The quaternion spherical linear interpolation method (slerp), which many researchers have adopted for numerical control, is inadequate for tool-axis-orientation planning in general five-axis computer numerical control machines. This paper explains why the quaternion slerp method fails to achieve constant rotational speed. The machining quality is degraded when the quaternion slerp method is used in five-axis machining. To compensate for this failure, this paper proposes a solution for tool-axis-orientation planning, the five-axis slerp method, which generates steady angular velocity of the interpolation curve. © 2018, Chinese Mechanical Engineering Society. All right reserved.