Based on the extended Hamilton principle, the finite element model of lateral vibration of drill- ing shaft is established using Timoshenko element model. Under the condition of maintain nonlinear anal- ysis precision, the linear degrees of drilling shaft system will be truncated using mode synthesis technique with free-interface but its nonlinear degrees are retained in the physical space, and simultaneously, the e- liminated eigenmodes are compensated with the residual attachment modes. So the DOF of the coupled system is reduced. Based on the updated model, the natural frequency influence rules of the drilling shaft system, as changing the distance of the intermediate supports, the length of drilling shaft and its the inner diameter, are discussed. By drilling the structural style of intersection hole as an example, the dynamic stable region and instability modes of the drilling shaft system are analyzed under the different rotating speed and drilling depth in drilling process. The numerical results show that the proposed methods for the dynamic design of complex drilling deep hole machine and the analysis of machining accuracy provide a theoretical basis.
