The present paper describes the role of the tool in vibrations of a machining system. The cutting force has been assumed to be constant. The shear deformation of the tool is considered. The quasi-static analysis of the situation yields a maximum allowable uncut chip thickness, which shows how the frictional interaction at the tool face and the ratio of the components of cutting forces alter this value. The relationship also expresses the effect of tool dimensions and work material on the vibration of the tool.