Screw rotors play a crucial role in the performance of compressors. For the large-batch production of small- or medium-sized rotors, continuously rotor hobbing or grinding may be more efficient than form machining. In this study, a general mathematical model was developed for the generating machining of screw rotors with a worm-shaped tool. A two-parameter enveloping theory was applied to simulate the cutting process as the tool conducts polynomial feed motion considering its cutting edge. The normal errors of the generated cutting lines were computed and presented on the rotor tooth surface topologies to show the correctness and practicability of the proposed model.