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I created some shape using 6 pixels stroke (no fill), then I expanded it and get the desired shape. Later, I need the same shape with thinner lines, but resizing does not work well for it.
So, the question is - Is there any way to get stroke (i.e. line connecting middle of thickness of the expanded line) from expanded shape?
(I cannot redo the actions because move far away from the expanding the path).
CS6
You could try this:
Your expanded stroke should be a compound path. So..
Using Object → Compound Path → Release. This will hopefully leave you with two shapes:
Each shape should be 3px either too small or too large. You can delete one of them (I deleted the smaller shape) and use Object → Path → Offset Path... to "shrink" it. If you use the larger shape, enter -3px.
This will create a "shrunken" shape for you that is hopefully what you need:
There are probably some cases where this will not work, I'd guess that it depends on how complex your expanded stroke is.
There is no method to recover the exact spline from an expanded path. Here's a link to Mike Kamermans' excellent Primer on Bézier Curves that explains the math with interactive examples.
The gist: You cannot compute the exact offset of Bézier Curves - Illustrator has to "guess" the best answer it can. If you're trying to recover a path from its offset, you'll be "guessing" twice.
The most accurate path you can generate will come from using a negative offset, as per JohnB's answer. Just be sure to use slightly less than half of the original stroke width (6pt stroke -> -2.9pt offset) to account for the errors introduced. Finish by using the Cut Tool to isolate one side of your offset path.
