This paper presents the development of a mathematical model describing the stiffness of a Stewart-platform-based milling machine. Matrix structural analysis is used to derive the stiffness matrix for each of the elements in the model and assemble them into a system-wide stiffness matrix. By incorporating the inverse kinematics of the machine tool, the system model is used to visualize the stiffness variation over the mill's workspace. Estimation of the system parameters is conducted through experimental stiffness measurements. Computer simulation is used to demonstrate how the developed stiffness model suggests an optimization process for tool-path planning.