The processing stability of machine tool system is studied in this work. In terms of the mass distribution and the need of sufficient energy to keep chatter, it is most possible that the chatter will occur on the final executive component of a machine tool. Based on it, the machine system may be simplified as a regenerative chatter model with double elastic bodies. This new model can adapt to improvement of component structure well. When there is a big difference between the receptances of two elastic bodies, the one with bigger receptance should become the active body of chatter, namely it is the weak one of the system. When the receptances of two elastic bodies are close, then the structure of two bodies should be optimized.