How to create variable spring curve

I want to make an extruder screw which is used in rubber industry. This screw has three areas with specific pitch in each area. All I need is to create a continuous spring curve with three different pitches.
I mean this spring curve needs to be more dense in one end and less dense in the other end.

Hi Alex - one way to do this is to CageEdit a helix with say. a degree 3 or 5 curve on its axis. See the attached file - turn on points for the white line and slide them up and down.
VariableHelix.3dm (241.1 KB)


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Thank you Pascal Golay, It was helpful.
How can I create a helix which has a low diameter in first end and a high diameter in the second end?