The rotors of three screw pumps are commonly machined using shaped milling cutters. The determination of the exact shape of the cutter is very important, since a high precision in the machining is required to obtain a high volumetric efficiency of the pump. This paper describes a method to determine the theoretical shape of the cutter, starting from the characteristic parameters of the pump. The rotors are modeled in space by helicoids. Then, the contact line between the tool and the workpiece is determined and this allows us to define the exact cutter profile, with a suitable reference system transformation.
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