frequency, and significantly improved thermal stability. The proposed design effectively expands the machining range while maintaining high precision, providing a practical engineering solution for advanced horizontal machining centers.