Method of machining a rotor with variable-lead screw

A method of machining a first rotor and a second rotor, with a formed tool, wherein the formed tool has a specific shape to impart a specific profile, wherein the first rotor and the second rotor have mutually complementary meshing threads, the first rotor being made from a first workpiece, the second rotor being made of a second workpiece, the method comprising:

• rotating the first workpiece about a longitudinal axis of the first workpiece;
• making one or more passes of the formed tool along the longitudinal axis of the first workpiece as the workpiece rotates so as to remove material to create the specific profile imparted by the specific shape of the formed tool to form the first rotor, thereby forming the flanks of each helix of the first rotor's thread, the value of at least one of the parameters that collectively define the relative position and relative movement of the first workpiece and formed tool being varied during each pass so as to vary the lead of the thread;
• and
• repeating the above steps for the second workpiece, thereby forming the second rotor, wherein adjustments are made to at least one of said parameters during one or more of the passes in order to maintain mutually complementary shapes of the threads of the rotors;
• wherein the adjustments made during each pass are made according to a global adjustment distribution, each global adjustment distribution being determined in advance by:
• theoretically calculating a clearance distribution that would e produced by counter-rotating a hypothetical pair of meshed first and second rotors of the shape that would be produced by machining them without any adjustments being made, at a plurality of locations along the longitudinal axes of the meshed rotors;
• deriving a location-specific adjustment set for each of said locations, each location-specific adjustment set comprising one or more parameter adjustments which would cause the clearance distribution at that location to be maintained within a predetermined range of values;
• and
• collating the adjustments to each parameter according to the location-specific adjustment sets.