A method is presented to evaluate the workspace variation of a Stewart platform based machine tool. Four sets of constraints, covering strut lengths, platform spherical joint angles, base spherical joint angles, and strut collisions, are formulated using inverse kinematics. Recognizing the need for varying the platform orientation during machining, an algorithm to efficiently calculate the workspace is developed. Computer implementation provides a powerful tool to study the dynamic variation of the workspace as the spindle platform rotates away from the horizontal orientation. A case study is presented on the workspace variation of an Ingersoll Octahedral Hexapod machine tool during machining. The results demonstrate the shift in size and location of the workspace as the platform orientation changes. Guidelines for NC coding are suggested to maximize the versatility of Stewart platform based machine tools, while avoiding the violation of constraint conditions on the workspace.