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The downhole positive displacement motor (PDM) plays a very important role in petroleum drilling engineering. The rotor-stator coupling is the key element in PDMs. Normally both the rotor and the stator are constituted of cylindrical helix surfaces. 3-axis and 4-axis NC milling machines are usually used to machine these helix surfaces. The current CNC milling technology for manufacturing large and complex spiral curved surfaces still retains the drawbacks such as 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 main strategy of the whirlwind milling method is to regulate the directions of principal curvatures of the surface being machined with those of the rotational surface of milling cutter so as to generate a perfect contact fit. Then, it can avoid the over-tolerance and gouging scores and enhance the productivity.