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The positive displacement motor (PDM) is a very important downhole motor in petroleum exploration and production. The key element in PDMs is the rotor-stator coupling which converts hydraulic potential of high-pressure fluid medium into mechanical energy. Normally both the rotor and the stator are constituted of cylindrical helix surfaces. The PDM's characteristics directly rely on the machining quality of rotor-stator coupling. 3-axis and 4-axis NC milling machines are usually used to machine the rotor helix surface or helix surface of stator mandrel, which is an injecting molding part utilized to shape the internal cavity of rubber stator. The current CNC milling technology for manufacturing large and complex spiral curved surfaces still retains the drawbacks such as consuming time of long duration, over-tolerance and gouging scores. A new 5-axis CNC machining scheme with the whirlwind milling method is proposed based on 5-axis CNC rotary-table horizontal milling machine specialized. The important strategy of the whirlwind milling technique is to optimize the directions of principal curvatures of the surface being machined with those of the rotational surface of milling cutter to generate a perfect fit. So, it can avoid the above-mentioned shortcomings and enhance the productivity. Some analytical characteristics of helical surfaces are discussed. The fundamental mathematical equations on whirlwind milling technique and its implementation are introduced.