The main shaft overhang a refers to the distance from the front end face of the main shaft to the middle point of the radial reaction force of the front support (generally the middle point of the front radial support), which is mainly determined by the structural design. However, a value has a great influence on the stiffness and vibration resistance of the main shaft components, so when determining a value, the value should be as small as possible on the premise of meeting the structural requirements. According to the structural requirements, the overhang a of xk2116 CNC gantry boring and milling machine spindle is 0.1M. Its structural layout has the following characteristics: