The roller gear cam surface is manufactured with nonequivalent milling method in many cases, by which the machining error is unavoidable. In order to simplify the tool position, we can transform one problem approximating the designed surface with the tool envelope surface to another problem approaching the theoretical tool axis trajectory surface with the actual one. Furthermore, the theoretical tool axis trajectory surface, which is an offset surface of the designed cam surface, is reconstructed by the NURBS ruled surface. In order to find the best tool axis vector to minimize the machining error, a simple least square approximation method is established to figure out all the control points of the NURBS tool axis trajectory surface. A numerical calculation and simulation example is described to verify the effectiveness of the tool position method proposed in the paper.