how to convert curve + helix sketch to unfold state

First of all you could create from the beginning everything in sheet metal even if it is not metal, to use unfold you would have to be original sheet metal or turn the piece into sheet metal, you really only occupy the length, although I understood you, so you should only add the length of all the arcs and you save the unnecessary work since it is not a piece of sheet metal in which you need the file flat, dxf or dwg for a laser cutter or similar. you have a total of 5710mm = 18.7336FT, the maximum to cut in a laser cutter is 12 ft, in a few words you do not occupy a flat file

Angel is right, if all you need is the length. No sense wasting a lot of time, when the answer is right in front of you.

But I have two suggestions:

1. The curve + helix should be drawn in the center of the leather width to provide a more accurate length.

2. If you do make the sheetmetal part, leave a small gap (.001mm) between the spirals

I have attached a tutorial I made for a clockwork spring. Using sheetmetal tools and for Alibre Design software. This method may provide what you require to model the leather coil in Solid Works with a flatenable part.

Thanks for the feedback.
I need to add some cut on this part, so flattening was a shortcut to achieve the final shape.
Lawrie thanks for the tutorial, I will try to reproduce your tutorial on solidworks, but I believe some function inside Alibre are not present into solidworks like "BendOnly".