How model worm (perpetual screw ) with variable pitch in SolidWorks?

I need to model worm with variable pitch (pitch came to zero !!) in SW 2012.
I can model worm with constant pitch.
I can model worm with variable pitch but:
when pitch is going under some value the operation is not succesful.....-
I add 4 pictures:
1. oryginal worm that i must to model
2. worm with constant pitch - operation OK
3. worm witch variable pitch - operation OK
4. error window when helix modified - pitch came to small values.

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