First, I would like to thank you for this great forum and all the people involved.
Regarding the problem of the arching in this self-adjusting spine system, I would like to offer some ideas I had recently.
Why don't you keep the string the closest as possible to the spine of the book?
I thought you could drill the cradle wings, and let the string pass through the drill.
The 2 drills should be done in the top of the lowest edge of the cradle wing. One drill in the middle of each of the wings.
As my English is a little poor, maybe some drawings would make myself clearer.
scanning first page
The drills are the hidden lines.
in the middle of the book
scanner in stand-by or scanning very thin material/loose paper
As my drilling habilities are equal to zero, I don't know of the possible difficulties in making these 2 drills, so maybe this is just a crazy idea.
I came with another idea too. Maybe you have noticed some kind of triangle piece just in the middle of the cradle wings, in the previous drawings.
I have added it so it would help the sytem rest when scanning thin material or loose paper, and let the system fit perfectly when in the lowest position.
Or keep the whole cradle steady when not in use, or while adjusting things, the string...
You can see this piece in this drawing. See that space in the middle: is occupied by the string, passing through. The amount of wood you remove from there and the way you do it is up to you.
Again, it's very easy to draw this piece, but I don't know how you would cut it in the real world.
And now from the top:
Another alternative would be to use a surface instead of a string
surface alternative_first page
surface alternative_middle of the book
surface alternative_view from the top
In the right cradle wing (red clamps) you would use adjustable clamps/L brackets (something like the ones used in the glass fixing in Daniel's assembly video in youtube http://www.youtube.com/watch?v=gdPOAcMi4pw
) to let more or less surface to be in the middle of the cradle wings (depending on the spine of the book). You would adjust them each time you scan a different book (books with different spine thickness)
Same clamps in the left cradle wing (clamps in yellow). In this case, the clamps would be fixed permanently.
Note that with this surface solution, the triangle piece doesn't need to be cut in the middle
Also note that the clamps (L brackets or whatever other fixing system you use, you name it) should be place in the lowest part, on the edge of the cradle wings. In the top of the wings you can place them wherever you want.
Maybe we could save materials by clamping just in the lowest part of the wings? (two yellow clamps in the lowest part of the left wing + two red clamps in the lowest part of the right wing)
How this surface will wear on those edges? I don't know...
So the surface needs to have some thickness (maybe a drawer liner?)
Ideally it would be kind of sticky, and black matte color.
And that was my brainstorming for today!
Finally, I guess I had the idea of this system right, but I haven't seen a video of it
So jck57 would you please post a short video?