The four-axis NC configuration is suitable for the manufacture of a part family (called a rotationally free surface, or RFS) requiring concurrent motions for the tool movement and part rotation. Algorithms are developed for handling tool interference in four-axis machining in which three types of interference exist: interference due to step size, tool size, and tool axis. The first two types are also present in three-axis machining, but the third type is unique and critical in four-axis machining. The performance of the algorithms is tested by graphic simulation and actual cutting. The results show that the algorithms can be readily incorporated into existing CAD/CAM systems as a module for interference-free four-axis NC machining of an RFS.>